| Plasma spray technology has been widely applied in industry. Unfortunately, the sprayed coating quality is not always perfect and predictable. Plasma jet instability is one of major causes for the inconsistent coating quality. This research has focused on investigating the causes of plasma jet instability, especially the arc instability in a spray torch. With combinations of electrical measurements, optical measurements and acoustic measurements, this research is designed to provide a complete picture of this arc instability.; The approach has been to determine the effects of the instability on the in-flight particle properties and the coating quality. The arc instability has been characterized by the arc voltage waveform. High-speed video imaging has been used to capture the arc dynamical behavior. A simple analytical model has been developed to quantitatively estimate the arc column diameter. The velocity of the plasma jet has been measured based on the arc fluctuation propagation.; The anode deterioration has been found having strong influences on the arc instability. These influences have been quantitatively described in terms of the boundary layer thickness and the arc operation mode. Fuzzy logic models have been used to diagnose the anode condition on-line and provide control strategies for constant particle heating. Effects of anode erosion on the jet turbulence have also been observed with a heating helium gas that simulates the jet.; The heating and cooling processes of a substrate exposed to a plasma jet have been measured, and the influence of the substrate temperature on the coating porosity has been investigated. The results of this research contribute to the understanding of the details of the plasma spray process and help to lay a solid foundation for process optimization and development of feedback control yielding a consistent coating quality. |
