Thermodynamic modelling and experimental studies 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 currently limited however by the oxidation of carbon fibres at temperatures above 400 degrees C. Coatings are therefore required which are capable of protecting these composites from oxidation at 1600 degrees C. Such coatings generally consist of several layers, each designed to perform a specific function since no satisfactory single layer coating exists [1]. The approach being taken is to use the results of theoretical modelling to predict the behaviour of potential systems and to select the most promising for further experimental work and testing. This paper describes the results of thermodynamic modelling and oxidation testing on one oxidation protection system. The system consists of an outer erosion protection layer, a functional layer with self-healing properties, and an inner bond layer. It is shown that the chemical stability and the result of exposure of the coating to oxygen can be predicted successfully using a thermodynamic modelling methodology developed at UMIST. Oxidation testing has been performed using a test rig which allows continuous mass change measurements to be made on samples at temperatures up to 1650 degrees C. This combination of thermodynamic modelling and oxidation testing is closely linked to other work at UMIST involving finite element analysis and mechanical testing. This combined approach enables potential coating systems to be fully characterised. It is shown that combining theoretical modelling and experimental testing is an excellent method for the design and assessment of integrated oxidation protection systems.