THE CUBIC - TO - HEXAGONAL TRANSFORMATION TO TOUGHEN SiC

摘要

Silicon carbide is a desirable high temperature structural material, however, its poor fracture toughness at room temperature has limited its practical application. Recent processing developments have toughened the microstructure with interlocking, plate-like grains and an ～1 nm thick amorphous grain boundary. Intergranular fracture around these elongated grains leads to crack deflection and elastic bridging behind an advancing crack tip, thereby giving rise to the fracture resistance. (K_c > 9 MPa m~(1/2).) Furthermore, the increase in interlocking that develops with the high aspect ratio results in an increase in room temperature strength even as grains grow. The plate-like microstructure arises from the cubic-to-hexagonal transformation in SiC, which must be judiciously controlled in order to insure full densification, to obtain a high aspect ratio, and to optimize the microstructure of the sintering additives. Thus this work evaluates this transformation using XRD, SEM and high resolution TEM. Although a multitude of literature discusses a transformation invoked by the motion of stacking faults and partial dislocations, this study has characterized a growth-induced transformation, with the initial beta grain acting as a seed on which the alpha grows. Further growth of dual-phase grains develops asymmetric faceting on the plate-like surfaces, which, coupled with crystalline triple junction phases, complicate the intergranular crack path and increase the effect of interlocking.