Subcritical Crack Propagation in 3Y-TZP Ceramics: Static and Cyclic Fatigue

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y₂O₃ (3Y-TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10^-12-10^-3 m/s in several environments, including air, water (in the temperature range of 3°-85°C), secondary vacuum (10^-5 mbar), and silicon oil. Analysis of the experimental results-three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs-shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static-fatigue/cyclic-fatigue sequences show that both stress corrosion by water and pure cyclic-fatigue effects are operative under alternative stresses.