COMPARISON OF WC-VC-Co AND WC-(W,V)C-Co CEMENTED CARBIDES SINTERED IN THE PRESENCE OF NITROGEN

摘要

In order to improve the mechanical properties of WC-Co based cemented carbide, a small amount of VC (about 0.5 wt.%) is added to prevent the formation of very large WC grains. During the analysis of these materials, steps were observed in the WC grains at the WC/Co interfaces. These authors observed small precipitates of a V-rich phase in the steps and they concluded that the steps were caused by the presence of VC in the materials. They suggested that the presence of VC in those steps might be the reason for the reduction of the WC grain size. In the current investigation large amounts (8.1 wt.%) of V were added to produce harder and more wear resistant materials. Two types of materials were produced: one with WC, VC and Co as starting powders (S1) and one with WC, (W,V)C and Co as the starting powders (S2) to produce a WC-(W,V)C-Co composite. In this paper the microstructures of these two samples are compared. The comparison is based on the (W,V)C (cubic carbide) and WC grain sizes. Furthermore preliminary results obtained of the WC/(W,V)C grain boundaries are also shown. The starting materials contained the powders of WC (1.45-1.55 μm), VC (1.2-1.8μm) (W,V)C (1.4 μm), and Co (1.3-1.6 μm). The materials were mixed in such a way that all had a composition of 8.1 wt.% V and 12 wt.% Co with the balance comprising of W and C. The carbon content was carefully monitored to avoid eta phase and graphite formation. The mixture of the starting powders, organic binder and milling liquid was ball-milled, spray dried and then pressed to form green bodies. The sintering of the green bodies was performed at 1410°C for 1 hour in 1000 mbar flowing N_2-gas. These materials were analyzed using back scattered electrons in a scanning electron microscope (SEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectrometry (EDX).