Damage mechanisms of metallic HVOF-coatings for high heat flux application

Abstract

The copper wall of regeneratively cooled liquid fuel rocket-combustion chambers is exposed to high thermomechanical loads. Although it is cooled by liquid hydrogen in internal cooling channels, surface temperatures of more than 800°C on the hot-gas side are reached. To lower this temperature and to protect the copper against oxidation, a metallic coating system is developed. It is applied with high velocity oxygen fuel spray (HVOF) and consists of a bond coat and a top coat. As top-coat materials, two candidates were tested in this study: A nickel-based and a cobalt-rhenium-based alloy. For the bond-coat, a new NiCuCrAl alloy has been developed previously.The purpose of this article is to investigate the failure mechanisms of the new metallic coating systems. For this purpose, two tests are applied. On the one hand, laser-cycling experiments are conducted to test the coatings with a thermal gradient between top-surface and substrate and to investigate the influence of high heating rates. On the other hand, the coatings were tested isothermally in a furnace to investigate the coatings without any thermal gradient. Interface delamination, buckling, formation of vertical cracks and kirkendall porosity are identified as relevant failure mechanisms and the conditions under which they will occur are discussed.