A study of Ru–Cr protective coatings for precision glass molding

This study evaluates the performance of Ru–Cr nanolayers used as protective coatings for precision glass molding (PGM) applications. Because the direct growth of Cr-oxide from Cr at high temperatures does not yield a dense oxide scale, Ru is employed as a diffusion barrier to stabilize the growth process. The fabricated coatings were subjected to annealing at 750 °C for different durations, with the coverage of aluminosilicate glass (ASG) pieces. During the annealing process, the outward diffusion of Cr resulted in the formation of thin and stable chromium oxide (Cr2O3) films, which remained unadhered to the glass pieces. Therefore, the Ru–Cr bilayer is a cost-effective coating design to protect mold surfaces in PGM. Detailed analysis using transmission electron microscopy (TEM) revealed that while columnar grains with vertical grain boundaries facilitate the outward diffusion of Cr, a thick Ru layer can effectively hinder Cr diffusion leading to a stable oxide film at a high temperature. A diffusion-oxidation phase-field model quantitatively elucidates oxide film thickness dependence on Ru thickness (>600 nm). Thinner Ru layers (<600 nm) display limited Ru thickness influence and some metallic Cr content in the scales, causing incomplete densification.