Oxidation in wire HVOF-sprayed steel

It is widely held that most oxidation in thermally sprayed coatings occurs on the surface of the droplet after it has flattened. Evidence in this pape r suggests that, for the conditions studied here, oxidation of the top surf ace of flattened droplets is not the dominant oxidation mechanism. In this study, a mild steel wire (AISI 1025) was sprayed using a high-velocity oxy- fuel (HVOF) torch onto copper and aluminum substrates, Ion milling and Auge r spectroscopy were used to examine the distribution of oxides within indiv idual splats, Conventional metallographic analysis was also used to study o xide distributions within coatings that were sprayed under the same conditi ons, An analytical model for oxidation of the exposed surface of a splat is presented. Based on literature data, the model assumes that diffusion of i ron through a solid FeO layer is the rate limiting factor in forming the ox ide on the top surface of a splat. An FeO layer only a few nanometers thick is predicted to form on the splat surface as it cools. However, experiment al evidence shows that the oxide layers are typically 100x thicker than the predicted value. These thick oxide layers are not always observed on the t op surface of a splat, Indeed, in some instances the oxide layer is on the bottom, and the metal is on the top. The observed oxide distributions are m ore consistently explained if most of the oxide forms before the droplets i mpact the substrate.