ASSESSMENT OF DIFFUSION-BARRIERS FOR TI-SIC COMPOSITES

The application of chemical vapour deposition and physical vapour depo sition coatings, either singly or in combination, onto SiC fibres is d iscussed in terms of their ability to enhance the high temperature sta bility of Ti-SiC composites. The thermal stability and success of pote ntial barrier layers was assessed by studying the fibre-matrix interdi ffusion and measurement of the mechanical properties of individual fib res following coating and thermal exposure. Measurements of the level of strength retention have proved to be a reliable method of assessing the effectiveness of potential diffusion barriers. Failures may resul t from one of three sources. For high strength fibres failures are SiC -core reaction zone initiated, for intermediate strength fibres failur es are surface defect (SiC) initiated and for low strength fibres, fai lures are fibre-matrix reaction zone or coating initiated. To ensure h igh strength (i.e. core failures) it is essential that a carbon layer is retained at the SiC surface. The most successful barriers have been shown to be TiB2 and PtAl2 coatings preventing outward diffusion of c arbon and minimising the interaction with the titanium matrix. From th ese results a life prediction model has been developed based on the fi bre-coating interaction which will predict fibre strength as a functio n of time at a given temperature. (C) 1994 The Institute of Materials.