Tribology | 3. Introduction of Wear and Wear Mechanism

Introduction of Wear

Undesirable removal of material from operating solid surface is
known as wear. There are two definitions :

(1) Zero wear : Removal of material which causes
polishing of material surfaces may be known as “Zero wear”. It
may increase performance. It is for betterment, so it is not

Zero wear is basically a polishing process in which the
asperities of the contacting surfaces are gradually worn off
until a very fine, smooth surface develops. Generally,
“polishing-in” wear is desirable for better life of tribo-pair.
Fig. 1 shows polished surface of helical gear which occurs due
to slow loss of metal at a rate that will have a little affect
on the satisfactory performance within the life of the gears.

Zero wear
Fig.1 Zero wear

(2) Measurable wear : Removal of material from
surface that increases vibration; noise or surface roughness
may be treated an “Measurable wear”. Often we measure wear in
volume/mass reduction. Undesirable removal of material occurs
in measurable wear.

Measurable wear
Fig. 2 Measurable

Measurable wear refers to a loss of material which must be
counted to estimate the life of tribo-pair. The extent of
measurable wear depends on the lubrication regime, the nature
of the load, the surface hardness and roughness, and on the
contaminants in the lubricating oil. A typical example of
measurable wear in helical gear is shown in Fig. 2 which is
typically known as pitting wear.

Pitting :

Pitting is a surface fatigue failure which occurs due to
repeated loading of tooth surface and the contact stress
exceeding the surface fatigue strength of the material.
Material in the fatigue region gets removed and a pit is
formed. The pit itself will cause stress concentration and soon
the pitting spreads to adjacent region till the whole surface
is covered with pits. Subsequently, higher impact load
resulting from pitting may cause fracture of already weakened
tooth. Sometimes impurities in materials provide nucleus for
crack generation as shown in Fig. 3 (c). Fig. 3 (d) shows
merger of generated cracks, which finally detaches from the
surface as shown in Fig. 3 (e). Such formation of pits (removal
of material) comes under measurable wear.

Formation Of Pitting
Fig.3 Formation Of Pitting

Removal of material from operating solid surfaces by solid
particles depends upon Load, Velocity, Environment, and
Materials. Removal of material from operating solid surface by
Fluid (liquid/gas) depends upon Velocity, pressure, Environment
and material.

As wear increases power losses increases, oil consumption
increases, rate of component replacement also increases.
Ultimately, it reduces efficiency of the system. Therefore, as
far as possible wear should be minimized.

Wear Mechanisms :

Wear can be classified based on the ways that the frictional
junctions are broken, that is, elastic displacement, plastic
displacement, cutting, destruction of surface films and
destruction of bulk material. There are many types of wear
mechanisms, but we shall discuss about common wear mechanisms,
which are:

Abrasive Wear : polishing, scouring, scratching, grinding,

• Adhesive Wear : galling, scuffing, scoring.

• Cavitation (interaction with fluid).

• Corrosive Wear (Chemical nature).

• Erosive Wear.

• Fatigue : delamination.

• Fretting Wear.

1. Adhesive Wear

Adhesive wear is very common in metals. It is heavily dependent
on the mutual affinity between the materials. Let us take
example of steel and indium [Fig. 4 (a)]. When steel pin under
load is pushed [Fig. 4 (b)] in indium block, and subsequently
retracted [Fig. 4 (c)], a thin layer of indium transferred on
the steel pin. Similar behavior is observed by pushing brass
metal in indium metal. This behavior demonstrates the loss of
indium material, which occurs due to high value of adhesive
force between steel and indium. If steel pin is subjected to
normal load as well as tangential load [Fig. 4 (d)] then severe
wear of indium material occurs. By introducing a thin layer of
lubricant at the interface of indium and metal, the severe wear
can be reduced to mild wear. Shear strength of lubricant layer
is much smaller than shear strength of indium metal, therefore
weak interface between steel and indium occurs which can be
sheared easily and wear rate reduces to mild value.

Adhesive wear.
Fig.4 Adhesive

Scoring wear, a severe form of adhesive wear, occurs due to
tearing out of small particles that weld together as a result
of overheating (due to high contact pressure and/or high
sliding velocity) of the tooth mesh zone, permitting metal to
metal contact shown in Fig. 5. After welding, sliding forces
tear the metal from the surface producing a minute cavity in
one surface and a projection on the other. The wear initiates
microscopically, however, it progresses rapidly. Scoring is
sometimes referred to as galling, seizing or scuffing.

Fig.5 Scoring.

Steps leading to Adhesive Wear :

  • Deformation of contacting asperities Fig. 6 (a).
  • Removal (abrasion) of protective oxide surface film.
  • Formation of adhesive junctions Fig. 6 (b).
  • Failure of junction by pulling out large lumps and transfer
    of materials Fig. 6 (c).

