Study of tribological properties of titanium-based thin films applied to the rubbing parts of internal combustion engines piston

PEOPLE'S DEMOCRATIC REPUBLIC OF ALGERIA
Ministry of Higher Education and Scientific Research

M'Hamed Bougara University Faculty of Hydrocarbons and Chemistry

Department of Hydrocarbon Transportation and Equipment
Master Degree Dissertation

Field : Technical Sciences
Sector : Hydrocarbons
Specialty : Mechanical Engineering
Option : Oil Fields Mechanics

Presented by: CHEMAA Khaled

CHABANI Yacine

THEME:

Study of tribological properties of titanium-based thin

films applied to the rubbing parts of internal combustion

engines piston

Defended publicly on the 07/04/2017

Before the jury:

Gaceb Mohamed (Professor UMBB) President of the jury

Gaceb Mohamed (Professor UMBB) Framer

Hassani Salim (Research Fellow CDTA) Co- framer

Examiners member:

Benbrik Abderrahmane (Professor UMBB) Examener

Harhout Riyad (Assistant Professor UMBB) Examener

Boumerdes 2017

«Allah will raise up to

ranks, those of you

who believe and who

have been granted

Knowledge»

The Holy Quran [58:11]

II

Acknowledgement

Acknowledgement

First of all we want to thank the almighty Allah for the blessing of health, knowledge and reason, and for huge opportunities gave us during our lifetime;

We want to thank our parents for the great sacrifices they made for us;

One of the biggest opportunities of Allah is that he made us studying in the faculty of hydrocarbons and chemistry of BOUMERDES (ex-INHC), where we had the pleasure of meeting great people, who sacrifice by their money, their time and fight just to make that historical monument (which was created in 1964) rise again despite the huge difficulties. People like Pr. M. TAIBI, Pr. S. AISSANI, Dr. A. GUEBLI, Pr. A. ABBASSOV, Pr. A. BENBRIK, Mr. T. HACHEMI Pr. A. BLAIDI and others, taught us indirectly what a man can do when he has the great will and the good well to change his surrounding society;

We want to thank, particularly Pr. M. GACEB our dissertation framer, and our honorable professor, first of all because he awakened us when we were in a deep sleep, then he guided and helped us (and anyone who seek for knowledge and they are few) with everything he can for the sake of science, we wish him good health to help our successors for years to come;

We want to thank Dr. S. HASSANI our dissertation co-framer and the responsible of Laser and Ionized Medium at the CDTA center, it was a great opportunity to work with someone who have such a knowledge, despite the short the time which didn't allow us to learn more from his great knowledge and experience;

We want to thank all people who help us to fulfil our dissertation, beginning by Pr. Abdi from the Mechanical Engineering Department of the USTHB who allowed us to use their tribometer, our professor Dr. A. GUEBLI who enriched us with his valuable advice, Mr. DRISS of CDTA Center who helped us with the thin film deposition process, and all the CDTA members and staffs;

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Abstract:

Because of their nanometric dimensions, thin films have a very high specific, a very high reactivity and exceptional mechanical, optical and electronic properties.

The application of this technology in the internal combustion engine industry reduces losses of friction and wear with a low loss of material, which directly influences on the cost, whether during production phase or during the service.

Keywords: Thin film, physical vapor deposition (PVD), tribological properties, pistons, internal combustion engines.

Résumé:

De par leurs dimensions nanométriques, les couches minces présentent une très grande surface spécifique, une réactivité très élevée et des propriétés mécaniques, optiques et électroniques exceptionnelles.

L'application de cette technologie en industrie des moteurs à combustion interne permet de réduire les pertes par frottement et usure avec une faible perte de matière ce qui influencer directement sur le coût, que ce soit a la phase de production ou de l'exploitation.

Mots clés: Couche mince, dépôt physique en phase vapeur (PVD), propriétés tribologiques, pistons, moteurs à combustion interne.

IV

List of symbols

List of symbols

??: Coefficient of linear expansion, Degree of ionization ratio, Stricking coefficient;

????: Melting temperature;

??: Thermal conductivity, mean free path, the wavelength;

??: Density;

????: Electrical resistivity;

??: Stresse;

u: Poisson's ratio;

??: Constant of friction;

w: Wave frequency;

?? : Molecular diameter;

A: Area;

A%: Breaking elongation;

B : Number of intermolecular impacts;

D??: Distance;

E: Modulus of elasticity; E??: Kinetic energy;

E??: Dissipated energy;

F: Force, load;

??: Rate of return of energy;

h: Planck constant;

H: hardness;

HB: Brinell hardness;

HV: Vickers hardness;

icorr : Corrosion current density;

k : Boltzmann constant;

????: Knusden number;

??: Molecule mass;

N: Molecules number;

?? : Number density of the molecules;

??: Pressure;

??0 : The pressure at the initial instant;

????: Saturated vapor pressure;

Q: Mass flow rat;

??: Radius;

??: Radius of the wear track;

????: Breaking strength;

R_e, Re0.2: Yield strength;

??: Time;

T : Temperature;

V

List of symbols

??: Rate of propagation of pits, molecules speed; ?? : The volume;

v??² : The-root-mean-square speed of molecules; ????: The most probable speed of molecules; W??: Wear rate;

VI

							Books list

Books list

[1] Aide-mémoire Science Des Matériaux, Michel DUPEUX, Dunod 2008;

[2] Aide-mémoire Métallurgie, Guy MURRY, Dunod 2010;

[3] Aluminum Alloys: Structure and Properties, L. F. MONDOLFO, Butterworths London, Boston 1976;

[4] Caractérisation par spectrométrie Raman? Sébastien REYMOND-LARUINAZ, université de bourgogne;

[5] corrosion aspects cinétiques de la corrosion, Université du sud Toulon-Var- institut des sciences de l'ingénieur de Toulon et du var;

[6] Dépôts physiques : techniques, microstructure et propriétés, Lech PAWLOWSKI, Presses Polytechniques Et Universitaires Romandes 2003;

[7] Introduction à la Diffraction par Rayon X, Simon HAZIZA and Emmanuelle DELEPORTE, école normale supérieure paris-Saclay;

[8] Lubrifiant et fluids pour l'automobile, AYEL, M.BORN, French oil institute,Technip edition;

[9] Nanomatériaux, traitement et fonctionnalisation des surfaces, Jamal TAKADOUM, Lavoisier 2008.

[10] Physique Des Plasmas Collisionnels, Michel MOISA, CNRS Éditions 2006;

[11] Physique des plasmas tome 1, Jean-Loup DELCROIX, CNRS Éditions 1994;

[12] Pistons and engine testing, MAHLE GMBH, springer vieweg - Stuttgart Germany;

[13] Techniques de l'Ingénieur : Traité Matériaux Métalliques Aluminium et alliages d'aluminium corroyés : propriétés métalliques, Roger DEVELAY;

[14] Techniques de l'Ingénieur : Traité Matériaux Métalliques Données numériques sur l'aluminium non allié, Roger DEVELAY;

[15] Techniques de l'Ingénieur : Traité Matériaux Métalliques Données Numériques sur les Alliages d'Aluminium de Transformation, Roger DEVELAY;

[16] Techniques d'ingénieur : Traité Matériaux Métalliques traitement de surface des métaux par voix sèche et en milieu fondu, Sylvain AUDISIO;

[17] Techniques de l'Ingénieur, traité Matériaux métalliques, Lubrification des surfaces lors de la mise en forme, M 597, E. Felder;

[18] Traité des matériaux Tome 4 : Analyse des surfaces, Collectif, Presses Polytechniques, Universitaires Romandes 2003;

[19] Traitements et revêtements de surface des métaux, Robert LEVEQUE, Dunod 2007;

[20] Vide poussé au laboratoire et dans l'industrie, G. GRIGOROV, Edition Masson & Cie 1970.

VII

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X

Figures list

List of Figures

Fig.1.1 MAHLE GMBH-pistons and engine testing 2^nd edition- springer vieweg - STUTTGART

GERMANY.

Fig.1.2 MAHLE GMBH - pistons and engine testing 2^nd edition-springer vieweg-STUTTGART

GERMANY.

Fig.1.3 MAHLE GMBH - pistons and engine testing 2^nd edition-springer vieweg-STUTTGART

GERMANY.

Fig.1.4 MAHLE GMBH - pistons and engine testing 2^nd edition-springer vieweg-STUTTGART

GERMANY.

Fig.1.5 MAHLE GMBH - pistons and engine testing 2^nd edition-springer vieweg-STUTTGART

GERMANY.

Fig.1.6 MAHLE GMBH - pistons and engine testing 2^nd edition-springer vieweg-STUTTGART

GERMAny.

Fig.1.7 MAHLE GMBH - pistons and engine testing 2^nd edition-springer vieweg-STUTTGART

GERMAny.

Fig.1.8 J.AYEL - M.BORN - lubrifiant et fluids pour l'automobile- French oil institute- Technip

edition.

Fig.1.9 J.AYEL - M.BORN - lubrifiant et fluids pour l'automobile- French oil institute- Technip

edition.

Fig.1.10 J.AYEL - M.BORN - lubrifiant et fluids pour l'automobile- French oil institute- Technip

edition.

Fig.1.11 J.AYEL - M.BORN - lubrifiant et fluids pour l'automobile- French oil institute- Technip

edition.

Fig.1.12 https://expertsdefaillances.com

Fig.1.13 https://expertsdefaillances.com

Fig.1.14 https://expertsdefaillances.com

Fig.2.1. Aluminum Alloys: Structure and Properties / L. F. Mondolfo / BUTTERWORTHS LONDON-

BOSTON 1976

Fig.3.1. Vide poussé au laboratoire et dans l'industrie

Fig.3.2. Traité des matériaux Tome 4 : Analyse des surfaces

Fig.3.3. Techniques du vide/ Simone Cassette/ cours sur internet

Fig.3.4. Vide poussé au laboratoire et dans l'industrie

Fig.3.5. Dépôts physiques : techniques, microstructure et propriétés

Fig.3.6. Dépôts physiques : techniques, microstructure et propriétés

Fig.3.7. Techniques d'ingénieur : Traité Matériaux Métalliques traitement de surface des métaux

par voix sèche et en milieu fondu

Fig.3.8. Techniques d'ingénieur : Traité Matériaux Métalliques traitement de surface des métaux

par voix sèche et en milieu fondu

Fig.3.9. institut-numerique.org

XI

Figures list

Fig.3.10. institut-numerique.org

Fig.3.11. institut-numerique.org

Fig.3.12. Institut-numerique.org

Fig.3.13. Institut-numerique.org

Fig.3.14. Traité des matériaux tome 4 : analyse des surfaces Fig.3.15. Traité des matériaux tome 4 : analyse des surfaces Fig.3.16. Traité des matériaux tome 4 : analyse des surfaces Fig.3.17. Traité des matériaux tome 4 : analyse des surfaces Fig.3.18. Traité des matériaux tome 4 : analyse des surfaces Fig.3.19. Nanomatériaux, traitement et fonctionnalisation des surfaces

Fig.3.20. Nanomatériaux, traitement et fonctionnalisation des surfaces

Fig.3.21. Nanomatériaux, traitement et fonctionnalisation des surfaces

XII

Tables list

List of Tables

Table1.1. Lubrification par les composés organo-soufrés en phase gazeuse : Approche de la lubrification dans la coupe des métaux, Johny TANNOUS, Thèse de doctorat de l'école centrale de Lyon, 2007

Table2.1. Aide-mémoire Science Des Matériaux / Michel Dupeux / Dunod 2008 Table2.2. Aide-mémoire Science Des Matériaux / Michel Dupeux / Dunod 2008

Table2.3. Techniques de l'Ingénieur : Traité Matériaux Métalliques Données Numériques sur les

Alliages d'Aluminium de Transformation / Roger Develay

Table2.4. Techniques de l'Ingénieur : Traité Matériaux Métalliques Données Numériques sur les

Alliages d'Aluminium de Transformation / Roger Develay

Table2.5. Aluminum Alloys: Structure and Properties / L. F. Mondolfo / BUTTERWORTHS

LONDON-BOSTON 1976

Table3.1. Vide poussé au laboratoire et dans l'industrie

Table3.2. Traitements et revêtements de surface des métaux

Table3.3. Handbook of chemical vapor deposition

Table3.4. Techniques d'ingénieur : Traité Matériaux Métalliques traitement de surface des

métaux par voix sèche et en milieu fondu

Table3.5. Dépôts physiques : techniques, microstructure et propriétés

Table3.6. Nanomatériaux, traitement et fonctionnalisation des surfaces

Table3.7. Traitements et revêtements de surface des métaux

XIII

Table of Contents

Table of Contents

Acknowledgement II

Abstract List of symbols Books list References List of Figures List of Tables

III IV VI VII X XII

Table of Contents XIII

Introduction 01

Literature review 04

Chapter 1: Overview on engine's piston 06

1.1. Piston types 07

1.1.1. Pistons for four-stroke gasoline engines 07

1.1.2. Pistons for diesel engines 08

1.2. Different forms of wear 10

Chapter 2: Aluminum and its alloys 21

2.1. Property Of non-alloyed aluminum (1000 series) 22

2.2. Aluminum alloys 22

2.2.1. Different classes of aluminum alloys 23

2.2.2. Physical properties of aluminum alloys 23

2.2.3. Mechanical properties of aluminum alloys 23

2.2.4. Standards defining aluminum alloys 26

2.3. Aluminum-Silicon Alloys (4000 series) 28

XIV

Table of Contents

Chapter 3: Vapor deposition and thin layer characterization techniques... 30

3.1. Physical bases of the vacuum technique...31 3.1.1. Vacuum and gaseous phase of the material... 31 3.1.2. Movement and speed of gaseous molecules.................................................................. 32

3.1.3. Basics of vacuum technology............................................................................................ 34

3.1.4. Duration of evacuation process... 35

3.2. The theory of plasmas... 36

3.3. Coating in the vapor phase (PVD, CVD)... 37 3.3.1. Chemical vapor deposition (CVD) techniques... 38 3.3.2. Physical vapor deposition processes (PVD)... 39 3.3.3. Physical mechanism of a thin layer formation .................................................................. 41 3.3.4. Thin film morphology... 44

3.4. Methods of microstructural characterization...46 3.4.1. Chemical characterization methods... 47 3.4.2. Crystallographic characterization techniques... 49 3.4.3. Microstructural characterization methods... 50 3.4.4. Characterization of mechanical properties... 52

3.5. Properties of thin films deposits 56

Chapter 4: Experimental Procedures.... 57

4.1. Determination of the sample grade... 58

4.2. Preparation of samples... 59 4.2.1. Samples Cutting... 60 4.2.2. Samples polishing... 60 4.2.3. Chemical cleaning... 60

4.3. Thin film elaboration process... 60

4.3.1. Description of PVD sputtering installation and working parameters.............................. 61

4.3.2. Description of PVD evaporation installation and working parameters........................... 62 4.4. X-ray diffraction... 64

4.4.1. Characteristics of the device... 64

4.4.2. Working Principle... 64 4.5. Raman Spectroscopy... 65 4.6. Scanning electronic microscope (SEM)...66 4.7. The nanoindentation... 67 4.8. The tribometer analysis... 69 4.9. Electrochemical techniques... 71

4.9.1. Equipment... 71

4.9.2. Establishment of EVANS diagrams... 72

4.9.3. Diagram E=f (log/i): EVANS Diagram.................................................................................. 73

XV

Table of Contents

Chapter 5: Results and Analysis 75

5.1. Analysis of XRD results 76

5.2. Analysis of RAMAN results 78

5.3. Morphological analysis 81

5.4. Interpretation of nanoindentation results 82

5.5. Interpretation of tribometer results 87

5.5.1. The coefficient of friction 87

5.5.2. Wear Rate 91

5.6. Corrosion test results 93

Chapter 6: Solidworks mechanical analysis of the thin layer 95

6.1. Initial data 97

6.2. Finite element analysis of the sample.................................................................................... 97

6.3. Analysis of the result 100

Conclusion 101

Appendices AI 1

Appendix I: nanoindentation charge-discharge curves AI 2

Appendix II: Nanoindentation results . AII 1

Introduction

2

Introduction

Introduction

The thin layer technology has developed with extreme rapidity in the last two decades whether in terms of implementation technology or industrial applications.

In mechanical industry, the objectives of thin films are mainly to improve the corrosion or wear resistance, to reduce the coefficient of friction, to modify the surfaces microstructure to improve their ability to retain lubricants and to optimize the lubrication of the rubbing surfaces and the improvement of the aesthetic qualities of the products.

In internal combustion engines, the current trend is to save energy, that is to say, the reduction of fuel consumption for equal or higher power levels. To do so it is necessary to reduce friction losses between surfaces in contact and increase their durability.

Several studies have shown that friction losses represent the major part of the energy consumption developed in an internal combustion engine (15 to 20%). Piston skirt friction, segmentation and bearings constitute 66% of total friction losses; Distribution (up to 20-25% at low rpm), crankshaft, transmission and gearbox contributing to the rest. Beyond the aspects of reducing friction and wear, reducing oil consumption and exhaust emissions represent ambitious challenges.

In this work, we propose to apply a thin layer of Ti-W-N on the piston skirt and the piston crown in order to increase the surface hardness and its corrosion resistance and to reduce the coefficient of friction.

The present dissertation contains six chapters;

The first chapter gives a brief illustration of the role of pistons technology, and the main degradations and problems observed in pistons. In the second chapter we present the mechanical properties of aluminum alloys, the alloys with which the pistons are generally manufactured. In the third chapter we describe the different thin film deposition techniques, the tribological properties of the Ti, W and N layers, and the main deposition characterization techniques.

In chapter four we give the different experimental procedures executed in CDTA center, starting with the research of the probable piston grade used as substrates in our study, passing by the preparation of the surfaces of the samples and the deposit installations, and arriving at the characterization of the tribological properties of our Layer (by XRD, SEM, Raman, nanoindentation, tribometer and corrosion tests).

3

Introduction

In chapter five, we give the experimental results obtained with their interpretation.

