DAMAGE PROCESSES IN CERAMICS RESULTING FROM DIAMOND TOOL INDENTATION AND SCRATCHING IN VARIOUS ENVIRONMENTS

Studies have been carried out to determine the influence of different chemical environments and tool shapes on damage produced during diamon d tool scratching and indenting of two advanced ceramics: chemical-vap or-deposited silicon carbide and a composite aluminum oxide-titanium c arbide. A nanoindentation/scratching apparatus was used in a controlle d-toad mode for the studies. Two diamond tool shapes were used: a wedg e with -45 degrees rake and 0.5 mm radius curved edge, and a Vickers p yramid. The environments studied were: air, water, mineral oil, minera l oil + stearic acid and two commercial water-based fluids. It was of interest to identify damage mechanisms, critical loads for initiation of severe damage, specific scratching energies and the effect on damag e of the different tool shapes. It was found that chemico-mechanical i nteractions during scratching in the different environments led to sig nificant differences in such parameters as damage threshold load, maxi mum tool penetration depth and specific scratching energy. In some cas es surface chemical films appeared to form and control the scratching process, especially at low loads. At higher loads significant chemical influences on the mechanical damage processes also were found. Damage morphology involved discrete surface cracking, spalling, grain pull-o ut and plastic deformation. In many cases considerable plastic respons e of the relatively brittle ceramics was observed. The proportions of cracking and plasticity varied substantially with tool shape, environm ent and material. Discontinuous surface cracking damage was observed i n some cases, suggesting that a sub-surface process of damage or strai n energy accumulation was involved.