Many mechanical equipments are subjected to sliding contact in real time applications. Pumps, valves, belt drives, bearings, machinery guide ways, piston- cylinder arrangements etc. are the few important sliding components which are continuously subjected to sliding wear. Much mechanical equipment’s failure occurred due to wear related problems.

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 6, Issue 11, Nov 2015, pp. 77-83, Article ID: IJMET_06_11_009
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=11
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
A CRITICAL REVIEW ON DIFFERENT
TYPES OF WEAR OF MATERIALS
Dr. A. Devaraju
Principal and Professor,
Department of Mechanical Engineering,
Adhi College of Engineering and Technology,
Affiliated to Anna University,
Kanchipuram-631 605, Tamilnadu, India
ABSTRACT
Many mechanical equipments are subjected to sliding contact in real time
applications. Pumps, valves, belt drives, bearings, machinery guide ways,
piston- cylinder arrangements etc. are the few important sliding components
which are continuously subjected to sliding wear. Much mechanical
equipment’s failure occurred due to wear related problems. Therefore,
understanding of different wear mechanism is important to design the
mechanical components. In this paper, various wear mechanisms have been
discussed with the help previous published research works and text books.
Key words: Wear Mechanism, Wear Rate, Mechanical Components,
Lubrication
Cite this Article: Dr. A. Devaraju. A Critical Review on Different Types of
Wear of Materials, International Journal of Mechanical Engineering and
Technology, 6(11), 2015, pp. 77-83
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=11
1. INTRODUCTION
When two solid surfaces are in contact, there is damage to the surface and/or
subsurface. Wear is the removal of solid metal from the one or both surface of which
are in solid state contact. Wear is quantified by the term 'wear rate' which is defined as
"the mass or volume or height loss of material removed per unit time or sliding
distance". The wear is characterized by mild and severe wear. The outcome of mild
wear, the worn surfaces is smooth and smaller in wear debris (typically 0.01µm to
1µm in particle size).
In contrast, the severe wear results in larger wear debris size (20 µm to 200 µm)
which can be seen in naked eye and roughened worn surface. The important wear
mechanisms are adhesive wear, abrasive wear, delamination wear, erosive wear,
fretting wear, fatigue wear and corrosive wear [1]. The wear behavior of materials is
important in tribology like frictional force [2]. The wear surfaces can be protected

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mainly in two ways: (1) separation of surfaces by applying a lubricant, and (2) surface
modification.
2. WEAR AND ITS MECHANISMS
As per available literature and current researcher’s knowledge concerned, seven
important types of wear mechanisms exhibit for different metal pairs. These seven
mechanisms are discussed as follows;
2.1. Adhesion
Adhesive wear is due to transfer of material from one surface to another surface by
shearing of solid welded junctions of asperities. It leaves pits, voids, cavities or valley
on the surface [3]. This wear occurs because of the adhesive bond. At the contact
points, the adhesive bond is stronger than the cohesive bond of the weaker material of
the Pair. Normally, adhesion occurs when two similar chemical composition metals
are in contact or contact surface are free from oxide layer (vacuum or an inert
atmosphere).Fig. 1.illustrates the adhesive wear mechanism of steel vs. indium Pair.
Figure 1 A schematic diagram illustrating adhesive wear mechanism [4].
When a clean steel or brass rounded end rod is pressed on the block of soft and
ductile metals such as lead and indium, strong adhesion will occur. When the rod is
removed, a fragment of soft metal (indium) adheres to the rod. It shows that the
adhesive strength of the contact junctions are stronger than the cohesive strength of
indium. The small addition of alloying element in the bulk material can alter the
adhesion between the solid surfaces. For example, the addition of sulfur in steel
enhances its machinability. Further, during sliding process, the iron sulfide comes out
of the surface and reduces friction as well as wear.
Similarly, the cast iron produces better tribological property than iron based
alloys. The reason is that the graphite becomes smeared out over the contact zone and
provides a lubricating film [3]. In the case of dissimilar metals, when the mutually
insoluble metals come in contact with each other, they would generally exhibit poor
adhesion [2,5,6]. However, if the surfaces are atomically clean, the adhesion would be
strong for this case also. Irrespective of solubility, the degree of softness also plays an
important role in adhesion. The soft metals exhibit a large real area of contact which
is responsible for high adhesion [7]. Although the use of lubricants at the contact
surfaces reduces the surface energy, the condensate of liquid film or pre-existing film
can significantly increase the adhesion [8,9].
2.2. Abrasion
Wear occurs due to hard particles or protuberances sliding along a soft solid surface.
It results in ploughing, wedging and cutting phenomena. In ploughing (also called
ridge formation) process, material is displaced at both the sides and forms a groove
with or without removal of material. The fundamental abrasive wear mechanism is
shown in Fig.2. There are two modes of abrasive wear: (1) Single body abrasive wear

