In a joint development project, TACR GmbH1 and H.C. Starck GmbH 2 took an approach to adjust the properties of an agglomerated and sintered YSZ powder to a process setup optimized for a HOSP type powder. Within the OEM approved process parameters, which could be changed within narrow limits only, the particle size distribution of the agglomerated and sintered powder was adjusted to meet process parameters and spray setup. Key parameters for the comparison of coating properties and spray behavior were coating porosity and deposition efficiency (DE).

This paper describes the development of an online process control system for spraying thick thermal barrier coatings as part of a Brite Euram research project. In addition to the evaluation of the substrate temperature, a system for acoustic emission analysis for particle impact and for controlling crack propagation is developed. The procedure, the problems overcome, and the results obtained are described. Paper includes a German-language abstract.

Within a Brite Euram project thick thermal barrier coatings for combustor applications were produced by plasma spraying of yttria partially stabilised zirconia (ZrO2 + 8 wt.% Y2O3). The material properties of such coatings strongly depend on their microstructure which can be altered by manipulating the parameters controlling the plasma spraying process. Covering a variation of possible microstructures, the coatings considered had a thickness of about 2 mm and were six to eight times thicker than the coatings currently in service. This investigation was concerned with an evaluation of the thermophysical and mechanical properties of these coatings and their correlation with the microstructure and the plasma spray parameters. Particular attention was paid to the influence of coating segmentation, microcracking and porosity. The experimental work included the measurement of the thermal diffusivity using the laser flash technique, thermal expansion measurements, and the determination of flexural strength and Young's modulus by means of a specially constructed four-point bend rig. Since some of the samples considered were sprayed according to a partially factorial test plan a statistical evaluation of the material data was possible yielding the correlation between process parameters and material properties.

The gasturbine industry is performing utmost efforts to allow increased operation temperatures to improve engine efficiency by a reduced cooling air/fuel-ratio. The reason is to save fuel and at the same time lower emission rates. A possible solution to improve the insulation of the Thermal Barrier Coating (TBC) is followed by the strategy of developing reliable thick TBCs. To be successful concerning the coating quality and to apply thick TBCs at reasonable costs and spray time it is necessary to apply the coating consistently by high deposition rates. The plasma spraying of thicker coatings leads to long spray periods which demands for a process control to keep the process parameters within the tolerances during the entire spray procedure. A major property is the control of substrate temperature with pyrometer systems and to define the process tolerances. Furthermore, a high deposition efficiency is of prime importance to shorten spray times. The optimisation of the gas composition in terms of viscosity and thermal conductivity (SPRAL 22) enables to increase dramatically this efficiency by 30% to 70%. Moreover, in order to reduce thermal expansion mismatch between substrate and ceramic top coat, a high amount of porosity is beneficial to lower young's modulus of the ceramic top. This can be achieved by adding a polymer powder to the yttria partly stabilised zirconia powder. It also leads to an increase in deposition efficiency.

For very large structures and parts in critical environments, a materials solution often cannot be found by using one material. The specific desired properties for those structures, like stiffness, ductility, high temperature stability, corrosion resistance, etc. are difficult to fulfill with only one material. In this case a solution may be found by using coatings and design their specific properties to replenish each other by their combination. The Thermal Spraying processes offer the necessary flexibility of producing thin to thick, ductile, soft to hard coatings while due to the wide range of process temperatures it is possible to process a wide range of materials, both as coating and structure. In this paper the some recent and important developments in Thermal Spraying to produce coatings for technical demanding structures will be described. These developments consist of High Power Plasma Spraying, powder- and process control development. To ensure process consistency during long spraying times and to apply reproducible coating quality a suitable process control is of great importance and the development of temperature control by Pyrometry and Thermography will be presented. The example will be drawn according to the application of a coating on a ball valve for off-shore and ship diesel engine parts (piston and valve).