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 repeated single-point scratching of alumina as a function of grain size, load, and number of passes. In the fine grain alumina, the lateral and median crack system is observed, together with intergranular microcracks and intragrain twin/slip bands distributed within the plastic zone, The distributed form of damage, namely twin/slip bands and intergranular microcracks, are also observed in the coarse grain alumina; but no evidence is found for well-defined median and lateral cracks in this material. The mechanism of material removal in alumina is identified as grain dislodgement resulting from grain boundary microcracking, irrespective of the grain size. Extension of lateral cracks is found to contribute to the material removal process only in the fine grain alumina scratched under a large load and after several passes. A model for the microfracture-controlled material removal process is proposed that relates the volume of material removed to the applied load and material properties including grain size, elastic modulus, hardness, and short-crack 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 obtained in scratching are used to describe the action of an individual abrasive grit in grinding and other abrasive machining processes.