Coating wear characteristic in heating tests under process conditions of Precision Glass Molding

Coatings in Precision Glass Molding experience severe thermo-chemical and thermo-mechanical loads during several hundred or thousand pressing cycles. Till now, little is known about the integrity loss of protective coatings for these applications. Gaining knowledge of fundamental mechanisms on the functional degradation of protective coatings is of great interest. Only they allow knowledge-based coating developments towards improved service life-time of the molding tools for the benefit of fabrication cost efficiency. Primarily microscopic glass sticking and corrosion phenomena at the coating surface are currently limiting the molds service lifetime. Hence, thermo-chemical mechanisms are of special interest. The work presented here investigates compositional changes at the surface-near region of precious metal PVD-coatings after being annealed in hot glass contact. Different characteristic wear zones identified on the samples have been analyzed by light microscopy, SEM/EDX as well as X-ray Photoelectron Spectroscopy (XPS). The results are compared to the wear stages observed on molds from industrial production. A qualitative model for the degradation of the substrate-coating system is proposed, ranging from the initial phases of material alteration to the final coating breakdown. The initial degradation is governed by diffusion and selective oxidation of material constituents. In particular, increased formation of oxides at the coating surface, but also throughout the coating microstructure, steadily deteriorates the non-stick property of the precious metal coating. As a consequence, strong local corrosive and oxidative attacks down to the interlayer lead to the loss of coating adhesion and the protective layer flakes off. At the final stage, the hard metal itself is attacked and shows characteristic corrosion patterns. In addition, indications that glass components might play a role in the overall degradation process were found, but could not be revealed in detail. With regards to highly precise molding tools for PGM, further work should be focused on the initial phase of wear, since the end of service life time is reached at early degradation stages, far before the coating flake-off.