Investigation of microstructure and porosity of columnar yttria-stabilized zirconia coatings produced by axial suspension plasma spraying

Abstract: Ceramic topcoats provide thermal barrier coatings (TBCs) with its heat insulating properties. The heat transfer is primarily occurring by conduction through the bulk material, therefore features such as interfaces, pores and grain boundaries are essential for thermal insulation as they act as scattering sites. In this work, yttria-stabilized zirconia (YSZ) coatings produced by suspension plasma spraying (SPS) were investigated with respect to their microstructure and especially their porosity. SPS coatings are built-up by fine powder particles and consist of pores in a wide pore size range, from a few nanometer up to micrometer sizes. The porosity in the coatings was measured using two techniques which have never before been applied on SPS coatings, X-ray microscopy and nuclear magnetic resonance (NMR) cryoporometry. With X-ray microscopy, the coating was visualized in three dimensions revealing a porosity that is heterogeneously distributed within the coating. The technique showed pores with a complex shape, which are part of a connected pore network as well as closed porosity. NMR cryoporometry measured pore sizes down to a few nanometer and provided valuable information regarding pore bottlenecks. The pore shapes were approximated using simple geometrical models. For all investigated coatings, the pore shape for the smaller pore sizes was best described with an elongated geometry while the larger pores had a more spherical geometry.  For SPS coatings, more information on the coating formation process is still required. A single splat analysis was performed and the first layer of deposited material by use of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) technique. The splats were examined with respect to their appearance, i.e. their size, thickness, presence of cracks and grain size. Splat volume was calculated and compared with the particles in the suspension.