Investigation of the Interfacial Reaction between Optical Glasses and Various Protective Films and Mold Materials

Abstract

The glass molding process (GMP) is regarded as a very promising technique for mass producing high precision optical components such as spherical/ aspheric glass lenses and free-form optics. However, only a handful of materials can sustain the chemical reaction, mechanical stress and temperature involved in the glass molding process. Besides, almost all of these mold materials are classified as hard-to-machine materials. This makes the machining of these materials to sub-micrometer form accuracy and nanometer surface finish a rather tough and expensive task. As a result, making mold life longer has become extremely critical in the GMP industry. The interfacial chemical reaction between optical glass and mold is normally the main reason for pre-matured mold failure. This research aimed to investigate the interfacial chemical reaction between various optical glasses, different anti-stick coating designs and several mold materials. The results showed that glass composition, coating design (composition, microstructure, thickness), environment (vacuum, air or in protective gas), reaction temperature and time could all have profound effects on the interfacial chemical reaction. Based on the results, a design developed specially for certain glasses is more likely to be the viable way of optimizing the effect of the protective coating.

Anti-sticking Re-Ir coating for glass molding process

“…6). In general, the glass loses its transparency and turns into translucent as the reaction progresses [8]. The glass on monolayer Re coating is translucent, while the glasses on monolayer Ir and multilayer Re/Ir coating stay transparent.…”

“…However, without a protective coating, those materials are also not suitable for the application of GMP due to the interfacial reactions. In order to reduce the interfacial reactions, various coatings have been employed to protect mold, such as hard carbon thin films [7][8][9][10][11][12][13] and precious metal alloy films [8,[14][15][16], including platinum-iridium (Pt-Ir), rhenium-iridium (Re-Ir) and so on. Previous studies [15] pointed out that a specially developed design for particular glasses was the most effective way to optimize the performance of protective coating.…”

Model of coating wear degradation in precision glass molding

“…In comparison to other industrial machines, e.g., the Toshiba GMP 106, this unit has only a minimal pressure of 40 mbar (Toshiba GMP 106: < 0.6 · 10 −2 mbar [6]) and lower lateral axis precision (industrial requirements: <1 μm form deviation of the final lens from specifications). The partial pressure of oxygen in the process atmosphere has major influence on the wear evolution [4]. However, the choice of a lower vacuum quality during initial pumpdown is reasonable for achieving time-efficient results.…”

“…However, the choice of a lower vacuum quality during initial pumpdown is reasonable for achieving time-efficient results. Since the service life time of molds can be several hundred and thousands of pressing cycles, working under low pressure instead of high vacuum is a widely used method to decrease the time for preparing worn-out samples which still correlate with pressing tests, especially with heating tests [4,5]. Coated CTN01L Following the heating cycles, photos of the sample-glass contact surface were taken with a Canon EOS 550D (Sigma DG Macro 1:2.8 50 mm).…”

“…One reason for this is the complexity of the respective materials in contact, e.g., hot glass melt with the nanocrystalline coating, another is the need for advanced measurement techniques resolving nanoscopic or microscopic alterations of the materials microstructure and composition. The correlation between surface reactants and glass adhesion has been reported before, e.g., by Chao et al [4]. The objective of this work is to explore the coating degradation of a PtIr coating with Ni adhesive layer in detail and summarize findings in a model.…”