Fracture toughness of polycrystalline silicon carbide thin films

Abstract

Thin film polycrystalline silicon carbide (poly-SiC) doubly clamped microtensile specimens were fabricated using standard micromachining processes, and precracked using microindentation. The poly-SiC had been deposited on Si wafers by atmospheric pressure chemical vapor deposition, a process which leads to residual tensile stresses in the poly-SiC thin films; we measured the residual stress adjacent each specimen via a micromachined strain gauge. The stress intensity factor, KI, at the crack tip in each specimen depends on the magnitude of these residual stresses and the precrack length. Upon release, those precracks whose stress intensity exceeded a critical value, KIc, propagated to failure, whereas no crack growth was observed in those precracks with K<KIc. The fracture toughness so determined was 2.8⩽KIc⩽3.4MPam1∕2. Our technique also allowed us to assess any susceptibility to moisture-assisted stress corrosion cracking, which proved to be essentially absent in poly-SiC.