23-09-2023

CHAPTER 2

FRICTION AND WEAR

FRICTION
 Friction is the resistance to motion during sliding or rolling, that is
experienced when one solid body moves tangentially over another
with which it is in contact.
 The resistive tangential force, which acts in direction directly
opposite to the direction of motion, is called the friction force.
 There are two main types of friction that are commonly encountered:
dry friction and fluid friction.
 As its name suggests, dry friction, also called “Coulomb" friction,
describes the tangential component of the contact force that exists
when two dry surfaces move or tend to move relative to one another.
 Fluid friction describes the tangential component of the contact force
that exists between adjacent layers in a fluid that are moving at
different velocities relative to each other as in a liquid or gas between
bearing surfaces.

1
 23-09-2023

INTRODUCTION
 Invention of Wheel – Difference between Rolling and Sliding
Friction
 Energy loss is much higher for sliding friction compared to
rolling friction, when the components are reasonably rigid.
 Highly counter-intuitive: static friction has almost no effect on
rolling friction!
 Combined effect of a number of energy dissipating effects

 We do not generally talk about a “rolling friction force”!

 Rolling friction could be a misnomer; Resistance to Rolling is

much better…

2
 23-09-2023

Sliding and Rolling Friction

• Sliding friction is friction in a pure sliding motion with
no rolling and no spin as sown in figure below-

RULES OF SLIDING FRICTION

 The first rule states that the friction force, F, is directly
proportional to the nominal load, W, that is, F = μW
 The second rule states that the friction force (or
coefficient of friction) is independent of the apparent
area of contact between the contacting bodies. Thus two
bodies, regardless of their physical size, have the same
coefficient of friction.
 Alternately, it is often convenient to express this rule in
terms of constant angle of repose or frictional angle θ
defined by μs = tan θ.

3
 23-09-2023

IN THIS EQUATION, Θ IS THE ANGLE SUCH THAT ANY BODY OF ANY WEIGHT, PLACED ON
A PLANE INCLINED AT AN ANGLE LESS THAN Θ FROM THE HORIZONTAL, WILL REMAIN
STATIONARY, BUT IF THE INCLINATION ANGLE IS INCREASED TO Θ, THE BODY WILL
START TO SLIDE DOWN, FIGURE 1

Figure 1: Force equilibrium diagram for a body on an inclined plane.

To these two rules, a third rule is sometimes added which is often attributed to
Coulomb (1785). It states that the kinetic friction force (or coefficient of friction) is
independent of the sliding velocity once motion starts.

BASIC MECHANISMS OF SLIDING FRICTION

 Amontons and Coulomb were the first to propose the mechanism of
friction.
 Coulomb pro-posed that metallic friction can be attributed to the
mechanical interaction of asperities of the contacting surfaces. In the
so- called Coulomb model, the action of the wedge-shaped asperities
causes the two surfaces to move apart as they slide from one position
to another and then come close again.
 Work is done in raising the asperities from one position to another
and most of the potential energy stored in this phase of the motion is
recovered as surfaces move back. Only a small fraction of energy is
dissipated in sliding down the asperities.
 Since friction is a dissipative process, the mechanical interaction
theory was abandoned. A realistic friction theory should include
mechanisms of energy dissipation.

4
 23-09-2023

 The dominant mechanism of energy dissipation in metals and

ceramics is plastic deformation.
 In engineering interfaces, even if deformation is primarily elastic,
some plastic deformation also occurs. Regardless of the type of
deformation, breaking of adhesive bonds during motion requires
energy.
 If we assume that there is negligible interaction between the
adhesion and deformation processes during sliding, we may add
them, and the total intrinsic frictional force (Fi) equals the force
needed to shear adhered junctions (Fa) and the force needed to
supply the energy of deformation (Fd). Therefore, we can write, Fi =
Fa + Fd or the coefficient of friction µi = µa+µd
 The distinction between the adhesion and deformation theories is
arbitrary. In both cases, there is local deformation, and the magnitude
of friction is influenced by the physical and chemical properties of
the inter- acting surfaces, the load, the sliding velocity, the
temperature, and so on.

Rolling friction

• Rolling friction is the friction generated by rolling

contact. In roller bearings, rolling friction mainly
occurs between the rolling elements and the
raceways, whereas sliding friction occurs between the
rolling elements and the cage.
• The main cause of friction in roller bearings is sliding
in the contact zones between the rolling elements and
raceways. It is also influenced by the geometry of the
contacting surfaces and the deformation of the
contacting elements.

