Analysis on cracking in hard thin films on a soft substrate under Berkovich indentation

• Amorphous carbon films are deposited on a PEEK by plasma deposition method.
• Non-closed picture-frame cracks outside contact region occur during indentation.
• Fracture mechanism of hard films on a soft substrate are studied by 3D FEA.
• Picture-frame cracks are induced by large tensile stress on the film surface.
• Radial cracks occur prior to picture-frame crack.

The present work studied fracture behavior of a thin amorphous carbon film by nano-indentation using a sharp Berkovich indenter. The film was deposited onto a poly-ether-ether-ketone substrate using the plasma deposition method. After indentation tests, both radial cracks and picture-frame cracks were observed. A three-dimensional finite element (FE) model was developed to understand the stress evolution in the thin film under Berkovich indentation. The stress analysis showed that the maximum tensile stress in the film occurs along the indenter edge which contributes to the formation of radial cracks. It was also found that the picture-frame cracks can be developed, depending on the modulus ratio between the film and the substrate. The FE simulation results agree well with experimental observations and shed new insights into fracture mechanics of thin films under Berkovich indentation.