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.
Highlights
In the plasma spray-physical vapor deposition (PS-PVD) technique, a fine powder is injected into a high-temperature plasma jet produced by a D.C. non-transferred plasma torch
The plasma jet undergoes an isentropic expansion, up to recompression through shock waves; its static pressure tends to equilibrate with the ambient pressure by means of expansion and compression zones, where the pressure oscillates around the chamber pressure
computational fluid dynamics (CFD) simulations of plasma jets under PS-PVD conditions were performed in order: (i) to understand the effect of the net electric power input to the plasma torch and deposition chamber pressure on the structure and characteristics of Ar–H2 and Ar–He plasma jet and (ii) evaluate potential continuum breakdowns in the computational domain
Summary
In the plasma spray-physical vapor deposition (PS-PVD) technique, a fine powder is injected into a high-temperature plasma jet produced by a D.C. non-transferred plasma torch. The powder particles are vaporized and re-condensed onto a substrate [1,2,3,4,5]. The coating is essentially formed from the vapor phase and may include clusters resulting from homogeneous nucleation in the gas phase [6]. This makes it possible to achieve coatings with various microstructures; e.g., dense, porous, columnar, which broadens the field of applications of coatings. Yttria-stabilized zirconia (YSZ) coatings with a columnar structure could be used as barrier coatings while dense YSZ coatings could be used as oxygen ion-conducting layer in solid oxide fuel cells [7,8,9,10]
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