Design and Tuning of a Vacuum Microplasma Spray System: Particle Entrainment

A vacuum microplasma spray system, operating on gross power levels of 1-3 kW, with powder feed rates below 1 g/min and using plasma gas mixtures of argon and hydrogen, was built and tested. In order to improve the deposition efficiency and quality of coatings, the system was analyzed and the matter of particle entrainment into the plasma jet core was identified as a critical sub-process. Particle entrainment is a strong function of carrier gas flow rate. Best carrier gas flow rates for entrainment of stainless steel, titanium, and tungsten powders, particle trajectories were determined by quantitative imaging with planar laser Mie scattering. It was found that particle distributions with greater average mass were less likely to overshoot the plasma jet core. This may be attributed to the particle accelerations within the particle injector. Introduction Motivation Plasma spraying is able to deposit coatings in a wide variety of materials and thicknesses [1, 2]. Protective coatings constitute one major class, as they are produced in commercial plasma spraying to improve the performance or lifetime of components in harsh environments of heat, corrosion, and wear. Newer applications for plasma spraying include biomedical coatings of titanium or hydroxyapatite. Strengths of the plasma spray process include the ability to deposit almost any material that has a liquid phase at moderate pressures, and the ability to independently control the temperature of the substrate’s bulk. Sprayed coatings often exhibit some porosity. Porosity can limit their use to conditions of low mechanical loading, but controlled porosity can also be advantageous for some applications, such as thermal barrier coatings, and biomedical coatings for bone ingrowth. The technology and the industry of plasma spraying have matured over several decades of effort to improve the quality and repeatability of the coatings. For the most part, these efforts have been limited to a conventional range of gross arc power (25-60 kW) and deposition rate (50-100 g/min) [1]. The success of many existing applications increases with increasing deposition rate. However, in recent years there have been increasing efforts in both research and industry to extend the process of plasma spraying into a new regime of gross arc power (approximately 500W-4kW) and deposition rate (4-40 g/min) [3-7]. This regime has typically been termed microplasma spraying, benefiting applications where small deposition spot size, 268 reduced standoff distance from spray torch to substrate, and other small-scale characteristics are desired over high deposition rate. This work focuses on the design and operation of a vacuum microplasma (VMPS) spray system operating at gross arc power levels of ~1.4 kW and deposition rates below 1 g/min. To date, there have been no published attempts at microplasma spraying in the vacuum environment. At the conventional scale, vacuum plasma spraying (VPS) has been under development for nearly as long as has atmospheric pressure plasma spraying (APS). VPS, in which the plasma plume and the coating are produced in a sealed, reduced-pressure chamber, allows greater control than does APS over chemical reactions that may occur between the process gases or ambient gases and the coating material. The vacuum condition also reduces losses in momentum and enthalpy of the plasma jet by reducing the effect of entrained ambient gas. These features of the VPS process render it especially appropriate for deposition of metals that are easily oxidized or are difficult to melt. The vacuum process also opens opportunities of reactive plasma spraying. Furthermore, if pushed to extremely low rates of spray deposition, VMPS offers the possibility of simultaneous co-deposition of sprayed material with vapor-deposited material in order to create novel composite coatings.