Deposition Mechanism Based on Plasma Spray-Physical Vapor Deposition

Thermal barrier coatings (TBCs) were prepared via plasma spray-physical vapor deposition (PS-PVD) technique at five spray distances by using agglomerated ZrO2-7wt%Y2O3 (7YSZ) powders. The microstructure and phase composition of these coatings under different spray distances were analyzed by field emission-scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). Besides, gas concentration of the powders in plasma jet was diagnosed with stand-off distances by Optical Emission Spectroscopy (OES). Eventually, three formation mechanisms of the PS-PVD coatings based on vapor deposition were proposed, and the relationship between the concentration of gas phase and the microstructure of the coatings was explained. The results reveal that: (1) the coatings prepared at spray distance of 350 mm and 1800 mm exhibit dense structure, while typical PS-PVD 7YSZ columnar coatings were formed at spraying distance of 650-1250 mm; (2) the coating formed at 350 mm spraying distance is composed of t' - and m-ZrO2, and others are mainly composed of t'-ZrO2 at over 650 mm stand-off distance; (3) At 350 mm spray distance, the coating is developed under the combination effect of the supersaturated spontaneous nucleation from high concentration of the gas phase and liquid/solid particles. At spraying distance of 650-1250 mm, the growth of columnar coatings are dominated by the non-spontaneous nucleation after the deposition of gas phase on the substrate, and supplemented by spontaneous nucleation in plasma jet. At spraying distance of 1800 mm, the coating is accumulated from spontaneous re-solidified gas phase particles.