The Preparation And Properties Of Less/No Binder Phase Ltrahne WCCemented Carbide

Ultrafine WC-Co cemented carbides meet the ever-increasing demands onhigh-performance carbide cutting tool materials，due to their high hardness, high transverserupture strength(TRS), high wear resistance and toughness at the same time. The low Coultrafine cemented carbides have unique advantages in the field of precision machinery,special wear-resistant materials, tooling and drawing dies, for their higher hardness，redhardness and higher wear resistance than conventional ultrafine carbide. In addition,binderless WC cemented carbides have excellent wear resistance，corrosion resistance，excellent polishing performance and oxidation resistance，so they have broad applicationprospects. In this dissertation, we prepared WC cemented carbides with low Co addition，byhigh energy ball milling and subsequent spark plasma sintering (SPS) or hot press (HP)sintering. The following researches were carried out:(1) High energy ball milling method was used to prepare WC-3%Co, WC-0.8%Co andWC-0.5%Co composite powder, analyzed the effects of high energy ball milling on themorphology and particle size of composite powders. The results showed that after45hours`high energy ball milling，the particles of composite powder became more uniform and fine，and WC grain size decreased; Using carbide milling pot and grinding ball could ensure thepurity of the composite powder after milling.(2) The effects of sintering temperature and sintering pressure of SPS process on themicrostructure and properties of the sintered WC-3%Co samples were researched. Theresults showed that the density of samples increased and WC grains grew up with raising thesintering temperature. With a sintering temperature up to1300℃, the WC-3%Co can besintered with a density of near99.3%of its theoretical density. After1300℃, WC grainsgrew up abnormally. Vickers hardness HV30, TRS and fracture toughness (KIC) increased withthe increase of temperature when it was among1200℃to1300℃. HV30and TRS decreasedwhen the sintering temperature was above1300℃, but little chang in KIC. When the samplewas sintered at1300℃, HV30, TRS and KICwere2257HV30,1906MPa and10.36MPa·m1/2,respectively. When sintered at1300℃, the density of sample increased with raising thesintering pressure, when it was among30MPa to50MPa, and the grain size decreased. Thebest mechanical properties were obtained sintered at1300℃for5min under50MPa. Thehardness, TRS and fracture toughness were2316HV30,2125MPa and11.03MPa·m1/2,respectively. (3) At the same time, the microstructure and properties of WC-3%Co samples preparedby HP sintering process were studied, and both SPS and hot pressing sintering process werecompared. The results showed that the grains of samples prepared by HP grew up fully, andsome abnormal grains were observed. The best mechanical properties were obtained sinteredat1450℃for30min under50MPa. The hardness, TRS and fracture toughness wererespectively2214HV30,1387MPa and10.6MPa·m1/2. Samples prepared by SPS and HP hadthe same density, but higher hardness, TRS and fracture toughness were obtained whenpowders were sintered by SPS. Therefore, because of its unique sintering and densificationmechanism, SPS technology has unique technical advantages in preparation of ultrafine grainand nanocrystalline carbide.(4) The effects of sintering temperature and cobalt addition on the microstructure andproperties of the sintered binderless WC samples were researched. The results showed thatWC grains grew up with raising the sintering temperature. After1900℃, grains began togrow up sharply; when the temperature rised to2000℃, the dimensions of most grains werelarger than10μm, and the large grains were plate-like. With little Co addition, thedensification process was accelerated significantly, when the WC-0.8%Co powders sinteredat1500℃, the sintered body was close to full densification. Besides, discontinuous WCgrain growth took place when added little Co. Below1800℃, Vickers hardness HV10increased with raising the sintering temperature, but decreased after1800℃. When sinteredat1800℃, the Vickers hardness HV10was2662HV10. Effect of temperature on the TRS wassimilar to Vickers hardnes. When sintered at1700℃, the TRS was1026MPa. With raisingthe addition of Co content, the mechanical properties of the sample overall increased, whensinerer at1500℃. The best mechanical properties were obtained sintered at1500℃for5min with0.8%Co addition. The hardness, TRS and fracture toughness were respectively2546HV10,1146MPa and8.01MPa·m1/2.