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Direct observation of contact damage around scratches in brittle solids

An experimental study has been made of the deformation and fracture produced around scratches in brittle solids such as soda-lime glass, sapphire, and poly-crystalline spinel. In the scratch experiments, a Vickers indenter was translated against a surface at velocities of a few mm/min with the normal load being kept in the range of 10 g to 700 g. The contact region was observed and photographed in-situ using a high resolution, optical microscope and video imaging system, while forces were measured with a piezoelectric force sensor attached to the indenter. The experiments have enabled several unique observations to be made of the contact damage such as the formation of plastic scratches, slip lines, and ribbon-like wear particles; the loads at which cracks form and their evolution during loading and unloading; and a new type of crack system around scratches in soda-lime glass which forms during unloading of the indenter. The implications of these results to contact damage and wear in brittle solids are discussed.