PhD Oral Exam - Saeid Amrollahybiouki, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Suspension plasma spraying (SPS) is a type of thermal spray technique that allows the deposition of sub-micron and nano-sized particles. In the SPS process, small particles are mixed with water or ethanol to form a suspension. 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. Compared to conventional coatings, SPS coatings can improve mechanical and thermal properties due to their higher interface density. However, the solid feed rate and deposition efficiency of the SPS process are significantly lower than other conventional plasma spray techniques. Using suspension with high particle concentration is one way to improve the feedstock deposition rate. However, nozzle clogging is an obstacle to injecting high-concentration suspensions for long periods of time because increasing the particle concentration increases the suspension viscosity, which leads to a decrease in suspension flowability. In this study, flash boiling atomization (FBA) is applied as a novel injection method in SPS to inject suspensions with high solids concentrations up to 70 wt.%.

Flash-boiling atomization uses a thermodynamic instability to break up a liquid jet. As the superheated liquid is accelerated through the nozzle, its pressure decreases, and if the pressure drops sufficiently below the saturation pressure, rapid boiling of the liquid can occur. Vapor bubbles rapidly expand within the liquid jet, bursting and shattering it into smaller fractions. FBA has many industrial applications such as fuel injection in engines, desalination, pharmaceutical industry, etc. The main objective of this study is to use FBA method to be able to inject suspensions with high solids concentration into the plasma flow and fabricate SPS coatings. In order to achieve this goal, the FBA of suspension was first studied under different experimental conditions, and suspension properties.

Suspension injection in SPS can be either axial or radial. In the axial injection, the suspension fragmentation happens inside the torch by interaction with the atomizing gas, and the disintegrated droplets are exposed to the plasma jet. In the radial injection, the suspension is injected from outside the torch into the plasma flow. Spray atomization and mechanical injection are two conventional methods used to inject the suspension radially into the plasma jet. The former is an ensemble of disintegrated droplets resulting from atomization and the latter is a continuous jet resulting from mechanical injection. In this study, the generated coatings with the FBA method were compared with conventional SPS coatings in terms of the microstructure of coatings, deposition weight per pass, deposition efficiency, coating thickness per pass, etc. The results show that there would be improvement in all of the parameters above.

Water and ethanol are commonly used as solvents to make a suspension. Poor atomization and resistance to evaporation are two major drawbacks of water-based suspensions that can affect coating properties, especially for aqueous zirconium suspensions. Water has a high resistance to fragmentation owing to its high surface tension and high mass density. In addition, water evaporation in the plasma jet consumes plasma energy, resulting in cooling of the plasma jet and a reduction in temperature during the process. The FBA technique can potentially improve the fragmentation performance and allows us to inject suspensions with high solids concentrations without clogging issues, because the suspension viscosity and surface tension would be significantly reduced 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 and showed that a dense coating microstructure with high solids deposition can be achieved under the condition of using 70 wt.% suspension and high power torch.