The chemical inertness of Ir–Re and Ta–Ru coatings in molding B2O3–ZnO–La2O3-based glass

• Ir0.77Re0.23 and Ta0.22Ru0.78 coatings applied for molding glass were evaluated.
• The failure mode of the compressive Ir0.77Re0.23 coatings was chemical attacking.
• The Zn outward diffusion from the glass material degraded the coating performance.
• Mechanical interlocking melded the tensile Ta0.22Ru0.78 coatings and glass material.

B2O3–ZnO–La2O3-based glasses with a high refractive index are widely used as optical glass materials for lenses. To evaluate the feasibility of applying protective coatings on glass molding dies, Ir–Re and Ta–Ru coatings were sputtered on cemented tungsten carbide or silicon substrates. B2O3–ZnO–La2O3-based glasses were placed on the surface of the coatings and heat-treated in a glass molding atmosphere of 15-ppm O2–N2 at a molding temperature of 600 °C. Up to 1000 thermal cycling annealing tests were performed at 270 °C and 600 °C. The holding time at the molding temperature was set at 1 min for each cycle. The variations in chemical inertness, residual stress, and coating characteristics after annealing were investigated.