Melting and solidification in plasma spray deposition — phenomenological review

Abstract

A == area covered by splatted particle An == nozzle cross -sectional area B == deposit-substrate material constant, defined below equation (41) Bi == Biot number CD == drag coefficient C == specific heat capacity at constant pressure d == splat diameter D == partic Ie diameter f o == correction factor in equation (24) hi == splat/substrate interface heat -transfer coefficient h o == effective heat -transfer coefficient through gas boundary layer H == plasma gas enthalpy 6.H m == latent heat of fusion K == thermal conductivity K == integrated average thermal conductivity L == particle heat content per unit volume relative to solid at room temperature == pp(C p 6.Tin +6.Hm) L' == heat required for fusion of particle == pp(4/3)1TR36.H m 111 == material constant in equation (24) M g == gas mass flowrate M p == particle mass Ma == Mach number Nu == Nusselt number Pi == pressure in arc gas chamber Po == ambient pressure 6.p == Pi -Po Pr == Prandtl number r == radial distance R .== particle radius R E == radius at which plasma ceases to be electrically conducting R m == radial distance to solid/liquid interface in partially melted particle Re == Reynolds number S == terminal thickness of splaUed particle

Melt Atomization

“…A schematic of a commercial hightemperature plasma gun showing its key features can be seen in Figure 3.32 (after ref. The core temperature within the plasma can exceed 10 000 Κ and often lie between 10 000 to 50 000 Κ [64,65]. For this sort of operation, the plasma is simply a source of a confined very high-temperature flame that is created by passing a gas through an electrical arc that is sustained between a thoriated tungsten cathode and a water-cooled copper anode.…”

“…Having a warm substrate also tends to promote adhesion of the droplets to the substrate material [65,76]. RF or DC plasma guns can be used as the melting and high velocity generating device, with the DC gun being preferred due to its ability to achieve higher gas velocities.…”

Beryllium processing technology review for applications in plasma-facing components

“…Advances in plasma spraying over the last 15 years, specifically the introduction of vacuum plasma spraying, have expanded the scope of plasma spraying from coatings to thick spray deposits ( > 15 mm) for potential structural applications. When plasma spraying is done under a reduced pressure condition, higher pressure ratios exist between the plasma spray torch and the operating environment, and they result in gas velocities in the range of Mach 2-3 [1]. Other advantages of vacuum plasma spraying over atmospheric and inert plasma spraying are the following:…”

“…Current state-of-the-art gettering furnaces for high-temperature and high-vacuum applications are available; they can provide ultrapure inert gases to the plasma spray torch. 1 10 (1) Chamber evacuated and backfilled with argon. Oxygen content about 0.5 volume percent.…”

Plasma Spray Deposition: A Need for Direct Process Control

“…Figure 1 also illustrates the "environmental" chamber controlling the inert background gas from 1 torr to atmospheric pressure. Depositions in environmental chambers has significant1y improved the metallurgical quality of deposits, increasing deposit densities and bonding while decreasing deposit oxide gas interstitial content [7,8]. Deposits with mechanical properties which meet or exceed the properties of materials consolidated by other means (casting, forging, PIM etc) are now able to be nroduced.…”