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

FRICTION, METALLIC TRANSFER AND WEARDEBRIS OF SLIDING SURFACE

Aadarsh Mishra1*

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

In this work the variation in coefficient of friction with sliding distance is presented. It includesdifferent sliding arrangements for both similar and dissimilar metals using pin on disk experiment.When similar metals slide on each other there is a significant change in friction, due to whichploughing occurs. In case of dissimilar metals, coefficient of metals is determined primarily bythe way metal transfer occurs between sliding surface.

Keywords: Coefficient of frction, Metallic transfer, Wear debris

INTRODUCTIONThe coefficient of friction (µ) is timedependent in case of two dry metallic surfacesliding against each other. This statementholds true experimentally in case of similarand dissimilar sliding metal surfaces. Theclassical theory of adhesion has failed toexplain the dependence of coefficient offriction (µ) on time as the shearing of weldedjunctions is dependent on the true area ofcontact which is a function of normal andtangential loads. Hence for a given set ofconditions we assume that µ is constant.However the metal transfer and formation ofdebris in sliding process affects the frictionalbehavior of sliding metals. Antler (1962) haverecently presented a new theory to explain the

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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.

time dependence of µ according to which thereare three frictioncomponents, i.e., surfaceadhesion, deformation of asperities andploughingby wear debris or hard asperities,which contribute to µ. Because of thechangingcontribution of these three components µvaries accordingly withsliding distance. Theyhave also attempted to generalize the frictionbehaviors for both similar and dissimilar metalssliding against one another. Mahdavian et al.(1982) and Bowden and Tabor (2001) carriedout their friction tests on a wide range of free-machining metalsincluding steel, aluminiumand brass. The metal transfer and back transferplay a significant role in determining the fictionof sliding metals. Mahdavian et al. (1982) areable to show that metallic transfer due to

Research Paper

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

adhesion at the early stage ofsliding controlsthe coefficient of friction. In case of soft metalsliding on a hard metal the coefficient offriction is dominated by adhesion. The weardebris generated in the process is formed fromthe softer metal. The other case includes dualtransfer of metals in which the wear debrisgenerated is of both the metals. The coefficientof friction is greatly influenced by the action ofhard asperities that are embedded in the softmetals. Here we intend to study the variationof coefficient of friction with sliding distance.

EXPERIMENTAL PROCEDUREAll friction tests were being conducted on pin-on-disk machine. Four pins were secured toan upper metal disk with equal angles betweenthem on the periphery of a circle with radius of35 mm. The pins were loaded by a deadweight of 35 N per pin against the rotating disk.The pins and rotating disk are maintained in acontinuous contact throughout the wearprocess. The rotation of the upper disksholding the pin was constrained by a springattached to the frame. The deflection of thespring could be measured continuously.Through this process coefficient of friction wasevaluated as a function of sliding distance. Therotating disk was coupled through reductiongears to a d.c. motor fitted with a variable-speed controller. The angular speed ismaintained at 2.5 rad/s.

Machining metals consists of grade mildsteel (85 HRB), Aluminum alloy (62 HRB) andbrass alloy (75 HRB) were used in a frictionexperiments. The pins of diameter 7 mm andlength 12 mm were used in the friction tests.All the surfaces of pin and disks were polishedby using different polish papers. An extreme

care was taken so that there is a continuouscontact between pin and the disks.

The geometric arrangements of similar anddissimilar metals were tested. The variationof friction with sliding distance was recordedand the wear debris was collected at thecompletion of test for microstructural analysiswith the help of Scanning Electron Microscope(SEM).Similar observations were alsoconducted on the wear trackof the disk andthe worn surface of the pin.

RESULTS AND DISCUSSIONTransfer of metal between two dissimilarmetals was studied by using ScanningElectron Microscope (SEM) operating in thescattered electron emission mode. Dualmetallic transfer takes place when thealuminium pin slides on the steel disk. This wasevident from the fact that steel debris wasembedded in the aluminum pin surface andthe aluminum which was transferred to thesteel disk appeared as dark patches on theScanning electron microscope. Thedebrisproduced from sliding the aluminium pinon the steel disk was mainly aluminium with asmall amount of steel which was in turnconfirmed by the scanning electron analysisof debris. It also gave additional evidence thatwear had been occurring in the steel disk andthe wear debris was in turn transferred to thesofter metal. On the other hand, when thealuminum disk was slid against a steel pin,metal transfer takes place from the softer metalto the harder pin. However the transfer of steelto the aluminum disk was insignificant andcould only be detected by using electronmicroscopy. Microstructural analysisconfirmed that particles were largely aluminumwith a very small amount of steel.

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

From the microstructural analysis we cansay the following points:

• There was a transfer of material from thebrass to the steel pin. The type of transferwas a dual metallic transfer.

• The debris generated from sliding wasmainly brass with some traces of steel.

• On reversing the sliding arrangementtransfer takes place from softer to hardermetal.

Figure 1: SEM Analysis Showing Transferof Aluminum to the Steel Disk

After Sliding

Figure 2: SEM Analysis Showing Transferof Steel Particles to the Aluminium After

Sliding

Figure 3: SEM Analysis of the Wear Debris

Figure 4: SEM Analysis Showing theTransfer of Brass to the Steel pin

Figure 5: SEM Analysis Showing Transferof Steel to the Brass Disk

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

• There were no traces of steel on the wornsurface of the brass pin and the debris.

CONCLUSIONThe pin on disk friction results confirm that thecoefficient of friction between sliding metalsis dependent on time. One of the importantreasons can be severe ploughing actionobtained in the pin-on-disk geometry. Themechanism of metal transfer plays a significantrole in determining coefficient of friction forsimilar and dissimilar metals sliding againsteach other. A detailed research work is neededto investigate the effects of embedding ofasperities and wear debris as well as the effectof thickness of metal transfer film on thecoefficient of friction.

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

REFERENCES1. Antler M (1962), “Wear, Friction and

Electrical Noise Phenomena in SevereSliding Systems”, ASLE Trans., Vol. 5,pp. 297-302.

2. Bowden F P and Tabor D (2001), “Frictionand Lubrication of Solids”, Vols. 1, 2,Clarendon, Oxford.

3. Glardon R and Finnie I (1981), “A Reviewof the Recent Literature on theUnlubricated Sliding Wear of DissimilarMetals”, J. Eng. Mater. Technol., Vol. 103,pp. 333-340.

4. Halling J (1961), “A Crossed-CylinderWear Machine and its Use in the Study ofSevere Wear of Brass on Mild Steel”,Wear, Vol. 4, pp. 22-31.

5. Mahdavian S M, Mai Y W and Cottrell B(1982), “Friction, Metallic Transfer andDebris Analysis of Sliding Surfaces”,Wear, Vol. 82, pp. 221-232.

6. Rice S L, Wayne S F and Nowotny H(1983), “Specimen Material Removal inPin-on-Disc Tribotesting”, Wear, Vol. 88,pp. 85-92.

7. Suh N P and Sin H-C (1981), “TheGenesis of Friction”, Wear, Vol. 69,pp. 91-114.

Figure 6: SEM of Collected Debris