A CRITICAL REVIEW ON DIFFERENT TYPES OF WEAR OF MATERIALS

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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

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ISSN Print: 0976-6340 and ISSN Online: 0976-6359

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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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Dr. A. Devaraju


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

A Critical Review on Different Types of Wear of Materials


(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

Dr. A. Devaraju


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].



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

Dr. A. Devaraju


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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[5] Keller, D. V. “Adhesion between solid metals”, Wear, Vol.6, pp. 353-365, 1963.

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modulated adhesion and the effect of adsorbates”, J. Adhesion, pp. 83-86, 1972.

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A CRITICAL REVIEW ON DIFFERENT TYPES OF WEAR OF MATERIALS