Preparation And Investigation Of Cemented Carbides With Cobalt-base Composite Binder Phase

The performances of cemented carbide are mainly improved by two methods.One is to adjust type and grain size of the hard phase,and the other is to continuously explore and optimize the binder phase.In recent years,the cemented carbide with new binder phase has attracted more and more attentions,and it has become a research hotspot of cemented carbide.In this study,several cemented carbides with cobalt-base composite binder phase were prepared by spark plasma sintering.Then their microstructure and mechanical properties were studied in detail through X-ray diffraction,TEM,SEM,optical hardness tester and universal tensile tester.The main contents are as follows:(1)WC-10(Co85-V-Ta)cemented carbides were sintered at different temperatures.It is found that the WC grains continue to grow up when increasing the sintering temperature,and the grains are prism or triangle.When sintered at 1200℃,Vickers hardness of the alloy reaches to 2079 kg/mm2,and then the hardness decreases with the increase of sintering temperature.The fracture toughness increases when increasing the sintering temperature,and it reaches to 11.12 MPa·m1/2 when sintered at 1350℃.The density achieves the maximum value of 14.18 g/cm3 at 1300℃.The variation trend of transverse rupture strength is similar to that of density,in which it reaches to the maximum value of 1351 MPa with the sintering temperature of 1300℃.Abnormal organizations of rich-V and rich-Ta between WC and binder phase were observed in the present studied cemented carbides.Two reasons may lead to that V and Ta migrate to WC side.One is the existences of a certain stress and defects,as well as large surface energy in the mixed powders after ball milling.The other is that V and Ta have higher reactivity at high temperature and they are more likely to spread into WC matrix.(2)WC-x(Co85.28-V8.88-Ti5.84)(x=5,8,10,15)cemented carbides were sintered at 1200℃.The study finds that the Vickers hardness gradually decreases with the increase of binder phase content.WC-5(Co85.28-V8.88-Ti5.84)specimen has the highest hardness up to 2204 kg/mm2.However,the fracture toughness and transverse rupture strength increase with the increase of binder phase content.The fracture toughness and transverse rupture strength of WC-15(Co85.28-V8.88-Ti5.84)specimen are 11.43MPa·m1/2 and 1455 MPa,respectively.Fracture mechanism of the alloys is mainly dominated by intergranular fracture.The diffusion and reaction between mixed powders are relatively violently during the sintering process,and some regions may exist excessive amounts of V/Ti,and meanwhile be lack of Co.Thus the brittle WC grains can not bond well in these regions.(3)WC-8(Co75.2i-Al3.71-W21.08)cemented carbide was sintered at 1200 ℃.The study finds that the formation of Co3 W3C is mainly due to excessive amounts of W in the binder phase of the alloy.The formation of Co3 W3C makes the mass fraction of Co in binder phase decrease,which results in voids and uneven distribution of the binder phase.The Vickers hardness,fracture toughness and transverse rupture strength of the alloy are 1942 kg/mm2,9.23 MPa·m1/2 and 14.33 g/cm3,respectively.