Analysis on multiple ring-like cracks in thin amorphous carbon film on soft substrate under nanoindentation

In the present work, the fracture toughness of thin amorphous carbon films was quantitatively characterized by nanoindentation using a conical indenter. The amorphous carbon films with different thickness, i.e. 140 and 400 nm, were obtained by plasma vapour deposition on a poly-ether-ether-ketone substrate. During indentation, it was noticed that multiple ring-like, through-thickness cracks occurred in the films. Correspondingly, 'sliding pop-ins' were observed in the load-depth curves. Using an energy method, the fracture toughness of the films could be therefore determined. The results showed that the fracture toughness of the amorphous carbon films was in the range of 0.99-2.87 MPa m ^0.5, which agreed well with the published data. Further, finite element analysis was also performed to evaluate the stress distribution in a thin film/soft substrate system, which showed that the ring-like cracks were caused by the tensile radial stress on the film surface.