Grain growth, densification and mechanical properties of nanocrystalline tungsten carbide-cobalt

Over two decades, attempts to produce cemented tungsten carbide with nanocrystalline grain structure have been made to obtain dramatically improved mechanical properties to extend the lifetime and robustness of tungsten carbide tools. The attempts have shown that the conventional methods by liquid phase sintering cannot retain nanoscale grain sizes while achieving full densification because significant grain growth of WC occurs during sintering. There have been many works that focused on developing alternative techniques to liquid phase sintering, such as Microwave Sintering (MS), Spark Plasma Sintering (SPS), High Frequency Induction Heated Sintering (HFIHS), and so on. In all of these investigations, densification is accompanied by significant grain growth. The finest average grain size that is achievable until now is still approximately 100-300 nm.;In this research, the challenges of sintering nanocrystalline WC-Co powders were further examined. The key challenge to the production of bulk nanocrystalline cemented tungsten carbide materials is to control the rapid grain growth during the early stage of sintering, especially during heat up stage. In order to understand the mechanisms of grain growth and densification during the early stage of sintering of nanocrystalline WC-Co powders, the sintering behaviors of nanosized WC during the early stages of sintering were studied as a function of temperature and time. The effects of other influencing factors, such as initial grain size, cobalt content, and grain growth inhibitor, were investigated. As a way to make nanocrystalline WC-Co materials, an ultrahigh pressure rapid hot consolidation process (UPRC) was developed. The effects of the UPRC process variables (including heating rate, temperature, holding time, and pressure) on grain growth and densification of the nano powders were studied. Based on the analysis of kinetics of the grain growth and densification and the microstructure evolution during sintering, the possible mechanisms of grain growth and densification were proposed. The mechanical properties of nanocrystalline WC-Co materials consolidated by UPRC were also evaluated and compared to that of samples produced by conventional liquid phase sintering and literature values.