Oxidation protection dynamics of NiAl droplet by inflight in-situ carbon deoxidation during atmospheric plasma spraying for high performance NiAl coatings

The oxidation of the metallic particles in plasma spraying leads to poor inter-lamellar bonding and thereby lower corrosion and high temperature oxidation resistance. In this study, a novel approach is proposed to exclude the oxide of in-flight Ni-Al droplets by introducing diamond as a deoxidizer in Ni/Al powder during plasma spraying. NiAl coatings were deposited with Ni/Al/diamond composite powder prepared by mechanical alloying by atmospheric plasma spraying at different spray distances to study the effect of the diamond addition on the oxidation behavior of in-flight droplets. The results show that the oxygen content in APS NiAl coating (3.23 wt%) was significantly reduced to 0.61 wt% by adding 2 wt% diamond to the Ni/Al powder. The measurement yields particle temperatures of higher than 2200 °C. Such high temperature contributes to the in-situ deoxidizing effect of C-containing droplets, resulting in oxide-free NiAl droplets for the deposition of the coating with low oxide content. This enhances both inter-lamellar cohesion and coating-substrate adhesion. Moreover, it was found that the size effect of a molten droplet on its oxidation kinetics significantly influences deoxidizing efficiency. Smaller particles require higher diamond content to ensure the in-flight deoxidizing to form oxide-free metal droplets. The result was further confirmed theoretically by numerical simulation. The present work provides a new approach to control the in-flight oxidation of metallic elements during plasma spraying, which has significant implications for the preparation of high-performance protective metallic coatings.