10

5
 23-09-2023

Laws of rolling friction

Coefficient of friction due to rolling (μr) is generally smaller than that
caused by sliding action. Therefore wherever possible rolling friction
compared to sliding friction is desired. μr is defined as the force
required to maintain steady rolling, divided by the load carried by the
roller.

Rolling friction coefficients often depend on hardness of contacting

solids. On increasing hardness, elastic deformation under load
decreases. Therefore, hysteresis loss and so the value of μr decreases.
For hard smooth steel rollers, the coefficient of rolling friction ranges
between 0.01 and 0.001. A roller or sphere made of soft material (as
shown in Fig. 1) when rolled over other soft surface, generates a higher
level of rolling friction.

11

Fig.1 - Rolling friction in rubber.

Examples of Rolling Friction : Ball bearings, Automobiles Tires
Rolling are made of high strength materials having hystereis losses
lesser than one percent. Due to such materials (μ = 0.001)

In practice, the balls must be surrounded by cage to separate them

and prevent the rubbing on one another. But sliding between the
cage and balls occurs, and this sliding friction is often far greater
than the rolling friction. Lubricants are used to reduce the sliding
friction between balls and cage and to prevent corrosion of the metal
parts.
12

6
 23-09-2023

Fluid Film Friction

In this form of friction, both surfaces are fully separated by a fluid
lubricant film (full-film lubrication) which is formed either
hydrostatically or, more commonly, hydro-dynamically. From a
lubricants point of view, this is known as hydrodynamic or hydrostatic
lubrication (Figure 2). Liquid or fluid friction is caused by the frictional
resistance, owing to the rheological properties of fluids. If both surfaces
are separated by a gas film, this is known as gas lubrication.

Figure 2. Hydrostatic lubrication as a form of fluid friction

13

HYSTERESIS
 A deformation (hysteresis) component of friction occurs in viscoelastic

materials (such as polymers) in the so-called elastic limit, because of elastic

hysteresis losses.

 For most metals, the fraction of energy lost in the elastic limit is less than 1%,

but for viscoelastic materials such as polymers (especially elastomers), it may

be large. During sliding, the material is first stressed and then the stress is

released as sliding continues and the point of contact moves on, Figure (a). Note

that tangential force produces an increase in deformation, Each time an element

of volume is stressed, elastic energy is taken up by it. Most of the energy is later

released as the stress is removed from the element of the body, but a small part

is lost (in the form of heat) as a result of elastic hysteresis losses.

14

7
 23-09-2023

Cont…

Figure (a) Sliding of a rough, hard sphere (representing a portion of an asperity)

over a polymer

Therefore, energy is fed before of an asperity, and some of the energy is restored
at the rear of the asperity. If we have same energy in the rear portion as we
expended on the front portion, the net work required in sliding would be zero.
However, polymers are not ideally elastic; during deforming and relaxing
polymers some energy is lost, as hysteresis losses.

15

WEAR: TYPES,
PREVENTION AND ITS
MEASUREMENT

16

8
 23-09-2023

DEFINITIONS OF WEAR:
 Wear is defined as the undesirable but inevitable removal
of material from the rubbing surfaces.
 It is also defined as the progressive deterioration of the
surface with loss of shape often accompanied by loss of
weight and the creation of debris. Though, at the outside
wear appears to be simple, the actual process of
removal of material is very complex.
 Wear is a process of removal of material from one or
both of two solid surfaces in solid state contact, occuring
when these two solid surfaces are in sliding or rolling
motion together.

17

FACTORS INFLUENCING WEAR

 Variables connected with metallurgy.
 Hardness
 Toughness
 Constitution and structure
 Chemical composition

 Variables connected with service.

 Contacting materials
 Pressure
 Speed
 Temperature

 Other contributing factors

 Lubrication
 Corrosion

18

9
 23-09-2023

FACTORS AFFECTING WEAR BEHAVIOUR

19

TYPES OF WEAR

20

10
 23-09-2023

ABRASIVE WEAR
 Abrasive wear occurs when a hard rough surface slides across a
softer surface.
 The harder surface asperities press into the softer causing
plastic flow to occur from softer to harder.
 Deformation process leads to attrition of material.

