Abstract

Suspension plasma spray (SPS) is an emerging coating process for making surfaces with superior properties. In the SPS process, ceramic particles are mixed with water or ethanol to form a suspension. A plasma torch provides the heat and momentum to evaporate liquid phase of the injected suspension, melt the coating particles, propel the in-flight particles toward a substrate, and eventually form a coating layer. However, the SPS process relies strongly on the coupon test and trials to find optimum spray conditions for plasma, suspension injection, and substrate location. At the end, an optimum spray condition set in a spray booth may not reproduce the same coating result in other booths. An effective control over the spray process improves the reproducibility of the spray conditions and consequently coating structures. Therefore, monitoring systems are employed to better understand and control the required spray condition. The monitoring included accessing state of droplets after the atomization of suspension and state of in-flight particles near the substrate. For further development of the SPS process, the suspension can be injected by an effervescent atomizer. This research aims to contribute in further improving the process and developing the diagnostic tools in SPS.
For a further improvement of the SPS process, an effervescent atomizer was investigated as an alternative way instead of the current methods of injection of the suspension in the plasma jet. Performance of the effervescent atomizer was investigated at room temperature by phase Doppler particle anemometry (PDPA). Size of droplets and shape of the atomized spray in a crossflow configuration was almost independent of the suspension concentration. Size of droplets depends on the atomization at the exit of the orifice and the breakup in the crossflow. Velocity of droplets at downstream is the velocity of the crossflow. It was found that the shape of spray was conserved in the crossflow and relatively smaller droplets were enveloped by the larger droplets.
As a contribution to adapt a diagnostic system for SPS, a two-color pyrometer was modified and investigated to measure temperature of in-flight particles. The in-flight particles are released after evaporation of the liquid phase of suspension droplets. A high cooling rate of the in-flight particles in terms of distance from the torch and radiation of plasma are main challenges for temperature measurement. To remove these limitations, the temperature was measured by a single-point measurement system based on thermal emission which equipped by readjusted bandpass filtering. The result of online temperature and velocity measurement was in a good agreement with the offline validation by collecting the splats and analyzing the samples. Moreover, the measurement condition has an impact on temperature, and the impact can be minimized by elimination of the stray radiation.
As a fundamental research work to develop a diagnostic system for SPS, a light diffraction (LD) system was adapted and investigated to measure size of in-flight particles. Refraction of the laser in the measurement volume and radiation from plasma were two main challenges of the size measurement. A shield of an optimized aperture was employed to control the condition of measurement volume. By applying a narrow bandpass filter at a right wavelength and selecting a right angle to collect the scattered signal from the in-flight particles, the size of particles was measured. A good agreement between the result of online measurements under the plasma condition and studying the feedstock particles in the wet cell unit in the room condition validated the size measurement.