MICROFRACTURE AND MATERIAL REMOVAL IN SCRATCHING OF ALUMINA

A bonded-interface sectioning technique is used to examine subsurface damage modes and to identify mechanisms of material removal in repeate d single-point scratching of alumina as a function of grain size, load , and number of passes. In the fine grain alumina, the lateral and med ian crack system is observed, together with intergranular microcracks and intragrain twin/slip bands distributed within the plastic zone, Th e distributed form of damage, namely twin/slip bands and intergranular microcracks, are also observed in the coarse grain alumina; but no ev idence is found for well-defined median and lateral cracks in this mat erial. The mechanism of material removal in alumina is identified as g rain dislodgement resulting from grain boundary microcracking, irrespe ctive of the grain size. Extension of lateral cracks is found to contr ibute to the material removal process only in the fine grain alumina s cratched under a large load and after several passes. A model for the microfracture-controlled material removal process is proposed that rel ates the volume of material removed to the applied load and material p roperties including grain size, elastic modulus, hardness, and short-c rack toughness. Removal rate is shown to be proportional to grain size I-1/2 and to load P-2. The model and the experimental results obtaine d in scratching are used to describe the action of an individual abras ive grit in grinding and other abrasive machining processes.