Investigations of High-Speed Friction at Metal-on-metal Interfaces

摘要

High-speed sliding at material interfaces plays an important role in the design of several tribo-elements including bearings, gears, and high performance brake liners for the automobile and aerospace industry. In the present study, plate-impact pressure-shear friction experiments were conducted to study time-resolved growth of molten metal films during dry metal-on-metal slip under extreme interfacial conditions. By employing tribo-pairs comprising of hard tool-steel against relatively low melt-point metals, e.g. 7075-T6 Al alloy, interfacial friction stress ranging from 100-400 MPa and slip speeds of over 200 m/s have been achieved. These relatively extreme interfacial conditions are conducive to the development of molten metal films 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 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 and the interfacial resistance approaches the shear strength of the molten Al alloy. The results of the study indicate that under these 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. The photo-micrograph of the cross-section of the 7075-T6 Al alloy reveals a thin region of severe shearing deformation in close vicinity of the sliding surface. The shearing deformation manifests itself as severely deformed grains in the direction of the sliding.