Figures showing Abrasive wear by a) Cutting, b) Fracture, c) Fatigue, d) Grain

pull-out.

21

TYPES OF ABRASIVE WEAR

Schematics of (a) a rough, hard surface or a surface mounted with abrasive grits sliding
on a softer surface (two body abrasive wear), and (b) free abrasive grits caught between
the surfaces with at least one of the surfaces softer than the abrasive grits (Three body
abrasive wear).
Mechanical Operations causing Two body abrasion : Grinding, Cutting & Machining
Operations causing Three body Abrasion: Free- abrasive Lapping & Polishing

22

11
 23-09-2023

ABRASIVE WEAR BY PLASTIC

DEFORMATION
• In the plowing (also called ridge formation) process, material
is displaced from a groove to the sides without the removal of
material. However, after the surface has been plowed several
times, material removal can occur by a low-cycle fatigue
mechanism. When plowing occurs, ridges form along the sides of
the plowed grooves regardless of whether or not wear particles
are formed. These ridges become flattened, and eventually
fracture after repeated loading and unloading cycles.

• In the wedge formation type of abrasive wear, an abrasive tip

plows a groove and develops a wedge on its front. It generally
occurs when the ratio of shear strength of the interface relative
to the shear strength of the bulk is high (about 0.5–1). In this
situation, only some of the material displaced from the groove is
displaced to the sides and the remaining material shows up as a
wedge.

• In the cutting form of abrasive wear, an abrasive tip with large

attack angle plows a groove and removes the material in the
form of discontinuous or ribbon-shaped debris particles similar to
that produced in a metal cutting operation, Figure 7.2.14c. This
process results in generally significant removal of material and
the displaced material relative to the size of the groove is
very little.

23

ABRASIVE WEAR (CONTD.)

 The controlling factors for three
modes of deformation are the
attack angle or degree of
penetration , and the interfacial
shear strength of the interface.
There is a critical angle for which
there is transition from plowing
and wedge formation to cutting;
and this critical angle is dependent
on the material being abraded.
The degree of penetration is
critical in the transition from
plowing and wedge formation to
cutting as the coefficient of
Schematics of plowed groove and formation of friction increases with an increase
wear particle due to plowing as a result of fracture in the degree of penetration.
of flattened ridge and propagation of surface and The mechanism of plowing,
wedge formation and cutting are
sub surface cracks. observed in ductile materials.

24

12
 23-09-2023

ABRASIVE WEAR (CONTD.)

 The controlling factors for three
modes of deformation are the
attack angle or degree of
penetration , and the interfacial
shear strength of the interface.
There is a critical angle for which
there is transition from plowing
and wedge formation to cutting;
and this critical angle is dependent
on the material being abraded.
The degree of penetration is
critical in the transition from
plowing and wedge formation to
cutting as the coefficient of
Schematics of plowed groove and formation of friction increases with an increase
wear particle due to plowing as a result of fracture in the degree of penetration.
of flattened ridge and propagation of surface and The mechanism of plowing,
wedge formation and cutting are
sub surface cracks. observed in ductile materials.

25

ADHESIVE WEAR
 It occurs when two nominally flat solid bodies are in sliding contact,
whether lubricated or not.
 Adhesion (or bonding) occurs at the asperity contacts at the interface, and
these contacts are sheared by sliding which may result in the detachment of
a fragment from one surface and attachment to the other surface.
 As the sliding continues, the transferred fragments may come off the
surface on which they are transferred and be transferred back to the original
surface, or else form loose wear particles.
 Some are fractured by a fatigue process during repeated loading and
unloading action resulting in formation of loose particles.

26

13
 23-09-2023

MECHANISM OF ADHESIVE WEAR

• In most cases, interfacial adhesion strength is expected to be small as

compared to the breaking strength of surrounding local regions; thus, the
break during shearing occurs at the interface (path 1) in most of the contacts
and no wear occurs in that sliding cycle.
• In a small fraction of contacts, break may occur in one of the two bodies
(path 2) and a small fragment (the shaded region) may become attached to
the other surface. These transfer fragments are irregular and blocky shaped.

27

MECHANISM OF ADHESIVE WEAR (CONTD.)

