Wear and Friction

Dr. Arnab Sarkar

E-mail: sarkararnab@nitrkl.ac.in
 Introduction
 Wear is the type of the failure that is related to the friction.
 Energy is wasted and consumed by friction.
 The majority of the power used by cars, trucks, buses, and airplanes is used to overcome friction.

Friction

Static Dynamic Sliding Rolling

Friction that Friction that Friction that Friction that
develops when develops when develops develops
the body is at the body is at when the when the
rest motion body slides body slides
over over
another. another.

Causes of friction: 1) Molecular adhesion,

2) Surface roughness, and
3) Deformation
 Tribology is the study of the science and technology that focuses on the friction, wear and lubrication of the interacting
surfaces in relative motion. Here the Greek word “tribos” means rubbing or sliding.
 When friction is useful?

Braking a car

Walking

Writing with a pen

When friction is not useful?
1) Pushing a heavy object up the hill
2) Engine efficiency
 Wear
Wear is the special type of the failure that is associated with the progressive damage involving the loss of materials due to the
undesirable removal of solids from one or both of the solid surfaces in solid state contact during sliding or rolling.
Factors affecting wear:
1) Surface geometry, 2) Applied load, 3) Sliding velocities, 4) Environment, 5) Initial microstructural features, 6) Mechanical,
Physical and Chemical properties and 7) Physical and Chemical properties of the lubricant and the impinged solid particles.

Different types of wear

 Abrasive Wear
 Adhesive Wear
 Erosive Wear
 Corrosive Wear
 Fretting Wear
 Surface Fatigue
Other wear types:
1) Polishing, 2) Electrical damage,
3) Cavitation damage, and 4) False Brinelling
 Abrasive Wear
When a hard material slides or rub against the softer material, leading to the
material removal.
This phenomenon occurs when foreign contaminants, like sand or metal fragments
comes in contact with both the surfaces.

Causes of abrasive wear:

1) Inadequate lubrication
2) Foreign particles in lubricants
3) Below average lubricant film.

Source: Lily Bearings

Abrasive wear Prevention of abrasive wear:
 Remove abrasives by improving the air or filtering the oil.
 The oil should be more viscous.
Gouging Grinding Scratching
Abrasion Abrasion Abrasion  Increase the hardness and strength of the metal surfaces
 Gouging abrasion Grinding abrasion

Source: Jiang et al. (2018)

Minerals Engineering Source: surfaceengineering.com/wear-modes/abrasion/

When big, hard abrasives like rocks or ore are handled with strong contact or impact pressures, gouging abrasion occurs in a
variety of comminution processes. For example: Cr white irons and tool steels displayed the best gouging abrasion resistance.
 Large particles.
 High Compression Load.
High stress or grinding abrasion: Grinding wheels use an abrasive material for various cutting and grinding purposes. It involves
 Smaller particles.
 High Compression Load. Scratching abrasion

Low stress or scratching abrasion: Scratching abrasion is essentially the primary

mechanism of wear damage when surfaces exposed to low stress abrasion reveal that
material has been removed by hard, and sharp particles.
For three-dimensional wear, the hard asperity or hard particles between the two contacting surfaces are responsible for the wear
failures.

WL
Wear Volume, V: V = αβ
Hv
α = shape factor of an asperity ~ 0.1
β = degree of wear by abrasive asperity,
it varies between 0 to 1, depending upon the depth of

Source: Calderon et al. (2022),

penetration by wear.
MethodsX
W = Load,
wear volume,V L = sliding distance.
Specific wear rate, ws = load,W × sliding distance,L
Wear coefficient, K =ws× Hv
 Adhesion Wear
 Adhesion wear occurs as the result of two flat bodies slides against one another owing to the large atomic bonding forces at
the surface compared to its surrounding.
 Adhesion occurs at the asperity contact of the rubbing surfaces.
 These contacts are sheared by the relative motion that initiates the removal of the material from one of the surfaces and
attachment on the another.
 During the sliding motion, relocated pieces have the potential to return to their original surface or may loose wear particles.
 The four main adhesive wear modes are scuffing, scoring, galling, and seizing.

Source: Zhang (2014)

Advances in ceramic matrix
composite. Source: Podgornik (2014), Advances
in ceramic matrix composite.

Mild adhesion = Assist in the removal of surface oxides.

Severe adhesion = Causes the removal of metal due to tearing, breaking and even melting of metallic junctions. Severe adhesion
causes scuffing, galling or seizure at the surface.
 Adhesion Wear
Scuffing: Scuffing is a particularly damaging form of wear in highly loaded lubricated contacts
Galling: A phenomena in which two metal surfaces in sliding contact unexpectedly stick together, causing frictional damage.
Galling is a severe form of wear that occurs during sheet metal forming operations often resulting in scratches on the sheet, leading
to significant damage to the tooling and decreased product quality.
Seizure: Seizure is the stopping of relative motion as a result of interfacial friction. It is identified by the stoppage of motion and
welds of contacting surfaces. It is the catastrophic failure of tribological systems.
Surface damage from accumulated seizures triggered by slipping under inadequate lubrication or harsh operation circumstances is
known as scoring.

