Deposition of oxidation-resistant coatings by vacuum-pulse-arc melting of NiAl-based granules to protect Ni superalloys

Here we report an innovative contactless method for vacuum-pulse-arc melting of single Ni41Al41Cr12Co6Hf0.25 (at.%) granules with a size of 160 ± 20 μm using a non-consumable electrode. A dense, uniform, and oxidation-resistant coating 100 μm thick, formed by a monolayer of molten granules, was obtained on the surface of a Ni-based superalloy fabricated by laser powder bed fusion. A distinctive feature of the proposed technology is the use of high-voltage breakdown to initiate a subsequent relatively low-voltage arc discharge with adjustable duration and energy. The utilization of granules of a certain size and precise control of the discharge energy makes it possible to control the monolayer thickness and the degree of substrate melting. A mechanism is proposed for melting a single granule located on the substrate surface. The resulting coating has a submicron structure consisting of NiAl and Ni3Al intermetallic compounds. The high resistance of the coating to oxidation at 1000 °C is evidenced by the formation of an oxide scale 500 nm thick, consisting of two sublayers: an upper layer about 200 nm thick with relatively large α-Al2O3 grains up to 150 nm in size and a lower layer consisting of a mixture of α-Al2O3 and θ-Al2O3 grains smaller than 40 nm.