• In another mechanism, plastic shearing of
successive layers of an asperity contact result in
detachment of a wear fragment.
• According to this theory, plastic shearing of
successive layers based on a slip line field occurs in
conjunction with the propagation of a shear crack, along
which the fragment detaches.
• This process results in thin wedge-shaped transfer
fragments. The fragment is
detached from one surface and transferred to the mating
surface because of adhesion.
• Further sliding causes more fragments to be formed
by either of the two mechanisms. These remain
adhering to a surface, transfer to the mating surface, or
to another previously attached fragment; in the latter
case a larger agglomerate becomes detached as a large
loose wear particle.
• These particles may be of roughly equal size in each
dimension.

28

14
 23-09-2023

PREVENTION OF ADHESIVE WEAR

 Mechanical
 Reduce load, speed and temperature.
 Improve oil cooling.
 Use compatible metals
 Apply surface coatings such as phosphating.

 Lubrication
 Use more viscous oil to separate surfaces.
 Use “extreme pressure” (anti-scuff) additives such as
sulphur phosphorous or borate compounds.
Note: Machine components affected by adhesive wear are: Piston/ cylinders,
swash plates, gear contacts, hypoid gear, cams and followers, rolling element
bearings.

29

FATIGUE WEAR
 Wear in which the surface damage of the material takes
place due to strain-induced on the surface for a particular
number of cycles to a certain critical limit.
 The fatigue process is generated differently in different
conditions, it depends on the type of materials or the type
of contact, etc.
 The repeated loading and unloading cycles to which the
materials are exposed may induce the formation of
surface or subsurface crack, which eventually will result
in the break-up of the surface with the formation of large
fragments, large pits in the surface.
 It is normally encountered in a pair of gears, ball and a
race or cam and the follower.

30

15
 23-09-2023

MECHANISM OF FATIGUE WEAR

 Crack Initiation and Propagation:

31

FATIGUE WEAR DUE TO SLIDING CONTACT

• It mainly occurs due to the unlubricated conditions and also due to the
sliding motion at the interface of the material reciprocating.
• The crack initiation takes place at the weaker region on the surface and in
this region, there is the orientation of the planes parallel to the sliding motion.
• The crack mitigates from the surface to the subsurface leading to laminar
wear particles along the plane

32

16
 23-09-2023

FATIGUE WEAR DUE TO ROLLING CONTACT

Properties of materials play an important role because of the concentration of
large number of stresses due to rolling.

33

PREVENTION OF FATIGUE WEAR

 The most effective method of preventing fatigue based
wear is to lower the coefficient of friction between two
interacting bodies, so that surface traction forces are
insufficient for de-lamination in sliding or contact fatigue
in rolling to occur.
 The other very important aspect in controlling fatigue
wear in both sliding and rolling is the material's
homogeneity. Homogeneous materials with minimum
imperfections or inclusions should be selected for sliding
and rolling contacts.

34

17
 23-09-2023

IMPACT WEAR

Erosion Percussion

• Occur by jet and streams of solid • Occurs from repetitive solid

particles, liquid droplets and body impacts. Repeated impacts
implosion of bubbles formed in result in progressive loss of solid
the fluid. material.
• Commonly found in Hydro • Percussive wear occurs by
turbines, boiler tubes, propellers, hybrid wear mechanisms which
pumps used in oil and gas combines several of the following
industries due to multi phase mechanisms: adhesive, abrasive,
sliding and colliding particles surface fatigue, fracture,
against the target surface and tribochemical wear.

35

EROSIVE WEAR
Classified as:
 Solid Particle Erosion
 Liquid Impingement Erosion

Solid Particle Erosion

 It is a form of abrasion that is generally
treated rather differently because the contact
stress arises from the kinetic energy of
particles flowing in an air or liquid stream as
it encounters a surface.
 The particle velocity and impact angle
combined with the size of the abrasive give a
measure of the kinetic energy of the
impinging particles, that is, of the square of
the velocity.
 Wear debris formed in erosion occurs as a
result of repeated impacts.

36

18
 23-09-2023

SOLID PARTICLE EROSION

 Ductile materials will undergo wear by a process of plastic deformation
in which the material is removed by the displacing or cutting action of
the eroded particle.
 In a brittle material, on the other hand, material will be removed by the
formation and intersection of cracks that radiate out from the point of
impact of the eroded particle.
 In ductile materials, two erosion mechanisms can be seen: cutting
erosion and deformation erosion.
 In cutting erosion, the detachment of crater lips occurs by one or several
impacts of the micromachining, plowing or lip formation type.
 In deformation erosion, the detachment of the material occurs by surface
fragmentation due to several impacts of the indentation type.
 Solid particle erosion is a problem in machinery such as ingested sand
particles in gas turbine blades, helicopter and airplane propellers, the
windshields of airplanes, the nozzles for sand blasters, coal turbines,
hydraulic turbines and the centrifugal pumps used for coal slurry
pipelines.
 It has useful application in processes such as sand blasting, abrasive de-
burring, and erosive drilling of hard materials.