The chapter six dedicated to a medialization of the thin layer, using Solidworks software, and we make mechanical test on it, in order to a better characterization of the layer.

4

Literature review

Literature review

Hardness of titanium base thin films

Hard coatings based on nitrides (TiN, CrN, AlTiN, etc.) or carbonitrides (TiCN) for mechanical applications came on the market in the mid-1980s. They are characterized by a high hardness (often higher than 20 GPa) and by great chemical inertia.

The deposited TiN/ZrN coatings have a very high hardness (> 30 GPa), much higher than the individual hardnesses of TiN and ZrN. It is noted that the hardness increases with the Zr content in the layers, which seems logical because the hardness of the ZrN is higher than the hardness of the TiN with the deposition installation and the conditions used. Another interesting result is the increase in hardness with the decrease in the individual thickness of the alternating layers.

TiAl(N,C,O) coatings exhibit a wear behavior considerably superior to the traditional DLC coating.

Corrosion resistance

It is well known that PVD layers generally have a columnar structure with numerous growth defects (porosities, grain boundaries, structural defects, etc.); thus, they do not provide adequate protection of the corrosion-sensitive substrates.

At present, the poor corrosion resistance offered by PVD coatings requires, on sensitive substrates, the deposition of a relatively thick electrochemically produced undercoat. The most commonly used solution is chromium or galvanic nickel with thicknesses of the order of 5-20

ìm.

To further improve the corrosion resistance, a Pd-Ni deposit is applied to the nickel before the final PVD deposition. This multi-layer process provides much better corrosion resistance, in particular, since the galvanic undercoat is much denser and does not have a columnar structure. However, today, and due to environmental considerations in some countries, there is an abandonment of galvanic coatings such as nickel, zinc, cadmium, chromium, gold and many other metals and alloys, unfortunately impossible to replace.

Other works describe the use of an ionic etching of Nb ions carried out by cathodic arc on ferrous substrate before PVD deposition [CHA 04; HOV 02, 05; REI07];

The TiAl(C,O) coatings represent an interesting improvement compared to the traditional Ti(C,O), especially since there nanohardness is close to 13 GPa. A SEM photograph shows a dense non-columnar and amorphous microstructure of this type of coating. This amorphous microstructure is particularly advantageous since it generally confers a higher resistance to corrosion of the substrate by preventing the electrolytes from passing through the coating by the intercolonial porosities.

5

Literature review

Thin films with oblique incidence and GLAD technical

The properties of thin layers depend primarily on their microstructure or their nanostructure. From the first experimental studies due to MOVCHAN and DEMICHISHIN [MOV69] and also to THORNTON [THO 74], to the recent structural models developed by simulation [ABE 97; MAL 96; TRO 03; WEI00], the majority of this work was mainly focused on the operating conditions influencing the morphology and structure of films deposited under normal incidence. Very few have been dedicated to thin films prepared under oblique incidence [DON 96; DIT 91; HOD 98; MBI 95; TAI 92].

All this work on the layers prepared under oblique incidence converges towards the same observation: an enlargement of the spectrum of the physicochemical properties of the deposited materials including their stress state, their density, their optical, electrical and magnetic anisotropy, etc.

The work of the teams of Brett and Robbie for the production of thin films under oblique incidence and on fixed or mobile substrate, allows the birth of a new technic: the GLAD technical: GLancing Angle Deposition.

The property of the GLAD technique in generating columnar architectures of various shapes at nanometric scales leads inevitably to questioning the mechanical behavior of these nanostructured films. Indeed, the structural shapes of some of these architectures (example: zigzags, spirals, etc.) give extended mechanical characteristics with respect to a conventional columnar structure.

Using the nanoindentation tests, the authors SETO et al [SET 01] clearly show that the helically-structured films have an elastic deformability of a higher order of magnitude compared with conventional columnar films.

Other studies on the elastic properties of GLAD films have also been carried out by LINTYMER and al [LIN 03a, 04, 05] on inclined and zigzag chrome layers. In particular, it has been demonstrated that the Young's modulus of zigzag films can be changed from single to double playing only on the number and size of the zigzags.

Economic considerations

It is customary to say that nanomaterials are expensive, but it must be shown that nanomaterials are additives to be used in small quantities, the cost per function can become minimal. For example, on a square meter, a 10 ìm coating will weigh about ten grams, adding 1% nanomaterial will lead to 100 mg of material, even with expensive material (1000 € / Kg for example), the cost of the coating is 1 € / Kg, which is negligible compared to the technical, technological and functional contribution of the layers thus realized.

Then it can be deduced that the use of nanomaterials in surface coatings is a technological reality, which is transforming itself more and more into economic reality.

Chapter 1

Overview on engine's piston

7

Chapter 1: Overview on engine's piston

1.1. Piston types

1.1.1. Pistons for four-stroke gasoline engines:

Modern gasoline engines employ lightweight designs with symmetrical or asymmetrical skirt profiles and potentially different wall thicknesses for the thrust and antithrust sides. These piston types are characterized by low weight and particular flexibility in the central and lower skirt areas.



Fig.1.1: Autotermique piston

1.1.1.1. Controlled-expansion pistons

Controlled-expansion pistons are pistons with struts that control thermal expansion. They are installed in gray cast iron crankcases. The main target of controlled-expansion piston designs, and many inventions in this field, was and still is to reduce the relatively large differences in thermal expansion between the gray cast iron crankcase and the aluminium piston

We distinguish in this category the Autothermic pistons, slotted at the transition from the piston crown to the skirt, at the height of the oil ring groove. They are characterized by their particularly quiet running behaviour.

The unalloyed steel struts cast in between the skirt and the piston pin boss, together with the light alloy that surrounds them, form control elements. They reduce the thermal expansion of the skirt in the direction that is critical for the



Fig.1.2: BOX-type piston

guiding of the piston in the cylinder.

1.1.1.2. Box-type pistons:

Compared with controlled-expansion pistons, this piston type is characterized by its reduced mass, optimized support, and box-like, often slightly oval skirt design. The box-type piston is compatible with both aluminium and grey cast iron crankcases. With a flexible skirt design, the difference in thermal expansion between the gray cast iron crank case and the aluminium piston can be compensated very well in the elastic range. If the box width is different on the thrust and antithrust sides, the piston is referred to as an asymmetrical duct piston. Box-type pistons are cast or forged.

1.1.1.3. Forged aluminum pistons:

In engines with very high power densities--such as highly loaded

turbocharged gasoline engines--cast pistons reach their limits. Forged pistons are a particularly good fit for this area of application; Figure 1.15. Their strength advantage in the temperature range of up to about 250°C improves the load carrying capacity for lateral forces, and increases the load carrying

8

Chapter 1: Overview on engine's piston

capacity of the pin bore and the fracture toughness. Forged pistons are therefore especially well suited for high-speed concepts and turbocharged engines. Because of the high ductility of the forged material, they also react more tolerantly peak pressures that can arise if an engine is operated very close to the knock limit. This allows lower ring land widths, among other things, and therefore lower compression heights. Since the manufacturing process is very stable, the forged pistons can be designed to the limit in order to minimize component weight.

1.1.2. Pistons for diesel engines 1.1.2.1. Ring carrier pistons Ring carrier pistons; have been in use since 1931. The first and at times even the second piston ring are guided in a ring carrier that is securely joined to the piston material by metallic bonding. The ring carrier is made of an austenitic cast iron with a similar coefficient of thermal expansion to that of the piston material. The material is particularly resistant to frictional and impact wear. The top ring groove, which is the most vulnerable, and the piston ring inserted in it are thereby effectively protected against excessive wear. This is particularly advantageous at high operating temperatures and pressures, which are particularly prevalent in diesel engines.

Fig 1.3: Ring carrier piston



Fig.1.4: Ring carrier piston with cooling gallery

1.1.2.2. Cooling gallery pistons

The cooling gap inside the piston head is generally used to optimize heat extraction. The high-speed collision of the engine oil on the gallery surfaces improves the heat transfer capacity due to the impact caused by the movement inside the cylinder. The heat flux removed by the cooling gallery can reach 60% to 70% of the total heat transmitted to the piston by the combustion gases, thereby effectively reducing the thermal loading of the piston. When the engine is running, the cooling oil is injected from the oil jet nozzle into the gallery through the inlet hole, flows in the circumferential direction and exits the gallery through the exit hole, the cooling oil does not fill the piston galley, the cooling oil exits the gallery at high speed under the inertia of the movement

However, the structure of the cooling gallery limits to some extent the resistance of the piston. With the steady increase in the maximum pressure inside the diesel engine there is also the prolonged exposure of the engine oil to higher temperatures in the piston cooling gallery which causes a degradation of the characteristics of the lubricant which will not only affect the piston but all engine components and carbon build-up, thereby reducing cooling efficiency and resulting in overheating.

9

Chapter 1: Overview on engine's piston

1.1.2.3. Optimized monobloc steel pistons: This piston type is a single-piece forged steel piston that is greatly weight optimized. With a small compression height (to less than 50% of the cylinder diameter) and machining above the pin boss spacing (internal), the piston mass, including the piston pin, almost corresponds to the mass of a comparable aluminium piston with piston pin. In order to improve the piston cooling, the outer cooling cavity is closed off by two cover plate halves. The MONOTHERM^® piston is used in passenger car and commercial vehicle engines with peak cylinder pressures of up to 20 MPa. 1.1.2.4. Composite pistons

Fig.1.5: Optimized monobloc steel piston

The composite piston enables the incorporation of cooling cavities and the combination of the properties of various materials in one piston. The performance range of four-stroke engines with composite pistons extends from 500 to 30,000 kW, with up to 20 cylinders. Areas of application include genets, main ship drives and auxiliary ship drives, and heavy construction and railroad vehicles.

Here are many variants of composite pistons. Common to them all is a design consisting of two main constituents: the piston crown with the ring belt (upper part of piston) and the piston skirt with the piston pin boss (piston skirt). The two parts are screwed together with appropriate threaded fasteners



With two contact surfaces With one contact surfaces

Fig. 1.6: Upper part of piston cooled by gallery (left) and by orifice (right)

The upper part of the piston is made of forged steel, while the piston skirt can be made from three distinct materials:

- Piston skirt made of forged aluminium-base alloy, forged aluminium skirts, are suitable for low and medium peak cylinder pressures, exhibit a low mass, and are easy to machine.

- Piston skirt made of nodular cast iron, Key features of a composite piston with nodular cast iron piston skirt, Figure 1.6, are low cold piston clearance and the resulting low secondary piston motion as well as high seizure resistance.

10

Chapter 1: Overview on engine's piston

The casting process, in contrast to forged steel, allows for undercuts and therefore a lighter design Superior to 200 bar. Compared with pistons with aluminum skirts, however, the mass is increased as a result of the higher material density.

- Piston skirt made of forged steel provides ultrahigh component strength and, related to the process, a material with extremely few defects. They are suitable for ultrahigh stresses greater than 24 MPa. Similar to pistons with nodular cast iron piston skirts, they provide the advantage of low cold piston clearance, resulting in low secondary piston motion.

1.2. Different forms of wear

Metal surfaces which are subjected to solid or fluid friction in the presence or absence of a lubricant may be affected by wear patterns which may range from regular, controllable wear (normal wear) to sudden deterioration causing the failure of the mechanism.

Surface deterioration is an isolated or simultaneous phenomenon classified into four basic forms: adhesive, abrasive, corrosive and fatigue.

1.2.1. Adhesive wear

Adhesive wear or wear by metal - metal contact is the most basic form of deterioration of surfaces. It is due to shear failure, during friction, micro-welds or junctions formed instantaneously between the opposing asperities of the two surfaces, in dry friction mode or in limited or mixed lubrication. The formation of these welds, which is very localized, is favoured by the thermal energy dissipated by friction, associated with very high contact pressure.

According to its nature, the weld will be a true weld or "hot weld", or a simple physical bond or "cold junction", therefore, depending on the shear strength of the junction, shear breakage occurs inside the softest metal or, on the contrary, at the junction.

The deterioration thus takes all forms ranging from severe scratching to light scratching or soft adhesive wear (Figure 1.8.).

In the first case, the deterioration, in its most serious form which is the seizure of the surface (Figure 1.7.), is characterized by metal stripping, transfer of metal from the softest surface to the hardest surface, (Burning) and the emission of relatively large dimensions (some ìm to some hundred ìm) of metal wear particles.



Fig .1.7: Piston seizure

11

Chapter 1: Overview on engine's piston

In the second case, the wear rate is very low and the wear particles consist of very small fragments of metallic oxides of generally lamellar shape (a typical dimension of the order of one to a few hundredths of a im).

A The shear strength of the welds TS is B The shear strength of the junctions

greater than the shear strength of the TS is less than the shear strength of the

softest metal TB softest metal TB



Wear particules Transferred fragments

Wear particules

The shear occurs inside the softest body B, an adhesive transfer and severe wear. ? Large wear particles, a few im diameters ? Important friction called "internal" or by welding

The shearing occurs at the junctions wear is moderate (soft adhesive wear) ? Very small wear particles (oxides) about 10 nm diameters ? Low friction known as "external" or by shear

Fig.1.8: Mechanisms of adhesive wear: A) severe adhesive wear - B) soft adhesive wear

In engines, adhesive wear mainly affects the components that come into contact with each other, such as cylinder and cylinder piston.

The introduction of antifriction additives which act chemically when the temperature of the contact rises, by attacking the metal in order to convert it superficially into a self-lubricating, easily shearable film, generally consisting of metal sulphides and phosphates.

1.2.2. Abrasive wear

Abrasion is a deterioration involving two phenomena

- A material removal from the surface by cutting;

- Plastic deformation of the surface (plowing) without ablation of material.



Fig. 1.9: Abrasion mechanisms: A) abrasion by cutting - B) abrasion by plastic deformation

Chapter 1: Overview on engine's piston

The abraded surface thus loses metal in the form of micro chips and is deeply modified by plastic deformation.

There are two types of abrasion:

- The two bodies abrasion caused by hard asperities or hard particles embedded on one of the surfaces and wears the other surface;



Two bodies HVA > HVB

Hard asperities or particles Three bodies

embedded on 1 of the 2 surfaces

and wears the other surface Abrasive grains conveyed between the

two rubbing surfaces.

Fig.1.10: The types of abrasive wear

12

- The three bodies abrasion generated by free abrasive particles (3rd bodies) conveyed between the friction surfaces, if the size of these particles is equal to the thickness of the oil film, the abrasion is said to be normal, it is manifested by scratches having the appearance of cutting grooves. On the other hand, if the abrasive particles are very small, the deterioration, whose size and appearance strongly depend on the angle of incidence of the particles with respect to the surface, is called abrasive erosion.



Fig.1.11. Diesel piston head deposit formation

Abrasive wear is countered by hardening of surfaces and especially by efficient filtration of air (in engines) and oil.

Polishing wear of the cylinders of heavily loaded diesel engines is a particular form of abrasive wear. The disappearance of the machining lines by polishing the cylinder bore is due to the combined abrasive actions of the carbon deposited on the piston crown (2-body abrasion) and the solid particles carried by the oil (These deposits and sediments contain, in greater or lesser quantity, microcrystals having a diameter of (0.1 to 0.5 ìm) of mineral salt, some of which are abrasive, resulting from the interaction of the products of thermal degradation of the organometallic additives of the oil.

13

Chapter 1: Overview on engine's piston

1.2.3. Corrosive wear

Corrosive wear is the friction removal of the reaction product formed on the surfaces by chemical attack. Among these, we can site:

- Hydrochloric (HCl) and hydrobromic (HBr) acids, which originate from the combustion of ethylene halides contained in the anti-knock additive of leaded gasolines;

- Sulfuric acid (H2SO4), which comes from the cold condensation of sulfuric anhydride (SO3) with the fuel combustion water in engines operating with sulfur fuel, which is particularly the case for diesel fuels.

- The organic acids coming from the partial oxidation of the oil or the fuel, the shortest in length of organic chain are the most corrosive. Corrosion is of particular concern when water and acids can condense on metal surfaces, i.e. when the temperature of these walls is relatively low, for corrosion to occur several conditions must be met:

- A corrodible metal (steel or antifriction alloy);

- A corrosive medium (aqueous electrolyte with a pH sufficiently low to corrode the steel); - An intimate contact.

The corrosion can be eliminated by one of the following methods:

- Elimination of sensitive metal (choice of a non-corrodible alloy or an organic material);

- Placing a barrier between the metal and the medium;

In general, the first means of action is only rarely economically or technically acceptable in the case

of internal combustion engines.

The second mean of action is specific to anti-rust and anti-corrosion additives.

But the most useful way of protection is the coating (for more information see the literature

review).

1.2.4. Fatigue wear

1.2.4.1. Fatigue fracture mechanism

By mechanical fatigue it is meant that the piston is subjected to external variable loading. The resulting stress causes cracks to nucleate and propagate in critical stressed areas. As shown in Fig 1.12, there are mainly two stress critical areas where crack initiates: piston pin holes and those regions on both sides of the bowl rim areas which are located on the same vertical plane that contains the pin holes. It is also observed that there is only one visible crack for mechanical fatigue.



Fig.1.12: Gasoline engine piston with a crack from one side of the pin hole to the head

14

Chapter 1: Overview on engine's piston

Under thermal fatigue, thermal gradients develop stress in two ways. Thermal stresses due to the distribution of the temperature along the vertical axis of piston - high temperatures at the top and lower temperatures at the bottom - and that due to the different temperatures on the piston head due to the flow of the hot air or fuel impingement. In the first case several fatigue radial cracks over the whole piston head can be observed.

The cracks so formed gradually grow under the fatigue conditions until it develops into a fracture on the crown. If severe enough, fracture continues across the crown surface to have the piston sliced to parts.

1.2.4.2. Fatigue damages to piston

Apart from piston crown, fatigue cracks are developed in other parts of piston as well and it includes piston rings and piston skirt.

- Piston ring

As the wear on cylinder walls increases, clearance between the piston and cylinder wall becomes high. Eventually, pressure acting on the ring increases (because the ring comes out of the groove) and consequently stress increase on the groove. The stresses at those fillet portions in the ring groove seem to be sufficient to initiate fatigue cracks on the piston.



