Chemical Reactions of Different Types in Detonation Sprayed Powders as Tools of Coating Design

This work shows that with computer-controlled detonation spraying, the phase composition of coatings can be changed relative to that of the feedstock powders. New phases can appear in substantial quantities due to chemical reactions of reduction, oxidation, and nitridation as well as interfacial interactions between phases in composite powders. The key advantage of computer control is that it precisely regulates the quantity and stoichiometry of explosive gas mixtures. It has thereby been found that TiO₂ experiences partial reduction to titanium suboxides and that chemical reactions with nitrogen are also possible. It has also been found that when nitrogen is present, titanium aluminides, Ti₃Al and TiAl, are likely to form nitrides in the sprayed coatings. Interfacial reactions between the phases of a composite have been studied, and in the case of the Ti₃SiC₂-Cu system, it has been found that deintercalation of Si can be prevented by maintaining relatively cold spraying conditions. At higher temperatures, coatings of an unusual phase composition form in which carbon-deficient TiCx inclusions are distributed in the Cu matrix as modified by the dissolution of silicon. The formation of new phases affects coating microstructure development and results in new microstructural features.