Interface Effects on Coating Failure of Diamond Coated Cutting Tools

Despite the superior tribological and mechanical properties, the advantages of diamond coated tools have been largely compromised by the insufficient adhesion. Interface characteristics play a vital role in the failure and performance of diamond coated tools. Thus, quantitative modeling of the coating-substrate interface is important to the design and usage of diamond coating tools. In this study, a cohesive zone model was incorporated to investigate the diamond coating on a tungsten carbide substrate. The cohesive zone model is based on the traction-separation law, and is represented by four parameters, determined from the tungsten-carbide fracture properties. The cohesive zone model was implemented in finite element codes to simulate the indentation process, using a spherical diamond indenter. The model was applied to examine the interface effects during indentation and the coating property effects on different coating failure modes. The simulation results are summarized as follows. (1) Normal mode delamination is the dominant mechanism of interface failure and takes place during unloading if the substrate yielding occurs during the loading stage. (2) The cohesive zone interface does not affect the critical load for coating surface tensile cracking, but affects the plastic strain during loading. In addition, for thin coatings, the maximum stress location changes between the perfect interface and cohesive zone interface cases. (3) Elasticity has a complex effect on coating failure. As the coating elasticity increases, the critical load for coating cracking decreases, but the critical load for substrate yielding increases slightly. Moreover, the interface delamination size will decrease with increasing coating elasticity.