Mechanical properties of SiC/SiC composites

2.5 dimensional SiC/SiC composites with pyrocarbon interlayer were fabricated by low pressure chemical vapor deposition (LPCVD). Effects of residual pores and interlayer thickness on mechanical properties of fabricated SiC/SiC composites were investigated. The flexural strength of the composites is mainly affected by the micro-pores between filaments instead of the macro-pores between yarns. Strength increases with the decrease of the amount and size of micro-pores. When the porosity is below 27%, the further decrease of the porosity does not change the amount and size of micro-pores and hardly contributes to the strength. The existence of a pyrocarbon interlayer of 90nm thickness increased composite strength from 174 MPa to 305 MPa, and changed the fracture behavior from catastrophic fracture to non-catastrophic fracture. The further increase of interlayer thickness results in fiber damage which reduces the mechanical properties of the composites. The strength of the composites with 180 nm and 310 nm thick interlayers is 274 MPa and 265 MPa, respectively. Their amount of fiber pullout is small and the fracture behaviors are similar to catastrophic fracture.