MICROSTRUCTURAL INVESTIGATIONS OF EBPVD THERMAL BARRIER COATINGS

This paper deals with the morphology, constitutive phases and fine microstructures of some EBPVD processed yttria partially stabilized zirconia thermal barrier coatings. This study has been performed by X-ray diffraction and scanning and transmission electron microscopy. It was shown that the coatings studied were constituted of a single metastable tetragonal t' phase, as expected if one refers to the well-known non equilibrium paths in the zirconia-yttria phase diagram, and not of a cubic solid-solution, as it is claimed in several papers in the literature. It was also shown that this tetragonality is not easily evidenced by X-ray diffraction because the t' grains exhibit a particular crystallographical texture (c axis generally parallel to the substrate/coating interface). The relationship between the coating tetragonality, its particular texture and its ability to resist crack propagation is discussed. It is suggested that the former could significantly contribute to the well-known toughness of EBPVD coatings, leading to increased resistance to thermomechanical fatigue.