1. Define Wear. Explain different types of wear mechanisms with relevant examples.

Ans.
Wear:- Wear is a process of interaction between surfaces, which causes the deformation and
removal of material on the surfaces due to the effect of mechanical action between the sliding
faces. Wear also refers to the dimension loss of plastic deformation.
There are different types of wear mechanisms, and they can be classified as follows:-
1 Adhesive wear
2 Abrasive wear
3 Surface fatigue
4 Fretting wear
5 Erosive wear
6 Corrosion and oxidation wear
1 Adhesive wear
Adhesive wear can be found between surfaces during frictional contact and generally
refers to unwanted displacement and attachment of wear debris and material
compounds from one surface to another. Generally, adhesive wear occurs when two
bodies slide over or are pressed into each other, which promote material transfer.
2 Abrasive wear
Abrasive wear occurs when a hard rough surface slides across a softer surface. ASTM
International defines it as the loss of material due to hard particles or hard
protuberances that are forced against and move along a solid surface. Example:-
Damage of crankshaft journals in reciprocating compressors.
3 Surface fatigue
Surface fatigue is a process in which the surface of a material is weakened by cyclic loading,
which is one type of general material fatigue. Fatigue wear is produced when the wear particles
are detached by cyclic crack growth of microcracks on the surface. These microcracks are either
superficial cracks or subsurface cracks. Example:- Wearing of piston sleeve surface.
4 Fretting wear
Fretting wear is the repeated cyclical rubbing between two surfaces. Over a period of
time fretting which will remove material from one or both surfaces in contact. It occurs
typically in bearings, although most bearings have their surfaces hardened to resist the
problem.
5 Erosive wear
Erosive wear can be defined as an extremely short sliding motion and is executed within a short
time interval. Erosive wear is caused by the impact of particles of solid or liquid against the
surface of an object.
6 Corrosion and oxidation wear
Corrosion and oxidation wear occurs both in lubricated and dry contacts. The
fundamental cause are chemical reactions between the worn material and the corroding
medium.[20] Wear caused by a synergistic action of tribological stresses and corrosion is
also called tribocorrosion.
 2. Explain the factors on which wear depends?
Factors affecting the rate of wear:
Hardness – particles with hardness lower than the surface cause little wear.
Shape– angular particles cause greater wear than rounded particles.
Size – larger particles cause more extensive wear as they carry more kinetic energy
Impact speed (for erosion) – particles with greater speed cause more extensive wear as they
carry more kinetic energy.
Impact angle (for erosion) – particles hitting at angles close to perpendicular to the surface
cause greater erosion.

3. Illustrate the significance of Archard’s equation for adhesive wear with Analytical expressions

The importance of wear losses leads to considerable effort in establishing theories and predictive
models of wear. Meng and Ludema  [1] have identified 182 equations for different types of wear. Among
them were empirical relations, contact mechanics-based approaches, such as Archards model, and
equations based on material failure mechanisms, which were found to get more popular recently
according to authors. In this review, empirical equations won’t be considered, as they are applicable for
very specific range of parameters. No unified fundamental theory of wear was established so far, and as
a consequence, there is no unique wear model, applicable in all cases.

One of the most famous and frequently used wear equations was developed by Holm and Archard in
1953[2]. The model considers adhesive wear and assumes the sliding spherical asperities to deform fully
plastically in contact. The area of contact then is circular with the contact area equal to   , where   is
the radius. The mean contact pressure in this case equal to hardness of the softer material, and
thus,   . After the asperity slides a distance of  , it is released from the contact and there
is a probability  , that debris will form. It is assumed, that if debris is formed, it is formed as a
hemisphere with the radius  , having a volume of  . Then the wear volume per sliding
distance    is  , and hence, as  ,  .
Introducing  , the total wear volume for a sliding distance  , equals
to  . The coefficient   is known as a wear coefficient and is frequently
used to compare the material wear resistance[2,3]. Most of the times, it is estimated experimentally.
Although the Archard’s equation was developed for the adhesive wear, it is widely used for modeling of
abrasive, fretting and other types of wear

4. Sketch a pin-on-disc apparatus and explain how it is used to measure wear

In a pin-on-disc wear tester, a pin is loaded against a flat rotating disc specimen such that a
circular wear path is described by the machine. The machine can be used to evaluate wear and
friction properties of materials under pure sliding conditions. Either disc or pin can serve as
 specimen, while the other as counterface. Pin with various geometry can be used. A convenient
way is to use ball of commercially available materials such as bearing steel, tungsten carbide or
alumina (Al2O3) as counterface, so that the name of ball-on-disc is used

5. What is the measurement method used to measure the following wear formats

A. Mass loss

B. Volume loss

C. Linear Dimension measureMass loss

Mass loss

Mass loss measurement by a precision balance is a convenient method for wear

measurement, especially when the worn surface is irregular and unsymmetrical in
shape. Sample to be measured is carefully cleaned, and the weight is measured before
and after a wear test. The difference in weight before and after test represents the
weight loss caused by wear. The unit can be gram (g) or milligram (µg).

Volume Loss

Wear volume is normally calculated from the wear track (scar) depth, length, width
and/or scar profile according to the geometry of the wear track/scar. A surface
profilometer, e.g. a stylus type, or sometimes a microscope with scale is used for the
measurement. The reporting unit of wear volume loss is mm3 or µm3. Wear volume loss
enables a better comparison of wear among materials having different densities.
However, it is not easy to measure volume loss when a wear track is irregular. In this
case, mass loss may be measured first, and the volume loss is calculated if the materials
is uniform and its density is known.
 Linear dimension loss

Measuring wear by linear dimension change is very useful in many engineering

situations, where certain dimension such as length, thickness or diameter is more
critical to the normal function of the system. A surface profilometer, e.g. a stylus type, a
micrometer or a microscope can be used. The unit for linear dimension loss can be µm
or mm.

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