Abstract:
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.
