Plasma Sprayed Ni–Al Coatings

Abstract

It is widely appreciated that Ni–Al can act effectively as a bond coat for thermal sprayed coatings. The origin of this technologically significant phenomenon is generally considered to lie in an ‘pexothermic process’ associated with solidification of the molten alloy droplets. This exothermic heat has been variously attributed to compound formation, yielding a high heat of reaction, or to heat of oxidation of aluminium. Experimental tests to date have led to ambiguous conclusions. The present study offers a re-examination of this subject using plasma spraying of Ni–Al, composite and prealloyed, in the presence and absence of oxygen. X-ray and electron microprobe analyses were used to characterise the phase structures of the coatings. Coatings plasma sprayed in air showed significant oxidation products, while those plasma sprayed in a vacuum showed the presence of compounds. The results indicate that for Ni–Al the heat evolved during conventional plasma spraying in air is principally due to partial oxidation of the components, and not to alum inide compound formation, as previously suggested. Mechanical property (tensile bond strength, hardness, and cavitation erosion) tests were performed to verify the microstructural findings.