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

热喷涂涂层可作为加热元件，提高系统效率，使加热元件小型化。热喷涂加热元件的一个基本技术挑战是由于涂层的非均质性引起的局部过热而导致的有限寿命。对于金属材料，高熵合金（HEA）具有很高的比电阻和正tcr值。考虑到这些性能，HEA合金是一种合适的替代涂层，可以满足加热元件的功能。到目前为止，HEA合金的这种潜力尚未得到研究。该提案旨在开发新型金属涂层作为加热元件，用于高达T=350°C的潜在应用，并延长使用寿命。基于HEA合金的新型原料将被设计和开发，以实现高比电阻。所设计的HEA合金将根据其对目标应用的适用性进行研究。采用高速热喷涂和等离子喷涂的方法将金属加热元件用细粉末部分沉积，以减少涂层的非均质性。改善涂层的非均质性可以减少局部过热。此外，低涂层厚度可以改善涂层系统的应力状态。因此，寿命可以增加。在前两年的研究中，将根据比电阻和热膨胀系数设计新型HEA合金。这些设计的合金将通过熔炼工艺生产。随后，这些样品将在微观结构、相组成、相稳定性、比电阻、热物理性能、机械性能和氧化行为方面进行表征和研究。将根据前两年的研究成果，从中选出两到三种最有前途的合金。在接下来的第三个研究年的项目中，将以选定的HEA合金为基础，开发热喷涂加热元件。研究了喷涂工艺和参数对涂层性能的影响。涂层的微观结构、相组成和电导率将被表征。涂层的寿命将通过循环加热试验来评估，并分析其失效机制。这样，新型加热元件的适用性将得到验证。

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.