The effect of heat treatment on the strength and toughness of carbon fiber/silicon carbide composites with different pyrolytic carbon interphase thicknesses

The effect of heat treatment on the strength and toughness of carbon fiber/silicon carbide composites (C/SiC) with different pyrolytic carbon (PyC) interphase thicknesses was investigated. It was found that as the heat treatment temperature (HTT) increases to 1900 °C, the strength and toughness of a low strength specimen (LSS, thin PyC ≈40 nm) increase by as much as 43.2% and 274.0%, while those of a high strength specimen (HSS, thick PyC ≈140 nm) show decreases of 25.1% and 14.8%, respectively. The elastic moduli of both LSS and HSS monotonically decreased with increasing HTT while the failure strains always became larger regardless of the initial interfacial bonding strengths (IBS). The mechanisms involved in the heat treatment of the C/SiCs were identified as (I) partial graphitization of the PyC that weakens the IBS, and (II) production of defects such as matrix cracks/delamination, interfacial debonding and fiber fracture/pull-out that lead to thermal residual stress relaxation. Thus heat treatment improves the strength and toughness of LSS with a relatively high IBS, but has a negative impact on both properties of HSS with a moderate IBS because the stress transfer efficiency onto the fibers is hindered by the too low IBS and the excessive stress relief.