Annealing and oxidation study of Mo–Ru hard coatings on tungsten carbide

As a protective hard coating on glass molding dies, Mo–Ru coatings were fabricated on binderless tungsten carbide substrates by RF magnetron sputtering. The Mo–Ru deposits were deposited at 550 °C, which revealed a columnar structure with a major axis perpendicular to the substrate. To evaluate the performance of hard coating in a realistic glass molding environment, the annealing effect was studied by heating the samples at 600 °C under the atmosphere constructed with a steady flow of nitrogen and controlled oxygen content of 12 ppm. The oxygen was absorbed into the Mo–Ru deposits, which affected the phase stability and mechanical properties. The Auger electron spectroscopy depth profiles certified the difference of oxidation resistance between Mo-rich and Ru-rich deposits. The X-ray photoelectron spectroscopy proved the valence variation of Mo in the near surface, accompanied by the introduction of oxygen ions. A scale of MoO3 and a Mo-depleted transition zone near the surface were pointed out by the transmission electron microscopy investigation on the Mo-rich deposit. On the other hand, the Ru-rich deposits absorbed oxygen in a less content and behaved higher hardness after annealing in the glass molding atmosphere, and became an appropriate protective coating for the die material.