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Examination of the Microstructure of Thermal Barrier Coatings


 The behaviour of plasma sprayed powder particles at impact is not adequately understood so that the microstructure of thermal barrier coatings cannot be predicted with the required accuracy. This is due to the complexity of the spraying process in which the shape of the impacting particles is influenced by the spraying conditions and the structure of the previously formed parts of the coating. In this paper we present some techniques to improve the understanding of coating formation. We investigate the impact of a single molten powder particle on a surface by performing splat tests, measuring the particle’s temperature, velocity, and diameter at the appropriate stand-off distance before the spraying process starts. The measured values are assigned to the splats. SEM-images of the splats are analysed by means of a image analysis algorithm. Thus we are able to evaluate the influence of particle properties on the splat shape. In order to get a better understanding of the mechanism of coating build-up, a three-dimensional series of cross sections is created. With this kind of examination the shape of splats can be characterised in three dimensions.