Porous Architecture of SPS Thick Y-PSZ Layers Structured at the Sub-Micrometer Scale

Suspension plasma spraying (SPS) is a recent technology able to process sub-micrometric-sized feedstock particles and deposit thinner (from 20 to 100 µm) layers than those sprayed by atmospheric plasma spraying (APS). SPS consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 µm. Due to the large volume fraction of the internal interfaces and reduced size of stacking defects, thick nanometer- or sub-micrometer-sized coatings exhibit better properties than conventional micrometer-sized ones (e.g., higher coefficients of thermal expansion, lower thermal diffusivity, higher hardness and toughness, better wear resistance, among other coating characteristics and functional properties). Compare to atmospheric plasma spraying, SPS exhibit very peculiar pore architecture: void average size exhibits the same scale than the structure; i.e., from sub-micrometer to nanometer sizes, which seems moreover non-connected. This pore architecture is dependent upon operating spray parameters (power parameters, suspension properties, injection parameters, substrate characteristics, etc.). Due to these specificities, the void content and its architecture can be hardly quantified using conventional techniques of too low resolution. Nevertheless, the discrimination of pore architecture in terms of size and shape distribution, anisotropy, specific surface area, etc., is critical in the understanding of the layer manufacturing mechanisms together with its properties. In this paper, different types of imaging experiments (AFM, FESEM, etc.) were performed to understand, describe and quantify the porosity level of thick SPS Y-PSZ ceramic coatings structured at the sub-micrometer scale.