Detaching mechanism for Mo–Ru hard coating on tungsten carbide

Abstract

Hard coating materials were widely applied on top surface of the molding dies to prolong lifetime and enhance the mechanical properties for glass molding mass production. The glass molding process can be recognized as both a cyclic thermal aging treatment and a high temperature oxidation process to the die materials. Thus, to remove damaged coating resulted from production injury is as important as to deposit it on the molding die. In this study, the Mo–Ru hard films selected as the protective coatings with Cr and Ti twin interlayers were deposited on tungsten carbide substrates. Chemical etching process was performed to remove the hard coating for recycling process. The stripping process was also monitored by the in-situ liquid mode atomic force microscopy. It is clarified that the Mo–Ru hard coating was etched to form some pits through surface defects in the beginning, then Cr interlayer was reacted with etchant to become soluble products. The Mo–Ru coating cracked and ejected due to the relaxation of residual stress and accumulation of etch products. On the other hand, the Ti interlayer was still effective to protect the tungsten carbide substrate from the etchant attack.