The role of transient oxides during deposition and thermal cycling of thermal barrier coatings

Abstract

High temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC) and a partially yttria stabilised zirconia (PYSZ) thermal barrier coating (TBC), were thermally cycled to failure for three different pre-oxidation treatments performed for 1 h at 1373 K and a partial oxygen pressure (pO2) of 20 kPa, 100 Pa and 0.1 Pa, respectively. These pre-treatments resulted in the formation of different thermally grown oxide (TGO) layers prior to TBC deposition with respect to the presence of the transient oxides NiAl2O4, θ-Al2O3, and Y3Al5O12 at the TGO surface. The TGO microstructures after TBC deposition and thermal cycling were investigated with a variety of analytical techniques and compared with those after pre-oxidation. For all pre-oxidation treatments, a double-layered TGO developed on the BC during thermal cycling. The TGO adjacent to the TBC consisted of small Zr-rich oxide crystallites embedded in an Al2O3 matrix when the TGO surface after pre-oxidation comprised of Y3Al5O12 plus α-Al2O3. When the TGO surface constituted of θ-Al2O3, the Zr-rich oxide crystallites were embedded in a NiAl2O4 spinel layer after thermal cycling. Zr was absent in the oxide layer when the TGO surface prior to TBC deposition was composed of NiAl2O4 spinel. The TGO contiguous to the BC consisted in all cases of α-Al2O3 with Y3Al5O12 crystallites. The roughness of the α-Al2O3/BC interface increased for a higher density of Y-rich oxide protrusions (i.e. pegs) along this interface.