Effects of density and heat treatment of C/C preforms on microstructure and mechanical properties of C/C‐SiC composites

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C-SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C-SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C-SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C-SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon-carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C-SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.