An investigation on the optimization, reliability and repeatability assessment of plasma spray coatings deposited using twin opposing injectors

In plasma spray process, the injection conditions crucially affect the particle properties and thus the characteristics of the deposited coating. The carrier gas flow has to be adjusted carefully depending on particle size and density, injector geometry and position, as well as on plasma jet characteristics to achieve optimum heat and momentum transfer. The variability of coating deposition due to parameter variations can be reduced by optimizing the injection. The situation becomes complicated when twin-opposing injectors are used to supply powder particles into the plasma jet. Further, the crossover of the two streams of particles makes it quite difficult to finalize the optimum injection set point. Hence, in this investigation a closed loop injection method was adopted to optimize the injection parameter of two opposing streams of particles. Three different opposing jet configurations namely the upstream (75°), orthogonal (90°) and downstream (105°) were employed and the methods to optimize injection were investigated systematically. The particle state - first order process map and the deposition state - second order process map were constructed using ensemble particle diagnostics method and in-situ beam curvature measurements respectively. The process property correlations were also fortified using reliability, repeatability and microstructure assessments of the coating deposits.