Plasma Spraying of Solid Oxide Fuel Cell Components

The central components for solid oxide fuel cells (SOFC) are the electrodes-electrolyte multilayer arrangement (PEN) and the separating bipolar plates. The PEN (Positive electrode- Electrolyte-Negative electrode) assembly consists of a dense gastight yttria-stabilized zirconia (YSZ) electrolyte and porous electrodes for which usually Ni-YSZ cermet anode and Sr-doped LaMnO3 cathode layers are used. The various PEN units are connected in a cell stack by bipolar plates which are either metallic or ceramic ones. Furthermore, a protective layer on the metallic bipolar plates consisting of a chromium alloy is required to prevent chromium evaporation leading to a rapid and strong degradation of the SOFC performance. At the DLR Stuttgart both the DC and the RF vacuum plasma spraying technique have been further developed and adapted to meet the requirements for the manufacture of the different SOFC components. The DCVPS process using specially developed Laval-like nozzles is especially appropriate to the production of thin and dense coatings as required for the electrolyte and the protective layers. However, applying special spray parameters and nozzles it is also possible to deposit porous electrode layers. The production of the entire PEN arrangement in one consecutive DC-VPS process is the objective of the actual development. On the other hand, the RF plasma spray technique is suitable for the near net-shape production of bulk components such as the metallic bipolar plate. The development of the deposition processes for the production of SOFC components using DC and RF plasma spray methods and the results obtained concerning PEN fabrication, deposition of protective layers and the near net-shape production of metallic bipolar plates are presented in the paper.

Simultaneous Independent Measurement of Splat Diameter and Cooling Time during Impact on a Substrate of Plasma Sprayed Molybdenum Particles

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed 2-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 microseconds after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 microseconds.

Mathematical Modeling of a Free Plasma Jet Discharging into Air and Comparison with Probe Measurements

Plasma spraying process modeling is useful to understand physical phenomena and to decrease the number of experiments. In this paper, a study of the external plasma jet is proposed: the PHOENICS™ CFD code was used with a 2D axisymmetrical geometry and a standard K-ε turbulence model. In a first step, thermodynamic and transport properties were calculated from chemical equilibrium composition, thermodynamic derivatives and kinetic theory of gases. Local Thermodynamic Equilibrium (LTE) was assumed for both plasma and surrounding gases. The proposed numerical results were computed for comparison with temperature measurements realized by Brossa and Pfender in the case of an argon plasma jet discharging into air, using enthalpy probes. The predictions were found reasonably accurate. The influence of the surrounding gas nature was also verified as the validity of the parabolic assumption.

Thermal Spray Hall of Fame

Listing of inductees in the ASM International Thermal Spray Hall of Fame, established by the ASM Thermal Spray Society in 1993.

Internal Plasma Spray Process for Cylinder Bores in Automotive Industry

In the Automotive Industry the need for lower manufacturing costs, the use of less strategic material, and easier, faster, and more flexible routes for manufacturing are being looked for continuously. The environmental concerns relating to the use of galvanic coatings is growing. This has led to the examination of the plasma-powder spray process for the application of coatings for surface modification. In the area of engine cylinder bore coatings a major advance is taking place in the use of a rotating plasma spray device. This paper covers the use of a plasma-powder spray process for the coating of aluminum-silicon cylinder block bores using a rotating plasma gun capable of producing coatings of reliable microstructure and integrity. Properties and microstructures of the applied coatings will be presented. Test results will be shown that the necessary bond strength of the coating can be achieved without the use of a bond coat. Surface preparation prior to coating and surface finishing methods after coating will also be discussed. Experience in Europe, Japan and the Unites States will be discussed which show that the plasma-powder spray process offers a performance proven and cost effective solution for the coating of cylinder bores, thus demonstrating the future application potential for this technology.

The Gas Permeability of Plasma Sprayed Ceramic Coatings

A simple test procedure, based on steady state flow through a membrane, has been developed for measurement of the gas permeability of specimens over a range of temperature. The reliability of this equipment has been verified by testing solid disks containing single perforations and comparing the measured flow rates with those expected on the basis of laminar flow. Coatings of yttria-stabilised zirconia have been produced by plasma spraying in vacuum and in air. The specific permeability of these coatings has been measured at temperatures ranging up to 600°C, using hydrogen gas. It has been found that permeability is increased for coatings produced with longer stand-off distances and at higher pressures. Porosity levels have been measured using densitometry and microstructural features have been examined using SEM. A model has been developed for prediction of the permeability from such microstructural features, based on percolation theory. Agreement between predicted and measured permeabilities is good, although it is clear that more comprehensive data are needed in order to validate the model systematically.

Influence of the Variation of Plasma Torch Parameters on Particle Melting and Solidification

A study of the flattening and cooling of particles plasma-sprayed on a substrate is presented. The characteristic parameters of the splats are linked to the parameters of the impacting particles by using an experimental device consisting of a phase Doppler particle analyzer and a high-speed pyrometer. However, during the long experiments required to get reliable correlations, it was observed that variations in plasma spray operating conditions may alter the particles behavior in the plasma jet. Therefore, a simple and easy-to-use system was developed to control in real time the spray jet. In this paper, the effect of carrier gas flow rate, arc current and powder mass flow rate is investigated. The results on zirconia and alumina powders show the capability of the technique to sense the particle spray position and width.

Vaporization and Ultra-Fine Particle Generation during the Plasma Spraying Process

The thermal spray process melts powder at very high temperatures and propels the molten material to the substrate to produce a coherent deposit. This heating produces a certain amount of vaporization of the feedstock. Upon exiting the plasma plume the fast cooling conditions lead to condensation of the vapor. An electrical low pressure impactor was used to monitor the concentration of ultra-fine particles at various radial and axial distances. Metal, namely iron powder, showed very high concentration levels which increase with distance. Ultra-fine particles from ZrO2-8Y2O3 reached a peak concentration at 6 cm. Use of an air barrier during spraying decreases the population of ultra-fine particles facilitating the production of a stronger coating.

Thermal Spray Forming Using the HVOF Technique

The feasibility of using the HVOF process for the thermal spray-forming of free-standing components has been investigated. HVOF spray forming offers a number of potential advantages compared to the established procedure of plasma forming, including increases in component density, and reduction in material decomposition during spraying. Using blends of carbide and superalloy powders in various proportions, HVOF spraying has been successfully used to form free-standing cylinders and cones of various lengths and thicknesses. Microstructural examination of the spray-formed material has shown a homogeneous distribution of carbides in the superalloy matrix, with very low levels of porosity. Refinement of the procedure has allowed reduction of the matrix content, and the forming of fragile materials.

Formation of Structure of HVOF Sprayed WC-Co Coating on a Copper Substrate

Mathematical modelling of the formation of the WC-Co coating structure and adhesion on copper substrate during high velocity oxygen-fuel (HVOF) spraying is provided. Smooth (polished) and rough (grit blasted) substrates are considered. Variations of solidification time, solidification velocity, thermal gradient, and cooling velocity in the coating and substrate interfacial region are studied. Formation of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is investigated. Behaviour of the crystal size and intercrystalline distance with respect to the thermal spray parameters and morphology of the substrate surface is analysed. Optimal conditions for the development of fine and dense crystalline structure are determined. Mechanical and thermal mechanisms of development of the substrate-coating adhesion are discussed. Results obtained agree well with experimental data.