Grain Growth Inhibition of Hardmetals During Initial Heat-Up

Ultrafine and nanoscaled hardmetals show a significant grain growth already before reaching liquid phase sintering temperature. To limit grain growth already during this stage of sintering known grain growth, mostly metal carbides like Cr3C2 or VC can be used. To investigate the grain growth inhibition and the dissolution of the carbides both the solid state sintering without Co (binderless tungsten carbide) and with 10 wt-% Co were investigated in the temperature range between 600 °C to 2000 °C and 1500 °C respectively. It could be shown that the i.e. Cr3C2 starts to dissolves already below 800 °C into a Cr-rich and C-rich phase. With and without Co this leads to earlier reduction of W-oxides and the formation of Cr-based oxides which alter the chemical processes during sintering and change the grain growth behaviour. By using thermoanalytic methodes as well as interrupted sintering experiments and the characterisation by FE-SEM, XRD and rietveld refinement the direct relation of C, Co and grain growth inhibitor content regarding the change in the chemical processes occuring during sintering were studied.