ABSTRACT

Increased strength during high temperature loading can result from crack healing and crack-tip blunting. Crack healing corresponds in effect to crack shrinkage, which is caused by thermally activated adhesion of the crack interface through diffusion or oxidation. Crack-tip blunting increases crack propagation resistance as a result of reduced sharpness, although crack dimensions do not change significantly. In cases where diffusion is responsible for the strength increase during high temperature loading, the threshold stress intensity is usually small. For instance, the threshold stress intensity for crack-tip blunting resulting in increased strength of sintered silicon nitride was reported to be 0.1 MPam112 at 1200°C in a nitrogen atmosphere. This indicates that strength may be increased at a very low stress intensity compared with a fracture toughness of 4.7 MPam 112 (14). In a Lucalox alumina having a fracture toughness of 2.0 MPam112, it was reported that cracks healed below a stress intensity of 0.2 MPam112 at 1400°C, and the negative crack velocity was shown to be a function of the stress intensity factor (15). When oxidation increases strength by bonding the crack interface, a higher threshold stress intensity can be expected. In fact, sintered silicon nitride shows a definite increase in strength in the region of stress intensity below 1.5 MPamV2 at 11 00°C in air ( 16).