Analysis on spiral crack in thick diamond-like carbon film subjected to spherical contact loading

We study the detailed mechanism of formation and propagation of spiral cracks upon spherical indentation on a thick diamond-like carbon (DLC) film deposited on a ductile steel substrate. Unlike the conventional ring cracks appeared just outside the impression, the non-axisymmetric spiral cracks are formed within the crater. Integrated acoustic emission (AE) and corrosion potential fluctuation (CPF) techniques are applied to monitor the crack process in situ. It is found that spiral cracking is possible only within a relatively narrow range of maximum indentation force, below which there is no fracture and above which the ring cracks are formed. Detailed stress analyses show that the spiral cracks are nucleated below the indenter when the equivalent stress is above a critical level, which then extends during unloading. The proposed theory agrees well with experimental observations and it is expected such mechanism can be extended to similar brittle film/ductile substrate systems.