Abstract

Suspension Plasma Spraying (SPS) is a thermal spray technique used to deposit sub-micron and nano-sized particles. The liquid is evaporated by exposing the suspension to the plasma jet. Then, the particles are melted and directed in the plasma jet to impact the substrate and form a coating. SPS has lower solid feed rates and deposition efficiency compared to other thermal spray techniques. Increasing particle concentration in the suspension can enhance the feedstock deposition rate, but high viscosity and nozzle clogging are issues.
To address these issues, this study explores flash boiling atomization (FBA) as a novel injection method in SPS for high solids concentrations, up to 70 wt.%. FBA uses thermodynamic instability to break up a liquid jet. When superheated suspension is accelerated through a nozzle and its pressure drops below the saturation pressure, rapid boiling occurs. Vapor bubbles expand within the liquid jet, causing it to fragment into smaller parts. FBA has applications in various industries such as fuel injection, desalination, and pharmaceuticals. The main objective is to use FBA to inject high-solids suspensions into the plasma flow to create SPS coatings.
Suspension injection in SPS can be axial or radial. In axial injection, fragmentation occurs inside the torch, while in radial injection, the suspension is injected from outside the torch into the plasma flow. Conventional radial injection methods include spray atomization, which creates disintegrated droplets, and mechanical injection, which produces a continuous jet. This study compared coatings made with FBA to those made with mechanical injection, assessing microstructure, deposition weight per pass, deposition efficiency, and coating thickness. Results indicated improvements across these parameters.
Water and ethanol are common suspension solvents. Water-based suspensions face challenges with atomization and evaporation resistance, impacting coating properties. FBA can improve fragmentation and prevent clogging by reducing viscosity and surface tension in the superheated state. The effect of high solids concentration and plasma power on coating microstructure, thickness per pass, deposition weight per pass, and deposition efficiency was investigated, showing that a dense coating microstructure with high solids deposition can be achieved using 70 wt.% suspension and a high power torch.