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

热喷涂涂层可用作加热元件，以提高系统效率并使加热元件小型化。热喷涂加热元件的一个主要技术挑战是由于涂层的不均一性导致的局部过热导致的有限寿命。对于金属材料，高熵合金(HEA)表现出很高的比电阻和正的TCR值。考虑到这些性能，HEA合金是一种合适的涂层替代材料，可以实现加热元件的功能。到目前为止，HEA合金的这种电势还没有被研究过。这项建议旨在开发新型金属涂层作为加热元件，用于温度高达350°C的潜在应用，并延长其使用寿命。将在HEA合金的基础上设计和开发新型的原料材料，以实现高比电阻。将研究所设计的HEA合金对目标应用的适用性。金属加热元件将采用高速热喷涂和等离子喷涂的方法，使用细粉沉积，以减少涂层的不均一性。涂层非均质性的改善可以减少局部过热。此外，较低的涂层厚度可以改善涂层体系的应力状态。因此，可以延长其使用寿命。在前两年的研究中，将根据比电阻和热膨胀系数设计新型的HEA合金。这些设计的合金将通过熔化工艺生产。随后，将从微观结构、物相组成、相稳定性、比电阻、热物性、力学性能和氧化行为等方面对这些样品进行表征和研究。将根据前两年的研究结果选出其中最有前途的两到三种合金。在接下来的第三个研究年的项目中，将在选定的HEA合金的基础上开发热喷涂加热元件。研究了喷涂工艺和工艺参数对涂层性能的影响。将对涂层的微观结构、物相组成和导电性进行表征。涂层的寿命将通过循环加热试验进行评估，并从失效机理方面进行分析。这样，新型加热元件的适用性就会得到鉴定。

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.