Creep of monolithic and fibre-reinforced silicon carbide

The mechanisms governing tensile creep and creep fracture are identified for siliconized silicon carbide (Si-SiC) and sintered silicon carbide (SSC), as well as for various SiC-matrix composites reinforced with interwoven bundles of different fibres. With Si-SiC and SSC, creep is controlled by the rate at which intergranular damage development allows grain boundary sliding. Fracture then results from the formation and link up of grain boundary cavities and cracks, unless premature failure occurs by rapid crack propagation from a pre-existing flaw. In contrast, with the woven SiC-matrix composites, immediate failure was never encountered on loading at stresses less than the UTS, despite the presence of macroscopic pores. Instead, the longitudinal fibres control the rates of creep strain accumulation and crack growth. However, the fracture properties are determined by oxidation-assisted fibre failure, because matrix cracking permits oxygen ingress during tests in air. By clarifying the processes limiting the creep capabilities of current product ranges, possible development avenues are suggested for fibre-reinforced composites displaying improved long-term service performance in oxidizing atmospheres.