Investigations on the nature of ceramic deposits in plasma spray-physical vapor deposition

In Plasma Spray-Physical Vapor Deposition (PS-PVD) process, major fractions of the feedstock powder can be evaporated so that coatings are deposited mainly from vapor phase. In this work, Computational Fluid Dynamics (CFD) results indicate that such evaporation occurs significantly in the plasma torch nozzle and even nucleation and condensation of zirconia is highly possible there. Experimental work has been performed to investigate the nature of the deposits in the PS-PVD process, in particular coatings from condensated vapor and nano-sized clusters produced at two spraying distances of 1000 mm and 400 mm. SEM images show that, at long spraying distance, many small nano-sized particles (<20 nm) which are significantly smaller than primary particles in the feedstock powder cover the coating surface in the center of the plasma jet. However, at short spraying distance, such nano-sized particles were hardly found. Besides, in the center area of the plasma jet, XRD patterns indicate only limited preferential growth orientation at long spraying distance which might be a consequence of the low mobility of nano-sized cluster deposits. Meanwhile, at short spraying distance, XRD patterns show obvious preferred growth orientations of (110) and (002). The results give a further insight in the formation mechanisms of high quality columnar structured PS-PVD thermal barrier coatings which have already shown excellent performance in cyclic life time tests.