Fig .1.13: Engine piston with damaged grooves

In case the rings are not fitted correctly in the grooves, the resulting misalignment stresses the ring material by the continuous reciprocating motion. This will accelerates the fatigue crack propagation which in turn stresses the ring lands and cause material to fracture away from the ring land areas, as depicted in Figure 1.13.

- Skirt fracture

If the clearance between piston and cylinder becomes too large, piston will be misaligned with cylinder, i.e., piston will be always at an angle with the cylinder walls. This causes the piston to flutter within the bore during the engine running. With larger clearances, the piston rotation angle also increases and makes contact with cylinder walls at two points; the bottom part of the skirt and the top part of the piston head. These contacts introduce a flexural load on the piston skirt. The

15

Chapter 1: Overview on engine's piston

consequence will be the formation of stress concentration areas where cracks initiate and propagate onto fracture, as shown in Figure 1.14.



Fig .1.14: Initiation and propagation of cracks onto fracture

1.3. Piston-cylinder Lubrication

1.3.1. The lubrication regimes in the piston-cylinder couple

Based on studies made by several researchers, it can be confirmed that:

- The main piston-cylinder lubrication regime is the hydrodynamic regime;

-boundary Lubrication and sometimes dry contact occur in the PMB and PMH areas, according to:

1. Courtney, Pratt, Tudor [PAN85] Using the measurement of the contact resistance of the segments-cylinder;

2. Wing and Sauders [PAN85] from the study of the rapid variations of the oil film thickness by adopting an indirect measurement of the piston-cylinder diametral clearances;

3. Furuhama in 1959 [FUR59] after he noticed the insufficiency of the hydrostatic lubrication which cannot explain a limit lubrication of the segments with the TDC which possess in this position a zero speed and consequently a zero film thickness;

4. Brown and Hamilton in 1975 [BRO76] by measuring the pressure distribution in the oil film and the thickness of the film in the vicinity of the TDC by capacitive proximity sensors. They demonstrated the limiting lubrication of the segments in the vicinity of TDC and BDC

5. Allen and al in 1978 [ALL84] From the measured values of the thickness of the oil film in the vicinity of the TDC, less than 0.25 ìm, they assert the existence of a metal / metal contact in these zones.

6. Richez and al [RIC82] carry out tests on an engine and show that the friction increases with temperature or with the decrease in viscosity. The measurement of the amplitudes of the roughnesses (2 to 4 ìm) and the thicknesses of the oil film (0.5 to 10 ìm) enabled them to demonstrate the existence of a metal / metal contact zone in the vicinity of the TDC.

16

Chapter 1: Overview on engine's piston

7. Uras and Patterson [URA87] Measuring the friction force of the piston and the measured friction force decreases as the piston velocity increases. This frictional force also becomes important in the vicinity of the TDC and BDC, indicating that the friction becomes limited in these regions.

8. Myers and al in 1990 [MYE90] Install capacitive gauges and surface thermocouples to study the influence of different parameters on the oil film in the vicinity of the TDC. They study the friction at engine start-up and thus notice the breakage of the oil film which causes the metal / metal contact.

- The boundary lubrication is located in the expansion and compression cycles:

1. Rogowski in 1961 [ROG61] after measuring the piston-cylinder friction force by means of a special device. He noticed that this force becomes important during the compression and relaxation phase;

2. Furuhama and al in 1978 [FUR59] with the improvement of their experimental set-up enabled them to show that the measured frictional forces are very high in the second half of the compression phase and the first half of the expansion phase at the TDC. The considerable increase in the friction force confirmed the existence of a metal / metal contact between the segment and the cylinder in these zones.

1.3.2. The main lubrication regime formulations in the piston-cylinder couple 1.3.2.1. Formulation of Furuhama and al

Experiments by Furuhama [RAY74] on driven engines and in normal operation led him to state that the main mode of lubrication of the piston and cylinder is of the hydrodynamic type. The Reynolds equation under these conditions is written:

???? (h3

?? ?? · ????

????) = 6 · ???? · ??h ???? + 12 ??h

????

with h: oil film thickness, ?? : Dynamic viscosity of lubricant, ??: Average hydrodynamic pressure, ????:

Speed of the piston.

The resolution of this equation makes it possible to determine the pressure and velocity

distributions in the oil film.

1.3.2.2. Formulation of Richez and al

Richez and al [RIC82] Introduce mixed lubrication of the segments and take account of the surface

roughness. By noting that the heights of surface's roughness of the segments and of the cylinder are

of the order of 2 to 4 ìm and that the thickness h of the oil film is of the order of 0.5 to 10 ìm, they

define a factor ë= h /??, and proposed the criterion:

?? = 1 The lubrication is mixed;

?? = 1 The lubrication is hydrodynamic.

They also give the law of distribution of pressure and speed in the oil film:

?? h3 ???? + ??h

???? (???? 12 ?? · ????

????) = ???2 ? · ??h ????

17

Chapter 1: Overview on engine's piston

???? = 1 - 0.9 exp (- 0.56 h

?? )

1.3.3. Generality on anti-wear additives (AW) in boundary regime 1.3.3.1. Boundary regime and tribofilm

Lubricating oil consists of one or more base oil which represents the framework of the product. However, for certain applications, and particularly at high temperatures and pressure (as in the case of pistons), the base oil is no longer efficient. In order to meet the various uses, multiple and different additives are added, specific to each use. Among the main types of additives are anticorrosive, anti-oxidation and anti-wear (AW) dispersant additives. These additives influence the behaviour of the lubricant by reinforcing its properties or by adding it others of which it is devoid;

Researchers in the field of lubrication develop different laboratory tests and each test fulfils a given function. These tests are Interesting on the mode of application or the behaviour of the lubricant. The formulation of lubricants requires control of the functioning of each additive (its reactivity to the metal surface and its mode of action) and their impact on the environment [TAN07].

In order to improve the performance of the lubricants in boundary regime in order to avoid seizure and to have reasonable friction, additives and in particular anti-wear additives (AW) and extreme pressures (EP) are incorporated in them. These compounds act directly on the metal surface by forming a protective film on surfaces called tribofilm by chemical reaction or physical adsorption.

Tribofilms formed in the boundary lubrication regime are designed to reduce friction and to protect the surface from wear due to lubricant/surface chemical reactions induced by temperature and pressure conditions.

Boundary lubrication is a complex phenomenon in which several parameters intervene such as solid-lubricant interactions, metallurgy, roughness, corrosion, temperature, pressure and kinetics of chemical reactions.

In the limit lubricating regime, the height of the surface roughness becomes greater than the thickness of the film at the interface, the oxide layer which protects the surface is removed and the two surfaces are brought into contact. This situation causes wear, increases friction and can cause welding of the roughness. The viscosity of the oil no longer intervenes under these extreme conditions, it is the additives present in the oil which will interact chemically with the metal surfaces to form the tribofilm whose thickness is a few tens of nm and which protects the Parts against wear [GEO79].

The additives (AW) by forming their own sacrificial film replace the oxide protective layer and considerably reduce the wear rate.

1.3.2.2. Types of anti-wear additives (AW)

The additives (AW) are classified according to their reactivity with respect to the metallic surface in two types, the active additives and the non-active ones [PAP98]. The active additives are those which chemically react with the metal surface to form a film. On the other hand, the non-active additives form films which are mainly unreacted with the surface.

18

Chapter 1: Overview on engine's piston

Molecules that contain sulfur, phosphorus and halides are considered active compounds while molecules containing boron, aluminum and graphite are classified as non-active compounds [DON94].

Table 3.1 shows the main anti-wear additives (AW) used in lubrication.

	Additive Families
sulfuric additives	dialkyldithiophosphates of zinc ZnDTP
	2-benzothioyle-tert-nonyle disulfur
	Phenyle sulfur
	Benzyle mercaptan
	Octadecyle mercaptan
	Dodecylebenzylemercaptan
	Ditert-octylemercaptan
Phosphoric additives	Alkyl d'acide phosphoré
	Alkyl phosphoré
	Phosphites
Borate Additifs	Esters boratés
	Borates de potassium
	Borates de calcium
mixed Additifs	Dithiophosphates métalliques (Zn,Co,Ni,..)

Table 3.1.The main anti-wear additives (AW) used in lubrication

The universally used additives (AW) are dialkyldithiophosphates and zinc (ZnDTP). This success is mainly due to its good antioxidant and anti-wear properties and its low cost [TAN07].

Several authors have shown that the chemical structure of sulfur-containing additives affects tribological performance, for example. Disulfides have been shown to perform better in anti-wear AW than monosulfides due to weak S-S bond in disulfides which is Easier to break than the C-S bond in monosulfides [FOR70].

Croszek [GRO71] Considers that the anti-wear (AW) activity of the disulfides depends on the electronegativity between the chemical elements of this molecule.

19

Chapter 1: Overview on engine's piston

Sakuraia Has shown that oxygen in oil plays an important role in the formation of oxide films and appears to have a good influence on tribological behavior [FEN61]. From the chemical reactivity of some additives, he concluded that oxygen and oxide films prevent wear on surfaces and reduce friction.

Wheeler [WHE78] differentiates two types of wear: soft wear and severe wear. Under severe pressure and temperature conditions, a significant amount of iron sulfide is detected in the wear traces in comparison with the mild pressure and temperature conditions, and concluded that under the effect of the elevation of Temperature and pressure, the sulfur compounds react by chemical corrosion with the metal surface to form iron sulfide.

Temperature affects the tribological behavior of sulfur additives and the formation of tribofilms. Temperature seems to have a limited effect on the decomposition of sulfur additives by promoting the formation of iron sulfide films which improves tribological performance [TAN07].

Chapter 2

Aluminum and its alloys

22

Chapter 2: Aluminum and its alloys

2.1. Properties of non-alloyed aluminum (1000 series)

There are many grades of unalloyed aluminum, known commercially as pure aluminum. These standard grades are part of the 1000 series of AFNOR standards. The purity of aluminum is always greater than 99.00% (grades 1100 and 1200), very often higher than 99.40% (grade 1040), but can reach 99.80% (grade 1080) or even 99.90% (grade 1090).

It is also possible to obtain a high purity aluminum of 99.95 % (grade 1095), 99.98 % (grade 1098), 99.99 % (grade 1199), and even an aluminum of very high purity (greater than 99.996%, or even equal to 99.999%).

The structure of the pure aluminum is face-centered cubic at any temperature below its melting point (660 °C). The main physicochemical properties of unalloyed aluminum are given in Table 2.1: (At 20 °C for a pure aluminum of 99.996%):

Melting temperature G??	660°C
Crystal structure	CFC
Density ??	2700 Kg/??^3
Thermal conductivity ??	237 ????^-1k^-1
Coefficient of linear expansion ??	23.1x 10-6 k-1
Electrical resistivity ????	0.0265 ??????

Table 2.1: Physicochemical properties of unalloyed aluminum

These properties are the reason why the unalloyed aluminum is widely used (in developed countries more than 40% of the aluminum consumption takes place in this state).

Table 2.2 gives the main mechanical properties of unalloyed aluminum:

Ultimate tensile strength ????	40 à 50 MPa
Yield strength Re	10 à 20 MPa
Proof stress Re0.2	20 à 35 MPa
Young's Modulus E	63 à 71 GPa
Elongation to rupture A%	50 à 70%
Poisson's ratio õ	0.33 à 0.35

Table 2.2: Mechanical properties of unalloyed aluminum

2.2. Aluminum alloys

The main alloying elements introduced into aluminum are copper, manganese, magnesium, silicon, zinc and lithium. These elements can be added individually or in combination. On the opposite, certain other elements are more or less excluded, in order to improve a particular property. For example the tenacity of alloys with magnesium and silicon is greatly improved by additions of manganese and chromium with limited contents (often Cr <0.4% and Mn <1%) and by a severe limitation of the contents of harmful elements such as iron.

23

Chapter 2: Aluminum and its alloys

2.2.1. Classes of aluminum alloys

The aluminum alloys are classified according to the main alloying element (s). We can

distinguish:

- Aluminum/copper alloys of the 2000 series;

- Aluminum/manganese alloys of the 3000 series;

- Aluminum/silicon alloys of the 4000 series;

- Aluminum/magnesium alloys of the 5000 series;

- Aluminum /magnesium/silicon alloys of the 6000 series;

- Aluminum/zinc alloys of the 7000 series;

- Aluminum alloys and other elements of the 8000 series.

We can also distinguish:

- Peening alloys (not hardening by heat treatment): which are aluminum/manganese alloys and

aluminum/magnesium alloys;

- Heat treatment hardening alloys: which are aluminum/copper alloys, aluminum/magnesium

alloys, aluminum/silicon alloys and aluminum/zinc alloys.

2.2.2. Physical properties of aluminum alloys

Table 2.3 gives the main physical properties of the different series of aluminum alloys (the mean value of different experimental results are considered):



Table 2.3: Physical properties of the different series of aluminum alloys

2.2.3. Mechanical properties of aluminum alloys

It is well accepted that the properties of aluminum alloys are determined essentially by the microstructure of the alloys obtained.

The microstructure is essentially conditioned by the nature, the quantity and especially the distribution of the elements of addition. Most metallic elements can be alloyed with aluminum, but only a limited number of those elements are used as the main addition and condition the properties of the alloys.

24

Chapter 2: Aluminum and its alloys

Table 2.4 below represents the mean values of the mechanical properties for different series of aluminum:



Table 2.4: Principal mechanical properties of different series of aluminum

2.2.3.1. Static strength

Table 2.4 gives the mechanical strength obtained for each of the series of aluminum alloys. In fact, those characteristics can be varied depending on many factors.

2.2.3.2. Tenacity

Three categories of aluminum alloy can be considered on the tenacity point of view:

- Tenacity of low- and medium-strength alloys: those alloys are of 1000, 3000, 5000 series and

all of other series in the annealed state;

- Tenacity of alloys of the 6000 series: Al-Si-Mg alloys with medium resistance of the 6000 series

are commonly used for working structures;

- Tenacity of high strength alloys: those alloys are of 2000 and 7000 series.

For the high strength alloys, it can be noted that:

- The tenacity increases when the yield point decreases.

- The newest alloys currently used for the construction of modern structures of aircraft are

designed to offer a particularly high tenacity:

? The 7475 is the aluminum alloy which has the best tenacity;

? The alloys 7010, 7050, and 7150 offer the best compromise between strength and

tenacity.

2.2.3.3. Endurance strength

The Wohler curves of aluminum alloys have the following characteristics:

- There is no asymptote up to 10⁸ cycles, so the fatigue test must be kept running up to 10⁸ cycles to determine the fatigue limit;

- The ratio between the fatigue limit at 10⁸ cycles (in alternating stresses) and the tensile breaking limit is equal to:

- 0.35 to 0.45 for Peening alloys (1000, 3000 and 5000 series alloys);

- 0.25 to 0.35 for heat treatment hardening alloys (2000, 6000 and 7000 series alloys); In general, this ratio decreases when the breaking load increases. Different parameters can influence the rate of fatigue cracking:

25

Chapter 2: Aluminum and its alloys

in

- The stress ratio R = Sm : the rate of fatigue cracking increases when R increases;

- The medium: humid or salty aggressive medium accelerates fatigue cracking;

- Overloads: during a fatigue test, if the stress intensity factor is temporarily increased by a tensile overload, a significant reduction in the rate of cracking is observed after suppression of the overload.

2.2.3.4. Corrosion resistance

In practice, aluminum has an excellent corrosion resistance, because of that it can often be used without coating. Adequately chosen, aluminum alloys without coating can resist to freshwater, sea water, and so many chemical products.

However, like the most common metals, corrosion damage can occur in different forms, depending on the class of the aluminum alloy, its metallurgical state, the exposure conditions, assembly and stressing conditions.

The principal forms of corrosion that can be encountered on aluminum alloys are:

- General corrosion or uniform corrosion

The exposed aluminum instantly covers with an oxide layer of 5 to 10 nm thick, and its corrosion resistance depends on the stability of this oxide layer in the corrosive medium. General corrosion is characterized by the uniform decrease of thickness over the entire metal surface; the oxide layer is dissolved in a regular manner by the corrosive agent.

In a natural, rural or marine atmosphere, the corrosion rate of aluminum is extremely low, even insignificant, and does not exceed 5 pm / year. In general, the general corrosion resistance of aluminum depends on the pH of the corrosive medium, that corrosion cannot practically occur in an acidic medium with pH <4 and in a basic medium with pH> 9.

In the most aggressive mediums such as marine mediums, copper-containing alloys (2000 and 7000 series) require protection; On the other hand, the alloys of the 3000, 5000 and 6000 series may have a corrosion resistance at least equal to that of the non-alloyed aluminum.

- Pitting corrosion

Pitting corrosion is a much localized form of corrosion, which appears by the formation of cavities or recesses on the surface, in which the diameter and depth depend on the class of the alloy, the corrosive medium and the exposure time.

The rate of deepening of the pits generally decreases due to the formation of hydrated alumina which hinders considerably the exchanges between the corrosive medium and the metal. Experiments show that the rate of propagation of pits v is related to time t by a relation of the type:

V= k
· t1/

3

All the aluminum alloys are sensitive to pitting corrosion, the best results being obtained with a content of 1% of iron (it is the case of alloys 8006, 8011 for example), likewise the presence of 0.1 to 0.2 % of copper improves the resistance for the penetration of pits (the case of alloy 3003).

26

Chapter 2: Aluminum and its alloys

- Galvanic corrosion

When two different metals are in electrical contact in a corrosive medium, they form a cell which produces current by consuming the most electronegative metal which constitutes the anode. Aluminum is the most anodic material compared to other metals (except zinc and magnesium). Aluminum will therefore often be the victim of mixed assemblies.

In fact, the attack rate of aluminum depends on: the difference of potential between the two metals present; on the electrical resistance between the two metals and on the corrosive medium and more particularly on its conductivity.

- Corrosion cracking or Intergranular corrosion

The intergranular corrosion encountered in various metals and aluminum alloys is the corrosion that propagates in the metal by consuming only the grain boundaries. This type of corrosion is due to the presence at the grain boundaries of a continuous and anodic zone compared to the interior of the grains.

