Model of coating wear degradation in precision glass molding

Coatings in precision glass molding (PGM) experience severe thermo-chemical and thermo-mechanical loads during several hundred or thousand pressing cycles. Until 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. The work presented in this paper investigates compositional changes in 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 used in 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. 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