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

摘要

Aluminas with four distinct microstructures havebeen fabricated to investigate the influence of grain size and grainmorphology on strength variability. The four microstructurescomprise two grain size scales and are characterized as either"equiaxed" with a narrow size distribution or "elongate" with ahigher aspect ratio and a broader size distribution. Indentation-strength tests indicate that only the coarse-grain, elongatemicrostructure exhibits a strong rising toughness-curve (T-curveor R-curve). Furthermore, in situ measurements demonstrate thatthe coarse-grain, elongate microstructure is the only one that dis-plays significant stable crack extension from annealed indentationflaws free of contact-induced residual stress. Strength tests onpolished specimens indicate that the highest mean strength isachieved in the fine-grain, equiaxed material with little or no T-curve. The lowest strength variability, however, is exhibited by thecoarse-grain, elongate alumina and is rationalized in terms of thestrong rising T-curve and its associated influence on crack stability.The study suggests that maximum reliability is achieved when theT-curvc is sufficiently strong to stabilize the propagation ofnatural flaws en route to failure.