Development of micro-heating elements based on bilayers PVD coatings and evaluation of their robustness under electro thermal and cavitation erosive load in nucleate boiling

在dfg项目“开发一种微结构方法来抑制PVD涂层中生长缺陷的影响”中，开发的铬基涂层满足了加热元件设计的基本要求。加热器由PVD双层结构构成，用于加热、温度测量以及气泡形成过程的研究，这对于研究核沸腾中的传热过程是必要的[FISH 2011 FISH 2012 Slom 2013a]。利用选择性功能化PVD涂层，将开发微加热器，使国际空间站的核沸腾过程分析和评估成为可能。因此，涂层必须承受电热，空化点蚀和侵蚀载荷。所述涂层所用的基材由透明材料制成，如氟化钡或硫化钡。这些加热器的几何设计必须高度适应国际空间站上的沸腾实验，例如最小的空间使用。氮化铬基层的制造方式，由于其高发射率，使其能够通过红外热像仪测量沸腾区的温度。直接沉积在氟化钡上的基材层需要高电阻。特别是高于铬的顶层。由于这个原因，只有铬的顶层作为加热器。为了保证流体所需的加热温度，涂层必须在所需的温度范围内具有规定的电阻。

In the DFG-project "Development of a microstructural approach to suppress the effect of growth defects in PVD coatings" developed chromium-based coatings meet the basic requirements for the design of a heating element. The heaters are constructed out of a PVD bilayer that is used for heating, temperature measurement and as well for the investigation of bubble formation processes, which are necessary to study heat transfer processes in nucleate boiling [FISH 2011 FISH 2012 Slom 2013a].Using selectively functionalized PVD coatings, micro-heater will be developed, enabling the analysis and evaluation of nucleate boiling processes at the International Space Station. Therefore the coating has to withstand terms of electro-thermal, cavitation pitting and erosive loads. The substrates used for the applied coating were made out of transparent materials such as barium fluoride or barium sulfide. These heaters have to be highly adapted in their geometric design to the boiling experiments on the ISS, for example minimum usage of space. The chromium nitride base layer has to be manufactured in the way that it enables, due to its high emissivity, the temperature measurement in the boiling zone by means of infrared thermography. The base layer which is deposited directly on the barium fluoride requires high electrical resistance. In particular higher than the Chromium top layer. For this reason, only the Chromium top-layer acts as a heater. In order to guarantee the required heating temperature of the fluid, the coatings have to possess defined electrical resistance in the required temperature ranges.