Thermal spray processes represent a cost effective and flexible method for the production of functional coatings by using metallurgical, ceramic and cermet materials. Due to the high kinetic energy of the impinging particles, the HVOF (High Velocity Oxygen Fuel) technique is able to produce extremely dense coatings with very low porosity. In this study, several yttria stabilized zirconia (YSZ) powders have been sprayed by HVOF for the fabrication of electrolyte layers for SOFCs (Solid Oxide Fuel Cells) applications. Coatings were characterized regarding their porosity, leak tightness and electrochemical properties. Results were compared to VPS sprayed coatings. The electrochemical behavior of the cells sprayed with the optimized set of parameters was determined applying U(i)-characteristics and impedance spectroscopy. With a destination thickness of about 40 µm, competitive leak tightness of the electrolyte and performance of the cells could be established.

This paper focuses on development of the anode layer for SOFCs by plasma spraying. The composite (cermet) anode, developed by thermal spraying, consisted of nickel and YSZ. The effect of different plasma spraying technologies on the micro structural characteristics and the electrochemical behavior of the anode layer were investigated. Coatings were fabricated by spraying nickel-coated graphite or nickel oxide with YSZ using a Triplex II plasma torch under atmospheric conditions as well as a standard F4-torch under atmospheric or soft vacuum conditions. The investigations were directed in order to have an open micro porous structure, higher electrical conductivity and catalytic activity of anode deposits. Porosity was investigated by measuring the gas permeability. SEM and XRD technologies were applied to examine the morphology, microstructure and composition of the layers. Electrical conductivity measurements were carried out to determine ohmic losses within the anode layer. Most promising layers were analyzed by measuring the electrochemical behavior to obtain information about catalytic activity and performance.

Using a D-optimal design of experiments (DOE), influence of feedstock powder and plasma gases were examined on deposition efficiency, gas tightness and electrochemical behavior of vacuum plasma sprayed YSZ for SOFC electrolytes. In-flight particle temperature and velocity, measured by on-line particle diagnostics, were correlated with plasma and deposit properties. Electrochemical testing of cells was performed to determine the influence of gas tightness and microstructure of electrolyte deposit on cell behavior.

Reproducibility of plasma spraying process and resulting deposits is one of the most essential requirements for its application in industrial environment. In most cases, only controlling process input parameters does not suffice for achieving stability and reproducibility of the process. Besides short time plasma instabilities such as arc root fluctuation, process deviations in longer time operations may arise due to electrodes wear and need to be identified by online diagnostics of the plasma spraying process. In this study two online diagnostic systems were employed to investigate the long-term behaviour of a F4-type plasma gun with a convergent and divergent Laval-nozzle. A Plasma Instability Analysis (PISA) system was applied to identify electrode wear and plasma fluctuation phenomena within the gun whereas Particle Flux Imaging (PFI) was used to correlate the electrode wear with the plasma jet characteristics at the nozzle exit. The long-term performance of electrodes was investigated under continuous operation and under periodic reignition of the torch. A good correlation between aberrations in the plasma process observed with PFI and changes in the frequency spectra detected by PISA was found.