Tailoring approach to predict the microstructure of thermal barrier coatings by adjusting the distribution of splat morphologies

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Atmospheric plasma sprayed Alumina and YSZ (Yttria stabilized zirconia) splats have been collected by different methods using APS process and investigated by different researchers. There is an agreement on the influence of in-flight particle and substrate conditions on the obtained splat morphologies. However, the impact of the interactions between the affecting conditions remains ill-defined. Furthermore, no clear relationships were established between the investigated distributions of splat morphologies and the resulted characteristics of pores and cracks within the produced coatings. Due to the fact that the efficiency of a TBC top coating depends mainly on the density, morphology and distribution of voids as well as on the density of segmentation cracks, there are optimal specifications that combine the required characteristics of pores and cracks for each field of application. In turn, the exerted thermal and mechanical loads determine the optimal required combinations.Therefore, the main aspect of this study is to collect YSZ splats and to produce the corresponding coatings under different conditions using DoE (Design of experiments) approach. The collection of splats will be achieved by means of the innovative and self-made beam shutter setup, which is available exclusively at the institute. The proposed work plan starts by identifying series of distributions of splat morphologies through a detailed splat analysis and establishing well-defined relationships between these distributions and the associated conditions. Afterwards, generally valid relationships have to be established between the identified distributions of splat morphologies and the resultant characteristics of pores and cracks. Furthermore, identifying the distributions of splat morphologies and segmentation cracks are optimized by investigating the interactions between the conditions that lead to the simultaneous coexistence of the desired microstructure. Even though, the optimization process will enhance the distributions of splat morphologies that control the coating porosity, the achievable level of porosity remains technically limited. Therefore, it is intended to add pore formers to YSZ spraying powders in order to increase the porosity to higher levels. The feedstock powders consist of up to 50 vol.% of pore former. A mixture variation will be utilized in conducting the experiments. SEM images of cross-sectional coatings will be used to investigate the effect of adding pore former on the obtained porosity within the obtained coatings. Finally, a direct relationship should be established between all these conditions, and the obtained microstructural features. The obtained results should allow us to produce APS coatings with predefined microstructure characteristics and enhance the reproducibility