For aluminum alloys, it mentioned that:

- Alloys of 1000, 3000, 4000 and 6000 series: are not sensitive to intergranular corrosion;

- Alloys of the 5000 series: Al-Mg alloys with less than 3.5% Mg are normally not susceptible to intergranular corrosion and Al-Mg alloys with more than 3.5% Mg may be sensitive to intergranular corrosion especially if the magnesium content is high;

- Alloys of the 2000 and 7000 series: have a good resistance to the intergranular corrosion obtained by a quick quenching.

2.2.4. Standards defining aluminum alloys 2.2.4.1. Wrought aluminum alloys

This family of aluminum alloys is the most widely used in industry. It includes all commonly

rolled products.

The NF EN 573 standard defines two European systems for the designation of these aluminum

alloys: the numerical system and the alphanumeric system.

The numerical system uses a 4-digit code which refers indirectly to the chemical composition.

According to this system the designation of an alloy includes successively:

4 letters: EN AW

EN refers to European standards;

A indicates that it is aluminum;

W specifies that it is wrought metal.

4 digits:

The first digit indicates the series of alloys

The second digit can be:

- 0 if it is the original metal,

- 1 to 9 if the alloy has certain particularity (purity of a unalloyed aluminum, variant of an alloy,

etc.);

27

Chapter 2: Aluminum and its alloys

The third and fourth digits have different meanings depending on whether they are:

- Of unalloyed aluminum; they are then the first two decimal of the number expressing the

minimum Al content,

- Of an alloy: these two numbers serve only to differentiate the grades of the same group.

Examples:

EN AW-1098 = aluminum with at least 99.98% purity;

EN AW-4015 = alloy wit 1.8% of Si and 0.9% of Mn;

In the alphanumeric system, unalloyed aluminum and aluminum alloys are distinguished. For

the unalloyed aluminum, the designation begins with the chemical symbol of aluminum Al

followed by the minimum content of the metal in % expressed in one or two decimal.

Example: EN AW-Al 99.98 = Aluminum with at least 99.98%.

Normally this designation should be preceded by a numerical designation and the minimum

content of the metal in % placed in brackets, the precedent example is designed as EN AW-1098

[A 99-98].

For aluminum alloys, the designation begins with the chemical symbol of aluminum Al,

followed by the chemical symbol (s) of the main alloying element (s), each followed by a number

representing the average content in % of the indicated element.

Examples:

Al Si2Mn = alloy of 1.8% of Si and 0.9% of Mn;

Al Si10 = alloy of 10% of Si.

Normally, this designation should be preceded by the numerical designation and be placed in

brackets such as the following:

EN AW-4015 [AlSi2Mn];

EN AW-4045 [AlSil0];

2.2.4.2. Cast alloys

Most of these alloys are easy to mold (permanent mold or sand casting). Many of these alloys contain sufficient silicon to cause a eutectic reaction, which gives the alloys a low melting point, good fluidity and good molding. The cooling in the die casting is faster than sand casting, which results a grain refinement and improved mechanical properties. Grain refinement is also caused by boron and titanium additives.

Designation according to the standard NF EN 1780, is given as: prefix EN; space ; Letter A (for aluminum); Letter B (for remelting ingots) or C (castings piece) or M (mother alloys); A dash ; followed by five digits representing the position of the chemical symbols giving the composition of the alloy ordered by decreasing content and limited to 4 elements.

Example:

EN AB-45400 [Al Si5Cu3] or EN AB-Al Si5Cu3; remelting ingots in aluminum alloy with 5% silicon and 3% copper.

28

Chapter 2: Aluminum and its alloys

2.3. Aluminum-Silicon Alloys (4000 series)

The equilibrium diagram of the Aluminum-Silicon alloy is given in Figure 2.1.



Fig.2.1: The equilibrium diagram of the Aluminum-Silicon alloy

The eutectic point varies from 11.7% Si to 14.5% Si. The most probable value is 12.5% Si. With rapid quenching the eutectic point can rise to 17% Si.

The main physical and mechanical properties of aluminum-silicon alloys have been given in Tables 2.3 and 2.4.

Hardness increases with increase in silicon content almost linearly and reach values between 500 and 700 MPa for 15% Si and 1000 MPa for 60% Si.

The modulus of elasticity has an average value between 85 and 95 GPa, this value increasing with temperature. The impact resistance also increases with increasing temperature.

The endurance strength is relatively low; it varies from 50 to 70 MPa for the hypoeutectic alloys and from 70 to 80 MPa for the hypereutectic alloys. The addition of cobalt and manganese increases the endurance strength. This strength abruptly decreases with the temperature increasing until 327 ° C.

The compression strength is 10 to 15% greater than the tensile strength.

The wear resistance is very good, especially for hypereutectic alloys. For alloys of 20 to 25% Si, the wear resistance is 10 times more than that of the non-heat treated steel.

The presence of calcium and sodium in the alloys increases their porosity. To reduce it, rare earths are added. Vacuum coatings are also used.

The silicon is an inert element in most corrosive mediums, the corrosion resistance of the aluminum-silicon alloy is the best compared to other aluminum alloys.

The coefficient of thermal expansion is relatively low compared to other aluminum alloys; it is decreased with the increase in silicon content.

29

Chapter 2: Aluminum and its alloys

According to Table 2.5 it is noted that the coefficient of thermal conductivity is decreased with the increase of the Silicon content.

Silicon content (%)	Temperature (K)
	300 à 400	300 à 600
5	22.1 à 22.5	24.1 à 24.5
7 à 9	21.4 à 21.7	23.1 à 23.5
12	19.9 à 20.6	21.6 à 23.1
> 14	18 à 19	20 à 21

Table 2.5: Coefficient of the thermal conductivity (10^-6 K^-1)

The alloy density is decreased with the increasing in Silicon content. An alloy of 6 to 9% Si has a density of 2650 to 2700 ????/ ??³ and with 10 to 13% Si has a density of 2600 to 2650????/??³.

Chapter 3

Vapor deposition and thin layer

characterization techniques

31

Chapter 3: Vapor deposition and thin layer characterization techniques

3.1. Physical bases of the vacuum technique 3.1.1. Vacuum and gaseous phase of the materials

Materials in the gaseous state are characterized by a great mobility of their molecules. Because of this; they always occupy the volume of the container in which they are enclosed. In physics and technology, we call vacuum, the state of a gaseous volume whose pressure is below the atmospheric pressure. For convenience, vacuum is divided into four ranges: low vacuum, fine or medium vacuum, high vacuum, and ultrahigh vacuum.

Table 3.1 gives the general characteristics of vacuum:

characteristics	Vacuum
	Low vacuum	Medium vacuum	high vacuum	ultrahigh vacuum
Pressure range (bar)	1 à 10^-3	10^-3 à 5 · 10-5	· 5 1012 a	< 10^-12
Number of molecules in ?? ??????	10^19 à 10^16	10^16 à 10^13	10^13 à 10^10	10^10
Mean free path ??	A « ??	A = ??	A > ??	A » ??
Flow regimes	Viscous flow	intermediate flow	molecular Flow	Molecular Flow
Application	Packaging, drying	CVD, plasma	PVD	Ion beams

Table 3.1 General characteristics of vacuum

According to kinetic theory, the molecules of a gas move in a continuous and chaotic manner. They collide and between their impacts their movement is rectilinear. The distance between two successive impacts is called the free path of the gas molecule. After each impact, the magnitude and direction of the velocities of the molecules change.

When the gas occupies a limited volume, there are molecular impacts on the walls of the container. So the gas pressure is the result of the statistical mean of these impacts.

A general criterion characterizing the vacuum in a system is the ratio of the mean free path ë of the molecule with a characteristic dimension ?? of the vacuum chamber, that is to say:

A

Kn = (3.1)
??

The Kn ratio is known as Knusden number, when Kn » 1, i.e. X » l the flow regime of the gas is molecular flow. When Kn « 1, i.e. X « l the regime is called viscous flow.

The limiting temperature above which the gas cannot be condensed, whatever the pressure, is called the critical temperature. This is a fundamental criterion for distinguishing gases from vapors.

Chapter 3: Vapor deposition and thin layer characterization techniques

Materials are in the gaseous state if the temperature is above their critical temperature. If the temperature is below their critical temperature, they are in the vapor state.

The pressure can be written in the form of the following equations:

P= ??
· k
· T (3.2)

?? : The number density of the molecules (m^-3);

k : The Boltzmann constant (k 1.38
· 10^-16 N
· m/K); T : The gas temperature.

1

P = 3 ??v² (3.3)

?? : The density of the gas (kg
· m^-3);

vv² : The-root-mean-square speed of molecules.

When the gas system is a mixture of different gases, each gas exerts its pressure Pi independently of the others. The total pressure PE is then the sum of the partial pressures (Dalton's law):

i=??

PE = ?Pi (3.4)
i=1

According to the kinetic theory, the mean kinetic energy of the translation of molecules is proportional to the thermodynamic temperature of the gas:

m · v^2	3 2 · k · T (3.5)
2

3.1.2. Movement and speed of gaseous molecules

The molecules of a gas have different speeds. During their thermal movement, the molecules collide. As a result the magnitude and direction of their speed change. The distribution of molecules according to their speed is described by a statistical law. It is generally assumed that this distribution follows Maxwell's law.

According to Maxwell, the number of molecules whose absolute speed is between v and v + ??v for any direction of motion is given by the equation:

m )^3/2 f (v) · ??v = 4??N · (2??kTl

????^2 ??^-2???? v^2??v

(3.6)

32

This function allows us to determine the most probable speed v_??. The speed v_?? corresponds to the maximum of the function f(v)and is determined by the condition:

33

Chapter 3: Vapor deposition and thin layer characterization techniques

?? ????²

????(?? 2???? ??²) = 0 (3.7)
Resolution of (3.7) gives ????:

2????

???? = (3.8)
??

In the calculations, the most often we use the arithmetic mean speed ??:

?? =v8???? = 1.128 ???? (3.9)

????

The root mean squared speed v??² can also be used:

v??^{2 =v3k??=} 1.223 ???? = 1.08 ?? (3.10)

??

Figure 3.1 shows the molecule speed distribution for two different temperatures;



Fig.3.1 Function of gas speed distribution for two different temperatures

The kinetic theory shows that each constituent of a gas mixture has an independent speed distribution.

At great distances, the real molecules attract themselves, while at smaller distances they repel each other. The number of intermolecular impacts B can be calculated by the following formula:

Chapter 3: Vapor deposition and thin layer characterization techniques

?? = v2??????²?? = v2????2v 8??

?????? (3.11)

?? : The molecular diameter (if we consider the molecule as a solid sphere). Finally we can give the mean free path ?? as:

?? =

1 = v2????^2??

???? v2??????2 (3.12)

34

This implies:

??

?? = (3.13)
??

The variation of the free mean path of some molecules with the pressure is shown in figure 3.2.



water vapor

nitrogen

oxygen, argon

Free mean path

oil

hydrogen helium

Fig.3.2 Free mean path of gas molecules as a function of total pressure

3.1.3. Basics of vacuum technology

Each vacuum installation contains a chamber the vacuum is obtained and maintained, a vacuum pump which evacuates the chamber and a piping which connects them together. When the pump is switched on, the gas in the chamber is directed towards the pump. The pump

35

Chapter 3: Vapor deposition and thin layer characterization techniques

delivers the gas it contains. New quantities of gas from the chamber replace the delivered gas and so on.

During pumping, the quantity of gas contained in the vacuum installation decreases, its pressure obviously falls also because the volume and temperature of the installation remain constants. In fact, the expansion of the gas produces a lowering of the temperature, but because of the high thermal inertia of the installation, it is considered that the gas temperature remains constant.

During pumping the pressure decreases throughout the installation, but the pressure in the chamber decreases more slowly than the pressure at the suction port of the pump.

The variation in mass flow rat as a function of the number of gaseous molecules passing through the section per unit time N is given by:

??= ?? · N^' · ?? (3.14)

N^': is The number of gaseous molecules crossing a section in a time unit (m^-2 · s)



Fig.3.3 Principle of a vacuum installation

3.1.4. Duration of evacuation process

The evacuation process of a vacuum system is generally non-stationary. During evacuation, the pressure and flow of each point of the installation vary. The evacuation duration is the time it takes to obtain a certain pressure in the vacuum installation.

For a piping with high conductance, we'll have:

V

??= ?? ????(??0 ?? ) (3.15)

V : The chamber volume (m³);

36

Chapter 3: Vapor deposition and thin layer characterization techniques

P0 : The pressure of the gas at the initial instant (N/??²);

P : The pressure of the evacuated gas (N/??²).

In the general case, the pumping time can be obtained using the nomogram shown in figure 3.4. To use it, first calculate the quotient V/?? by joining the points corresponding to V and Q on the respective straight lines. The obtained point on the line V/?? is joined to the point of the line P which corresponds to the pressure P which we wish to obtain. The point of intersection on the line t gives the necessary pumping time.



??/??

Fig.3.4 Nomogram for calculating the pumping time

3.2. The theory of plasmas

In their normal state, gases are electrical insulators. This is due to the fact that they don't contain free charged particles, but only neutral molecules. However, if they are given fairly strong electric fields, they become conductors. The phenomena that occur are called discharges in gases and are due to the appearance of electrons and free ions. The result of a discharge in a gas is the production of an ionized gas.

The degree of ionization of a gas is defined by the ratio á:

n

?? = (3.16)
n0 + n

n0 : The number of neutral particles per unit of volume;

n : The number of ionized particles per unit of volume.

The degree of ionization value varies from very low values (of the order 10^-10) to one. When the degree of ionization is equal to unity, it is said that the gas is totally ionized or that it constitutes plasma.

37

Chapter 3: Vapor deposition and thin layer characterization techniques

The set of concepts, methods and results of the study of this state of matter constitutes the physics of plasmas. In our environment, the plasma phase is totally absent in the natural state, whereas on the cosmic scale, more than 99.9% of the visible matter occurs in this phase.

When heating a gas at a sufficiently high temperature (of the order of 10⁴ K), the mean translated energy of its molecules can become of the same order as their ionization energy. Under these conditions, when two molecules collide, one of them can be ionized.

The probability of ionization as a function of energy is characterized by a threshold. This is the ionization energy. The ionization energies for all species are found in data banks.

Among all the weakly ionized gases we distinguish three families:

- Lowly ionized gases: in which some ions and electrons move in the middle of a sea of neutral molecules. Ions and electrons interact only with neutral molecules;

- Highly ionized gases without interactions between particles: in this plasma the charged particles follow without any collision a trajectory determined by external electromagnetic fields; - Highly ionized gases with interactions between particles.

Coating with plasma involves of modifying the state the surface by one of the three following methods:

- The deposition on the surface a thin layer of a few material;

- Chemical reaction with the surface itself (oxidation, nitriding) or physicochemical transformation thereof (modification of adhesion, surface energy);

- Erosion of the surface either by chemical action (formation of pores between one or more atoms of the surface) or ion sputtering due to plasma ion bombardment of the surface atoms or chemically-assisted sputtering, which combines ion bombardment and chemical erosion.

The main types of plasma are: hot plasma and cold plasma.

The plasma is called «hot» when the temperature of the ions and electrons is high (greater than 10⁷ k ). In this case, the medium is completely ionized.

The plasma is called «cold» when the electrons are hot (on the order of 10⁴ k), and the ions are cold (from 300 to 10⁴ k). The medium in this case is weakly ionized (ionization rate is close to 10^-6 until 10^-2 per neutral species).

3.3. Coating in the vapor phase (PVD, CVD)

Vapor deposition techniques allow us to have a deposit of thin layers (generally less than 10ìm) with very interesting physical and mechanical characteristics. Generally we distinguish two methods of deposition from the vapor phase: physical vapor deposition (PVD) and chemical vapor deposition (CVD).

In PVD processes, evaporation, sublimation or sputtering by ionic bombardment are used to transform the material of deposition in the vapor phase. These vapors are then condensed on the surfaces. The process can be summarized by the formula:

???????? s??l??d ??????s

??????s ??????ld s??l??d

38

Chapter 3: Vapor deposition and thin layer characterization techniques

The absence of a chemical reaction gave the designation: physical deposition.

In CVD, one or more vapors (two in general) are used, which react with each other on a surface to form a defined compound and volatile products of the reaction:

AG + _BGas + hot surface Csolide + _DGas + "'

The various processes of the PVD and CVD techniques are given in Table 3.2

Chemical vapor deposition(CVD)	Physical vapor deposition (PVD)
- Conventional chemical vapor deposition (CVD) - Metalorganic chemical vapor deposition (MOCVD) - Plasma-enhanced chemical vapor deposition (PECVD)	- physical vapor deposition by evaporation: - PVD by direct evaporation - PVD by Ionic deposition - physical vapor deposition by sputtering

Table 3.2 Various processes of the PVD and CVD

3.3.1. Chemical vapor deposition (CVD) techniques 3.3.1.1. Conventional chemical vapor deposition (CVD)

Two processes can be distinguished according to the gaseous environment:

- Static processes:

It is an isothermal process, in which the bar to be coated is placed in a closed chamber in contact with a mixture of a donor eventually alloyed (metal powder containing the element to be deposited), an activator (halogen compound) and an inert diluent which avoids self-sintering of the assembly. At 400 ° C, the activator reacts with the donor element to form the metal vapor. The subsequent heating at temperature between 800 ° C and 1100 ° C under a hydrogen atmosphere allows ensuring the production of the coating in a homogeneous manner.

- Dynamic processes:

For this process, the environment of the bar to be coated is continuously renewed by the circulation of a gaseous mixture of the material of the deposition formed outside the reaction chamber and carried by forced convection on the surface of the bar.

The need for high temperature in conventional chemical deposits to initiate the different chemical reactions leading to the metal transfer, several paths has been followed to do so, and two are the most developed for industry: the use of organic gaseous species (OMCVD) and the assistance of plasma (PACVD).

3.3.1.2. Metalorganic chemical vapor deposition (MOCVD)

The idea is to reduce the temperature of the conventional CVD by substituting the halogenated compounds which have a higher decomposition temperature than the new organometallic compounds.

