INVESTIGATION OF DIFFUSION BARRIER COATINGS ON SIC MONOFILAMENTS FOR USE IN TITANIUM-BASED COMPOSITES

The development of titanium-based composites has been hindered largely because of interfacial diffusion. When SiC is in contact with titanium at elevated temperatures, metal silicide (Ti5Si3) and carbide (TiC) are formed. The aim of our work is to reduce the fibre-matrix interaction at high temperatures by interposing compounds between the metal and SiC reinforcement. Procedures are described for the formation of thin layers on silicon carbide fibres (Sigma) by sputter deposition. Sputtering offers good control over density and stress state, thickness and composition. Past work has shown that sputtered Y2O3 deposited on SiC, SiC-Si or metal can reduce the degree of ceramic-metal reaction. Use of the Gibbs free energy of formation of TiO2 provides a simple basis for the assessment of this material for use in a titanium matrix. Since TiSi2 and silicon are also present in the Ti-Si diagram, they could act as diffusion barriers. Coatings of TiSi2, Y2O3 and ZrO2 have been produced by sputter deposition. The stoichiometry of the coatings has been revealed by Rutherford backscattering spectrometry. Coated fibres have been used to prepare titanium-based composites by hot pressing. Annealing between 900 and 1100-degrees-C has been performed. Scanning microscopy, Auger electron spectroscopy and transmission electron microscopy have been used to investigate the interfacial products.