TRAPPED CRACKS AT INDENTATIONS .1. EXPERIMENTS ON YTTRIA TETRAGONAL ZIRCONIA POLYCRYSTALS

Indentation radial cracking has been examined in a series of yttria-tetragonal zirconia polycrystals (Y-TZP) with a range of grain sizes. For all materials, two distinct types of cracks were found to exist. Above a threshold indentation load, short cracks of variable length were observed, ''trapped'' in a compressive, contact-induced transformation zone. Above a second threshold, longer, ''well-developed'' cracks of more consistent length formed, extending beyond the zone boundary. The stochastic nature of the formation of both types of cracks, and the existence of the transformation zone in general, generate many artefacts in toughness estimates made using unmodified indentation fracture mechanics analyses.