SURFACE

ENGINEERING of
MATERIALS
MSE 3131
Lecture 20-26 (Tribological
Properties of Materials)

Asrafuzzaman
Lecturer, Department of MSE,
RUET, Rajshahi.
• Greek word “ Tribos” means grazing, rubbing, abrading, scouring and
Ology means “The Study of”
• It is the science and engineering of interacting surfaces in relative
motions

Tribology

Friction Wear Lubrication

Tribology is a New Word…

•Coined by Dr. H. Peter Jost in England in 1966

• “The Jost Report”, provided to the British Parliament – Ministry for
Education and Science, indicated… “Potential savings of over £515
million per year ($800 million) for industry by better application of
tribological principles and practices.”

BUT…
Tribology is not a new field
First Recorded Tribologist – 2400 BC

Transporting the statue

of Ti – from a tomb at
Saqqara, Egypt

Figure taken from

“History of
Tribology”,
by Duncan Dowson.
The first recorded
tribologist –
pouring lubricant
(water?)
in front of the
sledge in the
transport of the
statue of Ti.
A more famous tribologist – 500 years ago
Tribology is all around us!
Assemblies or Products:
Manufacturing Processes
Construction and Exploration:
Natural Phenomena
High Friction
Low Friction – Good/Bad!!!
Watch
Out for
this
Sign!!!
 FRICTION

• Resistance to motion during sliding/rolling when one solid body

moves tangentially over another is Friction.

• Resistive tangential force which acts in a direction opposite to the

direction of motion is Friction force.

•2 Types of friction:

1) Dry/Coulomb friction

2) Fluid friction
(a)
• Friction that exists when 2 dry surfaces move relative to one another
is dry friction.

• Friction that exists between 2 adjacent layers in a fluid that are

moving at different velocities is Fluid friction.

 If solid bodies are loaded together and a tangential force (F) is applied,
the value of tangential force reqd. to initiate motion is Static friction
force (Fs). Few milisecs may be needed to initiate relative motion.

 Tangential force required to maintain relative motion is

Kinetic/Dynamic friction force (FK)
Fs  F K
 Surface contaminants/ thin films affect friction.
Benefits of friction
Friction makes possible:
•to walk
•to use automotive tires on roads
•to pick up objects
•………….

In some m/c applications [brakes, frictional power transmission (belt

drives)], friction is maximized.
Demerits of friction
•Energy loss
•Wear of moving surfaces in contact

In some sliding/rotating components (bearings), friction is undesirable

and minimized.
SOLIDSOLID CONTACT
Laws of sliding friction

2 basic laws  Amontons’ laws

1st law F = w = const. (coefficient of friction) When body is moving
on an inclined plane,
𝐹𝑆
𝜇𝑠 =
𝑊 𝐶𝑜𝑠𝜃
𝑊 𝑠𝑖𝑛𝜃
=
𝑊 𝐶𝑜𝑠𝜃
𝜇𝑆 =𝑡𝑎𝑛𝜃
2nd law : Friction is independent on the apparent area of contact. Thus 2
bodies, regardless of their physical size, have the same .

3rd law (often attributed) : K is independent of sliding velocity once

motion starts.

Fig 6.2.2 (a): Steel sliding on Al in air

= const. with variation of normal load.
For materials with surface films,  may not remain const. as a fn of load.
Fig 6.2.2 (a):
Steel sliding on Al
in air
Fig 6.2.2 (b): Cu sliding on Cu in
air
•= const. at low loads.
• A transition occurs
to a higher load
• = high and const
at higher loads.
Factors for low  are:
• Cu readily oxidizes in air so that at low loads, oxide film
effectively separates the metal surfaces & there is little or no
true metallic contact.

•Oxide film has low shear strength ( or low )

• At high loads, film breaks down resulting in intimate

metallic contact which is responsible for high  and surface
damage.
Fig 6.2.2 (c): AISI 440C SS
on Ni3Al alloy in air
•In high load regime,
↓es with N.

•Plastic deformation
& presence of a large
quantity of wear
debries are believed
to be responsible for
↓ in .
Fig 6.2.3:  for sharp diamond tip sliding on smooth Si, SiO2, natural
diamond
•Above some critical load,  as well as wear starts to ↑

•Below critical load,

 Low local hardness

 No plastic deformation

 Low 

• In case of diamond, transition does not occur because of

its very high hardness.
Fig 6.2.4:  as a fn of apparent area of contact
• of wooden sliders remains essentially constant with change
in apparent area of contact supports AMONTONS’ 2nd law.

•  may not remain constant for soft materials (polymers),

very smooth & very clean surfaces (where apparent area 
true area).
Fig 6.2.5: K as a fn of sliding
velocity
• K as a fn of friction
velocity has a –ve slope.

