| Cubic boron nitride(for short CBN)is one superhard material developed in the middle of the 1950*s that its hardness is only second to diamond. It has many advantage, such as high hardness, good thermotolerance,high chemical inertia and so on. However its hardness and wearability has strong directionality, which limits its application. In order to make up for the shortfall CBN mono-crystal, polycrystalline cubic boron nitride(for short PCBN)is developed in the 1970's. It not only has such advantages as high hardness and high wear resistance of CBN,but also overcomes the anisotropy of CBN mono-crystal.PCBN is sintered by CBN powder and binder according to a certain percentage under high temperature and pressure conditions.Defined amount of binders are added not only to promote sintering, reduce sintering pressure and temperature but also can improve the performance of sintering body. Therefore, it plays an important role for the synthesis of PCBN.In this paper, TiN,Co and Al are investigated as the binders to sintering PCBN under high pressure and high temperature.The sintering pressure is 5Gpa and the temperature is 1250℃to 1650℃. All the obtained samples are grinded and polished. SEM is used to observe the surface morphology of the sinteringbody, XRD test is used to analyze the sintering mechanism, and hardness test and cutting test are done to observe the wear morphology and to analyse the wear mechanism of the sintered samples, etc.Results show that there is strong relationship between hardness of sinteringbody and sintering temperature. The bonding between binders and CBN are worse at low sintering temperature, the binder phase can"t bond CBN firmly. All the components are mechanically pressed together that the consistency and microhardness of sinteringbody is low. At high temperature, some components in the binder react with CBN that it is conducive to forming good surface between binder and CBN, which will also activate CBN and promote their direct bonding. What is said above will finally improve the hardness of sinteredbody.XRD test results show that new reaction phases such as CoO and A1N are generated at high temperature, which will activate CBN and help to promote their direct bonding. In the meanwhile. A1N can inhibit the negative conversion from CBN to HBN. which will increase the content of CBN in sinteringbody.Wear morphology and wear mechanism of PCBN tools are analyzed by cutting test. Results show that the cutting performance and wear morphology of PCBN tools are different under different sintering conditions. The wear of rake face is mainly in form of crater wear, accompanied by the phenomenon of micro-chipping. The wear of flank face is mainly abrasive wear, diffusion wear and adhesive wear. Different sintering time is equated to different wear trend. Wear amount of flank face with long sintering time is essentially of linear relationship with cutting time. For tool with short sintering time, its wear is large at the beginning of cutting, but it decreases significantly with cutting time increasing. Finally, all the two kind of tools reach standard blunt nearly at the same cutting time.Results show that machined surface roughness in different processing stages are different, and the changes follow certain rules that with the increase of cutting time, roughness firstly decreases and then increases, finally it decreases again. |
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