Correlation between Chemical Glass Components and the Glass Sticking on Sputtered PtIr Physical Vapour Deposition Coatings for Precision Blank Moulding

Abstract

The increasing demand on high quality optical systems with complex geometries, low tolerances and a low installation space necessitates new replicative production systems for complex optical glass elements. The technology precision blank moulding shows promising properties to comply with these demands on an industrial bulk production. Due to the required high surface quality and low surface roughness of produced optical elements, moulding dies must have comparable low roughness and defect-free surfaces. To reduce wear and chemical interaction with the hot glass, moulding dies are often coated with a thin sputtered physical vapour deposition (PVD) coating. The objective of this research work was to analyze the diffusion behaviour inside different industrially used low-Tg (transformation point) glasses and their interaction with three different noble metal coating systems during an application oriented heating test. Therefore, three different PtIr coating systems with different interlayers (50 nm nickel as reference, 20 nm chromium, without interlayer) were deposited and tested in combination with six different industrially used low-Tg glasses. Using energy-dispersive X-ray spectroscopy (EDS) a diffusion of the light alkali and alkaline earth metals (sodium, potassium, calcium) was detected. The interaction between glass and coating was analyzed by EDS, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The different chemical compositions of the glasses have a significant influence on the interaction between glass and coating system. Several correlations between the chemical composition of the glasses and the amount of glass adhesion on the three coating systems were identified. The percentage of ions allocated to network modifiers lithium oxide, sodium oxide and potassium oxide correlates with the intensity of the interaction between coating and glass. The intensity of glass adhesion on the reference coating system PtIr/Ni is related with the zinc content in the glasses. Due to a diffusion process of the nickel interlayer, a direct correlation between the zinc content in the glasses and glass adhesion exists. The coating system with chromium interlayer showed comparable results to the system without interlayer.