Efect of fiber properties on mechanical properties of crystalline silicon carbide composites

Unidirectional SiC/SiC composites with three kinds of stoichiometric SiC fibers (Hi-Nicalon Type-S, Tyranno SA and SCS-9A ) and three kinds of fiber/matrix interphases (C, Multilayer C/SiC and 'porous' SiC), were prepared by isothermal chemical vapor infiltration. Tensile testing, double-notched specimen shear testing, single fiber push-out testing and transthickness tensile testing were carried out at ambient temperature to evaluate the mechanical properties of these materials. The microstructure and fracture surfaces of the test specimens evaluated were studied by scanning electron microscopy. Composites reinforced with SCS-9A fibers showed the highest ultimate tensile strength, more than 1 GPa, while the proportional limit stress of composites reinforced with Hi-Nicalon Type-S fibers was larger than that of the other composites. The composites reinforced with Tyranno SA fibers showed larger modulus of elasticity, although its fracture behavior was brittle due to the large interfacial shear strength and low fiber volume fraction. Among composites reinforced with the same fiber, those with multilayer C/SiC interphase showed brittle fracture behavior compared with the other composites due to large interfacial shear strength. The transthickness tensile strength of composites reinforced with Hi-Nicalon Type-S was larger than that of composites reinforced with Tyranno SA fibers, although the interlaminar shear strength of both materials determined by the compression of double-notched specimens was similar.