Development of EB-PVD TBC'S : the role of deposition temperature and plasma assistance

Gas turbine manufacturers have achieved continuingly improved engine efficiency and thrust-to-weight ratio by designing with increased Turbine Entry Temperature (TET). The protection of High Pressure Turbine (HPT) aerofoils with thin insulating ceramic coatings, referred to as Thermal Barrier Coatings (TBC's), has emerged as the next key technology to allow for further increases in TET. Electron Beam Physical Vapour Deposition (EB-PVD) is today's most promising processing route for the manufacture of TBC's applied on aerofoils. The purpose of this work was to generate a sound understanding of the relationship between the EB-PVD process and the structure of Zr02- 8wt%Y2O3 ceramic deposits, which could be exploited to achieve improved TBC performance. In particular, the role of deposition temperature and the potential benefits in using RF and DC plasma assistance of the EB-PVD process were investigated, together with their influence on the erosion performance of EB-PVD TBC's. The significance of particulate erosion as a degradation mode is assessed under conditions representative of the HPT environment. New explorable routes to achieve reduced thermal conductivity of EB-PVD TBC's are identified. It is shown that EB-PVD TBC's deposited at low temperature contain a massive content of microscopic voidage (-50%) which is responsible for their lack of thermal stability. The growth of EB-PVD TBC's at elevated deposition temperatures is explained in terms of dynamic sintering, whereby diffusion processes compete against the high rate arrival of vapour atoms to overcome the spontaneous defectiveness of the atomic build up. Modelling of the gas discharge physics has highlighted scope for improving the effectiveness of plasma assistance in causing ceramic structural damage, capable of modifying the coating thermal properties. The erosion rate of EB-PVD TBC's is shown to be controlled by their degree of plastic deformation upon particle impacts, which in turn depends on the ceramic column diameter and inherent porosity.