Crack stability and strength variability in alumina ceramics with rising toughness-curve behavior

Aluminas with four distinct microstructures have been fabricated to investi gate the influence of grain size and grain morphology on strength variabili ty. The four microstructures comprise two grain size scales and are charact erized as either "equiaxed" with a narrow size distribution or "elongate" w ith a higher aspect ratio and a broader size distribution. indentation-stre ngth tests indicate that only the coarse-grain, elongate microstructure exh ibits a strong rising toughness-curve (T-curve or R-curve). Furthermore, in situ measurements demonstrate that the coarse-grain, elongate microstructu re is the only one that displays significant stable crack extension from an nealed indentation flaws free of contact-induced residual stress. Strength tests on polished specimens indicate that the highest mean strength is achi eved in the fine-grain, equiaxed material with little or no T-curve. The lo west strength variability, however, is exhibited by the coarse-grain, elong ate alumina and is rationalized in terms of the strong rising T-curve and i ts associated influence on crack stability. The study suggests that maximum reliability is achieved when the T-curve is sufficiently strong to stabili ze the propagation of natural flaws en route to failure. (C) 2000 Acta Meta llurgica Inc. Published by Elsevier Science Ltd. Ali rights reserved.