Review on thin film coatings for precision glass molding

Precision glass molding (PGM) is an advanced glass manufacturing technology employed by the optical industry for the large-scale production of lenses and other precision glass components. One of the key breakthroughs is the use of anti-sticking coatings that extend the tool life and render a molded glass surface polish-free. In the literature, various protective coatings, including noble metals, ceramics, and diamond-like carbon (DLC) coatings, have been attempted. These coatings met with varying degree of success and have finite service life due to various degradation mechanisms. Noble metal coatings, although expensive, are the most preferred owing to their simplicity of fabrication and high operating temperatures. Their degradation starts from the diffusion and segregation of active elements along grain boundaries. Most of the attempted ceramic coatings are deteriorated because the oxidation of metallic elements—that occurs more easily than noble metals—develops an oxide scale that is adhesive to glass. DLC coatings exhibit poor thermal stability at high temperatures (> 600 °C) owing to the strong tendency of graphitization and inadequacy in developing high sp³-content; their application in PGM is limited. The improvement in oxidation resistance and anti-sticking of coatings is a continuous development task, particularly when the PGM technology is to be adapted for glasses with high glass transition temperature. This paper reviews the state of the art of these coatings with a focus on their deposition techniques, anti-sticking, microstructural and mechanical properties, and their degradation mechanisms in glass molding cycles. With these explorations, a perspective on further improving the reliability of PGM coatings is provided.