Shrouded plasma spray of Ni–20Cr coatings utilizing internal shroud film cooling

It is widely reported that oxidation during plasma thermal spraying leads to degradation in the coating corrosion and wear resistant properties. A number of shrouding systems have been developed to minimize oxidation to enable production of low oxide content coatings while spraying in air. The shroud design in this work was based on a detailed review of the mechanisms of oxidation during plasma spraying and the result of past shrouded plasma spray trials and simulations. It incorporated a novel film cooling concept within the shroud which was intended to cool the inner shroud wall and reduce energy loss from the plasma jet, while at the same time causing vaporized material to condense in the gas film and flow out of the shroud rather than building up on the shroud walls. Coatings were sprayed without the shroud, with the solid shroud alone (no shroud gas flow) and with various settings of internal and external shroud gas flows. The effect of these variables on the coating quality and oxide content was assessed by XRD, SEM and microhardness testing. The solid shroud was found to be effective in reducing in-flight oxidation but less effective in reducing layer to layer oxidation during spraying. High external shroud gas flows had the most significant effect on reducing coating oxidation. The internal shroud gas flow used to generate the film cooling effect was ineffective in significantly reducing internal dust build-up within the shroud and had no definitive effect on the coating properties. The shroud operation and mechanisms of oxidation are discussed to highlight areas of improvement for future designs.