Scuffing in gears Galling in three stages

Source: Devenport et al. (2023),

Source: Naokowe et al. (2008) Lubricants
 Factors affecting the wear rate
Wear Volume, V: V = wsWL  Materials microstructure.
WL  Surface roughness
V=K
Hv  Flash temperature
ws varied from 10-2 to 10-10 mm3/Nm  Local contamination
 Tribochemical Reactions.
Prevention
 Use the compatible materials.
 Improve the oil cooling.
 The oil should be more viscous to separate the surfaces.
 Reduce load, speed and temperature.
 Apply surface coating.
 Prevent the metal to metal contacts. This can be achieved either through a lubricant film or
suitable coatings.
 Erosive Wear
Impingement of particles (solid, liquid or gaseous) remove fragments of materials from the surface due to momentum effect causes
the erosive wear.
Erosive wear occurs as the result from the impact of the solid or liquid particles travelling at a high velocity against the materials
surface.
Example: Damage causes to the turbine blade of the aircraft wings when it passes through
the clouds of dust.

Causes of erosive wear:

1) Properties of the particle
2) Impact angle
3) Impact velocity.
4) Particle size

Source: machinerylubrication.com 5) Particle shape

 Erosive Wear
Erosive Wear

Solid particle erosion Liquid Drop erosion Cavitation erosion

 Solid particle erosion: Surface wear by the impact of solid particles. For example: Dusty environments cause wear on the
edges of helicopter blades.
 Liquid Drop erosion: Surface wear by the impact of liquid drops. For example: Raindrops erode missiles.
 Cavitation erosion: Cavitation wear is a type of surface wear on solid materials that occurs when the materials come into
contact with a liquid medium and the liquid bubbles burst under zero or negative pressure, causing damage to the surface. For
example: acid-eroded teeth enamel.
Prevention of erosion wear:

 Remove particles by improving the air.

 Increase the hardness and strength of the metal surfaces
 Minimize the impact angle to less than 15o.
 Cavitation damage

Courtesy: cpesystems.com/blogs/news/what-is-cavitation

Water Droplet erosion Bao et al. (2022) JMEP

Kuhlmann et al. (2022) Tribology letters

Archard's Wear Law

𝐕 𝐊𝐖
Wear rate = L = H
v
Wear Volume, V = KAiM
A = Impact angle, I =impact
velocity, M = function of particle
size, M = (particle size)3
i = (particle velocity)n, n = 2 to 2.5 for
metals, 2.5 to 3 for ceramics.
Mahdipoor et al. (2015) Scientific Reports
 Corrosive Wear
Corrosive wear = Corrosive wear is defined as the damage caused by the combined action of corrosion and wear in a corrosive
environment. It is also known as tribo-corrosion. For example: slurry pumps used for slurry transfer in the oil sands industry and
rubbing off the rust from the steel surface.

Source: Park et al. (2022) Applications in Structural Health Monitoring

Prevention of corrosion wear:

 Reduce the operating temperature.
 Use more corrosion-resistant metals.
 Use fresh oil.
 Use corrosion improved inhibitor.
 Fretting Wear
 Fretting wear is a wear process that causes damage when metal surfaces come in contact through continuous rubbing or cyclic sliding motion.
 It occurs because of environmental condition such as corrosion and small oscillating movements like fatigue. It is the type of wear that results
from vibration or low-amplitude oscillation under high contact pressure.
 It is a combination of adhesive and abrasive wear.
 The fatigue wear caused by oscillatory motion is intensified by the asperities adhering to the contact surface.
 However, the bond is just temporary, since the slight movements will eventually break it, leaving wear debris behind.
 If the debris and/or surface then experience a chemical reaction, mostly oxidation, then the process is known as the fretting corrosion.
 Fretting increases micropits and surface roughness, which lowers the components' fatigue strength even further.
 Fretting wear mostly occurs in all contacts subject to vibration. Many machine parts and systems are subjected to small oscillatory movement
experience fretting wear.

Source: Swain et al. (2020) In book: Tribology

 Fretting Wear
Causes of fretting wear:
1) Load: Localized stress concentrations at certain locations on the metal surface might result from uneven loads on mechanical components.
2) Environment: The consequence of poor temperature and relative humidity is responsible for fretting corrosion from oxidation.
3) Material Properties: Ductility, and surface roughness can cause the fretting wear.
4) Motion: The degree of fretting wear is influenced by minute variations in the sliding amplitude and number of cycles between two metal
surfaces.
5) Surface finish: Poor surface finish can cause the fretting wear.

Prevention of fretting wear:

 Changing the component design, dampening the vibration and ensuring that the joints are properly tightened can prevent the
fretting wear.
 Coating and lubrication increase fretting resistance by providing an additional layer of friction protection.
 Improving the hardness and strength can prevent the fretting wear.
 Coupling the soft and hard metals exhibits lesser fretting damage compared to two hard metals.
 Controlled environment during operation such as reducing the moisture to the material reduces the oxidation.
 Using loose metallic inserts to avoid. For example: using thin copper plates for titanium surfaces.
 Surface fatigue
 When the two bodies in contact with each other are weakened or damaged by cyclic loading (for example: during rolling), it
results in surface fatigue.
 Failure on the top layer of a body due to repeated loading.
 The cracking, pitting and spalling of a surface occurs in sequence due to cyclic stress.
 It is mostly occurs in rolling element bearing and gear teeths.
Prevention of surface fatigue:
 Lower the contact pressure and frequency of cycle stress.
 Use clean, dry and highly viscous oil free from abrasive particles.
 Increase the hardness and strength of the metal surfaces
 Using the high quality material can prevent the surface fatigue. Source: tribonet.org/

Under the influence of high impact, the surface fatigue became active. The wear process involves the material loss from the
surface by the external forces from another surface acting on it during a sliding motion, resulting in the delamination wear.
The plastic deformation on the surface involves the crack nucleation and propagation resulting in the separation of the material
from the surface.

Source: tribonet.org/