A kinetic model for iron aluminide coating by low pressure chemical vapor deposition: Part II. Model formulation

摘要

Formation of aluminide diffusion coatings on iron, nickel and cobalt by pack cementation and chemical vapor deposition (CVD) involves a series of similar steps, the slowest among them are the transport of aluminum bearing species from the vapor phase to the substrate by gas-phase diffusion and the solid-phase diffusion of aluminum into the substrate to form the aluminide phases. The former increases the surface concentration of aluminum in the coating while the latter decreases it. Modeling of the process is based on the observation that the surface composition of the coating tends to reach a steady state value in a short time after the commencement of the process, at which stage the rates of the above two processes are equal. However, in the case of iron aluminide coatings produced by low pressure chemical vapor deposition (1.33±0.13 kPa), the rate of transport of aluminum to the substrate is much faster than the solid phase diffusion of aluminum into the iron substrate since the diffusion coefficients of the vapor species are inversely proportional to pressure and the diffusion layer thickness is reduced considerably at low pressures. Under this condition, the vapor transport is no longer a rate-determining step and the composition of the aluminide coating is decided by the kinetics of the solid phase diffusion. This model could explain the kinetics of the process, surface composition and concentration profile of the coating.