Oxidation control in plasma spraying NiCrCoAlY coating

Atmospheric plasma spraying is usually accompanied by oxidation reactions, which result in the formation of oxides in the coatings. The presence of oxides in metallic coatings is usually undesirable because they cause the coating properties to deteriorate. This study highlights how the high temperature oxidation resistance of plasma sprayed NiCrCoAlY coating is influenced by both the oxidation behavior of NiCrCoAlY particles and by the shrouding gas during the spray process. It is shown that two different oxidation mechanisms are involved in the in-flight oxidation. One is diffusion oxidation, and the other is convective oxidation. The convective oxidation of NiCrCoAlY particles is the dominating oxidation mechanism when the plasma jet is at a distance of 55 mm from the torch nozzle exit; while diffusion oxidation was found to be the dominant mechanism when the spray distance is greater than 55 mm. Oxidation mainly occurs during in-flight and after impact on the substrate. In-flight oxidation is the dominant mechanism for NiCrCoAlY particles in plasma spray. Adding inert-gas shrouding is an effective method for decreasing the oxide content of the NiCrCoAlY coating, which significantly increases the coating's oxidation resistance. (C) 2012 Elsevier B. V. All rights reserved.