Thermodynamical analysis of Al and Si halide gaseous precursors in CVD. Review and approximation for deposition at moderate temperature in FBR-CVD process

In Chemical Vapour Deposition area, thermodynamic of precursors present in the reactor during process is a useful guide to design the experimental conditions of coating and can help to understand deposition mechanism. This paper, firstly, summarises studies from literature focussed on silicon and aluminium deposition obtain by conventional CVD, pack cementation and CVD-FBR process. Then, thermodynamic calculation was performed using Thermocalc-Software, to study the generation of Al- and Si-precursors at moderate temperature during CVD-FBR process in system Ar/H2/HCl:Al(s)/Si(s). Effect of temperature, input gas and powder mixture on equilibrium precursor partial pressures was discussed as well as effect input condition on chlorination of HCM12 substrate. Coating of such ferritic–martensitic steels requires low temperature process to avoid microstructure change (T < 700 °C) that makes CVD-FBR technique particularly attractive. Deposition mechanisms proposed in the literature were then shortly discussed in regards to the additional calculations performed.