Effective Parameters in Axial Injection Suspension Plasma Spray Process of Alumina-Zirconia Ceramics
Suspension plasma spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using significantly smaller particles as compared to conventional thermal spraying. Considering the complexity of the system there is an extensive need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/8 wt.% yttria-stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated using the Taguchi design of experiment. The microstructural features include microcracks, porosities, and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e., temperature and velocity were also measured. The role of the porosity in this multicomponent structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity content.
Investigation of microstructure and porosity of columnar yttria-stabilized zirconia coatings produced by axial suspension plasma spraying
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.
Electrochemical method to evaluate the connected porosity in ceramic coatings
Abstract The electrochemical behaviour of plasma-sprayed SS41 steels with the Al2O3−TiO2 system, Cr2O3 and ZrO2 stabilized by Y2O3 ceramics was studied using a potentiostat by measuring their natural potential and anodic polarization curves in 3.5% NaCl aqueous solution. The natural potential of ceramic-sprayed SS41 steels is the same as that of the substrate. The anodic polarization behaviour of ceramic-sprayed SS41 steels is also dominated by the substrate through connected porosity. However, the polarization current density of ceramic-sprayed SS41 steel is lower than that of the substrate because the current density is controlled greatly by the amount of porosity in the coating. The connected porosity of ceramic coatings was obtained quantitatively from the polarization current density ratio between the ceramic-sprayed specimen and the substrate at the corresponding polarization potential. The strong relation between the connected porosity and the porosity of cross-sections of the ceramic coatings, shown using micrographs, was recognized. By measuring electrochemically the connected porosity in coatings obtained under various spraying conditions, it would be possible to select the optimum spraying conditions to spray coatings which have less porosity.