Conventional thermal spray processes as atmospheric plasma spraying (APS) have to use easily flowable powders with a size up to 100 lm. This leads to certain limitations in the achievable microstructural features. Suspension plasma spraying (SPS) is a new promising processing method which employs suspensions of sub-micrometer particles as feedstock. Therefore much finer grain and pore sizes as well as dense and also thin ceramic coatings can be achieved. Highly porous coatings with fine pore sizes are needed as electrodes in solid-oxide fuel cells. Cathodes made of LaSrMn perovskites have been produced by the SPS process. Their microstructural and electrochemical properties will be presented. Another interesting application is thermal barrier coating (TBC). SPS allows the manufacture of high-segmented TBCs with still relatively high porosity levels. In addition to these specific applications also the manufactures of new microstructures like nano-multilayers and columnar structures are presented.

The suspension plasma spraying (SPS) technique implemented on DC plasma spray guns is a complex thermal spraying process. In order to gain a better understanding of this deposition technique a systematic “splat” study using a shutter mechanism and the line-scan test was conducted varying liquid feedstock properties (viscosity and surface tension) and injection parameters (stream velocity and mass-loading). Splat morphology revealed the degree of particle agglomeration within the droplet formed from the liquid/plasma interaction, as well as their impacting velocity and heating history. The droplet formation was correlated to the liquid feedstock injection velocity and its viscosity. A simple model was developed to explain the experimental results correlating suspension properties to suspension droplet fragmentation mechanism.

The innovative suspension plasma spraying (SPS) technique, in which the carrier gas used to inject particles (10…100 µm) into the plasma jet is replaced by a liquid feedstock, is currently under development procuring denser ceramic coatings due to the use of submicron particles. The suspension properties, as well as the most relevant injection parameters - injection angle and liquid velocity - are adjusted to improve the coating quality at acceptable deposition rates. In addition, the plasma jet instabilities are studied and correlated to the coating properties. In the present work, a feasibility study is conducted addressing the key factors influencing the coating morphological properties such as the porosity, cracks, molten-fraction and amount of over-spray. The experimental setup is adapted to different DC plasma torch architectures (Sulzer-Metco F4 and Triplex) operating under atmospheric conditions with the aim of correlating the plasma jet qualities to the properties of the feedstock under consideration. The metallographic probes and fractured surfaces of the resulting yttria stabilized zirconia (YSZ, 5 wt. % Y2O3) coatings are analyzed by means of light microscopy and scanning electron microscopy (SEM).