Steps leading to adhesive wear
Fig.6 Steps leading to adhesive

2. Abrasive Wear

Abrasive wear, sometimes called cutting wear, occurs when
hard particles slide and roll under pressure, across the tooth
surface. Hard particle sources are: dirt in the housing, sand
or scale from castings, metal wear particles, and particles
introduced into housing when filling with lube oil. Scratching
is a form of abrasive wear, characterized by short scratch-like
lines in the direction of sliding. This type of damage is
usually light and can be stopped by removing the contaminants
that caused it. Fig. 7 shows abrasive wear of a hardened

Abrasive wear of gear.
Fig.7 Abrasive wear of

Following are few well-known reasons of abrasive wear
mechanisms :

– Micro-cutting : sharp particle or hard asperity
cuts the softer surface. Cut material is removed as wear

– Micro-fracture : generally occurs in brittle,
e.g. ceramic material. Fracture of the worn surface occurs due
to merging of a number of smaller cracks.

– Micro fatigue : When a ductile material is
abraded by a blunt particle/asperity, the worn surface is
repeatedly loaded and unloaded, and failure occurs due to

– Removal of material grains : Happens in
materials (i.e. ceramics) having relatively week grain

Basic modes of abrasive wear are classified as two
body abrasion and three body abrasion

Two – Body Abrasion :

Two-Body and Three-Body Abrasion
Fig.8 Two-Body and Three-Body

This wear mechanism happens between two interacting
asperities in physical contact, and one of it is harder than
other. Normal load causes penetration of harder asperities into
softer surface thus producing plastic deformations. To slide,
the material is displaced/removed from the softer surface by
combined action of micro ploughing and micro-cutting.

Three Body Abrasion :

Three body abrasion is material removed from softer
surface by hard loose particles(Fig. 8), which are free to roll
as well as slide over the surface, since they are not held
rigidly. The hard particles may be generated locally by
oxidation or wear from components of tribological system. Iron
oxides wear debris produced during adhesive wear cause further
damage due to abrasion. Due to rolling action, abrasive wear
constant is lower compared to 2-Body abrasion.

Wear rate is lesser in three body abrasion than two
body abrasion.

3. Corrosive Wear

Chemical reaction + Mechanical action = Corrosive

The fundamental cause of Corrosive wear is a chemical
reaction between the material and a corroding medium which can
be either a chemical reagent, reactive lubricant or even

Stages of corrosive wear :

Sliding surfaces chemically interact with environment
(humid/industrial vapor/acid)

• A reaction product (like oxide, chlorides, copper

• Wearing away of reaction product film

Passivation of corrosion. Continuous corrosion.

Fig. 9  Passivation of corrosion. Continuous

The most corrosion films passivate (Fig. 9) or cease to
grow beyond a certain thickness. This is favourable as
corrosion process stops its own. But most corrosion films are
brittle & porous, and mechanical sliding wears away the
film. The formation and subsequent loss of sacrificial (Fig. 9)
or short life-time corrosion films is the most common form of
corrosive wear.

Sliding surfaces may wear by chemically reacting with the
partner surface or the environment, or both. The oxide layers
resulting from reactions with the environment are typically 10
microns thick, and they may have a protective role unless the
thickness tends to grow during the cyclic contact process. If
the oxide layer grows, it becomes liable to break in brittle
fracture, producing wear particles. Hard, broken-off oxide
particles may then profoundly affect subsequent wear life as
abrasive agents. If soft, ductile debris results, it may form a
protective layer on the surface.

4. Fretting Wear

Fretting Wear coined in 1927 by Tomlinson. It refers to
small amplitude(1 to 300 μm), with high frequency oscillatory
movement mainly originated by vibration. This generally occurs
in mechanical assemblies (press fit parts, rivet / bolt joints,
strands of wire ropes, rolling element bearings), in which
relative sliding on micron level is allowed. It is very
difficult to eliminate such movements and the result is
fretting. Fretting wear and fretting fatigue are present in
almost all machinery and are the cause of total failure of some
otherwise robust components.

Fretting wear

Fig.10 Fretting wear

Fretting occurs wherever short amplitude reciprocating
sliding between contacting surfaces (Fig. 10) is sustained for
a large number of cycles. The centre (Fig. 10) of the contact
may remain stationary while the edges reciprocate with an
amplitude of the order of 1 micron to cause fretting damage.
One of the characteristic features of fretting is that the
produced wear debris is often retained within the contact due
to small amplitude sliding. The accumulating wear debris
gradually separates both surfaces(Fig.11) and, in some cases,
may contribute to the acceleration of the wear process by
abrasion. The process of fretting wear can be further
accelerated by temperature. Reciprocating movements as short as
0.1 micron in amplitude can cause failure of the component when
the sliding is maintained for one million cycles or