The advantage of such a process is the lowering of the processing temperature compared with conventional CVD processes, taking the example of chromium deposition, the temperature is

39

Chapter 3: Vapor deposition and thin layer characterization techniques

reduced from 1000 °C for the conventional CVD to 500 °C using by bis-benzene chrome Cr(C6H6)2, the rupture of the radical bonding results in the formation of chromium carbide.

On the other hand, the disadvantages of this process are: the high cost, the toxicity for a large number of them and the lack of stability over time.

Electronics is the most favorable field for the development of the OMCVD technique, with layers of some ten of nanometers.

3.3.1.3. Plasma-enhanced chemical vapor deposition (PACVD)

A second way to reduce the temperature of the deposit to avoid any further heat treatment (see table 3.3), is to use the assistance of plasma and the use of chemically activated species such as ions and free radicals. These species are produced in the gaseous phase by electron-molecule collisions; the assistance of a plasma allows to obtain deposition speed varying from 1 to a few tens of ìm/h in a temperature range comprised between 150 and 500 ° C.

Deposition temperature (°C)
materiel	CVD	PACVD
Tungsten carbonate	1000	325-525
Poly silicon	650	200-400
Silicon nitride	900	300
Silicon dioxide	800-1100	300
Titanium Carbide	900-1100	500
Nitride of titanium	900-1100	500

Table 3.3 Typical temperature of deposition for CVD and PECVD

3.3.2. Physical vapor deposition Processes (PVD)

The PVD technique consists of the formation of a coating under reduced pressure in three distinct stages: vaporization of the species to be deposited, transport of these species and finally the condensation and the growth of the deposit. There are two main processes of this technique, those using evaporation and those using sputtering.

3.3.2.1. Physical vapor deposition by direct evaporation

Evaporated species (atoms, molecules) propagate in a straight line and condense on the cold zone (see figure 3.5). The transfer of the species takes place in molecular flow under a pressure of less than 10^-2 mbar, the rate of evaporation of the atoms can be found by the hertz-knudsen equation:

??(??) = av ??

2??R?? (P??- P) (3.17)

a: Stricking coefficient, it is the probability of having an adsorption; P: Gas pressure;

P??: Saturated vapor pressure;

R: Ideal gas constant.

40

Chapter 3: Vapor deposition and thin layer characterization techniques

The advantages of this process are:

- High deposition rates (more than 200 ìm / h);

- Low processing temperature allowing deposits on plastic.

The disadvantage of this process is that the deposits have a weak adherence and are often powdery, because they are made in a vapor phase with a low energy. It is often necessary to heat the substrate towards 300 to 400 °C, or to use subsequent heat treatments to improve the adhesion.



Fig.3.5 Schematic representation of physical vapor deposition by evaporation 3.3.2.2 Physical vapor deposition by ionic deposition

The ionic deposition differs from the direct evaporation by the phase of transfer and condensation of the vapor. In this case the vapor must pass through plasma to arrive into a state more or less excited and more or less ionized. The plasma used to improve the regularity of the deposit on bars with complex shape, energy exchanges are particularly important in the presence of a reactive gas (see figure 3.6).

The vapor phase is created by electron beam at a pressure less than 5 · 10^-4 mbar, the working potential difference in the deposition chamber is between 1 and 5 KV.

By introducing a reactive gas in the vapor phase such as nitrogen or methane, it is possible to produce defined compounds of carbide or nitride. The main application of this technique is the realization of deposition of TiN, (Ti, Al)N and TiC.

In competition with the CVD technique, this technique is widely used for protection against corrosion and erosion, the thicknesses are limited by some ìm due to the presence of residual compressive stresses.

			Chapter 3: Vapor deposition and thin layer characterization techniques



Source

41

Fig.3.6 Schematic representation of physical vapor deposition by ionic deposition 3.3.2.3. Physical vapor deposition by sputtering

Negative polarization in the order of 1 to 3 kV of an electrode (target) in the presence of a rarefied argon atmosphere at a pressure of about 1 to 10 Pa depending on the size of the reactor, leads to the creation of an electrical discharge between the target and the walls of the reactor. The walls of the reactor connected to the ground, act as an anode. The Ar + ions created in the discharge are accelerated in the cathodic cover and acquire the energy which they release when they impact on the surface of the target. This can cause the ejection of an atom by transfer of momentum, the implantation of the incident ion, the reflection of the incident ion neutralized by charge transfer or the emission of electrons that will serve to maintain the discharge (figure 3.7).

We can define two main characteristics that govern this mechanism:

- The spraying rate or yield Y (Table 3.4): it's defined as the number of spraying atom per incident ion. It grows linearly with the ion energy, and inversely proportional to the energy of sublimation of the material;

- The secondary electronic emission coefficient: defined as the number of electrons emitted during impact with incident ion. Its value is close to 0.1 for most metals but reaches very high values in the case of many oxides or nitrides.

42

Chapter 3: Vapor deposition and thin layer characterization techniques



Fig.3.7 Main mechanisms resulting from the interaction of an energy ion and a surface

element	Y	element	Y	element	Y	element	Y	element	Y
C	0,12	Ti	0,51	Cu	2,3	Pd	2,08	W	0,57
Al	1,05	Cr	1,18	Zr	0,65	Ag	3,12	Pt	1,4
Si	0,5	Fe	1,1	Mo	0,8	Ta	0,57	Au	2,4
in blue, Metals with very high sputtering yield

Table 3.4 Sputtering yield of different elements by argon ions at 400 eV 3.3.2.4. Magnetron effect

Two main problems result from the diode process. On the one hand, the low ionization rate of the discharge leads to low deposition speeds (<0.1 ìm / h) and, on the other hand, the high heat of the sputtered atoms leads to the synthesis of porous coatings. In order to avoid these two disadvantages, the target is generally equipped with a magnetron device consisting of two concentric magnets of opposite polarities (Fig. 3.8). A pole piece closes the magnetic circuit on one side, while the non-magnetic target to allow the magnetron effect; it leaves the field lines closed within the gaseous phase, which affect the trapping of the secondary electrons and thus increasing their possibility of encountering an argon atom in the context of an ionizing interaction. Dense plasma is then generated at the gap of the magnets, which leads, despite

43

Chapter 3: Vapor deposition and thin layer characterization techniques

heterogeneous erosion of the target, to increase considerably the discharge current and, subsequently, a deposition speed in the order of 10 ìm / h.



Fig.3.8 Principle of the magnetron device

3.3.3. Physical mechanism of a thin layer formation

The formation of thin layers by physical vapor deposition is the result of the condensation of the particles ejected from the target onto the substrate. It is carried out by a combination of nucleation and growth processes described in Figure 3.9.

At the moment of impact on the substrate, the incident atoms lose their kinetic energies and produce the limiting of their ability to diffuse into the substrate. This is true only if there is no external energy supplied to these particles by heating the substrate or ion bombardment. Since they are first adsorbed, they are known as adatoms. Those adatoms move on the surface until the thermal equilibrium with the substrate is reached.

During their displacement, the adatoms interact with one another; creating nucleus called clusters which continue their displacement by developing and colliding with each other. The clusters continue to grow in number and size until a nucleation density known as saturation density is reached.

The next step in the process of forming the thin layer is called coalescence. The clusters begin to agglomerate with each other by reducing the surface area of the uncoated substrate. The coalescence can be accelerated by increasing the mobility of the adsorbed species, for example by increasing the temperature of the substrate. During this step, new clusters may be formed on surfaces released by the approach of older clusters. The clusters continue to grow, leaving only holes or channels of small dimensions between them. Gradually, a continuous layer forms when the holes and channels are filled.

44

Chapter 3: Vapor deposition and thin layer characterization techniques



Fig.3.9 Layer growth process: nucleation and clusters growth

There are three modes of clusters nucleation and growth:

- A cluster called Volmer-Weber (fig.3.10. (a)):

During three-dimensional growth, or Volmer-Weber growth, clusters form and their coalescence forms a film. This mode of growth is usually favored when the atoms forming the deposited layer are more strongly bonded to one another than to the substrate.

- A cluster called Frank-Van der Merwe (fig.3.10 (b)):

The Two-dimensional (2D) growth, or Frank-Van der Merwe growth, is favored when the binding energy between the deposited atoms is less than or equal to that between the thin layer and the substrate. So, the films form atomic layer by atomic layer.

- The mixed type called Stranski-Krastanov (fig.3.10. (c)):

The third mode of growth, called Stranski-Krastanov, is a combination of the two preceding modes: after two-dimensional growth begins, a change in growth mode is observed, while clusters formation becomes energetically favorable.



Fig.3.10 Main growth modes of thin films

3.3.4. Thin film morphology

The growth mode of layers as well as the deposition conditions (substrate temperature, substrate nature, partial gas characteristics, etc.) influence on the crystallographic orientations and the topographical details of the clusters.

In fact, several models based on thin film growth approaches have been developed to study the influence of the deposition parameters on the microstructure of the layers. The first

45

Chapter 3: Vapor deposition and thin layer characterization techniques

description of the morphology of thin layers obtained by physical vapor deposition was presented by Movchan and Demchishin (fig.3.11) [MOV69]:

- First zone: the working pressure is greater than or equal to 1 mbar and the deposition temperatures are between 0.1 and 0.2 times the melting temperature of the deposited metal, this zone is favorable for applications or wettability of the surface is sought but not suitable for corrosion resistance or mechanical strength.

- Second zone: is called also the transition zone where the crystallization of the deposit is very fine with good properties in terms of resistance to corrosion and adhesion.

- Third zone: where the columnar crystallization is well identified, this zone is preferred for mechanical applications because of the good adhesion properties obtained.



Fig.3.11 Movchan and Demchishin structural model

Thornton [THO77] proposes a model that complements the previous one by considering the argon pressure in the sputtering. Its model shows a transition zone, called zone T, between zones 1 and 2 (fig.3.12). In this zone, the grains have a fibrous form without microporosities.

Later, Messier [MES84] showed that the fundamental process controlling the morphology of the T-zone was not only the gas pressure but also the effect of the gas pressure on the ionic bombardment of the growing film surface during the deposition by cathodic sputtering. He proposed a fifth zone M of morphology consisting of parallel columns with a domed surface (fig.3.13).

Chapter 3: Vapor deposition and thin layer characterization techniques



Fig.3.12 Structural modal of Thornton



Fig.3.13 Structural modal of Messier

46

3.4. Methods of microstructural characterization

After the application of the layer on the sample, this layer must be characterized. First, we must study its morphology using chemical, crystallographic and microstructural techniques (Table 3.5), and then we characterize one or more properties of this deposit (mechanical, optical, electrical, chemical properties, etc.). These properties are generally studied with all

47

Chapter 3: Vapor deposition and thin layer characterization techniques

results of morphological studies, in order to draw a conclusion either to modify the deposition process or to apply it in industry.

The greatest difficulty in the characterization phase is that these techniques are indirect. So, the engineer's art consists in crossing the results obtained, to constitute a global and synthetic vision of the most realistic product possible, in order to be able to produce a list of functional characteristics.

Chemical characterization methods	Crystallographic characterization methods	Microstructural characterization methods
- Auger Electrons Spectroscopy
(AES)		- Atomic force microscope
- Energy Dispersion Microscopy		(AFM)
(EDS)		- Scanning Tunnel microscope
- Electron Energy Loss	- Selected area diffraction	(STM)
Spectroscopy (EELS)	(SAD)	- Optical Microscope (LM)
- - Electronic microprobe analysis	- X-ray diffraction (XRD)	- Scanning electron
(EMPA)		microscope (SEM)
- Raman Spectroscopy (RS)		- Transmission electron
- X-Ray Photoelectron spectroscopy		microscope (TEM)
(XPS)

Table 3.5 the classification of main morphological characterization techniques of a thin layer

3.4.1. Chemical characterization methods 3.4.1.1. Auger Electrons Spectroscopy (AES)



Fig.3.14 Distribution of the primary, Auger and backscattered electrons, as well as X-rays under bombardment of a focused primary electron beam of a diameter D

48

Chapter 3: Vapor deposition and thin layer characterization techniques

This technique consists in bombarding the layer with an energetic primary electrons (3 to 30 KeV), which penetrates into the matter at depths of the order of some micrometers and excites the atoms of the layer, the de-excitation can be carried out by emission of the secondary electrons, and of the Auger electrons after the relaxation.

These electrons are characteristic for each atom, and the study of their spectrum makes it possible to identify the chemical composition of the deposit, the atlas of the spectrums of the Auger rays is available in the literatures.

The AES technique is used to analyze the most superficial atomic layers for conductive and semiconductive samples.

3.4.1.2. Energy Dispersion Microscopy (EDS)

This technique involves bombarding the layer by a beam of primary electrons; this beam can cause the emission of X-rays. The emitted spectrum breaks down into two radiations: a continuous radiation and a characteristic radiation which created after an inelastic impact between the atom and the electron. The characteristic radiation is specific for each element, and its detection allows for chemical analysis.

3.4.1.3. Electron Energy Loss Spectroscopy (EELS)

This technique involves exposing sample to a primary electron beam; some of these electrons will be subjected to inelastic impacts with the atoms of the sample, which causes the loss of their energy and a weak or random deflection of their trajectory. This loss of energy can be measured, and the spectrum can be used to determine the chemical characteristics of the sample.

3.4.1.4. Electron microprobe analysis (EMPA)

This technique is based on the analysis of the X-rays emitted by the atoms of the sample after inelastic impacts with the electron beam. The X-radiation is sent to a monochromator which is a single crystal having well-known crystalline planes. This radiation is then diffracted, and the diffraction angle varies as a function of the wavelength.

3.4.1.5. Raman spectroscopy (RS)

The Raman effect results from the slight change in the frequency of the monochromatic light projected on the sample, this modification is caused by the vibrations and / or the rotations of the molecules. The spectrum analysis of the scattered light gives information on the molecular composition of the sample, because the spectrums of the vibrations are characteristic for each molecule.

3.4.1.6. X-Ray Photoelectron spectroscopy (XPS)

Also called ESCA (Electron Spectroscopy for Chemical Analysis), this technique involves bombarding the sample with X-rays; the photons y of these rays enter the sample and ionize the atoms. The photons have kinetic energies E?? which correspond to the ionization energies E of the different orbits of the atom by the relation:

49

Chapter 3: Vapor deposition and thin layer characterization techniques

E = h?? - E?? (3.18)

h: Is Planck constant;

The energy of the electrons in each orbit is specific for each atom, so the knowledge of the energy of the projected photons h?? and the analysis of the kinetic energy of the photons makes it possible to identify the chemical composition of the sample. The average analysis depth by this technique is from 1 to 5 nm.



Fig.3.15 Principle of XPS analysis

3.4.2. Crystallographic characterization techniques

Crystallographic analysis techniques are based on the principle of X-ray diffraction (XRD) or electron-associated wave (SAD). These techniques make it possible to determine:

- Crystalline phase composition;

- The orientation and degree of organization of the crystal grains;

- Stress inside the repository.

3.4.2.1. Selected area diffraction (SAD)

A primary beam of electrons passes through a thin sheet; the wave associated with the electrons is submitted to diffraction on the crystalline planes of the sample. The transmission electron microscope TEM allows obtaining the image and the spectrum of this diffraction. The SAD technique makes it possible to obtain the information for very small crystalline grains.

3.4.2.2. X-ray diffraction (XRD)

The X-rays are diffracted on a crystalline plane according to Bragg's law: A = 2d sin?? (3.19)

The diffracted X-Rays wavelengths vary between ë = 0.071 nm (source with molybdenum anode) and ë = 0.154 nm (source with copper anode).

Chapter 3: Vapor deposition and thin layer characterization techniques

The diffraction experiments are done differently for thick layers (more than 10 ìm) and for thin layers. The thick layers are tested with a diffractometer in which the radiation source and the detector move circumferentially around a sample, placed at its center, the radiation penetrates into the interior of the layer and the signal of the substrate is low. This technique should not be applied to thin films because the signal of the substrate would be dominant compared to that of the deposit. In this case, the surface diffraction technique (GRXD) in which the incident beam interacts with the surface of the layer at a small angle varying from 1 to 10 ° so that to have a penetration depth of less than 1 ìm, the detector moves to obtain the diffracted signal.



Fig.3.16 Bragg law principle

3.4.3. Microstructural characterization methods

The microstructure analysis concerns the study of the layer surface (AFM, STM) as well as the study of grains inside the deposit (LM) or the internal grain morphology (TEM).

3.4.3.1. Scanning Tunnel microscope (STM)

The STM is used to characterize the surface of conductive or semiconductive materials. This microscope has a tip whose end has the size of an atom.



50

Fig.3.17 operating principle of the STM

Chapter 3: Vapor deposition and thin layer characterization techniques

The principle (figure 3.17), is to set a distance less than 1 nm (interatomic distance) between this tip and the surface of the sample, which allows the exchange of electrons and cause an electrical voltage of U = 2mV to 2V, the resulting current is called the tunnel electron current. Since this current is very sensitive to any changing in the distance between the tip and the surface, so this distance must be kept constant. This principle allows obtaining a map with a very good vertical resolution of the surface.

3.4.3.2. Atomic force microscope (AFM)

Unlike the STM, the AFM was developed to obtain surface profiles of non-conducting materials. The tip of the probe is equipped with a small diamond that touches the surface of the layer with a small force varying between F = 10^-6 and 10^-9 N to avoid any damage to it. The diamond is attached to a lever of which while the displacement occurs, the measurements are made by very sensitive sensors (fig.3.18).



51

Fig.3.18 Operating principle of the AFM 3.4.3.3. Optical or Light microscope (LM)

The use of the optical microscope requires special preparation of the layer surface. This microscope allows observing the porosity and the distribution of the pore size, the size of the unfused particles and their distributions, the thermal and mechanical deformation of the substrate at the interface with the deposit, the phase distribution in a composite deposit, etc. . .

