An experimental methodology for characterizing fracture of hard thin films under cyclic contact loading

Hard thin films are often employed as protective coatings for metal substrates and their fatigue/fracture property (especially that under contact) needs to be sufficiently understood. In this study, we present a combined experimental/computational framework for exploring the fracture characteristics of hard thin films upon both monotonic and cyclic contacts. The techniques of acoustic emission and corrosion potential fluctuation are combined to monitor the mode and initiation of crack, and numerical simulations based on the finite element method provide further information on the criterion of film fracture. For a model system of a TiN film physical vapor deposited onto a stainless steel substrate, ring cracks are produced when the stress in the film exceeds critical - such a critical moment arrives when the substrate undergoes excessive deformation, which makes the curvature of film bending critical just outside the contact zone. Since cyclic contact loading encourages large plastic deformation of substrate due to ratcheting plasticity, it is found that the critical contact force degrades, compared with monotonic loading. The present experimental/computational methodology is useful for obtaining the information of film fracture property under both monotonic and cyclic contact loads.