37

LIQUID IMPINGEMENT EROSION

 When small drops of liquid strike the surface of a solid at high
speeds (as low as 300 m/s), very high pressures are experienced,
exceeding the yield strength of most materials. Thus, plastic
deformation or fracture can result from a single impact, and repeated
impact leads to pitting and erosive wear.
 Mostly, the probable impact velocities and impact angles are such
that pure liquid impingement erosion is an unlikely mechanism; an
erosion-corrosion mechanism usually does more damage.
 In ductile materials, a single intense impact may produce a central
depression, with a ring of plastic deformation around it where the
jetting-out flow may remove the material by a tearing action.
 In brittle materials, circumferential cracks may form around the
impact site caused by tensile stress waves propagating outward
along the surface. In subsequent impacts, material can spall off the
inside surface due to the compressive stress wave from the impact
reflecting there as a tensile wave.
 Examples are: moisture erosion of low-pressure steam turbine blades
operating with wet steam, rain erosion of aircraft or missile surfaces
and helicopter rotors, nuclear power plant pipes, and heat
exchangers.

38

19
 23-09-2023

CAVITATION EROSION
 It is defined as the repeated nucleation, growth, and violent
collapse of cavities or bubbles in a liquid.
 Cavitation erosion arises when a solid and fluid are in relative
motion, and bubbles formed in the fluid become unstable and
implode against the surface of the solid.
 When bubbles collapse that are in contact with or very close
to a solid surface, they will collapse asymmetrically, forming
a micro-jet of liquid directed toward the solid. The solid
material will absorb the impact energy as elastic deformation,
plastic deformation or fracture.
 The damage created is a function of the pressures produced
and the energy released by collapse of the bubble. Thus,
reduction of surface tension of the liquid reduces damage, as
does an increase in vapor pressure.
 Damage by this process is found in components such as ships’
propellers and centrifugal pumps.
 Hard but not brittle materials that are resistant to fatigue wear
are also resistant to cavitation.

39

PERCUSSION EROSION
 It is a repetitive solid body impact, such as experienced by print
hammers in high speed electromechanical applications and high
asperities of the surfaces in a gas bearing.

• The impact wear is proportional to the slip factor because wear

primarily occurs during the portion of the impact spent in relative sliding.
•Normal impact on a harder substrate can
produce fracture, and repeated impacts can give rise to a subsurface
fatigue wear mechanism.

40

20
 23-09-2023

CORROSIVE WEAR
 It occurs when sliding takes place in a corrosive environment. In air,
the most dominant corrosive medium is oxygen.
 In the absence of sliding, the chemical products of the corrosion
(e.g., oxides) would form a film typically less than a micrometer
thick on the surfaces, which would tend to slow down or even arrest
the corrosion, but the sliding action wears the chemical film away,
so that the chemical attack can continue.
 The chemical wear requires both chemical reaction (corrosion) and
rubbing.
 Machinery operating in an industrial environment or near the coast
generally produces chemical products (i.e., it corrodes) more rapidly
than when operating in a clean environment.
 Chemical wear is important in a number of industries, such as
mining, mineral processing, chemical processing, and slurry
handling.
 Electrochemical corrosion may accelerate in a corrosive
environment because corrosive fluids may provide a conductive
medium necessary for electrochemical corrosion to occur on the
rubbing surfaces.

41

TRIBOCHEMICAL WEAR
 Friction modifies the kinetics of chemical
reactions of sliding bodies with each other, and
with the gaseous or liquid environment, to the
extent that reactions which occur at high
temperatures occur at moderate, even ambient,
temperatures during sliding. The wear occurring
as a result, is tribochemical wear.
 The tribochemical reactions result in oxidative
wear of metals, the tribochemical wear of
ceramics, formation of friction polymer films on
surface sliding in the presence of organics, and
the dissolution of silicon nitride in water during
sliding without fracture.

