Indentation pop-in as a potential characterization of weakening effect in coating/substrate systems

Thin hard coatings are generally used for surface enhancement. However, it was theoretically predicted that thin hard coatings may have a weakening effect of reducing the resistance to onset of plasticity in coating/substrate systems. Such weakening effect may also deteriorate the wear property of a coated system. In this study, we report a direct experimental validation of the weakening effect. Spherical nanoindentations were performed on both tungsten coated and uncoated single-crystalline silicon. The statistical distribution of indentation pop-in load reveals that plastic yielding initiates at a lower load in the coated silicon than in the uncoated one. A cross-sectional transmission electron microscopy (XTEM) observation demonstrates that the plastic deformation area in the coated silicon is larger than that in the uncoated one. These evidences prove the existence of weakening effect. Utilizing the recently reported finite element analysis models, the plastic yielding bearing capacity of coated and uncoated silicon was calculated based on the von Mises criterion. Good qualitative agreement is found between experimental results and the theoretical models.