Friction in glass forming: Tribological behaviors of optical glasses and uncoated steel near glass transition temperature

Friction critically influences the geometric precision and optical performance of glass optics during high-temperature forming processes. Heat-resistant steels, particularly high chromium martensitic steel, are frequently employed in these processes under elevated temperatures in open-air environments. This study presents a tribological analysis of the interactions between borosilicate glasses and uncoated steel molds. A high-temperature tribometer, operating on the pin-on-disc principle, was constructed to investigate frictional behaviors at the glass-steel interface under controlled temperature conditions. Two glass compositions were subjected to varying temperatures, sliding velocities, and pressures. The results reveal unique tribological behaviors for both glasses and the uncoated steel mold. Friction coefficients significantly increased with rising temperatures below the glass transition temperature, while exhibiting less variation within the glass transition regime as the glass becomes more viscous. This finding contrasts with previous studies and can be attributed to the combined effects of material strength at elevated temperatures, glass fracturing, and plastic-viscoelastic deformations. Furthermore, the study highlights the significant influence of temperature and pressure on friction coefficients within the glass transition regime, in contrast to the relatively minor impact of sliding velocity. These findings contribute to improved forming accuracy and prolonged mold lifetime in glass molding processes.