Cohesive zone modeling of initiation and propagation of multiple cracks in hard thin surface coatings

Surface coatings are increasingly used to improve the tribological performance of advanced products. The novel coating deposition techniques offer numerous possibilities for tailoring surfaces with different materials and structures. The tribological contact of loaded surfaces is, however, a complicated system itself, and further complexity is introduced when functionally graded coating structures are considered or improvement of specific micro- and nanostructural features is pursued. Furthermore, the mechanisms of damage in such a system are from a modeling standpoint highly complex and to great extent remain an active and open field of study. The focus of the current work is in the numerical modeling of graded thin hard coatings on a plastically deforming metallic substrate when loaded by contact that is typically exhibited during a scratch test. A finite element approach is implemented wherein a coating crack initiation and propagation are modeled using cohesive zone formalism. The cracks are considered to initiate and propagate within the coating and also within the coating to substrate interface. The results demonstrate how an optimization of the coating structure can enhance and exceed the performance of simplistic traditional coated systems. The material parameters of the problem and their significance in terms of fracture and failure behavior are discussed. The results are compared to fracture mechanical analyses and experimental information regarding the problem under study.