Effect of TaC and NbC addition on the microstructure and hardness in graded cemented carbides: Simulations and experiments

By using the gradient sintering process, a tough surface zone in the cemented carbides is usually created to prevent crack propagation from the hard-coating. The microstructure and hardness distribution in the gradient zone are crucial to crack propagation resistance. In the present work, the influence of TaC and NbC contents on the microstructure and hardness distribution in WC–Ti(C,N)–Co-based graded cemented carbides was chosen as the target and systematically studied via a hybrid approach of the simulations and key experiments. Under the guide of the thermodynamic calculation, the cemented carbides with different TaC and NbC addition have been designed and sintered at 1450 °C for 2 h in an N free atmosphere. The microstructure and concentration distributions were investigated using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Based on the thermodynamic and diffusion databases, the gradient zone formation of the cemented carbides was simulated by DICTRA software. The simulation can reasonably reproduce most of the experimental concentration profiles. Microindentation was used to measure hardness as a function of depth below the surface. On the basis of the simulations and experiments, key microstructural parameters in the hardness model were obtained to predict the hardness distribution in the gradient layer. The relationship among the composition, concentration and hardness distribution can be developed via the present approach.