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

摘要

The present paper describes results of plate-impact friction experiments conducted to study time-resolved frictional characteristics of sliding interfaces under extreme conditions. By employing pressure-shear impact of tribo-pair materials comprising hard tool-steels against low melt-point metals such as 7075-T6 Al alloys, interfacial normal pressures ranging from 1 to 2 GPa and slip-speeds of approximately 100m/s have been obtained. The resulting relatively large friction-stresses (100-400 MPa) combined with high slip-speeds generate conditions conducive to interfacial temperatures approaching the near-melt and fully-melt temperature regime of the lower melt-point metal (aluminum alloy) comprising the tribo-pair. During the early part of frictional slip the coefficient of kinetic friction is observed to decrease with increasing slip-velocity while during the later 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 conditions, the interfacial resistance approaches the shear strength of the molten aluminum alloy under hydrostatic pressures of approximately 1-3 GPa and shear-strain rates of ～ 10(sup)7s{sup}(-1). The results of the study indicate 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 aluminum to be smeared on the tribo-pair interface. Results of energy dispersive X-ray spectroscopy shows appreciable amounts of material transfer between the tribo-pair surfaces. Knoop-hardness measurements in 7075-T6 Al alloy at various depths from the slip interface, indicate that the hardness increases approximately linearly with depth and reaches a plateau at approximately 40 μm from the surface.