Phase distribution in, and origin of, interfacial protrusions in Ni–Cr–Al–Y/ZrO2 thermal barrier coatings

Interfacial morphology and reaction products in thermal barrier coating systems were investigated by scanning and transmission electron microscopy (SEM and TEM). The samples consist of yttria-stabilized zirconia (YSZ; 6–8 wt.% yttria) deposited by air plasma spraying onto either of two types of bond coats: a layer consisting of Ni–15.9Cr–5.3Al–0.6Y with 5 wt.% of alumina particulate added, or one that was only the base Ni–Cr–Al–Y composition. In samples thermally cycled to failure in a burner rig, numerous interfacial protrusions of several microns or more in size are observed. These have a complex microstructure and contain elemental Ni intermixed with Ni(Al,Cr)2O4 spinel, (Al,Cr)2O3, and other oxides. Unlike some prior studies, nickel oxide (NiO) was not detected. Protrusion microstructures were similar for the two bond coat systems, but interfacial protrusions for the case of the base composition (i.e. no added alumina particulate) did not contain any spinel phase. Comparison of cross-sectional samples before and after oxidation indicates that the protrusions arise from the encapsulation of isolated segments of the bond coat. The intermixing of metallic Ni grains with oxides in the reaction zone may contribute to failure by affecting local stresses during thermal cycling.