MICROSTRUCTURAL INVESTIGATIONS OF EBPVD THERMAL BARRIER COATINGS

This paper deals with the morphology, constitutive phases and fine mic rostructures of some EBPVD processed yttria partially stabilized zirco nia thermal barrier coatings. This study has been performed by X-ray d iffraction and scanning and transmission electron microscopy. It was s hown 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 c ubic solid-solution, as it is claimed in several papers in the literat ure. It was also shown that this tetragonality is not easily evidenced by X-ray diffraction because the t' grains exhibit a particular cryst allographical texture (c axis generally parallel to the substrate/coat ing interface). The relationship between the coating tetragonality, it s particular texture and its ability to resist crack propagation is di scussed. It is suggested that the former could significantly contribut e to the well-known toughness of EBPVD coatings, leading to increased resistance to thermomechanical fatigue.