Double Effect of Super-Hot Molten Droplets on the Formation of Oxide-Free Metal Droplets and Intersplat Bonding during APS using Deoxidizer-Containing Powders

With plasma spraying of metal coatings, severe oxidation leads to coatings with high content of oxide inclusions and, subsequently, limited interlamellar bonding. Recently, the strategy has been proposed to generate oxide-free molten droplets by air plasma spraying in ambient atmosphere through designing metal powders with specific deoxidizer. In this paper, the synchronous effect of the generation of super-hot molten droplets on the formation of oxide-free metal droplets and intersplat bonding by a spread-fusing self-bonding mechanism during APS using deoxidizer-containing powders will be examined. With Ni-based, Cu-based and Fe-based materials, boron can be used as deoxidizer, while with Al-containing alloys such as MCrAlY and NiAl, carbon can be used as deoxidizer. It was confirmed that the generation of molten droplets with a temperature higher than 2400^oC fulfills the thermodynamic deoxidizing conditions. By examining the collected powder particles passing through the plasma jet, and the splat morphology and splat-interface on samples prepared by FIB, it was revealed that oxide-free molten metal droplets are achieved through using super-hot deoxidizer-containing powders. Furthermore, the results also show that sufficient metallurgical bonding is achieved between splats within the coatings to yield a tensile adhesion over 150 MPa for NiCrB coatings. Therefore, with spray metal powders design and generation of super-hot droplets, dense metal coatings with high cohesion and low oxide inclusions can be deposited by APS.