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.
Due to the high cooling heat flux in the copper wall, a high thermal gradient and therefore high in-plane stresses as a result of different thermal expansion in the coatings were calculated with FEM-simulations. These stresses are compressive in the heating phase and become tensile after cooling if the compressive stresses relax at high temperatures. Therefore, different failure modes can be expected. To compare these loads to experimental results, the coatings are tested on the one hand isothermally in a furnace to investigate the coatings without any thermal gradient. On the other hand, laser-cycling experiments were carried out to test the coatings with a thermal gradient between top-surface and substrate and to investigate the influence of high heating rates. Depending on the loads and materials, different failure mechanisms like buckling, interface delamination or vertical cracks are observed.