3.4.3.4. Scanning electron microscope (SEM)

The operating principle of the SEM is the projection of a primary electron beam which scans the sample surface and which accelerates by a voltage of U = 1 to 30 kV. The beam of electrons interact with the atoms of the sample either by inelastic impacts followed by their ionization and by the generation of secondary electrons with a kinetic energy of E?? < 50 eV coming from the superficial region of the deposit (some nanometers), or By elastic impacts with these

52

Chapter 3: Vapor deposition and thin layer characterization techniques

atoms and the generation of backscattered electrons with a kinetic energy of E?? > 50 eV coming from the deeper regions (hundreds of nanometers).

The secondary electrons are detected by a scintillation detector equipped with a grid carried at the positive potential to attract these electrons at low energy. The backscattered electrons are detected by a semiconductor detector.

3.4.3.5. Transmission electron microscope (TEM)

The operating principle of this microscope is the emission of a beam of primary electrons passing through the sample. The image in this microscope is obtained by electrons that have not interacted with the sample. The electrons are accelerated by a voltage of several hundreds of kV to be able to cross slides, which must have a thickness less than 100 to 200 nm.

3.4.4. Mechanical characterization 3.4.4.1. Hardness

Three types of hardness are defined depending on the depth of indentation (Table 3.6). The loads used depend on the type of hardness. In nanohardness these charges range from some micronewtons to several hundredths of millinewtons, while microhardness requires loads ranging from a few tens of millinewtons to a few Newtons, in macrohardness the loads range from a few Newtons to several tens of Newtons.

	nanohardness	microhardness	macrohardness
Depth of indentation (rim )	0.001 - 1	1 - 50	50 - 1000

Table 3.6 Types of hardness

The use of each type of hardness depends on the material being tested, so the nanohardness is used to characterize thin layers with thicknesses of less than one micrometer, whereas the microhardness used to study certain surface treatments such as shot blasting or Thermo-chemical treatments (carburizing, nitriding, etc.). The macro hardness is used to measure the hardness of the steels after a heat treatment for example.

With the appearance of nanohardness since 1990, it was possible to understand different mechanical characteristics of materials at the sub-micron scale. However, although this test is fairly simple to carry out in principle, its technological implementation remains delicate, and the interpretation of the results is not simple. Indeed, it is a test, although extremely powerful, which requires a particular attention during the testing, and the analysis of the information.

The nanohardness test consists in the simultaneous measurement of the force F applied to the indenter and of its penetration h into the material under investigation. The representation F = f (h) for the charge and the discharge constitutes the nanoindentation curve.

The inverse analysis by finite elements analysis of the indentation curves allows, to some extent, to have mechanical properties such as: modulus of elasticity, hardness tensile strength, etc.

53

Chapter 3: Vapor deposition and thin layer characterization techniques

The interpretation of the measured characteristics (and in particular the hardness) by the nanoindentation test requires simultaneous consideration of the ISE (Indentation Size Effect) and the exact shape of the indenter tip (Berkovich or Vickers), and without forgetting the effects of roughness.

We can observe hardness variations of the order of few percents due to the presence of internal stresses in the materials tested, this is particularly true for thin films.

3.4.4.2. Adhesion of coatings

It is assumed that the adhesion of protective film is an indispensable condition, because of that, the deposition of a coating film is always preceded by a mechanical and / or chemical treatment intended to clean and activate it in order to optimize the adhesion of the film to the substrate. It is also often the practice to deposit an intermediate film (bonding layer) between the substrate and the final coating, for example, a layer of silicon is deposited on steels before being coated with a DLC layer.

For the characterization there are two main families of techniques: non-destructive and destructive techniques. The first uses an optical or acoustic probe that explores the coated material and detects any defect or inhomogeneity (cracks, porosities, bubble etc.) at the film-substrate interface. The most used amongst these methods are the ultrasonic methods.

Destructive techniques can be divided into two categories:

- Techniques used for ductile or weakly adherent coatings (paints, varnishes, polymers, etc.): like the peeling test, the swelling test...

- Techniques for metallic or ceramic coatings or when the adhesion is strong: as the scratch test.

- Peeling test

This test consists in applying a force F, at an angle è, to a band of length b, deposited on a substrate (Figure. 3.19). This force increases gradually over time until the force F?? corresponds to the initiation of the flow of the film. The operation is then continued at constant speed until the complete flow. At the end of the test, it is necessary to check the condition of the uncoated strip and that of the substrate in order to check that there is no elongation or plastic deformation and that all the energy expended during the flow of the band interface was used to break the bonds at the substrate-band interface.

The rate of release energy is given by:

G = (F??? ?) (1 - ????????) [??m^-2] (3.20)

Chapter 3: Vapor deposition and thin layer characterization techniques



Fig.3.19 principle of the peeling test

54

- Blister Test:

This test consists in making an opening in the substrate in such a way to retain the initial thickness of the film. A pressure P on the deforming film is then applied using an incompressible fluid. The film will begin to swell progressively up to a certain critical height ???? corresponding to a pressure P_?? and then begin to peel off from the substrate (Fig.3.20).

The rate of return of energy is given by:

G = ??P?????? (3.21)

?? : Constant having a value between 0.5 and 0.65;



Fig.3.20: Principle of the Blister Test

- Scratch Test

This test consists in applying, perpendicularly to the surface to be tested, a sharp Rockwell-type indenter (cone with an angle of 120 ° and a radius of curvature of 200 ìm) supporting a load of between 1 and 200 N which increases linearly over time with a loading rate of 10?? · m????^-1. When the indenter sinks into the surface of the material, it is made to slide with a velocity of 10 mm. m????^-1 (fig.3.21).

55

Chapter 3: Vapor deposition and thin layer characterization techniques



Fig.3.21 Principle of Scratch Test

3.4.4.3. Residual stresses in the coatings

In the production of a high-temperature coating ( PVD or CVD), at the time of return to ambient temperature, due to the difference in the coefficient of thermal expansion between the coating and the substrate, internal stresses can appear in the film, and the deposition-substrate assembly can deform if the substrate is sufficiently thin (some tens to a few hundreds of micrometers thick), in this case the direction of convex or concave deformation indicates, whether they are tensile or compressive stresses .

The internal stresses have an important role because they can influence both the hardness of the coating, its adhesion to the substrate and also its resistance to wear and cracking.

The two main techniques used for the determination of internal stresses are X-ray diffraction and the Stoney method. The two methods consist in reducing the thickness of the substrate by machining to such a value in which the action of the residual stresses is manifested.

- The Stoney method (the determination of the internal stresses by measuring the radius of curvature):

From this technique, the mean internal stresses ó can be calculated using the expression:

h?? ²????

?? = ????h??(1 - ????) (3.22)

h??, ????, ????: Are the thickness, the Young's modulus and the Poisson's ratio of the substrate;

h??: The coating thickness; ??: The radius of curvature.

This relation makes sense only when the ratio (h??/h??) is of the order 5 to10 % .

3.5. Thin films deposits properties

PVD and PACVD thin films are used to improve friction and wear resistance. Table 3.7 gives the main mechanical and tribological characteristics of the layers produced:

56

Chapter 3: Vapor deposition and thin layer characterization techniques

coating	TiN	(Ti,Al)N	TiCN	CrN	DLC
Color	Yellow gold	black	purple	silver	back
Hardness (HV)	2300 à 2500	2500 à 3200	3000 à 3400	1800 à 2200	3500 à 5000
oxidation résistance (°C)	400	800	300	600	400
Elaborating temperature (°C)	250 à 400	450	450	600	200 à 400
thickness (um)	2 à 5	2 à 5	2 à 6	3 à 8	1 à 4
Dry friction on 102 Cr6	0.55 à 0.65	0.50 à 0.60	0.45	0.40 à 0.55	0.05 à 0.07

- The coating of (Ti,Al)N is the most resistant to oxidation, so it will be used under severe thermal conditions.

- The DLC coating is the one which have the best tribological characteristics because of the presence in its microstructure of a large proportion of graphite bonds; it is therefore used for delicate lubrication conditions.

- The TiCN coating has a good hardness property, with good conductivity; it will therefore be used with strong mechanical stresses such as steels.

- The CrN coating has good ductility and good resistance to oxidation.

Chapter 4

Experimental Process

54

Chapter 4: Experimental Process

4.1. Determination of the sample grade

The piston used in our study as a substrate is HATZ E780. The materials used to make this piston are: Cast iron and aluminium-silicon eutectic alloy, the main production processes are: melding, forging [GER/https].



Fig.4.1: The piston HATZ E780 studied

According to the XRD analysis it can be ensured that the material used for piston manufacture is the eutectic Al-Si alloy fig. 4.2.



Fig.4.2: XRD spectra of a sample of the piston HATZ E780

55

Chapter 4: Experimental Process

The silicon content in aluminium for a eutectic Al-Si alloy is varied between 11.7 % and 14.5 % the average value is generally 12.5%. The most commonly used eutectic alloys for the manufacture of pistons are given in table 4.1 [EUR/http]:

Continent	4032 (AW-AlSi12,5MgCuNi)	4047A (AW-AlSi12 (A))	4045 (AW-AlSi10)
Si %	11-13.5	11.0-13.0	9.0-11.0
Fe %	1.0	0.6	0.8
Cu %	0.8-1.3	0.3	0.3
Mn %		0.15	0.06
Mg %	0.8-1.3	0.1	0.05
Zn %	0.25	0.2	0.1
Ti %		0.15	0.2
others %	0.2	0.2	0.2
Al %	Rest	Rest	Rest

Table 4.1: most commonly used eutectic alloys for the manufacture of pistons

4.2. Preparation of samples

4.2.1. Samples Cutting

The piston with cylindrical shape is cut in the workshop in order to obtain samples with dimension of (20×10×6) mm; the cutting operation is divided into two stages:

First stage: Consists of piston separation in two sections (Head and skirt) By means of a metal chainsaw

Second stage: Consists in cutting the upper part of the piston obtained by the preceding method with the aid of a hacksaw into samples of well-defined size.



Fig.4.3: cutting samples, on the left stainless steel, on the right aluminium

56

Chapter 4: Experimental Process

4.2.2. Samples polishing

In the process of surface preparation operations, polishing is essentially a finishing operation, the purpose of which is to:

- reduce roughness and surface topology by eliminating surface defects such as microcracks, porosities and inclusions.

Using a MECAPOL 230 polisher (fig.4.4), The 304 stainless steel substrates and the Al-Si substrate are polished, using abrasive papers with particles size varied from (200 to 1200) it is a kind of sandpaper, but with a larger particle size Fine and controlled.

Obtaining of the polished surface finish is done progressively (from the big size to the small one) in the presences of the water for the cooling and the evacuation of the debris.



Fig.4.4 : MECAPOL 230 polisher

Characteristics of the device

- The polishing head makes it possible to process up to 6 samples Ø 50 mm flanged in a plate by central pressure.

- Pressure force between 0.5 à 30 daN.

- Time between 10s to 99 minutes.

- Rotating speed between 20 à 600 rpm/mn.

4.2.3. Chemical cleaning

The purpose of chemical cleaning is to remove contamination or impurities formed after polishing to ensure good adhesion of the deposit.

The samples are immersed in an acetone bath for about 5 minutes; then rinsed with distilled water and then dried with the aid of compressed air.

4.3. Thin film elaboration process

In our work, we will deposit a thin layer on three types of samples with different substrates in

order to better characterize the layer obtained:

- Four samples of a substrate obtained from an Al-Si alloy piston HATZ E780;

- Four samples of 304 stainless steel substrate;

- Five samples of a 304 stainless steel substrate on which an Al-Si layer is deposited;

57

Chapter 4: Experimental Process

- In four of the previous five samples, a layer of Ti-W-N is deposited.

We use the PVD sputtering technique to deposit thin layers of Ti-W-N on Al-Si, stainless steel 304 and stainless steel with intermediate layer of Al-Si, the evaporation technique was used to create the intermediate layer

4.3.1. Description of PVD sputtering installation and working parameters



Fig.4.5. PVD sputtering installation of the CDTA

1) Bottle of nitrogen gas - 2) Nitrogen passage line - 3) Primary pressure gauge - 4) secondary pressure gauge - 5) Primary and secondary pump ignition switches - 6) Secondary Pump Cooling Water Pipe - 7) deposition reactor - 8) Secondary diffusion pump - 9) Pallet primary pump - 10) Gas pressure control valve (nitrogen and argon) - 11) Throttle valve - 12) Power supplies

The deposition chamber is cylindrical in shape, with a high of 230 mm, diameter of 210 mm, the interelectrode distance is 45 mm, and the lower part of the chamber is connected to the pumping system.

The primary pump is an Alcatel-type pump that can achieves a vacuum of up to2 · 10^-2???????? (2.67 · 10^-2????????) And a flow rate of0.5 à 70 ????³ /?? .

The secondary diffusion pump used allows a vacuum to reach 10^-5???????? (1.33 · 10^-5????????) And a flow rate of 250 ????³ /?? .

Chapter 4: Experimental Process

The primary pressure gauge is Priani type, it measures pressures up to 10^-3Torr (1.33 · 10^-3 mbar). The secondary pressure gauge is a Penning type gauge, the measured pressure can reach10^-5Torr (1.33 · 10^-5 mbar).

The power generator supplies the system with a maximum electrical power of 20 Watt, a voltage of 2 kV and an electrical current of 10 mA.

The working parameters are:

- The application of argon alone for 5 minutes with a pressure of 9.5 · 10^-2Torr ; (0.127 mbar). Argon ions spray directly the target, allowing surface cleaning;

- The introduction of nitrogen into the plasma for one hour with the pressure of 0.1 Torr (0.133 mbar);

- The percentage of nitrogen introduced into the plasma is 5% ;

- The working voltage is 2 kV.

a)		b)
Fig.4.6: the deposits obtained on the PVD sputtering installation: - a) on Al-Si, - b) on stainless steel, - c) on stainless steel with intermediate layer of Al-Si	c)	b)

58

4.3.2. Description of PVD evaporation installation and working parameters

The working parameters are:

- The limiting pressure of the primary pallet pump is 10^-2 mbar ;

- The limiting pressure of the secondary diffusion pump is 3 · 10^-5 mbar ;

- in order To have a thin layer on stainless steel similar to aluminium-silicon eutectic alloy (of 12.5 % Of silicon content see chapter 2), il faut Put on the crucible 60 mg of powder contains 52.5 mg of the aluminium powder and 7.5 mg of the silicon powder.

59

Chapter 4: Experimental Process



Fig.4.7: The PVD by evaporation installation of CDTA

1) Primary pressure gauge - 2) Secondary pressure gauge - 3) Primary and secondary pump ignition switches - 4) Secondary Pump Cooling Water Pipe - 5) Deposition reactor - 6) Secondary diffusion pump - 7) Pallet primary pump - 8) Throttle valve - 9) To power supply



a)

b)

The crucible of the evaporator

Al-Si powder To evaporate

Fig.4.8: Inside the reactor chamber: a) Al-Si powder on the crucible, b) The samples of stainless
steel on the substrate port

60

Chapter 4: Experimental Process

4.4. X-ray diffraction

An indispensable complement to elemental chemical analysis for the identification of compounds, X-ray diffraction allows the fine characterization of massive crystallized materials



Fig.4.10: Our sample at the interior of the diffractometer

Fig.4.9: Diffractometer D8 ADVANCE of CDTA 4.4.1. Characteristics of the device

· The diffractometer uses the BRAGG-BRENTANO assembly;

· Source (anticathode) of copper;

· Point scintillation detector;

· Acquisition range between 0 ° and 90 °. With precision of steps up to 0.01°;

· Grazing incidence configuration, with a minimum angle of incidence of 0.1 °;

· Eva operating software.

4.4.2. Working Principle [CDTA/http]

X-ray diffraction gives access to many information contained in the arrangement of atoms within a crystallized material and The 3D geometrical type of arrangement (network), the distances between atoms (mesh size, typically a few Å) which constitute schematically a "unique" identity card for each compound.

The BRUKER D8 Advance diffractometer is equipped with a BRAGG-BENTANO geometry goniometer. In this type of diffractometer, an incident monochromatic X-ray beam is diffracted by the sample at certain specific angles, According to Bragg's law: _2dhkl sinè = A, in which dhkl Refers to the interreticular distance of family plans (hkl), è Is the angle of incidence taken from the surface of the planes (hkl) ,ë The wavelength of the scattered photons.

61

Chapter 4: Experimental Process

As Lambda does not vary during a measurement, it is enough to vary the angle Theta to locate all diffraction angles. By means of a converter - a scintillation counter, the intensity of each point of the measurement is thus observed.

When the X-ray beam is diffracted, its representation is a peak. A Theta scan produces an X-ray diffraction pattern.

The recording of the signal by a suitable detector makes it possible to visualize the angles and Intensities of the diffraction peaks obtained. The indexing of these peaks is carried out using specific databases that have more than 350 000 records. Allowing the identification of the compound (s).

4.5. Raman Spectroscopy

4.5.1. Principle of RAMAN spectroscopy

During the interaction of a light beam with the material, several phenomena can occur. A part of the light beam is reflected, a part is diffused and a part can be transmitted through the sample (figure 4.11). During propagation in a dense medium, different phenomena appear: refraction, absorption, diffusion, and possibly other non-linear effects. The absorption may then induce photoluminescence or non-radiative de-excitation processes.

The scattering of light is manifested by the deflection of a part of the light beam in multiple directions. The majority of the scattered light is of the same energy as the incident light. This phenomenon of elastic diffusion is called Rayleigh scattering. However, a small portion of the scattered light (about one photon out of 106) present gain or loss of energy relative to the incident light. This is the phenomenon of Raman scattering. In a classical approach, this phenomenon of inelastic diffusion is explained by the creation of an induced dipole which oscillates at a frequency different from that of the incident light. Indeed, under the action of a monochromatic electromagnetic wave of frequency ù whose electric field oscillates according to:

??= ??0 cos(????) (4.1)



Fig.4.11: Spectrometer Raman HORIBA of CDTA

62

Chapter 4: Experimental Process

The Stokes diffusion and the anti-Stokes diffusion would be of the same intensity. However, experimentally the intensity of the Stokes diffusion is always (outside resonance) higher than that of the anti-Stokes diffusion.

