An investigation of sliding metallic wear in the presence of thin film boundary lubrication

摘要

An asperity interaction model is given in which the frictional force results from the pushing of waves of plastically deforming material in the softer surface ahead of asperities on the harder surface. Evidence is presented to show that the lubrication at the hard asperity-wave interface is elastohydrodynamic in nature. This means that the surfaces are separated in the regions of asperity contact by a highly effective lubricant film. As a consequence the possibility of metallic adhesion between the contacting surfaces is virtually eliminated and adhesive wear is highly unlikely to occur. It is becoming increasingly clear that in such situations wear results from the surface damage which occurs with the repeated straining of the softer surface by successive wave passes. From the slipline field model used to represent the wave formation process it can be shown that the strain cycle undergone by the deforming material consists of cyclic and unidirectional (ratchetting) components. Results of a metallographic study of the surface damage resulting from repeated wave passes in wedge tests (scaled-up hard asperity) are presented. These show how low cycle fatigue can lead to crack propagation into the softer surface and how ratchetting can rotate these cracks to be more or less parallel to the surface. With subsequent wave passes the tongues of material thus formed can then be broken off to form wear particles.