Growth kinetics of MoSi2 coating formed by a pack siliconizing process

Abstract

Growth kinetics of MoSi2 coating formed by pack siliconizing process of Mo substrate was investigated on the basis of the dependence of its growth rate on Si content in the pack powders. Pack siliconizing was carried out using (1-70) wt % Si-5 wt % NaF-bal SiC packs at 1100degreesC in a hydrogen atmosphere. The chemical vapor deposition (CVD) of Si on Mo substrate was also performed at 1100degreesC using SiCl4-H-2 gas mixture to determine the maximum growth rate of MoSi2 coating obtainable in the bulk Si/Mo diffusion couple. The growth kinetics of the MoSi2 coating formed by the pack siliconizing process obeyed a parabolic rate law irrespective of compositions of pack powders. The growth rates of MoSi2 coating increased with increasing Si content in the pack powders up to 40 wt % Si but attained a constant value in the pack powders with over 40 wt % Si, which was equal to that obtained by the CVD process. The rate-limiting step for growth of MoSi2 coating formed by the pack siliconizing process was subject to theoretical considerations. Three models such as a gas-phase diffusion controlled model, a solid-state diffusion controlled model, and a dynamic equilibrium model of gas-phase diffusion and solid-state diffusion were evaluated. The theoretically predicted results based on the dynamic equilibrium model and the experimentally obtained results were found to be in good agreement. (C) 2004 The Electrochemical Society.