Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings T. J. Nijdam,* G. H. Marijnissen,   E. Vergeldt,   A. B. Kloosterman, à and W. G. Sloof,*   Received September 21, 2005; revised July 31, 2006 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 treat- ments 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 *Department of Materials Science and Engineering, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands.   Sulzer Elbar BV, Spikweien 36, 5943 AD Lomm, The Netherlands. à Structures and Materials Division, National Aerospace Laboratory NLR, Voorsterweg 31, 8316 PR Marknesse, The Netherlands.    To whom correspondence should be sent. Tel.: +31-15-2784924; fax: +31-15-2786730 e-mail: W.G.Sloof@tudelft.nl Oxidation of Metals, Vol. 66, Nos. 5/6, December 2006 (Ó 2006) DOI 10.1007/s11085-006-9036-8 0030-770X/06/1200-0269/0 Ó 2006 Springer Science+Business Media, LLC 269