The deposition of aluminide and silicide coatings on gamma-TiAl using the halide-activated pack cementation method

The halide-activated pack cementation method (HAPC) was utilized to deposit aluminide and silicide coatings on nominally stoichiometric gamma-TiAl. The deposition temperature was 1000 degrees C and deposition times ranged from 2 to 12 hours. The growth rates of the coatings were diffusion controlled, with the rate of aluminide growth being about a factor of 2 greater than that of silicide growth. The aluminide coating was inward growing and consisted of a thick, uniform outer layer of TiAl3 and a thin inner layer of TiAl2, with the rate-controlling step being the diffusion of aluminum from the pack into the substrate. Annealing experiments at 1100 degrees C showed that the interdiffusion between the aluminide coating and the gamma-TiAl substrate was rapid. In contrast to the aluminide coating, the silicide coating was nonuniform and porous, consisting primarily of TiSi2, TiSi, and Ti5Si4, with the rate-controlling step for the coating growth believed to be the diffusion of aluminum into the gamma-TiAl ahead of the silicide/gamma-TiAl interface. The microstructural evolution of the aluminide and silicide coating structures is discussed qualitatively.