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Int. J. Mech. Eng. & Rob. Res. 2014 Aadarsh Mishra, 2014

ANALYSIS OF RELATION BETWEEN FRICTIONAND WEAR

Aadarsh Mishra1*

*Corresponding Author: Aadarsh Mishra,aadarshm9@gmail.com

To control wear and friction several methods like soft/hard film coating, multi-phase alloying andcomposite structuring have been developed. We can say that coefficient of friction and wear arenot the material properties but they are the responses of tribo-sysytem. They have a relationwith each other. Wear is defined as a set of adhesion, transfer, abrasion, fatigue and oxidation.Due to these processes transfer of matter takes place from one state to another. The finaltransformation in wear is a collection of free debris particles.

Keywords: Wear, Friction, Roughness, Hardness, Ductility, Oxide film, Transfer

INTRODUCTIONFriction and wear can be defined as theresponses of a tribo-system.Coefficient offriction and wear define the state of contact ofbodies not the material properties of thebodies. Although in some special cases ofcontact, they can be treated as the materialproperties of the bodies. Both friction and wearcan be treated as two kinds of responses fromone tribo-system and are related with eachother in each and every state of contact. Nowdays in addition to traditional method oflubrication methods like soft/hard film coating,multi-phase alloying and composite structuringhave been developed to control the friction andwear. To study the tribo-characteristics of

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engineering, Manipal Institute of Technology, Manipal University, Manipal, Karnataka 576104, India.

materials we should first understand the wearmechanisms in which roughness, hardness,ductility, oxide film, reaction layer and adhesivetransfer play a significant role. The purpose ofthis research paper is to understand the wearmechanisms by studying the characteristics ofwear and friction by using different materials.

EXPERIMENTAL STUDIESDue to partly transferring of the thin soft layerto counter surface by adhesion in tribo-elements, relative displacement takes placeat the interface which is between surface ofcoating and transfer layer. This layer is ofsmaller shear strength of the softer materialthan that of the underlying elemental material.

Research Paper

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Due to this low friction can be obtained andmoreover the wear of tribo-system can alsobe reduced. The examples can include Gold,Silver and lead. If we take any metal on whichcoating has been done the roughness ofsubstrate has a significant effect on the wearof that particular coating. With increase in thecritical number of rolling cycles the friction willalso increase. By reducing the surfaceroughness of substrate, the wear of thin softcoating can be reduced. This can be done bychanging the coefficient of friction. The mostcommon example may include lead film onwhich the analysis was done.

When the coating on the surface of tribo-element is harder than that of the element thenthere is a reduction in wear of the element till itexists on the surface as the wear in case ofharder materials is less than that of the softermaterials when the coefficients of friction issame. Delamination of coating takes placewhen the wear rate of coating is smaller in thesteady state because of high hardness. Otherreasons may include inappropriate contactpressure and shearstress at the early stage ofcontact cycles. It can also be introduced byusing a critical coating thickness after a certainamount of wear.

For maximum tensile strength at the edgeof sliding contact high coefficient of friction hasa significant effect. Possibility of crackpropagation may increase from the surfacedefects of coating. Decisive functions for thedelamination of coating may include Hightensile stress and local yield on thesurface ofcoating. Hard coating like diamond or DLCagainst softer material like ceramic duringsliding friction tends to be high since asperitieson the coating surface work as abrasive. In this

case the coefficient of friction may vary from0.5 to 0.04 which depends on their roughness.To avoid any delamination the surface coatingof a material should be minimized.

When a tribomaterial which is made ofsofter material like plastics is rubbed and isworn then by forming a transfer layer of the softmaterial on the counter surface of hardermaterial, its wear can be reduced. Wear rateof soft material in sliding against itself is verysmall in this case.

RESULTS AND DISCUSSIONWhen a triboelement is made of ductilematerial like Aluminum, copper, nickel, iron oran alloy with a combination of them then thematerial in the contact region deformsplastically under the combined stress ofcompression and shear. Larger wear rate iscaused due to large plastic deformationbecause the wear surface tendsto becomerough and protective surface layers areeasilydestroyed.The introduction of a harderreinforcing phasein the ductile matrix by acertain volume fraction canreduce ductility ofthe matrix material in the contact regionwithouthaving brittleness, and wear of matrix canbereduced.

In case of a triboelement being made of abrittle material the material in the contactregion tends to have brittle fracture ofmicroscopic scale underconcentrated largecontact pressure at each contact asperity. Italso results in large wear rate and rough wearsurface. Due to soft reinforcing phase in thebrittle matrix a certain volume fraction can givea certain amount of ductility to the brittle matrixmaterial inthe contact region, and wear ofmatrix can be reduced.

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For instance if we take SiC particles on awear of Ni matrix then it can be classifiedunder a hard reinforcing phase in a ductilematrix. We can say that volume fraction ofabout 10% of soft phase reduces the matrixwear by more than a half.

Many metallic alloys that have been wellused for triboelements. They are alloys basedon iron, copper, aluminum, tin, lead or othermetals. These types of alloys have more thantwo phases out of which one is harder and theother can be softer. In case a triboelement ismade of one of these alloys and its surface isrubbed in air by other solid surface, the rubbedsurface shows the following responsesof multi-phases as the result of repeated stress cycleandfrictional heat cycle:

• Oxide film growth takes place on the surfaceand fracture.

• Plastic deformation in the surface layer.

• Restructuring of the microstructure.

• Adhesive transfer of materials to the countersurface.

• Retransfer between the two mating surface.

• Generation of wear debris and wearparticles.

• Detachment and bending of wear particles.

• Smoothing of wear surface.

• Surface morphology between two matingsurfaces after roughening.

These responses may take place at thesame time in different phases at the contactinterface with different stages. They maydepend upon physical and chemical propertiesof each phase and of each oxide film.Extremehardening in the wear track and the

transfer layer can benoticed. It is due to theeffect of oxygen that forms oxide films ontheAl-alloys and the steel of counter surface.

Because of hardness and brittleness ofceramics it is recommended that they should beused in the mild wear state to avoid large scalefracture. Humid air or water is active enough toform silicon oxide under rolling contact. Aluminaforms aluminum hydroxide by sliding in water.These oxides or hydro oxides in humid air aresoluble and the resultant wear surfaces becomevery smooth and availablefor hydro-dynamiclubrication with air as well as water.

If the wear state is mild and tribo-chemicalwear mode is used then the generatedchemical products of the wear debris are softas well as soluble in water. Hence, a reactionlayer on a hard ceramic surface works as averysoft coating and its wear mechanism mustbe similar tothat of an artificial soft coating.

CONCLUSIONIn the near future for the technical developmentof wear control the characteristics of wearcoatings, material composites, metallic alloysand ceramics have been reviewed in relationto their frictional characteristics.the introducedobservationson wear and friction ofrepresentative materials arewell explained interms of roughness effect, hardnesseffect,ductility effect, oxide film effect, reactionlayereffect and transfer effect. New technologiesfor bettercontrol of wear will become possibleby combining theseeffects better.

ACKNOWLEDGMENTI would also like to dedicate this research workto my father late R S Mishra and mother K LMishra.

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4. Roberts E W and Price N B (1988), “InVacuum Tribological Properties of High

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5. Sherbiney W A and Halling J (1977),“Friction and Wear of Ion-PlatedSoftmetallic Films”, pp. 211-220.

6. Spalvins T (1992), “Lubrication withSputtered MoS2 Films”, pp. 267-274.

7. Zum Gahr K-H (1987), Microstructure andWear of Materials, pp. 174-176, Elsevier,Tribol.