CRACK RESISTANCE AND FATIGUE OF TRANSFORMING CERAMICS .1. MATERIALS IN THE ZRO2-Y2O3-AL2O3 SYSTEM

Crack resistance characteristics and fatigue properties have been studied in four types of Y2O3-TZP ceramics including one containing Al2O3. The largely linear-elastic behavior connected with the very small transformation zone (< 5-mu-m) explains the absence of any resistance-curve behavior and the flaw-controlled strength. The crack resistance shows high sensitivity to environment-induced subcritical crack growth. This influence is also operative in both types of fatigue experiments, i.e., under static and cyclic stresses, leading to reduced fatigue thresholds compared with K(IC). While for static conditions a benefit is observed from enhanced t-m ZrO2 transformation, cyclic stresses provoke an additional fatigue effect. However, if the cyclic stresses are restricted to subthreshold values, cyclic stress-induced effects in the process zone provide an improvement of the materials being visible as a strengthening effect.