THE KINETICS OF MULTILAYERED TITANIUM-SILICIDE COATINGS GROWN BY THE PACK CEMENTATION METHOD

The halide-activated, pack cementation coating method is a viable technique for forming silicide diffusion coatings that provide oxidation resistance for titanium alloys at high temperature. In this study, growth rates were determined for the five-layered silicide coatings on commercially pure titanium grown at 950 degrees C, 1050 degrees C, and 1150 degrees C using three halide activators. Solid-state titanium-silicon diffusion couple experiments were also made to determine the diffusional growth rates for the five silicide phases. Based on a model for growth of multiple layers, solid-state diffusion controls the growth rates in all of the silicide layers for coatings formed by packs with the less stable AlF3 and CuF2 activators. However, the growth of TiSi2 was influenced by gas-phase transport for coatings formed using the more stable MgF2 activator. The growth rates of the layers (TiSi2, TiSi, Ti5Si4, Ti5Si3, and Ti3Si) are parabolic; by assuming solid-state diffusion control, the diffusion coefficients in these phases were calculated.