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
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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
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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
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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
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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
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- 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.
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Chapter 1: Overview on engine's piston
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