Effects of temperature, rate, and cyclic loading on the strength and toughness of monolithic ceramics

Abstract

The concept of grain bridging and pullout is applied to monolithic ceramics to understand the effects of temperature, displacement rate, and load cycling on crack wake shielding. At low temperature, the pullout of completely debonded grains accounts for all the toughening. The importance of this process diminishes with temperature. This is because of the more uniform stress distribution along the crack plane and the softening of grain boundary glassy phases, both of which tend to reduce the incidence of complete grain boundary decohesion. At sufficiently high temperature, however, the softening of grain boundary phases may allow the sliding zone to extend to the grain's end, increasing the incidence of intergranular fracture. This “high temperature” pullout triggers a sudden increase in toughness. Our pullout model successfully explains the high temperature peak and the dependence of peak position on displacement rate in fracture toughness and strength observations for some monolithic and whisker-reinforced ceramics. Degradation of interfacial friction, as by cyclic loading, is seen to decrease the frictional work for low temperature pullout but increases the frictional work for high temperature pullout. Thus, this model also provides a rationale for the opposite effect of stress cycling on crack resistance at low and high temperatures reported recently for ceramics.