Oxidation characteristics and surface energy of chromium-based hardcoatings for use in semisolid forming tools

Dies for SSM-forming suffer from multiple combinations of wear mechanisms. The modes of wear are similar to those occurring in tools for pressure die casting. One of the most important problems for semisolid processes with aluminum is the sticking of the produced components in the die. Sliding contact of solid aluminum particles with the metallic die surface is one origin for sticking. Chromium-based PVD coatings are found to give satisfying solutions for pressure die casting tools. Their coefficients of thermal expansion are matched to those of tool steels as well as they form stable oxide scales protecting against surface interaction with the molten metal. The paper presents the effect of oxidation as a concrete means for reducing the sticking of aluminum on tools surfaces. Therefore, hardcoatings of CrC, CrN and CrAlN were characterized for their oxidation behavior. The surface energies of differently oxidized specimens were measured and set into relation to the oxidation temperature. Clear correlations between the oxidation state and the surface energies were found. The course of oxidation was investigated by in situ XRD measurements. Finally, the technological performance of the coatings was tested by pin-on-disk investigations. The oxidation was found to have a strong influence on the technical performance.