Thermal properties of Cf/HfC and Cf/HfC-SiC composites prepared by precursor infiltration and pyrolysis

Ultra-high temperature ceramic infiltrated carbon-fibre composites were prepared by precursor infiltration and pyrolysis (PIP) using a laboratory synthesized precursor. Microstructures and thermal properties including thermal expansion, thermal diffusivity, specific heat capacity and oxidative stability are correlated. XRD reveals the presence of C-f-HfC and C-f-HfC-SiC phases without formation of oxides. The CTE observed at 1200 degrees C is slightly higher for C-f-HfC (3.36 x 10(-6) K-1) compared to C-f-HfC-SiC (2.95 x 10(-6) K-1) composites. Lower thermal diffusivity of the C-f-HfC-SiC compared to C-f-HfC composites is attributed to a thermal barrier effect and cracks in the composites which formed due to the CTE mismatch between carbon fibre and the matrix as well as CO generated during graphitization. The thermal conductivity of C-f-HfC (4.18 +/- 0.14 Wm(-1) K-1) is higher than that of C-f-HfC-SiC composite (3.33 +/- 0.42 Wm(-1) K-1). Composites microstructures were coarse with some protruding particles (5 pm) with a homogeneous dense (similar to 70%) matrix (HfC and HfC-SiC) for both composites.