•Slope is very small i.e; K

changes only by a few %.
Summary:
•First 2 laws are obeyed for a few % of many cases

•  is strictly constant for a given pair of sliding materials under some

operating condition – temperature, humidity, normal pressure, sliding
velocity, apparent and real area of contact

•So, reported values should be used with caution

• Coulomb Model – Mechanical Interaction of asperities of the sliding
surfaces
• Considered, wedge shaped asperities – position a to b work is done
and potential energy is stored;
• From b to c this potential energy is recovered – very small/no energy
dissipation
•BUT, friction is an energy dissipative process – theory was abandoned
• According to Bowden and Tabor –two solids are in sliding contact –
high pressure at individual contact spots – Local welding – shearing
due to sliding
• Further improvement – welding of asperities not necessary –
Interfacial adhesion is sufficient for friction of ceramics and metals
(Widely accepted theory)
•So, energy required for
 Overcoming adhesion among asperity contacts
 Microscale deformation of contacting surfaces (due to relative motion)
Macroscale deformation (Plastic Deformation) – when one surface is harder
than the other it plows/ploughs through creating groove!
• For viscoelastic materials (Polymer) – energy dissipates by hysteresis
losses
• In enggineering interfaces elastic deformation occurs (very little due
to phonons) and plastic deformation cause final adhesion breaking
•Bowden and Tabor proposed –

Fi = Fa + Fd or i = a + d

• The magnitude of friction however is dependent on

 Physical and chemical properties of interacting surfaces
 The load
 Sliding velocity
 Temperature and so forth

• For brittle material – consider adhesive contact, deformation, and

additionally, fracture toughness

• We will focus mostly on analysis of ductile materials (Most of the

traditional coatings are ductile)
Adhesion
• 2 nominally flat surfaces are placed in contact under load,
contact takes place at tips of asperities.
• Discrete contact spots are formed – physical/chemical
interaction
Coefficient of adhesion friction
𝐹
a =
𝑊
𝐴𝑟𝑎
= 𝑃𝑟 =𝑀𝑒𝑎𝑛 𝑟𝑒𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝑃𝑟𝐴𝑟
𝑎
a =
𝑃𝑟

•Adhesion is ↓ed by ↓ing surface interactions

• Presence of fluid film (air, H 2O, lubricant) would reduce
adhesion friction.
• Interfaces in vacuum would exhibit strong adhesion & high
friction.
Adhesion Friction of Elastomers
•Viscoelastic materials do not follow classic adhesion theory
•Bulgin et al. proposed StickSlip event
 Basic Mechanism of Sliding Friction contd…
Deformation
•2 types of interactions during sliding of 2 surfaces.
1) Microscopic
2) Macroscopic
Microscopic
•Plastic deformation
•Displacement of interlocking surface asperities
•Local asperity interactions involve very thin region & large strains
• Adhesion & asperity interactions are always present in any relative
motion.
Macroscopic
 Harder material surface asperity plough grooves in softer
material surface via plastic deformation resulting in fracture,
tearing.
 Ploughing results large volume deformation & its magnitude
depends on
 Surface rougness
 Size, shape & hardness of wear debris.
• Two sliding surface – one harder than the other – plow/plough into
softer surface producing groove if shear strength is exceeded

•Ploughing can also occur by wear particles

• Ploughing not only increase friction but also creates wear particles
which in turn increase subsequent friction and wear

• For metal and ceramics with rough surface and/or presence of wear
particles – Deformation term consists of plowing, grooving and more
dominant than the adhesion component
Calculation of ploughing component of friction force for 4 model
rigid asperities or trapped wear particles:

1) Let, a circular cone of roughness angle or attack angle, ,

pressed into a softer body.
• During sliding only the front surface of the asperity is in
contact with the softer body.
Calculation of ploughing component of friction force for 4 model
rigid asperities or trapped wear particles continued…

2) Let, a spherical shape wear debry of roughness angle or

attack angle, , pressed into a softer body.
• During sliding only the front surface of the asperity is in
contact with the softer body.
 

Calculation of ploughing component of friction force for 4 model
rigid asperities or trapped wear particles continued…..

3) Let, a cylindrical shape

wear debry (upright
position) pressed into a
softer body.
Calculation of ploughing component of friction force for 4 model
rigid asperities or trapped wear particles contd…..

