Polyfunctional Interface Coatings for High Temperature Resistant Ceramic Composite of SiC/SiC Type

Preliminary results are presented of the fabrication of a protective MoSi2 coating and a so-called "rheological" SiC coating on both SiC/C and SiC-coreless fibres. The study was carried out in order to solve one of the major problems in creating high oxidation resistant ceramic matrix composites of the SiC/SiC type for long-term application at 1800-2100 K. Chemical vapour deposition (CVD), reactive vapour deposition (RVD) and preceramic polymer deposition (PPD) were explored. Nicalon and SiC/C type fibres were coated. CVD protective coatings deposited under a mild regime (below 1100 K) in the Mo(CO)6 and SiH4 system had a multiphase composition: MoSi2, Mo5Si3, Si and Mo were identified by XRD. A consecutive deposition of Mo- and Si-layers followed by a heat treatment demonstrated the impossibility of firstly coating the Nicalon with a Mo-layer. Molybdenum disappeared quickly at heat treatment working temperatures due to interaction with initial oxygen of coreless SiC-fibres. Experiments with Hi-Nialon (not more than 0.8 wt% of O2) showed that the best results were achieved by RVD with SiC/C-fibres, when a layer of Mo was silicided with SiCl4. At the same time, a new complication was revealed: the formation of pores in the layer induced by interdiffusion. The results demonstrate two interconnected problems: the protection of fibres from the environment and the perfection of their internal composition and structure. The rheological SiC- coating on the Nicalon fibre was successfully prepared by PPD. 15 refs.