Slow crack growth behavior in transformation-toughened Al2O3-ZrO2(Y2O3) ceramics

Significant increases in the critical fracture toughness (K/sub IC/) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO2 particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO2 particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO2 phase transformation by tailoring the Y2O3 stabilizer content not only increases the critical fracture toughness (K/sub IC/) but also the K/sub Ia/ to initiate slow crack growth. For example, crack velocities (nμ) greater than or equal to 10-9 m/s are obtained only at K/sub Ia/ greater than or equal to 5 MPa.m/sup 1/2/ in transformation-toughened (K/sub IC/ = 8.5 MPa.m/sup 1/2/) composites vs K/sub Ia/ greater than or equal to 2.7 MPa.m/sup 1/2/ for comparable velocities in composites where the transformation does not occur (K/sub IC/ = 4.5 MPa.m/sub 1/2/). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (greater than or equal to 1 μm) monoclinic ZrO2 particles which reside along the intergranual crack path