Melting and solidification in plasma spray deposition — phenomenological review

Abstract

Plasma spray deposition has been a viable process for the manufacture of thin-walled components and coatings for more than three decades. However, plasma spraying in air results in deposits which exhibit low densities owing to oxidation of the droplets. A recent modification has been the addition of an evacuated chamber and the process has been termed ‘low-pressure plasma deposition’ (LPPD). The use of the evacuated chamber permits higher pressure ratios, resulting in plasma gas velocities of Mach 2–3. LPPD offers several advantages compared to conventional plasma spraying; these are: (i) higher particle velocities which create > 98% theoretical density deposits, (ii) broad spray patterns which produce large deposit areas, and (iii) the capability to heat and clean the substrate using a transferred arc. The high densities of the deposits achieved using the process, coupled with the inherent high solidification rates, make LPPD a viable and attractive process for producing ‘near -net-shape’ components for high-performance applications. A clear understanding of the particle-plasma (melting) and particle–substrate (solidification) interactions during plasma spraying is required to control and optimize the process. Specifically, plasma arc/jet theory, the development of plasma spray deposition methods, and applications are reviewed. The literature on particle melting and droplet solidification has been critically reviewed; this mostly pertains to conventional plasma spraying at relatively low plasma gas velocities and ambient-pressure spraying conditions. Differences between atmospheric plasma spraying and LPPD, and thus the complexities which need to be considered for the low-pressure case, are discussed.