42

21
 23-09-2023

ELECTRIC- ARC INDUCED WEAR

 When a high potential is present over a thin air film in a sliding
process, a dielectric breakdown results that leads to arcing.
 Arcing causes large craters, and any sliding or oscillation after an arc
either shears or fractures the lips, leading to three-body abrasion,
corrosion, surface fatigue, and fretting. Arcing can thus initiate several
modes of wear resulting in catastrophic failures in electrical machinery
 Methods to minimize electrical-arc-induced wear are as follows:
 to eliminate the gap between the two surfaces with a potential
difference.
 to provide an insulator of adequate dielectric strength (e.g., an
elastomer or Al2O3 coating) between the two surfaces.
 to provide a low impedance connection between the two surfaces to
eliminate the potential difference.
 to have one of the surfaces not ground.
 Bearing manufacturers recommend that bearings should be press-fitted
to the shaft and conducting grease should be used to eliminate arcing,
for example in the case of rolling-element bearings, shaft and the inner
race, the inner and outer races, and rolling elements.

43

FRETTING AND FRETTING CORROSION

 Fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion
between asperities and oscillatory movement causes ruptures, resulting in wear debris.
Most commonly, fretting is combined with corrosion, known as fretting corrosion.
 It occurs where low-amplitude oscillatory motion in the tangential direction (ranging from
a few tens of nanometers to few tens of microns) takes place between contacting surfaces,
which are nominally at rest.
 Examples of vulnerable components are shrink fits, bolted parts, and splines. The contacts
between hubs, shrink- and press-fits, and bearing housings on loaded rotating shafts or
axles are particularly prone to fretting damage.
 Flexible couplings and splines, particularly where they form a connection between two
shafts and are designed to accommodate some misalignment, can suffer fretting wear.
 The fretting wear rate is directly proportional to the normal load for a given slip
amplitude. In a partial slip situation, the frequency of oscillation has little effect on the
wear rate per unit distance in the low-frequency range, whereas the increase in the strain
rate at high frequencies leads to increased fatigue damage and increased corrosion due to
rise in temperature.
 To reduce wear, the machinery should be designed to reduce oscillatory movement,
reduce stresses or eliminate two-piece design altogether.

44

22
 23-09-2023

WEAR TEST EQUIPMENTS

 Dry sand Rubber wheel Abrasion Test
Apparatus

45

DRY SAND RUBBER WHEEL TEST

 It is a standard laboratory test used to assess the abrasion resistance of
materials, such as rubber compounds, coatings, or various other
surfaces.
 This test helps determine how well a material can withstand abrasive
wear when it comes into contact with a dry abrasive medium, typically
sand.
 It is often used in quality control and material selection processes to
evaluate the durability and performance of different materials.
 Test Setup: The test involves a specialized machine that includes a
rotating rubber wheel. The wheel is made of a specific rubber
compound being tested.
 Abrasive Medium: Dry sand with a known grain size and hardness is
poured into a container or hopper.
 Contact: The rubber wheel is brought into contact with the dry sand.
The wheel's weight, rotational speed, and the duration of the test are
carefully controlled and standardized.
 Abrasion: As the wheel rotates, it moves over the sand. The abrasive
action of the sand against the rubber wheel causes wear on the wheel's
surface.

46

23
 23-09-2023

DRY SAND RUBBER WHEEL TEST (CONTD.)

 Measurement: After a predetermined period of testing, the wheel is removed, and
the extent of wear is measured. This can be done by measuring the weight loss of
the rubber wheel or by examining the changes in its surface properties.
 Data Analysis: The test results are analyzed to assess the material's resistance to
abrasion. This analysis may involve comparing the wear of the tested material to
that of a reference material or using specific performance criteria.

 The dry sand rubber wheel test is commonly used:

 in industries where abrasion resistance is crucial, such as the automotive industry
(evaluating tire compounds), the manufacturing of conveyor belts.
 in the development of various rubber products.

 It provides valuable information for materials engineers and manufacturers to

make informed decisions about material selection and formulation.
 The specific standards and testing procedures can vary depending on the industry
and the exact requirements of a given application.