4) Let, a cylindrical shape

wear debry (transverse
position) pressed into a
softer body.
• During sliding only the
front surface of the
asperity is in contact with
the softer body.
Ratchet Mechanism (Ride-Over):
• If asperity of one surface is smaller in lateral dimension than the other
and contact stress is lower than plastic flow stress
• Smaller asperities climb up and down over the broader asperities
without any interfacial damage
•Energy required to climb up and it decrease during climbing down
• Not totally a nondissipative process – energy lost due to impact
deformation and phonon generation
Deformation component of friction can be ed by:
•Reducing surface roughness
•Selecting materials of almost equal hardness
•Removing wear particles from the interface
Examp. 6.2.1
A hard ball is sliding against a soft & flat surface at 2 different loads. At one
load, coefficient of friction is 0.20 & the groove width is 0.5 mm and at
another load, coefficient of friction is 0.25 & the groove width is 1 mm.
Calculate radius of the ball & adhesive component of the coefficient of
friction. Assume that the dominant sources of friction are adhesion &
ploughing & these are additive.
 Other Mechanisms of Sliding Friction

Chemical Effects:
• Greater the % d-bond character, less active is the metal lower
the friction.
• ↓es with ↓ in chemical activity.
•Same thing both in vacuum & air is shown.
• Complex metal oxide is formed. Enhanced bond strength of
complex metal oxide establishes strong contact at the interface &
thus friction is ↑ed when exposed to air.
Structural Effects:
•HCP metals exhibit low  (30% less) & wear than FCC metals.
• One key factor which affects friction & wear is the No of slip planes.
•HCP metals have limited No of slip planes.

• HCP metals (Co) deform by slippage, leaving many air gaps at

each junction.
• Cubic metals (12 slip planes) have no air gaps Contact is
stronger friction & wear correspondingly high.
Grain Boundary Effects:
• Strained metal high dislocation concentration chemically more
active on the surface higher friction.
• A grain boundary is a strained condition in that there are many
dislocations.
•Grain boundary regions are high energy regions at the surface.
• Near surface dislocations are blocked in their movement by a grain
boundary. They accumulate at the grain boundary & produce strain
hardening in the surficial layers.
•Strain hardening makes the sliding more difficult   is ↑ed.
• for polycrystalline material is higher than  for SC material.
 Friction Transition During Sliding

1. Runin period: ↑ / ↓
2. Steady state – 1
3. Transition period: ↑
4. Steady state – 2
1. Runin/ Breakin/ Wearin Period:
•High asperities may be knocked off.
•Initial surface films may be worn out.
These changes result in friction either going up /
coming down.

3. Transition Period:
• Changes in interface (roughening, trapped
particles) result ↑ of 
 Up to II
Up to I
 Identical metals show pattern I. ↑ of  is associated with
ploughing.
 Significant ↑ of  in IV is associated with a poor material pair.

 Drop of  in II is associated with smoothening of the 2 hard

surfaces.
 Further ↑ of  in III is associated with generation &
entrapment of wear particles.
Static Friction
 Fs F k   s  k
 s is time dependent.

 Rest time affects adhesion & consequently  s. s can ↑ /

↓ with rest time.
 During rest, if contact becomes contaminated with lower
shear strength species, s ↓es.
 if contact is clean & more interfacial bond develops,  s
↑es.
s for steel on steel surface
as a fn of log t approximates
to a straight line.

• For small values of t, slope is

steep.
• For large values of t, slope is
small.
• For freshly cleaved rock salt, formation of surface films over time reduces
friction

• ↑ in s with rest time is undesirable for many industrial applications

requiring intermittent operations of remotely controlled mechanisms, such
as antennas & other moving parts in earth observing satellites.
Rolling Friction:
•Much easier to roll surfaces than to slide them.

• It is the resistance to motion that takes place when a surface

us rolled over another surface
• The body is restricted to have a continuous shape with very
small surface roughness
•With hard materials,

rolling = 5103 510 5

sliding = 0.11
Rolling Friction continued…
• During rolling, any motion can be regarded as a combination
of Rolling, Sliding & Spin.

Free Rolling:
•Rolling motion where no sliding.
• Occurs between 2 bodies having same elastic properties, that
are geometrically identical.

Tractive Rolling:
•Rolling is associated with sliding.
•Driving wheels of a train on the tracks.
Friction of Various Materials:
 depends upon
•The interface
•Mating material (material pair)
•Surface preparation
•Operating condition

• Wood, leather & stones were used in ancient time. Typical 

values are given in Table 6.4.1
•s is (20 – 30)% higher than k.
• Typical  of metals, alloys, ceramics, polymers are given in Table
6.4.2
Effect of Operating Conditions:
•Sliding velocity

•Contact pressure

•Temperature

•Gaseous environment

•Relative humidity
Sliding velocity:
• ↑in velocity results
interfacial heating.
• Formation of oxides
becomes favorable at the
heated interface &  drops.
• For low melting point
metals, formation of a thin
molten layer at asperity
contacts beginsreduces
its shear strength  drops.
Temperature:
•High T ↑es rate of oxidation  drops.
•↑in T may result in solid state phase transformation.
•Co exhibits a phase transformation at 4170C from HCP (low
) to cubic close-packed structure (high ). This phase
change is responsible for a peak of  ↑ at 5000C.
•Drop in  above 5500C may be due to:
↑in oxide thickness
Change from CoO (poor solid lubricant) to Co 3O4
(better solid lubricant).
Thank You!