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.

In the Brite Euram Project COMBCOAT Thermal Barrier Coatings for improved thermal protection with a thickness up to 2 mm have been developed for combustor applications. As a typical experiment to evaluate the quality of the coatings, life cycles to spallation by means of thermal cycling tests have been determined at ANSALDO Ricerche, BMW Rolls- Royce and Volvo Aero Corporation. To ensure that the ranking on the different cycling experiments are equivalent and to compare cycles to spallation reached in the different rigs, a comparison of 6 different coatings, highly porous as well as segmented ones, has been performed on the three different rigs. Results of the different tests demonstrated that a direct comparison of the result is not possible. The ranking and the number of cycles to spallation have been inconsistent for different specimen and rigs. Detailed simulation of experiments need to be performed to understand the differences between results.