A. Mehner

Diamond Machinable Sol-Gel Silica Based Hybrid Coatings for High Precision Optical Molds

The demand for high precision optical elements as micro lens arrays for displays increases continually. Economic mass production of such optical elements is done by replication with high precision optical molds. A new approach for manufacturing such molds was realized by diamond machinable and wear resistant sol-gel coatings. Crack free silica based hybrid coatings from base catalyzed sols from tetraethylorthosilicate (TEOS: Si(OC2H5)4) and methyltriethoxysilane (MTES: Si(CH3)(OC2H5)3) precursors were deposited onto pre-machined steel molds by spin coating process followed by a heat treatment at temperatures up to 800°C. Crack-free multilayer coatings with a total thickness of up to 18 µm were achieved. Micro-machining of these coatings was accomplished by high precision fly cutting with diamond tools. Molds with micro-structured coatings were successfully tested for injection molding of PMMA optical components. The wear resistance of the coatings was successfully tested by injection molding of 1000 PMMA lenses. Hardness and elastic modulus of the coatings were measured by nano indentation. The chemical composition was measured by X-ray photo electron spectroscopy (XPS) as a function of the sol-gel processing parameters.

Deposition, Machining and Measuring of Novel Hard Coatings

Optical molds for precision glass pressing are frequently coated to improve their wear and oxidation resistance, and to reduce sticking of the hot glass to the mold surface during the glass pressing process. Currently PVD (Ti,Al)N, CrN, ZrN and PtIr coatings are used as protective coatings for these molds. Novel nano crystalline PVD Ti-Ni-N and Ti-Cu-N coatings and thin sol-gel ZrO2 coatings were developed and tested as new protective coatings for glass pressing molds. Optical molds for injection molding of plastic lenses are frequently coated with thick diamond machinable electroless nickel coatings since steel cannot be machined with diamond tools. These nickel coatings have several drawbacks such as residual porosity, limited hardness and temperature resistance. Therefore, thick sol-gel silica based coatings were developed and tested as an alternative for the electroless nickel coatings.