Fracture analysis of diamond-like carbon films under conical nanoindentation

This article reports fracture in hard films on a soft substrate under conical indentation. A diamond-like carbon (DLC) film was deposited onto a poly-ether-ether-ketone (PEEK) substrate using plasma chemical vapor deposition. Nanoindentation was performed on the film surface, in the meanwhile, load and depth data were recorded, and ‘pop-in’, correlated with crack formation, was found in load-depth curves. Ring cracks and radial cracks in the film were observed by scanning electron microscopy and focused ion beam method after indentation. Finally, finite element analysis using cohesive elements was conducted to study the stress distribution of the hard film/soft substrate. It is found that ring cracks in the thin film are induced by high tensile radial stress on the film surface outside the contact region of the indenter, while radial cracks are caused by high tensile stress on the thin film near the interface. The results also show that the radius of ring cracks increases with the number of ring cracks, which agrees well with experimental observations.