Development of novel metallic feedstock materials for heating elements produced by thermal spraying

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Thermally sprayed coatings can be used as heating elements to increase the system efficiency and to miniaturize heating elements. An essential technical challenge for thermally sprayed heating elements is the limited lifetime due to local overheating caused by the heterogeneities of the coatings. For metallic materials High Entropy Alloys (HEA) exhibit a very high specific electric resistance and positive TCR-values as well. Considering these properties HEA alloys represent a suitable alternative as coating fulfilling the function as heating element. Up to now, this potential of HEA alloys has not been studied. This proposal aims for the development of novel metallic coatings as heating elements for potential applications up to T=350°C with prolonged lifetime. Novel feedstock materials will be designed and developed based on HEA alloys to achieve high specific electric resistance. The designed HEA alloys will be investigated with respect to their suitability for the aimed application. The metallic heating elements will be deposited using a fine powder fraction by means of high velocity thermal spray processes and plasma spraying to reduce the heterogeneities of the coatings. The improved heterogeneity of coating can reduce local overheating. In addition, the low coating thickness can improve the stress states in the coating systems. As result, the lifetime can be increased. In the first two research years, novel HEA alloys will be designed with respect to specific electric resistance and their thermal expansion coefficient. These designed alloys will be produced by melting processes. Subsequently, these samples will be characterized and investigated in terms of the microstructure, phase composition, phase stability, specific electric resistance, thermos-physical properties, mechanical properties and oxidation behavior. The two or three most promising alloys among them will be selected based on the results achieved in the first two research years. In the following project for the third research year, the heating elements by means of thermal spraying will be developed based on the selected HEA alloys. The influence of spray processes and parameters on the coating properties will be investigated. The coatings will be characterized regarding their microstructure, phase composition and electric conductivity. The lifetime of the coatings will be evaluated by means of cyclic heating tests and analyzed with respect to the failure mechanisms. In this way, the suitability of the novel heating elements will be qualified.