Thermochemical aspects on the formation of Ti-bearing coatings in a fluidized bed CVD process

A thermodynamic approach of a fluidized bed chemical vapour deposition process for the formation of Ti-bearing coatings on steels is attempted. For the thermochemical analyses, a system consisting of Al2O3 or SiO2, Fe2Ti, Fe and C as well as NH4Cl or NH4I as activators of the process was considered. The partial pressures of the equilibrium gaseous species and the condensed phases of the above closed system have been calculated for amounts of the above constituents analogous to those used in the experimental practice and for different amounts of NH4Cl activator. The analyses indicate, that in all cases, the Ti-halide vapours, which are necessary for the coating formation, exist in the examined temperature range 773-1473 K. Different modes for the coating deposition are discussed. With NH4Cl as activator higher deposition rates of the coatings can be expected, in comparison to the NH4I. SiO2 is not a good candidate as filler oxide as it reacts with the fluidizing medium. Although the process is feasible without hydrogen, addition of 10% of the latter is beneficial for the process as it increases the partial pressures of the Ti-chlorides.