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 during this stage of sintering the known grain growth inhibitors, mostly metal carbides like Cr3C2 or VC can be used. To investigate grain growth inhibition and dissolution of these carbides both the solid state sintering without Co (binderless tungsten carbide) and with 10 wt-% Co were investigated in the temperature range between 600 degrees C and 2000 degrees C and 1500 degrees C, respectively. It could be shown that e.g. the Cr3C2 starts to decompose already below 800 degrees C into a Cr-rich and C-rich phase. In both types of material 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 thermoanalytical methods as well as interrupted sintering experiments and their characterisation by FE-SEM and XRD, the direct influence of Co and grain growth inhibitors regarding the change in the chemical processes happening during sintering was studied.