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High-temperature coating systems, consisting of a Rene´N5 superalloy, a Ni-23Co-23Cr-19Al-0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal-barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl 2 O 4 spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl 2 O 4 /a-Al 2 O 3 interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl 2 O 4 , h-Al 2 O 3 , Y 3 Al 5 O 12 at the TGO surface after pre-oxidation was avoided. The a-Al 2 O 3 layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of h-Al 2 O 3 instead of a-Al 2 O 3 . Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.

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