Effects of diamond size and content in Ni/Al/diamond composite powders on in-flight NiAl particle oxidation behavior and microstructure of the coatings by atmospheric plasma spraying

Plasma-sprayed NiAl coatings exhibit excessive oxide inclusions, which significantly limits the coatings performance and applications. This study aims at clarifying the factors influencing in-flight deoxidizing kinetics of NiAlC particles using diamond as deoxidizer to promote the development of novel APS approach for metal coating using oxide-free molten droplets in ambient atmosphere. The influence of diamond particle size and content on the in-situ in-flight deoxidizing effect during atmospheric plasma spraying of NiAlC particles and microstructure of APS NiAl coatings are systematically investigated. Results demonstrate that the NiAl coatings prepared with Ni/Al/diamond powders present a much denser structure with significantly improved adhesive strength in comparison with conventional NiAl coating. When 1–2 μm diamond is used as a deoxidizer, the incomplete dissolution of larger diamond particles into molten NiAl limits the continuous supply of carbon to droplets surface. Therefore, the oxygen content of the coatings increases and bonding strength decreases with increasing carbon content. When 100 nm diamond is used as a deoxidizer, its dissolution into molten NiAl is promoted. The carbon within in-flight NiAl particles can be continuously supplied to surface by vortex motion to achieve in-situ in-flight deoxidation as carbon content is increased from 2 wt% to 4 wt%. The oxygen content of NiAlC coatings is significantly decreased to 0.65 wt% with increasing nano-sized diamond to 4 wt% compared with the oxygen content 3.32 wt% of NiAl coating deposited by Ni/Al powder.