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

EFFECT OF HARDNESS ON SLIDING BEHAVIOROF MATERIALS

Aadarsh Mishra1*

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

Hardness is one of the key factors which influence the sliding behavior of materials. In thetribological pairs, the softer hardness among two materials is considered. Effects of hardnessare much more complex in tribological condition. Hardness varies with position and time. It alsodepends on sliding speed, temperature and chemical environment and has a significant effecton sliding behavior. Mechanical mixing strongly influence the local hardness. Hardness can alsoeffect transition phases in friction and wear and can help us to understand the geometric effectswhen the materials are interchanged in the tribological system.

Keywords: Hardness, Wear, Abrasion, Sliding, Mechanical mixing, Variable and local hardness

INTRODUCTIONThe wear volume can be calculated by usingthe equation:

V = kSL/H

where V is the wear volume; L is the normalload; S is the sliding distance; H is thehardness of one of the interacting materialsand k is the wear coefficient.

Wear coefficient can be defined as:

• Probability that an asperity contactgenerates a wear particle.

• Ratio of the volume worn to the volumedeformed.

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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.

• Factor reflecting the inefficienciesassociated with the process generatingwear particles.

The Linear wear equation can be used tocalculate wear volume.The coefficient k isassumed to be constant but can change byorders of magnitude. Hardness is the propertythat appears explicitly in the linear wear equation.Other materials properties are also included inthe coefficient k.The main focus of this paper isthe role of hardness in tribological systems.

EXPERIMENTAL STUDIESThere are various forms of abrasive and slidingwear. These are both part of a continuous

Research Paper

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

spectrum ranging from microcutting to smoothsliding. When the wear coefficient is large,abrasive wear is a relatively efficient way toremove the materials while sliding wear is a lessefficient way of removing materials when thewear coefficient is smaller. Generally abrasionranges between these two extremities.Microcutting occurs when hardness differencesaresufficient and when local rake angles areappropriate. It can be recognized by the debrisparticles generated. Cutting involvespenetration of the workpiece, and this dependson the hardness. The hardness in turndependson composition and microstructure. The role ofhardness in sliding situations are not very clear.Fixed abrasives become less effective inremoving materialas the sharp points fracture,wear and becomecoated. A transition occursfrom one side of spectrum to the other. In certainmaterials the transition is in a reverseorder.Most common example is 304 stainlesssteel.

It is commonly observed that transfer ofmaterial from one surface to its counterfaceoccurs during sliding in which the process ofAdhesion is involved. When there isappreciable transfer, as in many tribo-systemsinvolving metals, it is common to callthe wear processadhesive wear. Localcontacts createlarge plastic strains andassociated changes in the near surfacemicrostructure. The resulting heterogeneitiesmake the material susceptible to localizedshear. The transfer can be in either direction,but it is generally larger for the material withsmaller cohesive strength transferring to thematerial with larger cohesive strength.Deformation, adhesion and fractureaccomplish mechanical mixing.

The mechanically mixed material on thesliding surface and the resulting weardebrisnormally have the same microstructure andhardness. Sliding wear debris are notgenerated directly from the base material.

In case of stainless steel, the mechanicallymixed material is very hard. Thecorresponding material having very fine grainsize is effectively softer than the adjacent work-hardened base material. The MechanicallyMixed Layer (MML) can vary considerably fromone system to another. It may consist of two or

Figure 1: TEM Image of LongitudinalSection of a Worn Copper Block After

Sliding

Figure 2: SEM Image of Debris Particles

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

more than two phases. These phases mayconsist of wearing materials, from coatings orfrom components of lubricants present.

Composition, crystal orientation andmicrostructure are responsible for localvariations in materials. Some of theseproperties are present in materials evenbefore sliding begins. In sliding tests withaluminum it has been possibleto correlatedeformation substructure dimensions withthethickness of lamellar features.

At higher loads, deformation substructurecan develop further below the sliding surface.

hardness and geometrical factors, in particularthe degree of penetration. The importance ofthe relative hardnessof the pin and the diskwas thus emphasized. The ratio Hd to Hp is usedfor subsequent sliding tests. When the initialvalue of Hd/Hp is less then one severe wearoccurs and friction is high. On the other hand,when the value of Hd to Hp is greater than onethen there are two cases. In one of the casefriction is low and sliding is smooth. In the othercase transition of materials may occur.Generally if the value of Hd/Hp is less than onerough sliding and severe wear occurs. Formany metals and alloys, sliding at or near roomtemperature produces a hardened surfacelayer by work hardening and other processes.However, there are other materials, particularlythoseinvolving at least one phase with lowmelting temperature, in which sliding producesa negative hardnessgradient.

Figure 3: SEM Image of a Partof Extruded Material

The source of shear instability would thenbe absent, so transfer of fragments wouldoccur less readily, deformation to a depth ofone substructure unit is needed beforeslidingwear becomes appreciable.

RESULTS AND DISCUSSIONThe sliding behavior may change significantlyif the materials in a sliding system areinterchanged. This is particularly likely if onecomponent is in steady contact while the otherhas intermittent contact in pin on diskexperiment. The operating mode depends on

Figure 4: Secondary-Electron Imageof Longitudinal Section of Pb Disk After

Sliding in Vaccum

Despite the attention already given to thetribological characteristics of steels, there isstill much to be learned. Many results show thatvariations in local hardness values, arising fromvarious sources strongly affecting slidingbehaviour.

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

CONCLUSIONIt is well recognized that a basic problem ofusing solid lubricant films is replenishment.Ideally, the materialitself will continue to supplythe appropriateamount of easily shearedmaterial to assure smoothsliding with lowfriction and wear. The results provide someusefulguidelines for selecting promisingcompositions and microstructures. Therelative hardness or flow strengthof the twophases is crucial. The advancing asperitydeforms the matrix phase plastically and thiscausesthe softer phase to be extrudedsufficiently so it canbe spread over the surfaceby the asperity. If the matrixis too soft, then bothphases deform appreciably, and the desiredcondition of a soft film on a hard base isnotestablished. If the matrix is too hard, then thereis insufficient deformation of the matrix, for agiven load and microstructure, to extrude theproper amountof soft phase. Thus, an optimumratio of hardness orstrength is expected.

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

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3. Khruschov M M and Kragelskii I V (1965),Friction and Wear, Translated by LRonson and J K Lancaster, Butterworths,London.

4. Kuo S M and Rigney D A (1992), Mater.Sci. Eng. A, Vol. 157, pp. 131-143.

5. Rigney D A, Chen L H, Naylor M G S andRosenfield A R (1984), Wear, Vol. 100,pp. 195-219.

6. Sliney H E (1992), Solid Lubricants, in PJ Blau (Ed.), ASM Handbook, Vol. 18, pp.113-122, ASM Int., Materials Park, OH.

7. Yang Z Y, Naylor M G S and Rigney D A(1985), Wear, Vol. 105, pp. 73-86.