Influence of Al/(Al+Cr) ratio and doping effects on wear and molten aluminum attack resistance in AlCrN-based PVD coatings for lube-free aluminum die casting

Abstract

AlCrN-type PVD coatings are excellent candidates for die coatings; they reduce or eliminate the need for conventional lubricants in aluminum die casting. A variety of AlCrN-based coatings are commercially available, and an understanding of their differences is needed to ensure optimum performance in the die casting process. In this work, the effects of Al/(Al+Cr) ratio, TiCN-doped layers, surface roughness, and coating defect population were evaluated for five AlCrN-based PVD coatings. Characterization was performed to determine the structure, microstructure, roughness, nature and distribution of surface defects and coating adhesion to the substrate. Also, tests were conducted to evaluate wear, oxidation, and resistance to molten aluminum attack. In the modified molten aluminum test, coatings with higher roughness and higher fractions of microparticles displayed greater adhesion to the solidified aluminum. As-deposited coatings with higher Al/(Al+Cr) ratio provided better protection (little or no adhesion) when in contact with the molten aluminum, but the protection decreased when post deposition polishing was applied to the coating, where some aluminum reaction was observed even without measurable adhesion. Combining the AlCrN coating with alternating nanolayers of TiCN not only provided higher protection to molten aluminum attack compared with the higher Al/(Al+Cr) ratio coatings, but also promoted the lowest coefficient of friction, smallest wear rate, and a similar air oxidation resistance. Thus, for the five coatings examined, the combination of results indicates the multilayer (Al0.34Cr0.11N0.55)0.84/(TiC0.4N0.6)0.16at.% coating is an excellent candidate for industry in-plant trials geared towards moving aluminum die casting towards a lube-free process.