Tribology of high-speed metal-on-metal sliding at near-melt and fully-meltinterfacial temperatures

The present paper describes results of plate-impact friction experiments co nducted to study time-resolved frictional characteristics of sliding interf aces under extreme conditions. By employing pressure-shear impact of tribo- pair materials comprising hard tool-steels against low melt-point metals su ch as 7075-T6 Al alloys, interfacial normal pressures ranging from 1 to 2 G Pa and slip-speeds of approximately 100m/s have been obtained. The resultin g relatively large friction-stresses (100-400 MPa) combined with high slip- speeds generate conditions conducive to interfacial temperatures approachin g the near-melt and fully-melt temperature regime of the lower melt-point m etal (aluminum alloy) comprising the tribo-pair. During the early part of frictional slip the coefficient of kinetic frictio n is observed to decrease with increasing slip-velocity while during the la ter part transition in interfacial slip occurs from near-melt to the fully- melt temperature regime of the 7075-T6 Al alloy. Under these fully-melt con ditions, the interfacial resistance approaches the shear strength of the mo lten aluminum alloy under hydrostatic pressures of approximately 1-3 GPa an d shear-strain rates of similar to 10(7) s(-1). The results of the study in dicate that under these extreme conditions molten aluminum films maintain a shearing resistance as high as 100 MPa. Scanning electron microscopy (SEM) of the sliding surface reveal molten alu minum to be smeared on the tribo-pair interface. Results of energy dispersi ve X-ray spectroscopy shows appreciable amounts of material transfer betwee n the tribo-pair surfaces. Knoop-hardness measurements in 7075-T6 Al alloy at various depths from the slip interface, indicate that the hardness incre ases approximately linearly with depth and reaches a plateau at approximate ly 40 mum from the surface. (C) 2001 Elsevier Science B.V. All rights reser ved.