Microfracture about scratches in brittle solids

Abstract

Observations of the microcracking about scratches in a number of brittle solids including sapphire and a variety of glasses are presented. The scratching was carried out primarily with a Vickers pyramid indenter under various loads at one scratching velocity. It is observed that the nature of the cracking is very similar to that occurring about a quasi-static pointed indenter. Directly beneath the indenter a well-defined median crack is formed, behind the indenter small partial Hertzian cracks develop within the track, and subsurface lateral cracking initiates from the region of the plastically deformed zone. With a Vickers pyramid the cracking phenomena may be conveniently divided into three regions with increasing load. At low loads ( P < 0.05 N) no cracking is observed, intermediate loads (0.1 N < P < 5 N) well-defined median and lateral cracks occur, at higher loads ( P > 5N) the plastically deformed track appears to shatter and the extent of lateral and median cracking is less than that occurring for the higher loads in the intermediate region. The present observations and those of others in the literature have been interpreted in terms of an approximate indentation fracture mechanics analysis, and there is reasonable agreement between theory and experiment. The paper concludes with a discussion of the implications of this work to abrasive wear rate and to the residual strength of scratched bodies.