Periodic crack formation in brittle thin film

Abstract

Crack, failure of materials, is recognized as separation pattern of materials. Throughout human history we have struggled to avoid and prevent this phenomenon because crack occurrence inherently means that objects already lost its original integrity and functionality. Moreover microscopic flaws placed in the materials body will easily alter its growth characteristic. So by their catastrophic growth nature, crack patterns are usually chaotic and unpredictable. But Mother Nature always has some exceptions. These periodic or ordered crack patterns are quite distinctive because not only for their rarity but also for unexplained mechanism in most of cases. Here simple but noble generation of ordered crack from thin film system is demonstrated by using conventional semiconductor fabrication techniques and its governing conditions of periodic pattern are discussed. Strong confinement of thin film crack and arbitrary steering of its propagation are realized by inserting thin metallic layer, thin polymeric layer and even atomically thin graphene layer in between substrate and brittle thin film. This thin interlayer-mediated controllability comes from local debonding of brittle medium or effective analogues of debonding due to relatively weak adhesion or elastic softness of interlayer. Following experiments and numerical calculation using Extended Finite Element Method (XFEM) from our study reveal that this wavy crack is intrinsically analogous to so called brittle adhesive crack. Previous studies of periodic cracks such as metallic insertion layer, self debonding sol-gel film, and even drying colloidal film, could be explained by similar mechanism. This finding could provide simple analogy on various ordered periodic crack systems existing in nature. And also this simple but noble method to control crack path could lead to the development of new concept of nano fabrication techniques.