The microstructure of alumina scale grown at the CoNiCrAlY / Y-PSZ (yttria- partially stabilized zirconia) interface upon oxidation in air at 1100 degreesC for 100 h has been investigated by XRD, SEM, TEM and EDS. The oxide scale microstructure of the plasma-sprayed thermal barrier coatings is characterized by a discontinuous evolution of grain size throughout the scale thickness. The columnar alumina grains in contact with MCrAlY as well as the important porosity observed close to Y-PSZ indicate that the scale growth was evolved by an anionic diffusion mechanism. The observation of a well-defined faceted interface between the innermost alumina grains and metallic bond-coat, as well as the occurrence of epitaxy between these phases definitively prove that the better adhesion of alumina to MCrAlY alloy was achieved by the joining of the oxide and metal lattices, rather than by a keying effect due to the internal oxide precipitates. These latter can contribute to alumina adherence by pinning the emitted dislocations resulting from the displacement of interfacial ledges during alumina growth, and thus increase the metallic strength near the interface.