Mechanisms of oxidation and its role in microstructural evolution of metallic thermal spray coatings—Case study for Ni–Al

Thermal spray coatings are formed by successive impingement and interbonding among the splats (solidified individual molten particles). Depending on the processing conditions employed during the spray process, deposits are produced with an assortment of microstructures and properties. This study brings out how the basic microstructural differences are influenced by mechanisms involved during the spraying processes. The Ni–5 wt.% Al metallic system is chosen for a systematic study of cross comparison across different spraying techniques because of the potential variety in coating microstructure and phases possible through application of the different spray processes as indicated in the literature. Spray techniques such as Air Plasma spraying (APS), Wire Arc Spraying and, High velocity oxy-fuel spraying (HVOF), each differ with respect to their feedstock injection and melting methods, spraying parameters and deposition efficiency as well as oxidation involved during spraying. The goal of this study is to address the mechanisms of oxidation involved and present schematic models to explain their role in the microstructural evolution of Ni–5 wt.% Al coatings in case of different processes. In-flight oxidation and post-impact oxidation occurring on top surface of splats are discussed in detail and the effects of these mechanisms on intersplat contact and coating buildup are addressed.