Nano-scale investigation of the degradation mechanism of multilayer protective coating for precision glass molding

Precision glass molding (PGM) is widely applied to produce high-quality optical products with complex geometries, where the raw glass is heated until molten and then pressed into the shape predefined by the mold. Due to the harsh molding condition, detrimental processes such as oxidation, corrosion, glass sticking on the mold surface will occur rapidly, strongly limiting the mold's service lifetime. Thus, designing high-performance and high-durability mold as well as developing concept to impede their degradation becomes a top priority for the PGM industry. To achieve this goal, understanding the fundamental degradation mechanisms of mold is very important. In this work, I studied the degradation of the PGM molds, made of cemented tungsten carbide with a 650-nm-thick PtIr surface protective layer and a 20-nm-thick Cr or Ni adhesive layer. Laboratory exposure treatments under well-defined conditions as well as industrial PGM processes were conducted. To reveal the evolution of degradation at the nanometer scale, atom probe tomography (APT) and scanning transmission electron microscopy (STEM) were mainly utilized. The results show that the degradation process was initiated by the outward diffusion of the interlayer (Cr or Ni) through the PtIr layer to the surface, which was facilitated by high diffusivity paths, mostly grain boundaries. Depending on the oxygen content in the atmosphere, Cr and Ni were either selectively oxidized and gradually formed a stable oxide scale or just accumulated on the surface. Consequently, the surface properties of the coating were altered. Internally, extensive interdiffusion between different regions took place, leading to the formation of intermetallic compounds as well as Kirkendall voids, which deteriorate the cohesion of the interface.