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

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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.