Detonation spraying behavior of TiCx–Ti powders and the role of reactive processes in the coating formation

In this work, the formation of coatings by chemical reaction-accompanied detonation spraying of TiCx–Ti composite powders differing in the microstructure and carbon content was studied. The powders were synthesized by a reaction between titanium and carbon in the presence of excess of titanium via self-propagating high-temperature synthesis followed by mechanical milling with an additional amount of titanium (which produced a composite product containing 4.2 wt% of carbon) or by thermal annealing of mechanically milled mixtures containing 2.1 wt% of carbon. It was found that in the detonation coatings, the relative amount of metallic titanium dramatically decreased in comparison with the powders due to its reactions with carbon and nitrogen contained in the spraying atmosphere and the formation of titanium carbonitrides TiNvCw. It was possible to distinguish regions in the cross-section of the coatings with a composite microstructure inherited from the powders and with a microstructure in situ formed by the products of chemical reactions that occurred during spraying. The volumes of the material with the in situ formed microstructure contained the major fraction of the porosity of the coatings. The average hardness of the TiCx–TiNvCw–Ti coatings ranged between 400 and 480 HV. Despite higher porosity of the coatings formed at higher degrees of chemical transformation, their average hardness increased. No significant differences in the hardness of the coatings produced from two feedstock powders were observed. The similarity between the coating formation processes from two TiCx–Ti powders of different microstructures and the similarity of those to the coating formation process from a metallic titanium powder show that the reactivity of titanium during detonation spraying is a crucial factor determining the coating microstructure and properties.