High-speed frictional slip at metal-on-metal interfaces

In this study plate-impact pressure-shear friction experiments are employed to investigate dynamic slip resistance and time-resolved growth of molten metal films during dry metal-on-metal slip under extreme interfacial conditions. By employing a tribo-pair comprising of a hard tool-steel against a relatively low melt-point metal (7075-T6 Al alloy), interfacial normal stress of up to 5 GPa and slip speeds of approximately 250 m/s have been achieved. These extreme interfacial conditions are conducive to the development of molten metal film at the tribo-pair interface. A Lagrangian finite element code is developed to understand the evolution of the thermo-mechanical fields and their relationship to the observed slip response. The code accounts for dynamic effects, heat conduction, contact with friction, and full thermo-mechanical coupling. At temperatures below the melting point of the tribo-pair materials are described as isotropic, thermally softening, elastic-viscoplastic solids. For material elements with temperatures in excess of the melt temperature a purely Newtonian fluid constitutive model is employed. The results of this hybrid experimental-computational approach provide new insights into the thermo-plastic interactions during the high-speed metal-on-metal slip. During the early part of frictional slip the coefficient of kinetic friction is observed to decrease with increasing slip velocity. During the later part, transition in interfacial slip occurs from dry metal-on-metal sliding to the formation of molten Al film at the slip interface. During this slip phase, the interfacial resistance approaches the shear strength of the molten Al alloy under normal pressures of approximately 1-4.5 GPa and shear-strain rates of similar to 10(7) s(-1). The results of the study indicate that under such extreme interfacial conditions molten aluminum films maintain a shearing resistance as high as 50-100 MPa. Scanning electron microscopy of the slip surfaces reveal molten aluminum to be smeared on the tribo-pair interface at high impact velocities. Also, the extent of the molten area, as observed from the SEM micrographs, increases with increasing impact velocities. (C) 2008 Elsevier Ltd. All rights reserved.