Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings
(TBCs) have been used in gas turbine engines for a number of years. The primary
mode of failure is attributed to oxidation of the bond coat and growth of the
thermally grown oxide (TGO), the alumina scale that forms on the bond coat and
to which the ceramic top coat adheres. Once the TGO reaches a critical
thickness, the TBC tends to spall and expose the underlying substrate to the hot
gases. Erosion is commonly accepted as a secondary failure mechanism, which
thins the TBC thus reducing its insulation capability and increasing the TGO
growth rate. In severe conditions, erosion can completely remove the TBC over
time, again resulting in the exposure of the substrate, typically Ni-based
superalloys. Since engine efficiency is related to turbine entry temperature
(TET), there is a constant driving force to increase this temperature. With this
drive for higher TETs comes corrosion problems for the yttria stabilised
zirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from molten
calciumâ  magnesiumâ  aluminaâ  silicates (CMAS) which degrades the YSZ both
chemically and micro-structurally. CMAS has a melting point of around 1240 à °C
and since it is common in atmospheric dust it is easily deposited onto gas
turbine blades. If the CMAS then melts and penetrates into the ceramic, the life
of the TBC can be significantly reduced. This paper discusses the various
failure mechanisms associated with the erosion, corrosion and
erosionâ  corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d,
where D is the contact footprint diameter and d is the column diameter, as a
means of determining the erosion mechanism is introduced and discussed for E
