Application of diffusion barriers in composite materials

Application of diffusion barriers in composite materials is discussed from the point of view of thermodynamic and kinetic stability of the reinforcement/matrix interface. Two types of interface protective coating, i.e. inert diffusion barriers and reactive diffusion barriers can be distinguished. The application of one coating or the other depends on the type of damage the material suffers as a result of the interface reactions. A double-coating concept for SiC fiber-reinforced titanium matrix composites (TMC) is discussed from the viewpoint of this classification of diffusion barriers. Application of the C/TiB2 coating system for SiC fiber-reinforced TMC is considered in detail, and some approaches to improve the performance of this coating were suggested and examined experimentally with TiB2 coatings produced by magnetron sputtering deposition from a TiB2 target. The effect of the composition and the microstructure on barrier properties of TiB2 layers was examined. A crystalline microstructure with relatively large grains and of stoichiometric composition was found to be optimal for TiB2 protective coatings in SiC fiber-reinforced TMCs. A strong impact of layer stoichiometry on the diffusion of metal atoms through a crystalline TiB2 layer was found. However, stoichiometry did not significantly affect the diffusion of carbon. Crystalline TiB2 of proper stoichiometry can provide excellent protection for carbon coated (SCS6) SiC fibers in titanium aluminide based metal matrix composites (MMC).