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 deposition1 (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 trans-ferred 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^ubstrate (solidification) interactions during plasma spraying is required to control and optimize the process. Specifically, plasma arc/jet theory, the de-velopment 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. © 1983 The Metals Society.