ROLE OF GRAIN-SIZE IN STRENGTH VARIABILITY OF ALUMINA

Relationships between grain size and strength variability in alumina were investigated. Results are presented for an ultra-high-purity alumina with a narrow grain size distribution and an equiaxed grain morphology at three grain sizes ranging from 5 to 27 mum. Vickers indentations are used to introduce controlled flaws into the specimens. It is found that increasing the grain size leads to enhanced flaw tolerance owing to a rising R-curve. However, contrary to recent theoretical predictions, no reduction in strength variability is found with increasing grain size. It is proposed that increasing variability in local fracture toughness with grain size offsets any improvement in reliability. This suggests that an upper bound to the reliability of grain-bridging materials may exist.