Effects of Grain-Boundary Structure on the Strength, Toughness, and Cyclic-Fatigue Properties of a Monolithic Silicon Carbide

An in situ -toughened silicon carbide (ABC-SiC) has been examined in the as-processed condition, where the grain-boundary films are predominantly amorphous, and following thermal exposure at a temperature of 1300°C, where the films become fully crystalline. Previous work has shown that, at elevated temperatures (up to 1300°C), after the grain-boundary films crystallize in situ , only a marginal reduction in strength, fracture toughness, and cyclic-fatigue crack-growth properties is observed, in comparison with those of the as-processed microstructure at 25°C. In the present study, the effect of such crystallization on the subsequent strength, toughness, and fatigue properties at 25°C is examined. Little or no degradation is observed in the room-temperature properties with the crystallized grain-boundary films/phase; in fact, although the strength shows a small reduction (∼3%), the fracture toughness and fatigue-crack-growth threshold both increase by ∼20%, compared with that of the as-processed structure with amorphous grain-boundary films.