Fundamentals of sintering nanoscaled binderless hardmetals
By reducing the metallic binder in cemented carbides, so-called binderless hardmetals or pure tungsten carbide ceramics can ultimately be achieved. Nanoscaled tungsten carbide has a very high hardness of above 3000 HV1 and still offers quite a good fracture toughness of up to 7.5 MPa * m1/2. Since only solid state sintering can be used, sintering temperatures as high as 2000 °C or higher are normally needed to completely densify these materials. At these temperatures abnormal grain growth which leads to a decrease in hardness is an often reported problem. However, using pressure assisted techniques such as SPS, ROC or HIP, WC ceramics can be produced at lower temperatures, although productivity and freedom in size and form are limited. The aim of this work was to investigate the drivers for densification, grain growth as well as the microstructure and chemical phases occurring during solid state sintering in a conventional SinterHIP furnace. A WC starting powder with DB ET = 115 nm was milled to nanoscaled size (50 to 60 nm), sintered at different temperatures (interrupted sintering experiments) and then analysed by using FESEM, XRD, Rietveld analysis, dilatometry and other methods. It could be shown that three main processes during sintering of slightly under-stoichiometric binderless WC hardmetals occur: first, the reduction of surface oxides at 800 °C after which densification starts due to cleaned surfaces and the existence of W in a nascent state; second, the secondary carburisation of WC and W at around 1400 °C; and, third, grain growth and pore annealing at around 1600 °C when a closed porosity exists. By adding Cr3C2 as grain growth inhibitor the secondary carburisation step was lowered to around 1000 °C and a solid solution of (W,Cr)2 instead of pure W2C was formed.