Deposition Mechanisms of Thermal BarrierCoatings (TBCs) Manufactured by PlasmaSpray-Physical Vapor Deposition (PS-PVD)

Plasma spray-physical vapor deposition (PS-PVD) is a promising technology to produce columnar structured ceramic thermal barrier coatings with excellent performance at high deposition rates. In the PS-PVD process, major fractions of the feedstock powder can be evaporated so that coatings are deposited mainly from the vapor phase similar to electron beam-physical vapor deposition (EB-PVD). But, unlike conventional PVD processes, the interaction between plasma flow and vapor species incombination with the higher chamber pressure makes non-line of sight deposition possible to deposit coatings on shadowed parts of the substrate. The different processing parameters can definitely affect the coating growth mechanisms in PS-PVD. However, their relations to deposition mechanisms which are significant for coating development are still not very clear and relevant reports are limited. In this work, the characteristics of plasma jets generated in the PS-PVD process by standard plasma gases, Ar, He and H$_{2}$, have been studied by optical emission spectroscopy. Abel inversion was introduced to reconstruct the spatial characteristics. In the central area of the plasma jet, the ionization of Ar was found to be one of the reasons for low emission of atomic Ar. The excitation temperature of Ar was calculated by the Boltzmann plot method. Its value decreased from the center to the edge of the plasma jet. Applying the same method, a spurious high excitation temperature of He was obtained, which could be caused by the strong deviation from local thermal equilibrium of He. The addition of H$_{2}$ into plasma gases leads to a lower excitation temperature, however a higher substrate temperature due to the high thermal conductivity induced by the dissociation of H$_{2}$. A loading effect is exerted by the feedstock powder on the plasma jet, which was found to reduce the average excitation temperature considerably by more than 700 K in the Ar/He jet. This characterization of plasma jets under PS-PVD conditions was an important basis for the following studies of the columnar structured YSZ coatings. They were investigated with respect to the powder feeding rate, the agglomeration of feedstock, deposition rate, substrate surface temperature, vapor incidence angle, and flow condition. With increasing powder feeding rate, the efficiency of heat transfer from plasma to the powder declined gradually followed by a lower evaporation rate of the feedstock. Hence, a moderate powder feeding rate and agglomeration of feedstock by organic binder should be used in PS-PVD to achieve effective feedstock evaporation and thus vapor deposition. The observation on initial deposits indicates that faceted crystals are deposited from vapor phase. Based on electron backscatter diffraction [...