47

PIN ON DISC SET UP FOR FRICTION TESTING

48

24
 23-09-2023

PIN ON DISC (CONTD)

 The Pin-on-Disc (POD) friction test is a widely used experimental method for
studying the frictional behavior of materials, especially in the field of tribology.
 The POD test involves a simple setup where a pin or a small cylindrical specimen is
pressed against a rotating disc, and the frictional forces and wear characteristics are
measured and analyzed.
 Components of Pin on Disc Friction Test:
 Pin Specimen: This is the test material, typically in the form of a pin or a cylindrical
rod. The material's properties, such as hardness and composition, are often the focus
of investigation.
 Rotating Disc: The disc is usually made of a different material than the pin
specimen. It rotates at a constant speed or variable speed during the test.
 Load and Pressure: A known load is applied to press the pin against the rotating
disc, creating contact and friction between the two surfaces. The contact pressure is a
crucial parameter and is calculated as the force applied divided by the contact area.
 Friction Force Measurement: Sensors or transducers are used to measure the
frictional forces acting on the pin specimen. These sensors provide data on the
coefficient of friction, which is the ratio of the frictional force to the applied load.
 Wear Measurement: The wear of the pin specimen and the disc is monitored during
the test. This can be done using various techniques, such as weighing the specimens
before and after the test, using profilometers to measure surface roughness changes,
or observing the wear debris generated.

49

PIN ON DISC FRICTION TEST (CONTD.)

 Purpose of the Pin-on-Disc Test:
 Friction Coefficient Measurement: Determine the coefficient of
friction between the two materials under specific conditions (e.g., load,
speed, lubrication).
 Wear Behavior Study: Investigate the wear mechanisms and wear rates
of materials in sliding contact.
 Material Characterization: Assess the tribological properties of
materials, such as hardness, wear resistance, and frictional behavior.
 Lubricant Evaluation: Evaluate the effectiveness of lubricants or
coatings in reducing friction and wear.
 Surface Treatment Assessment: Study the impact of surface treatments
(e.g., heat treatment, coatings) on friction and wear properties.

 The Pin-on-Disc test can be customized for various applications by

adjusting parameters like load, speed, and environmental conditions
(e.g., temperature, humidity, and lubrication).
 It's a valuable tool for understanding how different materials and
lubricants perform in real-world situations, helping engineers and
scientists optimize designs and materials for reduced friction and wear
in industrial and mechanical systems.

50

25
 23-09-2023

WEAR TESTING EQUIPMENT (CONTD.)

 Air Jet Erosion Testing

51

AIR JET EROSION TESTING

 It is a laboratory technique used to evaluate the resistance of materials to erosion
caused by high-velocity air or gas flow carrying abrasive particles. This type of
testing is particularly relevant in industries where components or materials are
subjected to erosive wear in real-world applications, such as aerospace, automotive,
mining, and oil and gas.
 Setup: In a typical air jet erosion test setup, there is a nozzle that directs a high-
velocity stream of air or gas containing abrasive particles toward the material being
tested. The abrasive particles can vary in size and type, depending on the specific
application.
 Specimen Preparation: The material being tested is usually in the form of a flat or
curved specimen. The specimen is mounted in a test chamber in a way that exposes it
to the eroding jet.
 Erosion Test: The high-velocity jet is directed at the specimen for a predetermined
period of time. The erosion rate, or the material loss over time, is typically measured.
This can be done by weighing the specimen before and after testing or by measuring
the depth of erosion.
 Parameters and Variables: Various test parameters can be adjusted to simulate
different erosive conditions. These parameters include air/gas velocity, abrasive
particle size and concentration, test duration, and nozzle design. The choice of
parameters depends on the specific application and the type of erosion being studied.
 Data Analysis: After the test is completed, data such as erosion rate, material loss,
and surface wear characteristics are analyzed. This data can help in assessing the
material's resistance to erosion and in making decisions regarding material selection
for actual components or structures.

52

26
 23-09-2023

AIR JET EROSION TESTING (CONTD.)

 Air jet erosion testing serves several purposes:
 Material Selection: It helps engineers and material scientists choose the
right materials for applications where erosion is a concern. By evaluating
different materials under controlled conditions, they can identify those that
offer better erosion resistance.
 Quality Control: Manufacturers can use this testing method to ensure that
their materials or components meet certain erosion resistance standards and
specifications.
 Research and Development: Researchers use air jet erosion testing to
develop new materials or coatings that can better withstand erosive
conditions, leading to improved product performance and longevity.
 Failure Analysis: When components fail due to erosion in the field, air jet
erosion testing can be used in the laboratory to replicate the conditions and
understand the failure mechanisms.