4.5.2. The information accessible by Raman spectrometry

The information provided by Raman spectroscopy is relatively extensive:

- Identification of phases or chemical compounds ·

- Characterization of materials ·

- Determination of the molecular structure ·

- Study of amorphous and crystalline systems.

The Raman spectrum of a compound indicates both the type of binding of a compound and its crystalline structure.

4.6. Scanning electronic microscope (SEM)

The scanning electron microscope provides surface images of virtually all solid materials at scales ranging from the magnifying glass (x10) to that of the transmission electron microscope (x 500,000 or more).

Conventional SEM operates in an ordinary vacuum (10-5 to 10-6 mbar); The samples may be massive, ranging in size from a few ìm (particles) to about tens of cm in diameter, or even more (industrial samples). They must endure the vacuum. The preparation is generally simple.

The SEM at low pressure allows observation in a vacuum of up to 30 mbar, making it possible



fig.4.12: scanning Electronique microscope JEOL JSM 6360LV de CDTA

Chapter 4: Experimental Process

to examine moist or oily samples, insulators without prior metallization (ceramics, corroded metals).

Characteristics of the device

? A maximum resolution of 50 nm.

? The SEM is coupled to the EDS for the elementary microanalysis.
? Maximum voltage 30 kV.

4.7. The nanoindentation



Fig.4.13: The nanoindentation device CSM_NHT of CDTA

The indenter used on our characterization equipment CSM_NHT (Nano-Hardness Testers is a Berkovich indenter (pyramidal with triangular base geometry). We can apply a normal force between 0.3mN and 500mN.

h cos65.27° = b a = 2v3 h ??a??65.3°

63

Fig.4.14: The geometry of Berkovich Point

Chapter 4: Experimental Process

	v3 ?? = 2 ??

64

Fig.4.15: the footprint of the Berkovich tip

During an indentation test an acquisition system records the applied force as a function of the penetration depth of the tip. These two parameters are measured during a charge phase and a discharge phase. The result is a load-displacement curve.

The two main properties measured by the nanoindentation are the modulus of elasticity (E) and the hardness (H). The model used for the calculation of these two properties is that developed by Oliver and Pharr. According to this model the hardness is given either geometrically (fig.4.16) or analytically by:

????????

H = (4.2)

????



Fig.4.16: The determination of the hardness from the charge-discharge curve

Fmax : Maximum applied load;

A_c : The contact area between the indenter and the sample which is given in the case of a Berkovich indenter by:

65

Chapter 4: Experimental Process

A?? = 24,56 · hC 2 (4.3)

????????

h_C= h?????? - ??· (4.4)
??

???? 2

?? = ??h = · ???? · vA_C (4.5)

vit

E_r : reduced Modulus of elasticity.

The modulus of elasticity of material is given by:

?? = (1 - v²) ( ???? · ???? ) (4.6)

??_??(1 - v??²) - ????

ui: Poisson's ratio of the indenter; u: Material Poisson's ratio;

Ei : Modulus of elasticity of the indenter.

In our nanoindentation tests, we used the Oliver-Pharr model with the following parameters:

- Spherical diamante Berkovich indenter ( r = 10ìm) in diameter

- approaching speed: 2500 nm/min

- Time of charge and discharge: 10.0 s

- Linear load increment

- Slope to contact: 80%

- Load Speed: 2.00 mN/min

- Speed of discharge: 2.00 mN/min

- Material Poisson's ratio u = 0.30

This device is equipped with an optical microscope to select the area to be indented. An X-Y

motorized table with a repositioning accuracy of 1 ìm allows the programming of complex

indentation networks.

4.8. The tribometer analysis

On the CSM tribometer (fig.4.18), a ball, a tip or a plane is placed in contact with the surface of the sample under a predetermined load. The device is mounted on a lever arm, and associated with a displacement sensor.

The coefficient of friction is determined during the test by measuring the deflection of this elastic arm. The wear rates for the ball and the pawn are calculated by determining the loss of volume during the test. The formula for determining the wear rate WS is given by:

??

????= ?? · ???? (4.7)

66

Chapter 4: Experimental Process

??: The volume of lost matter (mm³) ;

D: The covered distance (mm) ;

F??: The normal force applied to the sample (N).



fig.4.17 : Tribometer of the USTHB

The volume of the waste material is calculated by the formula:

?? = 2??R [r² aresi?? (d2r) - (₄^d) · v4r² - d²] (4.8)

R: Radius of the wear track (mm);

r: Radius of the ball (mm);

d: Width of the wear track (mm).

Tribometer analysis facilitates the study of friction mechanisms for a wide variety of material couple with or without lubricating agent. In addition, the control of test parameters such as speed, contact pressure, frequency, test duration and environmental parameters make it possible to reproduce the actual stresses of use of these materials.

Instruments can provide rotational or alternative movement of the sample. A particular point of these instruments is based on the possibility of interrupting the test as soon as the coefficient of friction reaches a predefined value or, when a number of cycles is realized. In addition, the tribometer is equipped with a containment enclosure in order to use the instrument under controlled atmospheric temperature conditions.

The Tests performed by tribometer conforming to ASTM G99 & DIN 50324.

The specifications of the CSM tribometer are:

67

Chapter 4: Experimental Process

	Nano	Micro
Normal Strength Range	50 ìN - 1 N	until 60 N
Maximum tangential force	10 ìN - 1 N	10 N
Maximum temperature	-	1000 oC
Rotation speed	1 - 100 rpm	0.3 - 500 rpm
Rotating test radius	30 ìm - 10 mm	30 mm
Linear travel speed	10 - 500 ìm	60 mm
Length of linear travel	Until 10 mm/s	until 100 mm/s
Frequency	0.1 - 10 Hz	1.6 Hz
Penetration depth measurement	20 nm - 50 ìm	until 1.2 nm

Table 4.2: The specifications of the CSM tribometers

The characteristics of our work:

- Geometry of the tip: Ball;

- Material of the tip: alumina????2??3 ;

- Radius of the ball: 3.00 mm;

- Linear speed: 0.50 cm/s;

- Normal load: 1.00 N;

- Stop condition: 10.00 Meters travelled or if ii > 1.00 ;

- Temperature: 20.00 °C ;

- Humidity: 40.00 %.

4.9. Electrochemical techniques

4.9.1. Equipment

The polarization measurements were carried out in a three-electrode glass cell: a working electrode, a platinum counter electrode and a saturated calomel reference electrode (SCE). This cell, shown in figure 4.18, is designed to maintain a fixed distance between the three electrodes. The passage of the current in the cell is carried out through the counter-electrode.

68

Chapter 4: Experimental Process



Figure 4.18: the Cell used in electrochemical test

Electrochemical measurements were conducted using a computer-controlled, potentiostat, model parstat 4000 (Fig.4.19).



Fig.4.19: Assembly for electrochemical testing

4.9.2. Establishment of EVANS diagrams

The purpose of the EVANS diagram is to rapidly obtain, by a laboratory measurement, the corrosion rate and the corrosion current density denoted _icorr from the polarization curves

69

Chapter 4: Experimental Process



Fig.4.20 Diagram of the electrochemical cell

The potential measurement is carried out in a given aggressive medium at a given temperature by means of a potentiostat to which are connected three electrodes.

? The working electrode E.T constituted by the metal studied,

?
· The reference electrode E.R which serves as a reference for the dissolution potential, ?
· The counter electrode C.E, which is a platinum electrode used to drain the major part

of the current flowing through the circuit when the working electrode is subjected to an

overvoltage.

4.9.3. Diagram E=f (log/i): EVANS Diagram



Fig.4.21: a polarization curve modal

To measure the corrosion current density, the representation E = f (i) is modified, we switch
coordinates to E = f (log / i) or EVANS diagram in order to linearizes the polarization curve and

Chapter 4: Experimental Process

makes it possible to determine Experimentally the value of corrosion current density _icorr. THE CURRENT is obtained by extrapolating the linear parts of the anodic and cathodic polarization curves. This point of assistance, in cases where it is quite clear, gives access to the characteristics of corrosion (corrosion potential and the corrosion current density), these two linear parts, respectively called the anodic and cathodic Tafel branches.

We see clearly on this diagram the Tafel domain in which the potential varies linearly with the logarithm of the intensity according to the equations of Tafel

For the anodic process:

i

(4.9)

i0(Fe)

i

(4.10)

i0(H2/Fe)

E = EFe + b_a log For the cathodic process:

E = EH + bC log

70

EF_e,i0(Fe),EH, i0(H2/Fe) are respectively The equilibrium potentials and the exchange current of the couples Fe2+/Fe et H+/H2 ; b_a et b_c Are the Tafel constants of oxidation and reduction, (ba>0)

This graphic construction therefore provides access to these two important corrosion data. The calculation of Tafel's equations is based on equality:

i i

Ecorr = EFe + b_a log _i0(Fe)= EH + bC log i0(H2/Fe) (4.11)

Chapter 5

Analysis of the results

Chapter 5: Analysis of the results

72

5.1. Analysis of XRD results

- Stainless steel bars:

The phases of the thin film were evaluated by the XRD analysis, and the results are shown in the figure 5.1.

We can see that the deposited layer is an amorphous layer which does not have a crystallographic alignment with a random orientation of the grains, so it is inaccessible by X-ray diffraction.



Fig.5.1: XRD spectra of the Stainless steel bars with and without a layer of Ti-W-N

According to fig. 5.1, a bump can be observed on the angular value 2è = 23.5°, which corresponds to tungsten silicon (Si-W) in its amorphous form, which is requiring a subsequent heat treatment to obtain the crystal structure of layer.



Chapter 5: Analysis of the results

2000

Fe y

vierge

SS without a layer

ss evapo Al-Si+ Ti-W-SS with double layer

1800

1600

1400

1200

1000

800

600

400

200

0

Intensity

Si-W

Fe y

Fe á

73

0 20 40 60 80 100

2-théta-scale

Fig.5.2: XRD spectra of the Stainless steel bars with and without double layers of Al-Si and Ti-W-N

In the XRD spectra of fig.5.2, we can see again the bump correspond to the silicon of tungsten (Si-W) at 2è = 23.5 °, we notices also iron peaks belonging to the substrate which are iron á in 46 ° and iron y at 44 °, which means that the covering of the surface is not perfect and the formation of an amorphous layer.



SS withTi-WN Ti

-W-N layer

ss evapo AlSi+ TiWN

SS with Al-Si and Ti-W-N double

layers

700

600

500

400

300

200

100

0

Intensity

800

0 10 20 30 40 50 60 70 80 90 100

2-théta-scale

Fig.5.3: XRD spectra of the Stainless steel bars with double layers of Al-Si and Ti-W-N and with a layer of Ti-W-N

74

Chapter 5: Analysis of the results

Figure 5.3 shows that the intermediate layer deposited by evaporation between the substrate and the Ti-W-N layer influences in the layer diffraction properties in a manner which permits the appearance of peaks corresponding to the substrate.

- Aluminum bars:

The diffraction spectra of Ti-W-N layer are shown in figure 5.4.



700

l-i vierge

Al-Si without a layer

Ti-W-N

Al-Si with a Ti-W-N layer

600

500

400

300

200

100

0

Intensity

0 10 20 30 40 50 60 70 80 90 100

2-théta-scale

Fig.5.4: XRD spectra of Aluminum-Silicon bars with and without a layer of Ti-W-N

The spectra obtained by the XRD analysis of the Ti-W-N layer on the aluminum substrate shows that a layer with peaks similar to that of the substrate with an amorphous structure was formed.

5.2. Analysis of RAMAN results

The results obtained by the RAMAN on the Ti-W-N layer for different substrates are given by the following curves:

Chapter 5: Analysis of the results



Fig.5.5: RAMAN specter of Al-Si bar with Ti-W-N layer

Fig.5.6: RAMAN specter of stainless steel bar with Ti-W-N layer

75



76

Chapter 5: Analysis of the results

Fig.5.7: RAMAN specter of stainless steel bar with double layers of Al-Si and Ti-W-N

It is good to notice that the spectrum obtained for different bars are almost the same, which means that the same elements have been observed in the deposited layer for different substrates.

We can identify the spectrums peaks as the following:

- Peaks with the wavenumber 125 ????^-1 and 675 ????^-1 can be corresponded to titanium dioxide ????02 peaks [IOP/http];

- Peak with a wavenumber close to 62.5 ????^-1 can be corresponded to a titanium nitride ?????? peak [CHE94];

- Peak with a wavenumber close to 807 ?`??^-1 can be corresponded to a tungsten oxide W2⁰3 peak [COU64];

- Peak with a wavenumber close to 172 172 ????^-1 observed in the spectra of stainless steel with double layers of Al-Si and Ti-W-N can be corresponded to an aluminum oxide ????203 peak [AND02].

From the RAMAN spectrum results, it can be said that our layer contains titanium Ti, tungsten W and nitrogen N, in the form of tungsten oxide, titanium nitride and titanium dioxide. All of these peaks (except the ????02 peak in stainless steel substrate) are observed in different bars.

In the contrary of the XRD results in which amorphous results are obtained, the RAMAN results are identified all the elements of the thin layer, which can be explained by the operating principle of each instrument. The XRD gives results for a crystallographic analysis, so it allows us essentially to determine the crystalline composition of the layer, which is practically difficult without a recrystallization of the deposition by an appropriate heat treatment.

77

Chapter 5: Analysis of the results

RAMAN, on the other hand, is used for chemical analysis. It allows us directly to observe the chemical composition by the diffusion of the light at the frequency of the radiation caused by the vibrations of the molecules.

5.3. Morphological analysis

Figure 5.8 shows pictures from the Scanning Electron Microscopy of a stainless steel bar with the double thin layer of Al-Si and Ti-W-N.

In the picture b) we observe the formation of a layer made up with a small nanoparticle of the order of 50 nm in diameter, and particles of large diameter which can reach 500 nm incorporated in the surface, the interaction of these particles creates a form of deposition called the clusters, those clusters collide one another in order to increase their number and size until the coalescence density where the agglomeration commences, and the uncoated substrate surface reduces.

a) b)



c) d)

Fig.5.8: SEM pictures, magnification of the double layers Al-Si and Ti-W-N on stainless steel with apparition of

nanoparticles

78

Chapter 5: Analysis of the results

The morphology and the topography of the layer were studied by the SEM on a cross section of a silicon bar with the thin layer (fig.5.9). The cross section of the bar shows a columnar structure terminating with domes at the surface, with grain diameter between 50 and 400 nm.

The mean thickness measured is about 1.1 rim, which lasted one hour of deposition, what means that the growth rate is equal to 0.30 nm/ s. That rate is a very large, which means that the deposition is not an atom by atom layer. It is reasonable to assume that non-uniform nucleation at the substrate surface at the beginning of deposition is the main cause of the nonuniform growth of the layer, which cause to the random Nano-crystallization, and this can explain the XRD results.

a) b)



c)

d)

Fig.5.9: SEM pictures; cross section of a silicon bar with a Ti-W-N layer

5.4. Interpretation of nanoindentation results

The charge-discharge curves of different bars for different depths and effort are given in the

annex, the mechanical properties obtained for each test are given in the following curves, besides the tables of the results are given in the Annex II.

Chapter 5: Analysis of the results



Modulus of elasticity E (G Pa )

140

120

100

40

80

60

20

0

0 50 100 150 200 250 300 350 400

Max depth (nm)

79

Fig.5.10: Scatter plot of modulus of elasticity as a function of the max depth (nm)



Hardness (GPa)

3000

2500

2000

1500

1000

500

0

0 50 100 150 200 250 300 350 400

max depth (nm)

Fig.5.11: Scatter plot of hardness as a function of the max depth (nm)

Chapter 5: Analysis of the results



Modulus of elasticity E (G Pa)

4000

8000

7000

6000

5000

3000

2000

1000

0

0 50 100 150 200 250

max depth (nm)

80

Fig.5.12: Scatter plot of modulus of elasticity as a function of the max depth (nm)



Hardness H (MPa)

350

300

250

200

150

100

50

0

0 50 100 150 200 250

max depth (nm)

Fig.5.13 Scatter plot of hardness as a function of the max depth (nm)

Chapter 5: Analysis of the results



Modulus of elasticity E (GPa)

300

250

200

150

100

50

0

0 50 100 150 200 250

max depth (nm)

81

Fig.5.14 scatter plot of modulus of elasticity as a function of the max depth (nm)



Hardness H ( M Pa)

4000

9000

8000

7000

6000

5000

3000

2000

1000

0

0 50 100 150 200 250

max depth (nm)

Fig.5.15 scatter plot of hardness as a function of the max depth (nm)

There are in fact four factors to optimize in order to increase the hardness of a layer: the nature of the material of deposition; its microstructure and its architect which are more or less interdependent, and also there is the ionic bombardment during the growth of the film.

If we make a comparison between The hardness values obtained and the hardness values of layers with titanium, we find that our values are of low hardness, this can be explained by the low content of interstitial element (nitrogen) which is 5%, what means a large percentage of

82

Chapter 5: Analysis of the results

substitutional elements (the transition elements of titanium and tungsten). Indeed, when the content of transition elements increase, the average size of the crystallites decreases [TAK08/A], the decrease in the size of the crystallites is the origin of the decrease in the hardness. This effect is found in the Hall and Petch law, which reflects the fact that the tensile stress of material is inversely proportional to the square root of the mean crystallite size [TAK08/A].



Modulus of elasticity E (G Pa )

170

165

160

155

150

145

140

0 20 40 60 80 100 120 140 160 180 200

max depth (nm)

Fig.5.16: Scatter plot of modulus of elasticity as a function of the max depth (nm)



Hardness H (MPa)

20000

18000

16000

14000

12000

10000

4000

8000

6000

2000

0

0 20 40 60 80 100 120 140 160 180 200

max depth (nm)

Fig.5.17: Scatter plot of hardness as a function of the max depth (nm)

We can see in this latter bar, that its hardness is very high compared to that measured in the preceding bars and reach a value of 17172 MPa. Indeed, large harnesses are measured in the

83

Chapter 5: Analysis of the results

case of multilayers, and the cause of this phenomenon is generally attributed to a combined effect including the blocking of the dislocations at the interfaces between the layers and the intrinsic structural differences in each layer [CIE99].

