Adhesive interlayers' effect on the entire structure strength of glass molding tools' Pt–Ir coatings by nano-tests determined

Precision glass molding is a technology for the medium to large scale production of complex optical components with high surface quality and form accuracy. However, the process is only economically viable if a long lifetime of the molding tools can be guaranteed. This can be achieved by using protective coatings on the optical surfaces of the molding tools. The most commonly used coatings for this application are based on noble metals, as they show reduced interaction with the glass during molding. The coatings must have excellent mechanical and chemical properties at high temperatures to withstand the stresses during molding and simultaneously extreme low surface roughness and defect density. The form accuracy of the molding tools is in the sub-μm range and must be maintained even after the coating deposition. Therefore, very thin films of approximately 300nm thickness are used. High film adhesion and strength properties are necessary for preventing surface defects and coating delamination. In the described investigations, platinum (Pt)–iridium (Ir) coatings were deposited directly on cemented carbide samples by Physical Vapour Deposition (PVD) process. Moreover, for improving the adhesion, different materials such as of Ni and Cr were employed as adhesive interlayers at various thicknesses. These interlayers were deposited on the substrate before the Pt–Ir film, during the same PVD process. Appropriate experimental procedures were conducted for characterizing the coatings' mechanical and adhesion properties such as nanoindentations, nano-impact and nano-scratch tests. FEM calculations simulating the films' loadings during nano-impact test explain the effect of the adhesive interlayer on the entire coating substrate structure strength. ► Cr or Ni nano-interlayers on HM glass molding tools improve the film adhesion. ► This enhancement is more significant in the case of a Cr interlayer. ► The effectiveness of the adhesive interlayers depends on their thickness. ► The latter has to be adapted to the substrate roughness Rt.