UNIT 2
Wear and Friction
Unit Content
1. Wear and Friction
2. Types of wear
3. Wear mechanisms
4. Lubrication processes
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Unit Objectives
1. Understand interaction between wear, friction, and lubrication
2. Differentiate between the various types of wear and their
mechanisms
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Learning Outcomes
1.Identify the different types of wear and how they occur
2.Note the effect of friction and wear on the performance of materials
3.Have fair knowledge of lubrication processes and its effect on the
performance of materials
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4
What is Tribology?
• Tribology is a science and engineering of friction, wear and
lubrication of elements in relative motion
• Examples: hinges on doors, joints (hips, knees, ...),
workpiece/tool, car windshield wiper, tyres on the road, brakes
• Tribology in industries and companies???
• Economic effects in businesses???
Tribology more than just lubrication and friction!!!
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Wear, Friction, Lubrication
• Wear is the main cause of material wastage. It occurs
when there is material loss.
• Friction is the main cause of energy dissipation. It is
the force that resist motion.
• Lubrication reduce friction and consequently wear.
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Wear, Friction, Lubrication
• Friction may be defined as the resistance encountered
by one body in moving over another
• Two important classes of relative motion: sliding and
rolling
A force, F, is
needed to
overcome friction
and cause motion
by (a) rolling or (b)
sliding
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Wear, Friction, Lubrication
• A tangential force F is needed to move the upper body
over the stationary counterface
• The coefficient of friction μ is the ratio between this
frictional force and the normal load W
• This is given by μ = F/W
• μ can vary over a wide range, from about 0.001 in a
lightly loaded rolling bearing to greater than 10 for two
identical clean metal surface
• Sliding in air in the absence of a lubricant ranges from
about 0.1 to 1
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Wear
• A process of removal of material from one or both of two solid
surfaces in solid state contact
• Occurs when two solid surfaces are in sliding or rolling motion
together.
• Most tribological contacts are lubricated to reduce CoF and
associated wear and damage.
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Wear
• Wear needs to be evaluated, measured, and classified.
• Measuring includes loss of material: mass or volume
• There are five main categories of wear. Each has a specific wear
mechanisms that occur.
• In practice, more than one mechanism are involved simultaneously
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Classification of Wear
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Classification of Wear
Abrasive wear:
It is defined as the wear due to hard particles or hard protuberances forced against
and moving along the solid surface
Adhesive wear:
Wear due to localized bonding between contacting solid surfaces leading to
material transfer between the two surfaces or the loss from either surface
Erosion:
Removal of material from a surface due to mechanical interaction between that
surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles
Fatigue wear:
Fraction of material from a solid surface caused by the cyclic stresses produced by
repeated rolling or sliding on a surface.
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Abrasion - Low stress abrasion
Examples:
Particles sliding on chutes, plowing sandy soils, sliding systems in
dirty environments, ash handling equipment, mineral handling
equipment.
Applicable surface treatment:
Hard plating, case hardening, selective hardening, CVD coating
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Abrasion - High stress abrasion
Examples:
Milling of minerals, rollers running over dirty tracks, earth
moving equipment, heavily loaded metal to metal sliding systems
in dirty environment
Applicable surface treatments:
Heavy carburized case, cemented carbide wear tiles, heavy flame
hardening, cast white iron wear plates
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Abrasion - Gouging
Examples:
Hammer mills, gyratory crusher parts, ball mill parts, agricultural
equipment in rocky soils.
Applicable surface treatment:
Hardfacing
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Abrasion - Polishing
Example:
Mixing device for grains and fine solids, high concentration of soot
in engine oil
Applicable surface treatment:
Hard plating, thin film hard compound, case hardening, selective
hardening
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Control of Abrasive Wear
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Erosion - Solid particle impingement
Examples:
Fans in dirty environment, abrasive blasting, aircraft operating in sand
or dirty, exhaust systems carrying particles
Applicable surface treatment:
Carbide and ceramic wear tiles
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Erosion - Liquid impingement
Examples:
Rain impingement on aircraft,
liquid spray deflectors, steam
turbine vanes
Applicable surface treatment:
Ceramic and carbide wear tiles,
elastomer and plastic cladded
surface, corrosion resistant
plating
Process of materials’ damage by liquid
impingement erosion
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Erosion - Cavitation
Examples:
Ship propellers, pipelines, pumps,
mixing device, ultrasonic agitators
Applicable surface treatment:
Ion implantation, ceramic tiles,
corrosion resistant plating
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Erosion - Slurry
Examples:
Slurry pipelines, slurry pumps, mineral floatation system, cement
handling equipment
Applicable surface treatment:
Hard plating, ceramic and carbide wear tiles, chromized steel, plastic
lined pipe
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Effect of impact velocity on erosive wear
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Effect of angle of impingement on erosion wear
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Adhesive - Fretting
Examples:
Bearing on shafts with a loose
fit, clamping faces of injection-
moulding cavities, metal parts
vibrating in tract or rail transit
Applicable surface treatment:
Hardfacing of co-based alloys
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Adhesive - Seizure
Examples:
Hinge pins, overheated autoengine
causing seizure by thermal expansion
of pistons in cylinders, valves,
unlubricated sliding system
Applicable surface treatment:
Lubricating thin film coating, case
hardening, selective hardening.
Insufficient clearance
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Adhesive - Galling
Examples:
Fitted sliding members, plug
valves, gate valves
Applicable surface treatment:
Chromium plating, hard coating,
case hardening, ceramic and
carbide coating
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Adhesive - Oxidation
Examples:
Dry sliding systems on gauge and fixtures, hinge assemblies,
conveyors, sliding parts on machine tools
Applicable surface treatment:
Case hardening surface, thermal barrier coating
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Mechanism of Adhesion Wear
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Fatigue - Pitting wear
Examples:
Cam paths, gear teeth, rail and metal tires
Applicable surface treatment:
Carburizing, selective hardening
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Fatigue - Spalling
Examples:
Coated cam and gears, plated mechanical stops, thin plating on
reciprocating systems
Applicable surface treatment:
Hardfacing
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Fatigue - Impact wear
Examples:
Hammer heads, riveting tools, pneumatic drills
Applicable surface treatment:
Hardfacing
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Fatigue - Brinelling
Examples:
Static overload on mating surface: wheels on rails, on rolling element
bearing, on mold faces
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Wear Rate
The loss of material per unit of sliding distance is measured.
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Visual Classification of Wear of Metals
Mild wear Severe wear
High (100 – 1000 times
Rates of wear Low greater than those observed
in mild wear)
Surface
Smooth Rough
morphology
Heavily deformed metallic
Oxides (from both material (from both
Zone 3 material
surfaces) surfaces) with incorporated
oxide particles
Fine oxide
Debris Coarse metallic flakes
particles
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High Low www.knust.edu.gh
resistance