 Air jet erosion testing is a valuable tool as:

 It is used for assessing the durability and performance of materials in
environments where erosion is a critical factor.
 It helps ensure the reliability and safety of products and components in
industries where erosion is a concern.

53

SLURRY POT EROSION TEST

54

27
 23-09-2023

SLURRY POT EROSION TEST

 It is a laboratory experiment used to assess the resistance of
materials to erosion caused by the impact of abrasive particles
suspended in a liquid. This test is often employed to evaluate the
durability of materials used in industries such as mining, oil and gas,
and transportation, where components can be subjected to abrasive
wear.
 Test Setup: A slurry pot erosion test involves a test apparatus
consisting of a slurry pot or erosion cell, which is designed to
simulate the erosive conditions experienced in the field.
 Slurry Preparation: A slurry is prepared by suspending abrasive
particles (such as sand, gravel, or other abrasive media) in a liquid.
The choice of abrasive particles and the liquid medium depends on
the specific application being simulated. For example, in the oil and
gas industry, the slurry may consist of sand suspended in a liquid
that mimics the properties of crude oil or drilling mud.
 Sample Holder: The material to be tested (e.g., a metal alloy,
polymer, or coating) is usually machined or prepared into a
standardized shape or sample holder. This sample holder is then
positioned in the slurry pot.

55

SLURRY POT EROSION TEST (CONTD.)

 Erosion Test: The slurry is circulated in the pot, and the abrasive
particles impact the surface of the test material. This simulates the
erosive wear that the material would experience in an actual operational
environment. The test may be run for a specified duration, and erosion
rates are typically measured during and after the test.
 Measurement and Analysis: After the test, the sample is removed
from the slurry pot, and the extent of erosion or material loss is
measured and analyzed. Various parameters, such as erosion rate,
erosion depth, and surface morphology, may be assessed to evaluate the
material's resistance to erosion.
 Data Interpretation: Test results are used to assess the performance of
materials and coatings under erosive conditions. Engineers and material
scientists can use this data to make informed decisions about material
selection, design improvements, and maintenance strategies in
industrial applications.
 Slurry pot erosion tests are important:
 It is used for optimizing material selection and design in industries
where erosion is a common problem.
 By subjecting materials to controlled erosive conditions in the lab,
researchers can develop more erosion-resistant materials and extend the
lifespan of critical components in various industrial systems.

56

28
 23-09-2023

CORROSION TEST

57

CORROSION TEST
 Purpose: To determine how well a material can withstand corrosion in
conditions that mimic those encountered during service, such as in the oil
and gas industry, chemical processing and power generation.
 Test setup: Autoclave corrosion tests involve placing material samples,
often in the form of coupons or specimens, into a high pressure vessel
(autoclave) along with the corrosive medium. The autoclave is sealed,
pressurized, and heated to the desired temperature.
 Test environment: The autoclave is filled with a corrosive environment that
simulates the conditions the material will encounter in the real world. This
environment can include chemicals, gases, and liquids depending on the
application. Common corrosive agents include saltwater, acidic solutions,
and sour gas.
 Temperature and Pressure Control: The autoclave is heated to the desired
test temperature and pressurized to the specified level. The temperature and
pressure are monitored and controlled throughout the test to ensure they
remain constant.
 Testing Duration: The specimens are exposed to the corrosive environment
for a predetermined period, which can range from days to weeks or even
months, depending on the specific requirements of the test.

58

29
 23-09-2023

CORROSION TEST (CONTD.)

 Evaluation: After the test duration is complete, the specimens are removed from
the autoclave, cleaned, and carefully examined for signs of corrosion. This
evaluation can include measuring corrosion rate, corrosion depth, and other
relevant parameters.
 Data Analysis: Test results are analyzed to determine the material's corrosion
resistance and performance under the specified conditions. This information is
crucial for making informed decisions about material selection and coating
processes.
 Importance of Autoclave Corrosion testing:
 It provides accelerated testing, allowing engineers to assess how materials
will perform in corrosive environments over extended periods in a relatively
short time.
 It helps identify weaknesses in materials and coatings, allowing for
improvements or changes in design.
 It assists in selecting the most suitable materials and coatings for specific
applications, ensuring the longevity and reliability of products.
 It can be used to meet industry standards and regulations for corrosion
resistance.

59

30