WEAR

FRICTION

WEAREROSION

LUBRICATION

Damage to a solid surface, generally involving progressive loss of material, due to relative motion between the surface and a contacting substance or substances

Substances that separate rubbing surfaces and readily shear while adhering to the surfaces

Impeding motion in sliding

Progressive loss of original material from a solidsurface due to mechanical interaction between that surface and a fluid, a multi component fluid, or an impinging liquid or solid particles

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FRICTION

Manufactured surfaces are not perfectly flat or smooth

When these imperfect planes are put on another, they will contact at ‘spots’

When motion is attempted, the resistance to the motion is called “friction”

WEARReal surfaces do not only contain surface asperities, also contain errors of form: wave form machining features patterns resulting from machine tool used

When a solid surface rubs on another solid surface – deformation, wear, oxide removal and film shear occur.

SURFACE ROUGHNESS FRICTION

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0.03 : cold working 0.7 : hot working

2.0 : machining Plastic : very low or self lubricating

Friction cannot be eliminated

Reducing Frictioni) The selection of materials that have low adhesion. e.g. carbide , ceramicii) The use of surface film or coating. Lubricant (such

as oil) and solid film (such as graphite) iii) Subjecting the tool or die-workpiece interface to ultrasonic vibration (~20kHz)

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TYPES OF WEAR

ADHESION ABRASION SURFACE FATIGUE

AdhesiveGalling/SeizureScuffing/Scoring

OxidativeFretting

Low StressHigh Stress

GougingPolishing

SpallingBrinelling

Impact WearPitting

Requires adhesion of one surface to the other

Requires hard, sharp surfaces imposed on softer surfaces

Requires repetitive compressive stresses

A) TYPES OF WEAR (Sliding Contact) - ADHESION1. ADHESIVE WEAR Due to localised bonding between contacting solids leading to a material

transfer or loss from either contacting surface. All material systems are susceptible to adhesive wear – factors control this

type of wear is wear coefficients

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Examples of wear coefficients:•Clean lubricated soft metals – 1 to 3 x 10-3

self-mated•Clean unlubricated hard metals – 2 to 5 x 10-5

self-mated•Clean unlubricated hard metal – 1 to 3 x 10-6

to plastic•Clean unlubricated plastic - 1 x 10-3 to 3 x 10-6

to plastic • Well-lubricated hard metals – 1 to 3 x 10-7

self mated

eg : gears, cam, slides and valves

Adhesive wear can be reduced by :i. Selecting materials that do not form strong adhesive bondsii. Using harder material as one of the pair

iii. Applying hard coating

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2. GALLING

Occurs when adhesive wear becomes severe Formation of macroscopic excrescences ~ material flows up from the surface

this large up-feature often leads to seizure in systems where the sliding members rub with little clearance.

Soft single phase metals are particularly susceptible to galling ~ stainless steel

Propeller shaft

3. SCUFFING and SCORING Scuffing : Moderate form of adhesive wear ~ macroscopic scratches or

surface deformation aligned with the direction of motioneg: If a piston in a cylinder working properly, only mild wear occurs, the cylinder

stay smooth, and its diameter just gets larger as wear occurs Scoring : Leads to unacceptable high wear rates (surface is roughened)eg: Overloaded or unlubricated gears often score

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4. OXIDATIVE WEAR (Mild Wear) The least severe form of adhesive wear. Starts by adhesion at the real areas of

contact, then wear particles form and with repeated rubbing, these particles react with the environment an usually form an oxide

eg: Ferrous systems – rubbing surfaces appeared to be rustedAlmost all household door hinge pins develop oxidative wear – neglect to oil

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5. FRETTING Due to oscillatory motion of small amplitude (less than 300 µm) which can

produce fretting wear or fretting corrosion ~ surface that looks gnarled and pitted.

With oscillating rubbing , contacting surfaces locally adhere at asperities or up-features, the junction is broken, and, with repeated rubbing, wear particles roll back and forth in between the contacting surfaces.

If wear particles react with ambient environment, the damage is called fretting corrosion. If the surfaces and particles do not react with the environment, the result is fretting wear.

eg: Ferrous metals : always display frettingPlastics : exhibit fretting wear

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“ALL WEAR PROCESSES IN THE ADHESION CATEGORY AT LEAST START BY SURFACE ADHESION. TYPIFIED BY CONFORMING SURFACES IN SLIDING CONTACT. MOTION CAN BE UNIDIRECTIONAL , ROTATIONAL OR OSCILLATING. LONGER TIMES AND HIGH LOADS LEAD TO WEAR”

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B) TYPES OF WEAR - ABRASION Abrasive wear : Unintentional wear produced by a hard/sharp particle

imposed on and moving on a softer surface. The harder/sharper the particle, the greater the wear.Particles : in sand paper, pavement or grinding wheel.

1. LOW-STRESS Fine scratches of the surface due imposed/moved sharp/hard particles Materials can be removed through chip formation from a fixed grain sliding on

a surface (grinding wheel) or that grain could just produce a scratch

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2. HIGH-STRESS More severe form of abrasion in which the abrasive substance is imposed on

the surface with sufficient stress to cause the abrasive to fracture or crush.eg : surface grinding – the abrasive grains or the grinding wheel fracture during

process. The mechanism of material removal is usually scratching. Furrows are plowed

in the material from the sharp edges of the abrasive, and chips are formed

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3. GOUGING ABRASION Damage to a solid surface characterised by macroscopic plastic deformation

from a single impact The damage is in the form of a gouge groove or deep scratch. Damage

produced by dropping a large rock on a rigid metal surface or pressing the rock against the surface with sufficient force to crush it.

eg: normally occurs in mining equipment and rock crusher and on power shovel teeth and buckets

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4. POLISHING Produce by fine abrasives, and yet there are no scratches or furrows produced

by the polishing abrasive.eg : Polishing of steel with aluminium oxide or diamond particles ( 1 µm Ø). They

remove scratches and polish. The abrasive particles remove oxides, the polishing fluids corrode the surface

and this removes material.

