Strategy to Generate Oxide-Free Molten Metal Droplets for Fully Dense Corrosion-Resistant Coatings by Atmospheric Plasma Spraying

Abstract

Thermal spray techniologies are very efficient to deposit metal coatings, which have been applied to different industrial fields for protection of metals from wear and corrosion. However, severe oxidation during inflight of spray particles introduces large amount of oxide inclusions in the coating which limits lamellar bonding formation and thus full utilization of coating material performance potential. In this project, the spray powders containing the deoxidizers such as carbon and boron are designed to develop in-situ deoxidizing effect to generate oxide-free molten metal droplets by air plasma spraying (APS) in ambient atmosphere for depositing dense coatings with sufficiently bonded splats. The thermodynamic and kinetic conditions for continuous deoxidization during whole in-flight molten droplets are presented. The experiments were conducted for NiCr, NiCrCu, CuNi coatings with boron as deoxidizer and for NiAl and FeAl coatings with dispersed diamond as deoxidizer. Results show that through powder design and spray condition optimization different spray particles can be heated to temperatures from 2100°C to 2500°C. It was found that the oxide contents in all coatings decreases with increasing spray distance, which indicates that deoxidizing effect of deoxidizer is maintained during whole spray particle in-flight. The examination shows that all APS coatings present dense microstructure. The electrochemical test reveals that the corrosion only occurs to coating surface and no trace of corrosive solution penetration into coating. As a result, the corrosion-resistant metal coatings can be realized by APS in ambient atmosphere through developing in-situ deoxidizing effect and subsequently oxide-free molten droplets with using deoxidizer-containing spray powders.