Formation of Y2SiO5 Layer in Y2SiO5 Oxidation Protection Coating for Carbon/Carbon Composites

As an oxidation protection system for carbon/carbon composites beyond 1973 K, authors have proposed a Y2SiO5 oxidation protection coating system, which is composed of a Y2SiO5 outer layer and a SiC inner layer. This work tried to form the Y2SiO5 layer on CVD-SiC layers by atmospheric plasma spraying (APS), and to adhere both layers with chemical bonding. YSix is composed of Y, Si, C and a few O, and is expected to be useful for adhesion between the Y2SiO5 layer and the CVD-SiC layer. As a surface treatment of CVD-SiC layer for APS, YSix/SiC layers composed of YSix and SiC were formed on the surface of CVD-SiC layers. With the YSix/SiC layers, the surface of CVD-SiC layer can be coarsened, and Y2SiO5 layers can be formed on CVD-SiC layers by APS. The Y2SiO5 layer as formed by APS was not composed of crystallized Y2SiO5 phase. Therefore, the Y2SiO5 layer is expected not to have the same oxygen permeability and thermal expansion coefficient as the crystallized Y2SiO5 phase. But, the Y2SiO5 layer crystallizes by some heat treatments in an oxidizing atmosphere. A heat treatment at 1873 K for 1 hour in Ar atmosphere is an effective method to adhere the Y2SiO5 layers to the YSix/SiC layers with chemical bonding.