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C) TYPES OF WEAR - SURFACE FATIGUE1. PITTING One of the common forms of surface fatigue. Formation of cavities/pits when regions of tribosurface spall. Pits initiated as substrate cracks and the cracks propagate to allow ejection

of a fragment. Once a metal or hard material fragment contaminates the rolling /moving surface, it is rolled into the surface, other pits form and the surface is ruined.

Eg : All heavily used rail-road tracks

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2. IMPACT WEAR Wear that occurs on surfaces subjected to repeated impact

eg : chisels, impact drivers and hammers of all typesPlastic flow like that on the end of a well-used cold chisel. Some times, a subsurface crack occurs and fragments can spall from the surface.

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3. SPALLING Fracture of a portion of the surface of a material that is subjected to repeated

stresses – usually compressive When a hard brittle coating or hardened layer is applied to a significantly

softer substrate, repeated loading cause spalling. Usual solution : back up thin hard coatings with a surface that is only about

20% softer

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EROSIONProgressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multi component fluid, or an impinging liquid or solid particles

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Erosion

Solid Particles Liquid Impact Cavitation

Slurry Liquid

Types of Erosion :1. Solid Particle Damage to a surface caused by impingement of solid particles carried by a gas

usually air The mechanism of material removal can range from spalling in brittle materials

to cutting in ductile materials. Abrasive grains in sand blasting, when they impact the target surface, they behave as tiny cutters to form chips

A particle forms a crater, repeated strikes hit the extended material around the crater and the lips eventually fracture

eg : fans used in dirty areas, jet engine blade, soot blowers, cyclone separators

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2. Slurry Erosion Material removal caused by slurry motion across a solid surface Mechanism : similar to solid particles – however involve mostly parallel

flow of slurry and the action of the particles is like scratchingeg : water pumps handling drilling and mines, conveying slurries in pipelines

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3. Liquid Impact Erosion is produced by impingement of very high velocity liquid droplets on a

solid surfaceeg : rain erosion of aircraft, steam turbine when condensate is present.

4. Liquid Erosion Removal of material by the action of a liquid impinging or moving along the

surface of a solid Mechanical action of the liquid removes protective oxide.

eg : metal pipes – steel, stainless steel and copper

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5. Cavitation Damage to a solid surface by implosion of bubbles near a surface When a bubble collapses, the surrounding liquid rushes in to fill the void. A

jet is form that can develop stresses that exceed the yield strength of materials – produce microscopic fracture to form pitseg : fluid propulsion devices : pumps, on ship propellers

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To reduce erosion (minimisation of friction)i. Surface hardened componentsii. Minimize surface roughnessiii. Improving the quality of materials to minimize local point for crack initiationiv. Ceramic and cermets ideal wear resistant

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LUBRICATIONSLubricants – substances that separate rubbing surfaces and readily shear while adhering to the surface

3 major categories : Oils, Greases and Solid-film lubricants Oils (mineral oils) and greases (synthetic oils) can be made from crude oil or

from chemical feedstock1. Oils Viscous fluid : eg, mineral oil – very low friction EP (extreme-pressure) Lubricant (Synthetic) - Chemicals (usually organic

materials with S, Cl and P content) reacts at elevated temp with metal surface. Requires continual checking of oil supply

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2. Greases combination of an oil (80-90%), a thickener ( 3-D sponge that releases oil

when the device is in use and retains oil when not in use) and an additive eg : fatty acid, fats, waxes

Convenient and the easiest to use, and with seals, possible to lubricate over 20 years

• Solid lubricants – applied as coatings to metals and compounded (mixed) into plastic. Develop films between rubbing surfaces which reduce friction.

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3. Solid lubricants Applied as coatings to metals and compounded (mixed) into plastic.

Develop films ( a few microns to more than 50 microns) between rubbing surfaces which reduce friction.

Solving lubrication problems where oil and grease do not work. eg : Molybdenum disulfide, Tungsten disulfide, Graphite, Silver etc

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Self lubricated plastic

Graphite

Self lubricated bearings

Pin-on-disc test To determine material’s wear rate The wear is determined using pin which is press onto the rotating disc under

a defined load.

The pin can also be replaced by a ball (ball-on-disc.)

Either the pin or the disc can be the test piece of interest. The contact surface of the pin may be flat, spherical, or of any convenient

geometry, including that of actual wear components. The loading condition is defined by the normal load, the sliding velocity and

the initial temp. of the test medium

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Wear units :mass loss ( mg/hr)

Thickness loss ( mm/hr)Distances (mg/m)

SpecificationsConforms to the specifications of: ASTM G G99Sliding Speed Range: 0.26-10 m/secDisc Rotation Speed: 100-2000 rpmMaximum Normal Load: 200 NPin Size: 3-12 mm diagonal/diameterDisc Size: 160 mm x 8 mm thick

The wear volume can either be determined by the changed geometry of the specimen:

• Shortening of the pin• Determination of the wear track volume of the of the pin on the disc) or• By the mass reduction of the specimens.

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