Droplet-Plasma Interactions in Suspension Plasma Spray (SPS) and in Solution Precursor Plasmas Spray (SPPS)

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Thermal spray coating process is an enabling technology which is employed to protect components from harsh environments and significantly enhances the performance of these components. Due to a combination of factors, such as rapid progress in materials science, development of additive manufacturing and 3D printing, as well as the increasing environmental challenges being brought on by climate change. Protective coatings and the related surface technologies are an indispensable part of strategic development roadmaps in many industrial sectors in which problems linked to wear, erosion, corrosion and thermal efficiency significantly increase manufacturing, maintenance and operating costs. As an example, in 2013, the US National Association of Corrosion Engineers estimated that the cost of corrosion, wear and other materials deterioration in the US alone exceeded $US 276 B. Such massive costs can be substantially reduced through deposition of better protective coatings. Suspension and solution precursor plasma spraying (SPS and SPPS), which is the subject of the proposed study, are two new emerging and very promising coating technologies that have produced very high-quality coatings. In the solution plasma spray (SPS) process, fine powders are suspended in a liquid and are sprayed into a high temperature plasma jet. By suspending the powder in a liquid, normal feeding problems associated with feeding fine powders into a thermal plasma are avoided. In the solution precursor plasma spray (SPPS), a precursor is formulated by dissolving salts in a liquid and spraying them into a plasma jet. As the liquid evaporates, the droplets become supersaturated, resulting in the nucleation of fine particles that will be melted and deposited onto a substrate. SPS and SPPS coatings have shown considerable advantages over the traditional (powder) thermal spray coatings. These processes deposit coatings with a particle size ranging from a few tens of nanometers to just a few micrometers. It has been shown that finely structured coatings have better mechanical, thermal and chemical properties. However, compared to the traditional powder spray coating process, SPPS and SPPS are far more complicated and much research is needed to better understand the details of these two processes. In particular, how do SPS and SPPS droplets interact with plasma is fundamental to understanding of these processes. This 5-year research program proposes the development of two accurate mathematical/computational models of the interaction of a single SPS and SPPS droplet with a thermal plasma under controlled conditions. The models will be validated by introduction of single droplets into a newly developed radio frequency inductively coupled plasma torch (RF-ICP). Employing particle image velocimetry (PIV), the droplet's trajectory will be monitored, the generated particles will be collected on a substrate at different distances from the torch exit for analysis and characterization.