A Methodology to Control the Microstructure of Plasma Sprayed Coatings

The aim of this thesis is to enhance the overall understanding of the plasma spray process and increase control and reproducibility of the coating properties. This was performed by establishment of relationships between controllable process parameters, in-flight properties of the injected particles (velocity and temperature), and microstructure properties. The relationships were used to establish a process control strategy. The research was performed in two areas. The first area is numerical simulation using computational fluid dynamics to model the plasma, the particle in-flight properties, and the plasma particle interactions. The second area is applied experimental research, focused on observations, measurements and statistical modeling techniques. The experimental work included measurements of particle properties, and evaluation of coating microstructures, using scanning electron microscopy and an in-house image analysis code. It was shown that the relationships were satisfactorily modeled by statistical linear regression. Particle velocity and temperature could be independently controlled by the arc current and the primary gas flow rate. The variables having the largest influence on the microstructure were found to be particle velocity, particle temperature, substrate temperature, and spray angle. The process control strategy was developed by creation of process maps, describing individual microstructure features, and deposition efficiency, as functions of particle velocity and temperature. Based on these maps a process control methodology, called process windows, was established. In this method each desired coating criteria are specified and corresponding particle velocity and particle temperature ranges determined. Keeping the particle properties within these ranges ensures the coating to meet its requirements. It was also shown that prior to implementation of the process window methodology, enhanced reproducibility can be achieved by reduction of spray gun tolerance limits. This work should be regarded as a foundation for a process control tool by which coating properties can be controlled and optimized on-line in industrial production.