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

Aluminas with four distinct microstructures have been fabricated to investigate the influence of grain size and grain morphology on strength variability. The four microstructures comprise two grain size scales and are characterized as either "equiaxed" with a narrow size distribution or "elongate" with a higher aspect ratio and a broader size distribution. indentation-strength tests indicate that only the coarse-grain, elongate microstructure exhibits a strong rising toughness-curve (T-curve or R-curve). Furthermore, in situ measurements demonstrate that the coarse-grain, elongate microstructure is the only one that displays significant stable crack extension from annealed indentation flaws free of contact-induced residual stress. Strength tests on polished specimens indicate that the highest mean strength is achieved in the fine-grain, equiaxed material with little or no T-curve. The lowest strength variability, however, is exhibited by the coarse-grain, elongate alumina and is rationalized in terms of the strong rising T-curve and its associated influence on crack stability. The study suggests that maximum reliability is achieved when the T-curve is sufficiently strong to stabilize the propagation of natural flaws en route to failure. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ali rights reserved.