A study on phase stability observed in as sprayed Alumina-13 wt.% Titania coatings grown by detonation gun and plasma spraying on low alloy steel substrates

AT-13 wt.% type powders, depending on the process used for spraying has shown variable type of resultant phases in the as sprayed conditions. In a detonation gun process it appears that freezing of the molten plume with the same composition as the liquid was possible, in contrast to this in a typical plasma spraying process preferential evaporation and phase separation of the solute from the molten plume was prevalent. These variations to the solidification paths has resulted in a totally metastable γ phase in the former process and in the latter process heterogeneous nucleation of α phase in addition to the normal nucleation of metastable γ phase was observed. In all cases the titanium present in the solute appears to be merely trapped in the metastable γ phase and a newer type of metastable “X” phase which has high solubility for titanium, as reported in the literature, did not form in any of the processes studied. The imposed rapid solidification condition had caused all the phases stabilized to be of nanocrystalline sizes which could be verified by Hall–Williamson analysis. Owing to the solute trapping of titanium the lattice of metastable γ phase was severely micro strained and titanium cation was nearly insoluble in α phase.