Study On The Hot Press Sintering Of Nanocomposite WC-Co Powder

WC-Co cermet is a kind of tool materials of good performance. Especially, the ultra-fine WC-Co has both high strength and high hardness. Until now, many methods have been applied in the preparation of nanosized WC-Co powder, but it is difficult to get nanocrystalline WC-Co cermet by sintering. This thesis reports some results on the hot press sintering of nano-composite WC-Co powder, which is prepared by spray pyrogenation & continuous deoxidization and carbonization process.The tests of TG&DSC and electron microscope under high temperature of nano-composite WC-Co powder indicate that the sintering temperature is about 1320 "C. But the further practical sintering work shows that the sintering temperature is 1350℃. The sintering time also affects the specimens' properties. Nano-composite WC-Co powder can be hot press sintered to cermet whose HRA is 94.6 and TRS is 1183 MPa under 1350℃ for 2 hours .Considering the growing of nano-sized powder during sintering and to improve the physical performance, some sintering research about the addition of inhibitor VC has been done. The result indicates that the addition of VC makes the HRA lower and the TRS higher. Especially when 1.2wt %VC is added, the effect of inhibition is most obvious. At this time the crystal size is about 200nm-300nm. And the TRS comes to be 1293 MPa.Analyzing the results of volume density, HRA, TRS, SEM and XRD, the conclusion is that:The sintering mechanism of nano-composite WC-Co powder is liquid phase sintering. And the flow of liquid phase makes the loose powder into bulk materials. The growing process of crystal influences the TRS and the HRA. The relationship of hardness vs the crystal size follows the Hall-Petch experience expressions. The crystal structures of sintered specimens are decided mainly by non-cubic characteristic of WC, the solution of WC in Co and the interface energy between WC and Co. The result of X-ray diffraction indicates that during the sintering no phase changes and no brittle solid-solution appear. From the effect of VC on the WC growing, we can conclude that the solution - deposition process is the main way of WC crystal growing. TRS come higher after adding VC into nano-composite WC-Co powder, just because the finer crystal retards the extension of crack. At the same time, the strength of materials is also decided by the intergrate strength among crystals.