Preparation And Cutting Performance Of High Performance Polycrystalline Cubic Boron Nitride Insert

Polycrystalline cubic boron nitride?PcBN?has high hardness,good thermal conductivity and excellent chemical stability,that is often used as cutting tool materials.It is suitable for cutting quenching steel,ductile iron and high temperature alloys.However,its application is limited by its shortcomings such as brittleness, difficult densification and short service life.In present work,a in-situ reactive-sintering and spark plasma sintering?SPS? have been used to promote the densification of PcBN.Moreover,the in-situ formation of SiC whiskers during sintering process can improve the toughness of PcBN,thus achieving high-performance and low-cost PCBN bulk materials.The ion implantation process was also used to improve the surface characteristics of PcBN cutting tools, further improving the service life of PcBN tools.The main results are as follows:?1?SiO₂ was coated on the surface of cubic boron nitride?cBN?particles by the sol-gel method.Then,SiCw coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1,500°C for 2 h.Then,cBN-SiCw complex powders were sintered at 1450°C for 10 min under a pressure of 5GPa by high pressure and high temperature?HPHT?sintering technology using Al,B and C as sintering additives.It was found that the SiCw and Al₃BC₃?ABC?had been fabricated by in-situ reaction,which cannot only promote densification but also improve mechanical properties of the PcBN composites.The Vickers hardness,fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density.The composite contained 20wt%of SiCw exhibited the highest Vickers hardness and fracture toughness of 42.7±1.9 GPa and 6.52±0.21 MPa·m^1/2,respectively.At the same time,the flexural strength reached 406±21 MPa.?2?cBN particles were coated by SiO₂ using a sol-gel method.Nearly full dense PcBN composites were fabricated by spark plasma sintering at 1650?under a pressure of 40 MPa for 10 min using Al,B and carbon as the sintering additive.The combination of particle activation of SPS,in-situ reaction and the liquid phase formed at low temperature by the Al improves the sintering driving force of cBN,accelerates the material transfer and promotes the densification of cBN.?3?The surface treament of PcBN cutting tools,which have been fabricated by high temperature and high pressure sintering,was carried out by the combination of N ion implantation and Al ion implantation.The service life of PcBN cutting tools at different implantation doses was compared.The results show that the service life of PcBN cutting tools can be improved by 1.3 times when the implantation doses of N ion and Al ion are 9.0×10¹⁷ions/cm².The main mechanism is that on the one hand,ion implantation forms compressive stress on the surface of the blade;on the other hand,it can significantly improve the surface finish and hardness of the PcBN cutting tools.?4?Through the cutting experiments of QT800-2 ductile iron,the cutting performance,wear morphology and wear mechanism of the PcBN cutting tools were studied.The results show that the main wear is mechanical wear in the ductile iron by PcBN cutting tools.Under the same cutting conditions,the cutting force of the ion implantation tool is always less than the tools prepared by HTPH and SPS.With the increase of cutting speed,the three direction force decreased first and then increased.The radial force of the three kinds of cutting tools is quite different.When the cutting speed is 300m/min,the three forces of the three kinds of cutting tools reach the minimum.The surface roughness of the workpiece produced by ion implanted PcBN insert is the best,and the roughness range is between 0.2 and 3.2.With the increase of cutting time,the fluctuation of the roughness curve of the workpiece becomes more and more intense,which indicates that the wear of the tool back surface leads to the increase of the surface roughness.