Thickness-dependent fracture behaviour of amorphous carbon films on a PEEK substrate under nanoindentation

The present work studied the fracture behaviour of a brittle film/ductile substrate under a sphero-conical indenter. To understand the effect of film thickness, the amorphous carbon films were prepared with different thicknesses, i.e. 140 nm, 400 nm and 1300 nm, on a poly-ether-ether-ketone substrate. It was found that multiple ring-like cracks occur for all the films under nanoindentations. However, film spalling between ring cracks was noticed for the film with 1300 nm thickness; whereas spalling was observed at the center of indent for the thin film with the thickness of 140 nm. Finite element analysis revealed that different crack patterns were resulted from different stress distributions in the film-substrate systems, depending on the film thickness. In particular, when the film is thick the cracks were mostly caused by the contact stress induced by the indenter. For a thin film, however, the ring cracks could be formed due to the bending stress caused by the plastic zone in the substrate. Moreover, a dimensional analysis was preformed to quantitatively determine the ratio of bending stress and contact stress in different brittle film/ductile substrate system. The new findings of the thickness effect are important for fracture analysis of brittle films using nanoindentations.