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 al uminum is the sticking of the produced components in the die. Sliding conta ct of solid aluminum particles with the metallic die surface is one origin for sticking. Chromium-based PVD coatings are found to give satisfying solu tions for pressure die casting tools. Their coefficients of thermal expansi on are matched to those of tool steels as well as they form stable oxide sc ales protecting against surface interaction with the molten metal. The pape r presents the effect of oxidation as a concrete means for reducing the sti cking of aluminum on tools surfaces. Therefore, hardcoatings of CrC, CrN an d CrAlN were characterized for their oxidation behavior. The surface energi es 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 investigat ed by in situ XRD measurements. Finally, the technological performance of t he coatings was tested by pin-on-disk investigations. The oxidation was fou nd to have a strong influence on the technical performance. (C) 2000 Elsevi er Science B.V. All rights reserved.