Large heat flux exposure of metallic coatings for rocket engine applications

Abstract To protect the copper wall of the combustion chamber of regeneratively cooled liquid rocket engines, a coating system may be applied. Due to the high cooling heat flux in the combustion chamber wall, a large temperature difference between the hot coating and the cold substrate leads to in-plane stresses as a result of different thermal expansion. On the hot side of the wall, these stresses are compressive in the heating phase and become tensile after cooling if the compressive stresses relax at high temperatures. To investigate the influence of these loads and to test possible coating systems for the use in rocket engines, laser-cycling experiments were carried out previously. The coatings failed mainly by buckling in these experiments, whereas vertical cracks were observed in coatings in full scale rocket combustion chambers. The present work elucidates this different behaviour with FEM simulations of the laser cycling experiment and of a coated rocket combustion chamber. It was found that the heat flux through the coating as well as the shape of the substrate have a great influence on whether the mechanical loads are tensile or compressive. Based on these results, an improved laser cycling experiment was designed to reproduce the loads in the rocket combustion chamber in small scale experiments.