The use of Finite Element Analysis in the design of integrated oxidation protection systems for ceramic matrix composites

Ceramic matrix composites reinforced with carbon fibres are candidate materials for use in high temperature applications such as gas turbine components and structural components of re-entry vehicles. The performance of these materials is limited, however, by the oxidation of carbon fibres at temperatures above 400 degrees C. Coatings are required which are capable of protecting these composites from oxidation for 5000 hours at 1200 degrees C or 100 hours at 1600 degrees C. The most successful coatings are multi-layer, where each layer is designed to perform a specific function. An approach has been developed in which the results of Finite Element Analysis in conjunction with thermodynamic modelling to identify systems which have both the necessary mechanical and thermodynamic properties and to select the most promising for further experimental work and testing. This paper describes the results of Finite Element Analysis on simple example systems to illustrate the methodology which has been developed for the modelling of the mechanical properties of potential systems. The systems consist of an outer erosion protection layer, a functional layer with self-healing properties, and an inner bond layer. Large stresses build up in these coated composites because the substrate has such a low coefficient of thermal expansion compared with the coating layers and this leads to cracking. The modelling enables protection systems with low residual stresses and, therefore, reduced cracking to be identified. It is also possible to model changes in composition and thickness of individual layers to optimise the protection system. It is shown that the stresses that are generated in the various coating layers on thermally cycling to high temperatures can be predicted using a finite element analysis methodology developed at UMIST.