Investigation of Plasma Gradient Coatings with a Top Hafnium-Oxide Layer by Nuclear Backscattering Spectrometry

Abstract

Gradient coatings with a thickness up to 160 μm are obtained by the low pressure plasma spraying of NiCoCrAlY, ZrO₂–Y₂O₃(7%), and HfO₂–Y₂O₃(9%) powders and their mixtures. The coatings are studied by scanning electron microscopy and nuclear backscattering spectrometry of protons. According to the spectrometry data, the thickness of the gradient coatings (so-called “mass” thickness) is significantly less than the geometric thickness. It indicates a rather high porosity of the coatings (20–30%) and is explained by the presence of nanostructured areas in them. The difference between the geometric and mass layer thicknesses is less significant in the case of a thicker coating due to sintering of the particles during the spraying of subsequent upper layers. The mass thicknesses of the NiCoCrAlY + ZrO₂–Y₂O₃(7%) and ZrO₂–Y₂O₃(7%) + HfO₂–Y₂O₃(9%) layers are greater than the thicknesses of the main layers due to the formation of additional transition zones of mixed composition at the interfaces. The NiCoCrAlY + ZrO₂–Y₂O₃(7%) layer is thicker than the ZrO₂–Y₂O₃(7%) + HfO₂–Y₂O₃(9%) layer due to the higher porosity of the ceramic layer.