Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. The as prepared YSZ TBCs were characterized by scanning electron microscopy (SEM), Electron backscatter diffraction (EBSD) and focused ion beam (FIB) systems before and after heat treatment. A thermally grown oxide (TGO) layer was formed between YSZ topcoat and bond coat after heat treatment. EBSD results revealed that the TGO layer was composed of alpha-Al2O3 and cubic Al2NiO4 layers. Mechanism controlling the growth of the TGO layer were analyzed in detail. It was found that the AlNi phase gradually disappeared from bond coat during heat treatment, which provided Al and Ni sources for the formation of TGO. Meanwhile, EBSD results confirmed that the growth of the TGO layer was dominated by the diffusion of Al and Ni rather than 0 penetration. Furthermore, direct evidence on the diffusion pathways of Al and Ni was also found, indicating that Al and Ni proceeded along the pores, cracks and YSZ grain boundaries to promote the further growth of TGO layer with increased heat treatment time. (C) 2016 Elsevier Ltd. All rights reserved.