Oxidation behaviour of C/C–SiC coated with SiC–B4C–SiC–cordierite oxidation protection system

In order to improve the oxidation as well as the hot gas corrosion behaviour of C/C–SiC composite materials a new multilayer coating was developed. It consists of an external cordierite (2MgO × 2Al2O3 × 5SiO2) layer applied via vacuum plasma spraying (VPS) and an internal CVD-BoraSiC (SiC–B4C–SiC) layer. In this paper the oxidation protection properties of the multilayer system are compared to those of the basic BoraSiC coating. It was found that the additional cordierite coating is able to decrease the oxidation rates and increase the residual strength of C/C–SiC samples. This improvement is based on a mechanism that stops part of the cracks running through BoraSiC, mainly at the SiC–cordierite interface. This crack density reduction can be mainly attributed to the extraordinary low coefficient of thermal expansion and the uneven distribution of temperature during plasma spraying.