5.5. Interpretation of tribometer results 5.5.1. The coefficient of friction

The coefficient of friction is not an intrinsic property for a single material, but it is a value which depends on a couple of materials. In our study, the antagonist material of our bars is the aluminaAl2O3. The conditions of the study are given in the previous chapter.

Figure 5.18 shows the coefficient of friction variation for the Al-Si bare in terms of distance traveled, and Table 5.5 shows the statistical results of the coefficient of friction obtained.



Fig.5.18: Curve of the coefficient of friction variation for the Al-Si bare as a function of distance traveled

u min	u max	u moyen	Standard deviation
0,00753824645653367	0,845997452735901	0,233906209468842	0,138767600059509

Table5.1 statistical results of the coefficient of friction obtained

According to the curve, it can be seen that, from 0 m to 7.4 m, the coefficient of friction value is low (between 0.06 and 0.26), after that it increases abruptly to reach higher values (between 0.62 and 0.85), this is due to the existence of an aluminum oxide (alumina) layer on the substrate surface with very low thicknesses, and which has a low coefficient of friction. If the values corresponding to the alumina are eliminated, the following results are obtained:

84

Chapter 5: Analysis of the results



Fig.5.19: Curve of the coefficient of friction variation for the Al-Si bar as a function of distance traveled
without the alumina values

u min	u max	u moyen	Standard deviation
0,1744	0,846	0,5253	0,1154

Table5.2 statistical results of the coefficient of friction obtained

The curve in Figure 5.20 shows the friction coefficient variation for the Al-Si bar with the Ti-W-N layer, and Table 5.7 gives the statistical results obtained.



Fig.5.20: Curve of the coefficient of friction variation for the Al-Si bar with Ti-W-N layer as a function of distance

traveled

85

Chapter 5: Analysis of the results

u min	u max	u moyen	Standard deviation
-0,0074	0,1644	0,0857	0,0145

Table5.3: Statistical results of the coefficient of friction obtained

Comparing the results obtained for a piston without and a piston with a thin layer of Ti-W-N, we noticed that the coefficient of friction decreases from 0.5253 to 0.0857, that is to say the use of this thin layer Allows us to reduce piston friction (if we consider that the antagonist material is alumina) by about 613%.

By way of comparison of the coefficient of friction of the layer for different substrates, we can give the results obtained from this layer deposited on the stainless steel 304 and on a layer of Al-Si deposited by evaporation on the stainless steel 304.



Fig.5.21: Curve of the coefficient of friction variation for the stainless steel bar with Ti-W-N layer as a function of

distance traveled

u min	u max	u moyen	Standard deviation
0,0031	0,2477	0,0839	0,0182

Table5.4: Statistical results of the coefficient of friction obtained

86

Chapter 5: Analysis of the results



Fig.5.22: Curve of the coefficient of friction variation for the stainless steel bar with double layers of Al-Si and Ti-W-N as a function of distance traveled

u min	u max	u moyen	Standard deviation
0,0031	0,2477	0,0839	0,0182

Table5.5: Statistical results of the coefficient of friction obtained

From these results it can be estimated that the mean value of the coefficient of friction for the torque Ti-W-N / Al2O3 is 0.08, which represents in fact a low value.

If we compare the results obtained between the bar with and a without the thin layer of Ti-W-N in terms of dissipated energy in the contact during the work, using the Coulomb's law:

E_s= u
· Fn
·Ds

u represent the sliding friction coefficient which has a value of 0.52 for the Al-Si bar without Ti-W-N layer and 0.08 for the Al-Si bar with Ti-W-N layer;

F_n is the normal load applied on the contact and has a value of 1 N;

D_s is the distance traveled between the torque and it's equal to 10 m.

The dissipated energy in the torque Al-Si/Al2O3 is equal to 5.2 Joules for every 10 meters of piston movement i.e. 0.52 J/m, but in the torque Ti-W-N/Al2O3 it's equal to 0.8 joules for each 10 meters that is to say 0.08 J/m.

It should be noted that the most of this dissipated energy in the torque (more than 95%) [COU64] is a heat energy, which requires for the piston without layer more expense in lubrication and cooling. The reduction of these losses results a gain in the output and therefore in the size and mass of the cooling systems and so of the engine.

87

Chapter 5: Analysis of the results

We can add that the propagation of this heat inside the piston under the effect of the thermal conduction causes the initiation of the cracking and the creation of thermal stress inside the piston.

The reduction of the friction coefficient by about 613% can be explained by the studies done by BUCKLEY [BUC67] and COURTEL [COU64] which point that the friction is minimal on the crystalline planes with the maximum atomic density, and on these planes in the directions of maximum atomic density.

If we make a comparison of the friction factors in different atomic structures we can notice that the crystalline materials with a hexagonal structure have generally lower values of the coefficient of fiction than those in cubic structures.

The origin of this phenomenon is that the atoms of the hexagonal structures are strongly bound with all their neighbors and have just some freedom to establish interfacial bonds with the atoms of the antagonistic materials. On the other hand, the coefficient of friction depends on the force required to break the interfacial binding between the torque surfaces [GRA08].

The structure of aluminum is a face-centered cubic structure, but Titan and nitrogen have a hexagonal structure, and tungsten has a centered cubic structure, that structure has sliding planes less than those of face-centered cubic structure.

5.5.2. Wear Rate

The method allows us to obtain the wear rate was given in chapter 4. This method based on the measurement of the wear track using the optical microscope. The representation of the wear track for the different bars is given in the following figures, and the values of volume of tearing mater and that of wear rates are given in Table 5.6.



Fig.5.23 the wear track on the Al-Si bar without layer



88

Chapter 5: Analysis of the results

Fig.5.24 The wear track on the Al-Si bar with the layer of Ti-W-N

Fig.5.25 The wear track on the stainless steel bar with the layer of Ti-W-N

Fig.5.26 The wear track on the stainless steel bar with double layers of Al-Si and Ti-W-N

89

Chapter 5: Analysis of the results

Al-Si without layer		Al-Si with Ii-W-N
volume of tearing mater (mm^3)	Wear rate (mm2/N)	volume of tearing mater (mm^3)	Wear rate (mm^2/N)
2325.52	0.2326	1309.18	0.1309
Stainless steel with Ii-W-N		Stainless steel with Al-Si and Ii-W-N
volume of tearing mater (mm^3)	Wear rate (mm2/N)	volume of tearing mater (mm^3)	Wear rate (mm^2/N)
798.79	0.08	3614.29	0.3614

Table 5.6 Volume of tearing mater and the wear rate of different bars

We can see from these results that the wear rate for a piston without a layer has an acceptable value of 0.2326 mm2 / N, but it can be reduced by almost the half when we add the Ti-W-N layer.

For the Ti-W-N layer, the wear rate value is varied between 0.08 and 0.13, which represents very good results for a torque of metal / metal which has an average of values between 1 to 0.1 mm²/N [HOL58].

5.6. Corrosion test results

In this test, we search to resolve the question of whether the thin layer of Ti-W-N is able to protect the metal substrate against corrosion in a solution of 5% NaCl at 25° C.

The results presented in this paragraph concern the thin layer of Ti-W-N produced by the cathode sputtering process which is shown in the EVANS diagrams in order to obtain the corrosion rate of the layer.

The polarizability curve log(i) = f(i) allow us to determine the corrosion potential E(I = 0) using the Tafel lines and also the corrosion current. This quantity has a huge importance to evaluate the corrosion rate by the Faraday law.

We can notice that the two curves are parallel, which indicate that the two bars have an identical corrosion mechanism.

Nevertheless those results are totally in contradiction with the fact that the different compounds formed in the layer (TiN, TiO2, W2O3...) have good corrosion resistance characteristics in aggressive medium. We can explicate this contradiction by saying that this layer is deposited in the form of column with significant porosity between the grains formed, and this will cause to concentrate the phenomenon of corrosion on small zones, that type of corrosion called pitting (see chapter 2), which acceleration the degradation of the layer .

These results can be overcome by:

? Variation of the layer deposition parameters, the increasing of the deposition energy lead to more compact and denser structure;

Chapter 5: Analysis of the results

? Variation of nitrogen concentration.



SS + TI-W-

N

1

0,1

0,01

log(i), i en A

0,001

0,0001

,00001

0,000001

0,0000001

1E-08

1E-09

1E-10

90

-0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4

Potential (V)

Fig.5.31: Curve of polarizability of the bars of the stainless steel with and without the layer on a logarithmic scale

Chapter 6

Piston's loads study

92

Chapter 6: Piston's loads study

6.1. Mechanical analysis of the internal combustion engine piston using SOLIDWORKS

The piston is one of the most important components of the internal combustion engine. Piston fails mainly due to mechanical stresses and thermal stresses. In this paper is determined by using the finite element method, stress and, deformation and displacement distribution.

6.1.1. Initial data

The study considers a flat sample of aluminium - silicon alloy with dimension of (20X20 mm), subjected to 50 KN of normal force which is equivalent to 100 bar.



Fig.6.1. Dimensions of the piston (a) and 3D model of the piston (b)

The material used for the piston is aluminium -silicon eutectic alloy. The properties used are in table 1.

properties of the material value

Density 2670 kg/m3

Young's modulus 95.88 Gpa

Poisson `s ratio 0,33

Tensile yield strength 66.13 Mpa

Table 6.1.properties of the sample

93

Chapter 6: Piston's loads study

The material properties used for the Ti-W-N thin layer, are in table 2.

properties of the material value

Density 4810 kg/m3

Young's modulus 122.73 Gpa

Poisson `s ratio 0,33

Tensile yield strength 160.8 Mpa

Table 6.2 Properties of the Ti-W-N thin layer

6.1.2. Finite element analysis of the sample 6.1.2.1. Sample with thin coating



Fig.6.3. shear stress due to mechanical load for the sample with the thin film coating



Chapter 6: Piston's loads study

94

Fig.6.4. VON MISES stress due to mechanical load for the sample with the thin film coating



Fig.6.5. Normal stress due to mechanical load for the sample with the thin film coating

95

Chapter 6: Piston's loads study

6.1.2.2. Sample with thin coating



Fig.6.6. Shear stress due to mechanical load for the bare sample



Fig.6.7. VON MISES stress due to mechanical load for the bare sample



Chapter 6: Piston's loads study

96

Fig.6.8: Normal stress due to mechanical load for the bare sample

	Sample without coating	Sample with coating
Shear stress	Sc = 2	Sc =4.9
VON MISES stress	Sc = 2.2	Sc = 5.4
Normal stress	Sc = 1.6	Sc = 3.9

Table.6.3. table summarizing the minimum values of safety coefficient 6.1.3. Analysis of the result

Table 6.3 shows clearly the impact of the thin films to the resistance characteristic of the sample, just by using a 1um of hard thin films the minimum value of safety coefficient jumped with 2.5 times to a much safer zone

Analysing the figures 6.4 and 6.8 we noticed a small drop on the VON MISES stress in the sample with the thin coating, even if this result is not what we hoped for, but it proof that a thin film can provide much greater characteristic just by using a small amounts of materials.

Conclusion

103

Conclusion

Conclusion

In this study our main purpose is to have very thin coating with high mechanical and thermal properties ( hardness and low friction coefficient) in order to use it in the car industry to limit the energy losses by friction between the piston and the cylinder , the used samples are taken from a real piston to have more realistic results.

In the way of searching to upgrade the mechanical characteristics of ordinary surfaces (the wear and the abrasive resistance ), we chose to deposit a thin layer based on titanium and tungsten in a medium containing 5% of nitrogen gas and the rest is argon gas, the choice in its se is innovative because this coating is not a very well-known one in the industrial field , the combination of titanium which is relatively a light material with excellent corrosion resistance and in the other side the hardness of the tungsten and its very high melting point provide the necessary improvement for the engine pistons to reach a very high efficiency .

We noticed that The thin film layer grow mainly from a nanoparticles with a diameter less than 1um, the interaction of this nanoparticles with each other form a micro particles called clusters, the growth of this clusters will form a columnar structure which will end by creating a granular structure with an inter-granular space which is in direct relation with the used energy and the deposition speed.

The measurement of the thin film thickness indicate that the deposition speed have a value of 0.30nm/s which is a very high speed if we take in consideration that the diameter of the atoms is a few Angstroms , so the growth of the thin film layer is taking place by accumulation of macro molecules with different orientation on the top of the surface which will cause a none Crystallographic alignment to appear, so in order to have a uniform structure we recommend to increase the energy of deposition in in order to break the molecules bond to a small one which will increase significantly the density and the porosity of the thin film enabling us to reach a higher mechanical and thermal properties

The obtained structure present a low hardness of 2 Gpa with a value of 0.9 for the friction coefficient which is very good if we take in consideration that the deposited thin layer have an amorphous and heterogeneous structure and low density.

Even if the result of the Ti-W-N thin layer deposited on a sample of aluminum -silicon sample are not what we hoped for because they didn't reach the hardness of the usual hard thin films, in the other side the samples of stainless steel with the same deposition condition of temperature and pressure gave much greater value than the aluminum samples

104

Conclusion

In order to a better characterization of the surface we established a corrosion test for the samples with and without thin layer , the results were significant , and they confirmed the previous results , the corrosion rate for the coated area were much greater than the bare ones which is mainly due to the high porosity of the surface

This study enable us to break into the nanomaterials world and to discover the and to discover the large potential of this field and for our study it's still promising in order to achieve the hardness of the ultra-hard materials, and also the part that we didn't get into it which is the thermal conductivity of the surface mainly because the lack of the necessary equipment to measure the thermal conductivity which is a very important parameter to determine if the coating represent a thermal barrier between the combustion chamber and the aluminum piston, and if its right it will decrease the energy losses through the piston enabling us to reach high thermal efficiency.

Appendices

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Appendix I: nanoindentation loading-unloading curves

Appendix I: Nanoindentation loading-unloading curves

1. Aluminum-silicon without a layer



Appendix I: nanoindentation loading-unloading curves



Fig.I.1: loading-unloading curves for 7 tests of aluminum-silicon sample without a layer

2. Aluminum-silicon with a Ti-W-N layer



AI 3

AI 4

Appendix I: nanoindentation loading-unloading curves





Appendix I: nanoindentation loading-unloading curves

AI 5

Fig.I.2: loading-unloading curves for 15 tests of aluminum-silicon sample with a Ti-W-N layer

3. Stainless steel with a Ti-W-N layer



AI 6

Appendix I: nanoindentation loading-unloading curves



AI 7

Appendix I: nanoindentation loading-unloading curves





Appendix I: nanoindentation loading-unloading curves

AI 8

Fig.I.3: loading-unloading curves for 25 tests of stainless steel sample with a Ti-W-N layer

4. Stainless steel with double layers of Al-Si and Ti-W-N



Appendix I: nanoindentation loading-unloading curves



AI 9

Fig.I.4: loading-unloading curves for 05 tests of stainless steel sample with double layers of Al-Si and Ti-W-N

AII 1

Appendix II: Nanoindentation results

Appendix II: Nanoindentation results

1. Aluminum-silicon without a layer

Tests	Modulus of elasticity E (GPa)	Hardness H (MPa)	Hardness HV
#01	95.921	2550.3	236.18
#02	100.03	2105.9	100.03
#03	115.81	2615.5	242.23
#04	101.34	2309.9	213.93
#05	86.902	1774.4	164.33
#06	69.493	2051.5	189.99
#07	101.67	2615.5	242.23

Table II.1 results of 7 tests of the aluminum-silicon sample without layer

2. Aluminum-silicon with a Ti-W-N layer

Tests	Modulus of elasticity E (GPa)	Hardness H (MPa)	Hardness HV
#01	150.87	4214	390.26
#02	89.186	3795.2	351.48
#03	184.44	6037	559.09
#04	316.43	4546.2	421.03
#05	109.92	6358.7	588.89
#06	142.71	6218.1	575.86
#07	109.32	3810.1	352.86
#08	153.16	5774.6	534.79
#09	112.85	5957.7	551.75
#10	103.58	6354.5	588.49
#11	117.62	7143	661.52
#12	79.703	4012.2	371.57
#13	121.8	4289.5	397.25

AII 2

Appendix II: Nanoindentation results

#14	127.34	4149	384.24
#15	106.49	5476.3	507.17

Table II.2 results of 15 tests on the aluminum-silicon bar with a Ti-W-N layer

3. Stainless steel with a Ti-W-N layer

Tests	Modulus of elasticity E (GPa)	Hardness H (MPa)	Hardness HV
#01	125.53	6187.7	573.05
#02	131.93	6494	601.41
#03	118.86	6714.9	621.87
#04	142.77	7271.3	673.4
#05	138.92	7634.4	707.03
#06	122.4	7072	654.95
#07	113.57	6949.6	643.61
#08	128.51	6735.1	623.74
#09	134.99	6289.6	582.49
#10	162.25	7032	651.24
#11	145.59	5856.7	542.4
#12	128.83	6301.8	583.61
#13	176.64	6328.8	586.12
#14	148.6	6880.3	637.19
#15	158.82	7079.3	655.62
#16	160.43	6146.1	569.19
#17	175.3	7308.6	676.85
#18	115.72	6513.4	603.21
#19	149.03	6701.1	620.6
#20	145.26	6627.9	613.81
#21	147.96	5767.6	534.15
#22	155.61	6252.9	579.09
#23	159.42	5783.1	535.58

AII 3

Appendix II: Nanoindentation results

#24	148.23	5736.4	531.26
#25	239.02	7450	689.96

Table.II.3 results of 25 tests on the stainless steel bar with a Ti-W-N layer

4. Stainless steel with double layers of Al-Si and Ti-W-N

Tests	Modulus of elasticity E (GPa)	Hardness H (MPa)	Hardness HV
#01	151.25	8561.6	792.9
#02	143.94	8780.6	813.18
#03	164.64	17172	1590.3
#04	161.26	10408	963.9
#05	149.71	11397	1055.5

Table.5.4 results of 05 tests on the stainless steel bar with double layers of Al-Si and Ti-W-N