Three-dimensional study of indentation-induced cracks in an amorphous carbon coating on a steel substrate

An amorphous carbon coating, approximately 1.7 μm thick, deposited onto a ductile steel substrate, was indented using a 5-μm radius spherical-tipped diamond indenter to a maximum load of 150 mN. Displacement discontinuities were observed during loading, indicative of crack formation in the system. After indentation, a focused ion beam instrument was used to prepare cross-sections, which revealed the presence of ring, radial, and lateral cracks. A three-dimensional reconstruction of the deformation zone beneath the indent was conducted using a dual ion/electron beam instrument, assisted by a commercial three-dimensional visualization software package. These three-dimensional images enabled a detailed analysis of indentation-induced cracking in this film at submicron resolution. In contrast to traditional understanding and modeling, it was observed that the development of ring cracks was asymmetric for this type of coating/substrate system. Based on this observation, it can be deduced that the strain energy release rate was different for the growth of the spiral ring crack compared with the traditional concentric-ring model. Consequently, the concentric ring-crack fracture model may not be appropriate for the evaluation of the fracture toughness of this type of coating. Hence, three-dimensional images have presented a requirement that the asymmetrical nature of ring and radial cracks should be addressed and carefully considered in the fracture mechanics analysis of this type of hard coating system.