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(Fig. 2(a)) in which abrasive marks will occur on one surface. The practical example
for single body abrasive wear is grinding, cutting and machining. (2) Two body
abrasive wear (Fig. 2(b)) in which abrasive marks will occur on both surfaces. In
tribological systems, the debris becomes entrapped between the contact surfaces and
makes grooves on one or both the contact surfaces.
In some practical applications like polishing process, the abrasive particles are
beneficial or desirable since it produces polished surfaces. The ridges formed during
abrasion or ploughing process become flattened after some sliding distance and
fractured due to repeated cyclic system [10, 11]. It also causes subsurface deformation
and surface as well as subsurface crack nucleation. The hardness is an important
property to control the abrasive wear. The experimental evidence reported that the
wear rate of two body abrasions is inversely proportional to the hardness [12] and
proportional to the normal load and abrasive particle size for many pure metals [13].
However, the complex behavior has been observed for alloys [14-16]. Wear
resistance of annealed pure metals are also directly proportional to their hardness but
more complex for alloys [12, 17, 18]. The reason for decrease of wear rate for longer
sliding distance experiments has been reported as (a) result of blunting of abrasive
surfaces and (b) clogging of the abrasive surface by wear debris [2].
Figure 2 A schematic diagram of abrasive wear mechanism (a) Single body abrasive
(b) Two body abrasive [3]
2.3. Erosive wear
Wear due to mechanical interaction between solid surface and fluid, or impinging
liquid or solid particles is called erosive wear. When particles with some velocity are
impacted on the surface of metal, the pits and large scale subsurface deformation
occur on the metal surface. The best example is when the rain droplets with different
velocities hit normal earth surface; it removes the surface and causes erosive wear. In

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plasma nitriding process, the sputtering is done to clean the specimens. In sputtering,
the argon ion which is in the gaseous form strikes the specimen surface and removes
the oxide layer.
From the practical point of view, the erosive wear is important. However, in some
experiments conducted with ceramic surfaces, the impingement of silicon carbide
particles with high velocity causes localized surface melting [19]. There is a
fundamental relationship between material loss and cohesive binding energy of the
metal. It has been proved that the cohesively stronger metals exhibit lower erosive
wear than cohesively weaker metals [20].
2.4. Fretting wear
Wear due to small amplitude of oscillatory or reciprocating movement between two
surfaces is known as fretting wear. It is a two step mechanism. Initially, the adhesive
wear occurs due to rubbing of two surfaces and then they become oxidized due to
large quantity of energy stored in wear particles.
2.5. Fatigue/ Delamination wear
Wear caused by fracture arising from surface fatigue due to cyclic loading is called
Fatigue/ Delamination wear. It results in a series of pits or voids. It usually occurs in
rolling or sliding contact bodies such as bearings, roads, etc. After repeated cyclic
loading, a crack is observed on the subsurface or the surface. The subsurface cracks
propagate, connect with other cracks, reach the surface and generate wear particles.
Similarly, the surface cracks move downward into bulk, connect with other cracks and
liberate a wear particle. The crack propagation is influenced by a number of factors.
The relative humidity in the air is one of the important factors. It has been
experimentally reported that the crack growth occurs rapidly in high moisture
environment rather than in dry air [21].
2.6. Corrosive/ Oxidative wear
Corrosive wear occurs when sliding takes place in corrosive or oxidative
environment. During dry sliding also, the oxygen from the normal environment or
other gases present in the environment can react with the solid surface. The excessive
presence of antiwear additives or other chemical agents also can bring corrosive wear.
At elevated temperature, oxygen can interact with sliding surface and form oxides
called oxidative wear. For example, oxidation of Inconel (nickel –chromium alloys
containing some iron) occurs at 100ºC resulting in the formation of nickel oxide
(NiO) and chromium oxide (Cr2O3). However, when the temperature is increased to
280ºC, the surface contains spinel of NiFe2O4 near the surface and Cr2O3 near the
metal interface [22]. It results in the formation of weak, mechanically incompatible
corrosive/oxide layer.
2.7. Deformation and Ploughing
When hard rough surface slides over a soft metal surface, the frictional resistance is
mainly developed by the asperities of hard surface ploughing through soft material
[23]. The force required for plastic flow of softer material represents the friction
coefficient. The ploughing of the surfaces by hard asperities and wear particles is
found to be the most important mechanism in most sliding situations [24].

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3. LUBRICATION
Lubrication is the process of introducing lubricants between contact surfaces to
reduce the frictional force. The main property of the lubricant is that it should produce
very lower shear strength and form a layer between the sliding surfaces [25]. In some
lubricating systems, although the lubricant film may not completely separate the
asperity contacts, it reduces the strength of the junctions formed. In other cases, the
lubricant film completely separates the surfaces and no asperity junctions are formed
at all. Regimes of lubrication are normally associated with dominant lubrication
mechanism involved in the mechanical system. The three main methods of lubrication
are: (1) hydrodynamic (or full film) lubrication, (2) boundary lubrication, and (3)
mixed lubrication [26].
Figure3 Methods of lubrication (a) Hydrodynamic lubrication (b) Boundary
lubrication and (c) Mixed lubrication
In hydrodynamic lubrication (Fig. 3(a)), the adequate pressure of fluid is supplied
between two contact surfaces which are in relative motion. The layers of fluid
completely separate the contact surfaces and support the load. In boundary lubrication
regime (Fig.3(b)), thin mono-layer of fluid film is formed between the frequent
asperity contact that leads to high values of coefficient of friction and wear compared
to hydrodynamic lubrication. Mixed film lubrication (Fig.3(c)) is the combination of

6. Dr. A. Devaraju
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full film lubrication and boundary lubrication. Boundary lubrication can be defined as
the regime in which average film thickness is less than the composite roughness.
4. CONCLUSION
The various types of wear mechanism and different lubrication process have been
discussed in detail. This review concludes that wear cannot be completely eliminated
between the sliding surfaces. However, it can be reduced (1) by applying lubricants
between sliding surfaces, (2) hardening the contact surfaces by mechanical and
chemical process and (3) designing the component material according to sliding
contact conditions. Wear is occurred by combination of two or more wear
mechanisms. Hence, understanding of wear mechanisms exhibited between sliding
surfaces are important while designing the any mechanical component.
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