An Improved Approach to Fracture Toughness Assessment of Brittle Coating on Ductile Substrate Systems under Indentation

Fracture toughness is an important material property that determines the structural integrity of a component with pre-existing or service-generated flaws. In the present research, an indentation-based method and the associated fracture mechanics model are proposed for fracture toughness assessment of brittle coating/ductile substrate systems.;A series of indentation tests are conducted on a WC/10Co/4Cr coating/1080 low carbon steel substrate specimen, which is a brittle coating on a ductile substrate system, in order to obtain the required information for validating the proposed fracture toughness model. It is found that radial/median cracks are generated beneath the indenter within the coating layer in an elliptical shape and stopped at the interface under the indentation loads being used in the present experiments. Optical microscopy is employed to examine the surface (radial) crack lengths and the focused ion beam (FIB) technique is applied to dissecting the coating/substrate specimen and thereby allows for measuring the crack penetration depth; thus the full crack profile is determined. These tests have also verified the non-linear relationship between c3/2 (where c is a radial crack length) and the applied indentation load P, which is exhibited in the proposed fracture toughness model.;The fracture toughness of the tested WC/10Co/4Cr coating/1080 steel substrate system is determined in terms of the proposed fracture toughness model, utilizing the experimental results. The fracture toughness value for this coating/substrate system is then compared to the known fracture toughness value of a similar brittle coating/ductile substrate system (WC/12Co coating/1020 low carbon steel substrate) and is found to be comparable. The proposed fracture toughness model best describes the experimental observation of cracking/fracture behavior of brittle coating/ductile substrate systems under indentation, compared with other existing models, which can be utilized to assess the fracture toughness of these coating/substrate systems. The developed approach makes a significant improvement in the existing fracture mechanics methods for fracture toughness assessment of brittle coating/ductile substrate systems.;The proposed models consider well-developed radial/median cracks generated under sharp indentation, despite that the crack formation process may have gone through crack initiation and propagation phases. For generality, the geometry of a well-developed crack is assumed to be semi-elliptical in shape. The driving force of the crack is considered to stem from the residual plastic zone expansion under the indenter, as well as the far-field Boussinesq (elastic) stress. Three well-defined configurations are studied. For the first configuration, a crack with a depth of less than 7% of the coating thickness is considered. In this case, the problem is treated as the one for the monolithic material with the coating material properties. For the second configuration, a crack that runs deeper than 7% of the coating thickness but is still within the coating layer is analyzed. In this case, the composite hardness is introduced into the analysis to account for the influence of the substrate material properties; and furthermore, an interface correction factor is proposed to take into account the presence of the coating/substrate interface and its influence on the stress intensity factor of the well-developed elliptical cracks. For the third configuration, a crack penetrating into the substrate is considered. In this case, based on the condition of deformation compatibility across the coating/substrate interface, the bulk modulus for the coating/substrate system is introduced into the analysis.