The very low pressure plasma spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with large area coverage based on plasma spraying. At typical pressures below 500 Pa, the characteristics of the plasma jet change compared to conventional low pressure plasma spraying processes (LPPS, often called vacuum plasma spraying, VPS) operating at 5-20 kPa. By VLPPS, quite thin and dense ceramic coatings can be obtained for special applications like solid oxide fuel cells, gas separation membranes, and wear protection. The combination of VLPPS with enhanced electrical input power has led to the development of the plasma spray-physical vapor deposition process (PS-PVD, initially called LPPS-TF process, TF = thin film). At appropriate parameters it is even possible to evaporate the powder feedstock material providing advanced microstructures and non-line of sight deposition e.g. of thermal barrier coatings for turbine components. Since relative large deposition rates and areas can be achieved, PS-PVD has the potential to fill the gap between PVD on the one hand and conventional plasma spraying on the other hand. To exploit the potential of such novel gas phase deposition, the plasma characteristics and their dependency on process conditions must be better understood. Besides power, spray distance and plasma flow conditions, the characteristics of the plasma gas are important parameters which are dependent particularly on its composition and chamber pressure. Thus, plasma conditions were investigated by optical emission spectroscopy. Based on the results, the distributions of plasma temperatures and electron densities were determined. Some experimental examples are given to demonstrate the impact on coating microstructures.
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