Interface delamination study of diamond-coated carbide tools considering coating fractures

Interface delimitation is one of the major failure modes of diamond-coated carbide tools in machining. On the other hand, diamond coatings are prone of cracking easily due to its brittleness, which may affect interface delaminations. To study any influence between the two failure modes, micro-scratch testing on diamond-coated carbide tools was conducted and finite element (FE) modeling was developed to simulate the scratching process. In scratch testing, normal and tangential forces as well as acoustic emission signals were recorded to detect coating delaminations and crack initiations. Scratched samples were also observed by optical microscopy to determine the corresponding critical load of delaminations and cracking initiations. In the FE scratch simulation, a cohesive-zone interface and the extended finite element method (XFEM) were applied to investigate delamination and coating fracture behaviors, respectively. The cohesive elements were based on a bilinear traction separation model and XFEM was implemented to model cracking behavior in a diamond coating with a damage criterion of the maximum principal stress. The major findings are summarized as follows. The coating fracture energy has a negligible effect on the critical load for interface delaminations, and similarly, the interface fracture energy has no effect on the critical load for coating cracking, indicating that the two failure modes are mostly uncoupled for the testing range in this study. From the experiments and simulations, it is estimated that the coating fracture energy of the samples tested in this research is in the range of 120 to 140 J/m(2), and the diamond-carbide interface fracture energy is from 77 to 192 J/m(2). Moreover, increasing the coating Young's modulus will increase the critical load for coating delaminations, but decrease the critical load of coating cracking. (C) 2014 Elsevier B.V. All rights reserved.