Fracture toughness of polycrystalline silicon carbide thin films

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, K I , 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, K I c , propagated to failure, whereas no crack growth was observed in those precracks with K < K I c . The fracture toughness so determined was 2.8 ⩽ K I c ⩽ 3.4 MPa m 1 ∕ 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.