Oxidation study of Cr–Ru hard coatings

Cr–Ru alloy coatings with Cr content ranging from 47 to 83 at.% were deposited at 400 °C by direct current magnetron co-sputtering with a Ti interlayer on silicon substrates. With a total input power of 300 W, the Cr content in the Cr–Ru coatings increased linearly with the increasing input power of Cr. The intermetallic compound phase Cr2Ru with columnar structure was identified for the as-deposited Cr56Ru44 and Cr65Ru35 coatings, resulting in an increase of hardness up to 15–16 GPa. To evaluate the performance of Cr–Ru coatings as a protective coating on glass molding dies, the annealing treatment was conducted at 600 °C in a 50 ppm O2–N2 atmosphere. The outward diffusion and preferential oxidization of Cr in the Cr–Ru coatings resulted in the variations of the crystalline structure, chemical composition distribution, and surface hardness after annealing. X-ray diffraction and transmission electron microscopy (TEM) proved that an oxide scale consisting of Cr2O3 formed on the free surface. Scanning electron microscopy and TEM observed the surface morphology and structural variation. The chemical composition depth profiles were analyzed by Auger electron microscopy, verifying the presence of a Cr-depleted zone beneath the oxide scale. The hardness of Cr56Ru44 and Cr65Ru35 coatings decreased to 11–12 GPa after annealing, accompanied by the replacement of the Cr2Ru phase by the Ru phase.

► We prepared crystalline Cr–Ru alloy coatings by direct current magnetron sputtering.
► Cr–Ru coatings were annealed at 600 °C for 2 h in a 50 ppm O2–N2 atmosphere.
► Cr diffused outwardly and oxidized to form a stable and protective oxide scale.
► The original columnar grains recrystallized to polycrystalline grains.