Oxidation protection coatings for C/SiC based on yttrium silicate

Abstract

The factor which currently precludes the use of carbon fibre reinforced silicon carbide (C/SiC) in high temperature structural applications such as gas turbine engines is the oxidation of carbon fibres at temperatures greater than 400°C. It is, therefore, necessary to develop coatings capable of protecting C/SiC components from oxidation for extended periods at 1600°C. Conventional coatings consist of multilayers of different materials designed to seal cracks by forming glassy phases on exposure to oxygen. The objective of this work was to develop a coating which was inherently crack resistant and would, therefore, not require expensive sealing layers. Yttrium silicate has been shown to possess the required properties for use in oxidation protection coatings. These requirements can be summarised as being low Young's modulus, low thermal expansion coefficient, good erosion resistance, and low oxygen permeability. The development of protective coatings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic computer calculations and finite element analysis have been used to design the coating. C/SiC samples have been coated using a combination of chemical vapour deposition and slip casting. The behaviour against oxidation of the coating has been evaluated.