Oxidation protection coatings for C/SiC based on yttrium silicate

The factor which currently precludes the use of carbon fibre reinforced sil icon carbide (C/SiC) in high temperature structural applications such as ga s turbine engines is the oxidation of carbon fibres at temperatures greater than 400 degrees C. It is, therefore, necessary to develop coatings capabl e of protecting C/SiC components from oxidation for extended periods at 160 0 degrees C. Conventional coatings consist of multilayers of different mate rials designed to seal cracks by forming glassy, phases on exposure to oxyg en. The objective of this work was to develop a coating which was inherentl y crack resistant and would, therefore, Mot require Expensive sealing layer s. Yttrium silicate has been shown to possess the required properties for u se in oxidation protection coatings. These requirements can be summarised a s being low Young's modulus, low thermal expansion coefficient, good erosio n resistance, and low oxygen permeability. The development of protective co atings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic com puter calculations and finite element analysis,sis have been used to design the coating. C/SiC samples have been coated using a combination of chemica l vapour deposition and slip casting. The behaviour against oxidation of th e coating has been evaluated. (C) 1998 Elsevier Science Limited. All rights reserved.