Generating Oxide-free Molten Metal Droplets by Air Plasma Spraying Enabled by Deoxidizer Addition to the Feedstock Powders

Thermal spray metallic coatings usually present a lamellar structure with limited lamellar interface bonding due to inevitable oxidation involved. Such microstructure not only degrades the mechanical performances of the coatings significantly but also cannot provide effective corrosion protection to the substrate. In the present paper, the recent progresses to generate oxide-free molten metallic droplets by air plasma spraying will be summarized toward to deposition of highly dense metal coatings. It is revealed that by designing the metallic spray powders containing deoxidizers such as boron or carbon based on thermodynamics theory of oxidation, the oxide-free in-flight molten metal droplets can be created in open atmosphere during air plasma spraying. The thermodynamics and kinetics for the in-flight deoxidization are presented for the deoxidizers of boron and carbon. The tests with Ni-based and Cu-based alloy coatings using boron as deoxidizer showed that the oxygen content in the coatings can be reduced to less than 0.6 wt.%. It was demonstrated with NiCrAlY, NiAl and FeAl alloys that contain element Al the coatings with low oxide contents can be deposited by APS using carbon as deoxidizer. The post-impact oxidation is mainly responsible for the introduction of oxide inclusions in these coatings. Besides deoxidizer addition, ultra-high temperature is the other necessary condition for the generation of oxide-free molten droplets. It was revealed that a minimum deoxidizer content is necessary to maintain continuous oxidation protection of alloying elements along with rapid mass transfer mechanism within molten metal droplets during the whole spray distance.