Multilayered chromium/chromium nitride coatings for use in pressure die-casting

Chromium nitride coatings are known to give reasonable solutions to the requirements of semisolid forming tools and of pressure die-casting of low-melting-point metals and alloys. These hard coatings have good mechanical behavior when working at high temperatures. They show enhanced hardness and good wear and corrosion resistance, as well as reduced adhesion to the molten or semisolid metal. We have developed a related hard coating based on multilayered stacking of CrN and Cr metal layers with bilayer thickness down to 22 nm. This coating is obtained by both RF magnetron sputtering and reactive cathodic-arc physical vapor deposition (PVD) on hardened tool-steel substrates. Multilayered coatings are characterized with respect to their structure, hardness and adhesion, and compared for performance to standard CrN single-layer coatings. The Cr metal inter-layer and the multilayered film structure improve the adhesion of the coating to the steel substrate by reducing the stress and film brittleness, and by better matching of the thermal expansion coefficients. When the bilayer thickness was reduced, a reduction in residual stress, and an increase in hardness and critical load were observed. In particular, our Cr/CrN multilayers with a bilayer thickness less than 60 nm surpass all CrN single-film properties.