Numerical Simulation of Plasma Jet Characteristics under Very Low-Pressure Plasma Spray Conditions

Abstract

Plasma spray-physical vapor deposition (PS-PVD) is an emerging technology for the deposition of uniform and large area coatings. As the characteristics of plasma jet are difficult to measure in the whole chamber, computational fluid dynamics (CFD) simulations could predict the plasma jet temperature, velocity and pressure fields. However, as PS-PVD is generally operated at pressures below 500 Pa, a question rises about the validity of the CFD predictions that are based on the continuum assumption. This study dealt with CFD simulations for a PS-PVD system operated either with an argon-hydrogen plasma jet at low-power (<50 kW) or with an argon-helium plasma jet at high-power (≥50 kW). The effect of the net arc power and chamber pressure on the plasma jet characteristics and local gradient Knudsen number (Kn) was systematically investigated. The Kn was found to be lower than 0.2, except in the region corresponding to the first expansion shock wave. The peak value in this region decreased rapidly with an increase in the arc net power and the width of this region decreased with an increase in the deposition chamber pressure. Based on the results of the study, the local Knudsen number was introduced for detecting conditions where the continuum approach is valid under PS-PVD conditions for the first time and the CFD simulations could be reasonably used to determine a process parameter window under the conditions of this study.

Numerical Analysis of the Interactions between Plasma Jet and Powder Particles in PS-PVD Conditions

“…The latter was chosen because of its ability to control the production and dissipation of turbulent energy at low Reynolds numbers, and, especially, in the viscous sublayer of near-wall regions without using near-wall laws. The model is described in detail in [27,28,33].…”

“…The computational domains are shown in Figure 2. They included the nozzle-anode of the plasma torch and a conical frustum that corresponded to the expansion of the plasma jet in the deposition chamber [27,28]. The plasma was supposed optically thin and in local thermodynamic equilibrium (LTE).…”

“…The hot plasma jet expands first through the divergent part of the nozzle where it accelerates up to 6000-7800 m/s depending on the torch power. It then exits the nozzle with the characteristics of an under-expanded flow as described in [28], as the chamber pressure is much lower than the pressure at the torch nozzle exit (about 15 kPa). The plasma flow exhibits an expansion zone ended by a Mach disk and followed first by a subsonic zone and then a supersonic zone accompanied by brutal changes in velocity and temperature when the plasma jet goes through a shock.…”

“…The problem is then to calculate the temperature and velocity fields of the plasma jet in the low-pressure (<200 Pa) chamber. A recent study showed that computational fluid dynamics (CFD) could be used under PS-PVD conditions provided that the plasma net power is high enough [27,28] as no continuum breakdown was noticed under such conditions.…”

Plasma–Solution Junction for the Formation of Carbon Material

“…, where k B , T, d, and P are the Boltzmann constant, temperature, diameter of molecules, and pressure, respectively [51]). High-density plasma is generated around the probe, and the sheath layer on the surface becomes thicker [42,47].…”