Micro-structure investigations on gas flow sputtered thermal barrier coatings

Zirconia based thermal barrier coatings are usually fabricated on high-temperature gas turbine components by Electron-Beam PVD or by thermal spray processes. Reactive sputter processes on the other hand could provide additional process parameters (e.g. ion density and energy) and are advantageous as complex coating compositions can be produced regardless of element vapor pressures. However, most sputter techniques suffer from low deposition rates, especially in the reactive mode. Gas flow sputtering (GFS) is a stable high-rate reactive sputter process operating at fine vacuum conditions and using a hollow cathode glow discharge. Sputtered particles are transported to even complex parts by an intense gas flow therefore enabling a local coating of part areas with the highest thermal load.

50-100 µm thick PSZ films (4 mole % Y₂O₃) have been deposited by GFS on bulk FeCrAl substrates which act as a model for a typical MCrAlY bond coat. The coating temperature was varied between 500 and 850 °C. Film morphology and crystallographic properties of the columnar films have been investigated and cyclic ageing experiments up to 1050 °C were performed. As both thermal conductivity and failure under thermal load are closely correlated to the intra- and inter-columnar microstructure, FIB (focused ion beam) experiments were performed. These revealed the diameter distribution, internal microstructure, porosity and properties of the feather arms of the PVD columns.