Sintering of binderless tungsten carbide

The most widely used mold materials for optical glass molding processes are cemented tungsten carbide and silicon carbide. In this research, tungsten carbide with minor addition of TiC and TiN as the second phase has been studied. The powders were ball-milled and pre-formed under a temperature of 200 °C and a pressure of 130 MPa. The specimens were sintered in a graphite lined furnace at a temperature of 1600 °C. A density of 15.43 g/cm3, a Vickers hardness number of 23.14 GPa, and a fracture toughness of 6.56 MPa m1/2 was found for the sintered specimen fabricated by this process. The result of X-ray analysis revealed no trace of precipitated graphite during sintering, nor the brittle eta-phase as a result of decarburization. Through scanning electron microscopy, spherical air bubbles have been found to precipitate inside the grains, because the activation energy for grain-boundary diffusion is lower than that of the air inside the grains. Therefore it is advisable that the pre-form process is carried out in vacuum.