Study On Preparation And Properties Of The Polycrystalline Cubic Boron Nitride

Polycrystalline cubic boron nitride（PcBN）tools materials have the advantages of high hardness,good wear resistance,good thermal conductivity,excellent stability under high temperature and good chemical stability.PcBN tools materials are widely used in cutting hardened steel,gray cast iron,titanium alloys,high temperature alloys and other hard processing materials.To produce high performance PCBN tool materials,the high energy ball milling process was used to refine the cubic boron nitride（cBN）grains.Next,the organic polysilazane（PSN）and the metal aluminum were used as composite bond.Dense PcBN bulk materials have been prepared by high-temperature and high-pressure methods and spark plasma sintering（SPS）,respectively.The amount of impurities introduced by XRD analysis,the particle size of the cBN tested by the laser particle size analyzer,the oxygen content of the cBN tested by the oxygen-nitrogen-hydrogen analyzer,and the microstructure of the cBN particles observed by SEM and TEM.The effects of different highenergy ball milling time on the cBN particle size,impurity introduction and oxygen content have been analysized.The thermal stability of PcBN was analyzed by TG-DSC,the phase of PcBN was analyzed by XRD,the microstructure of PcBN was observed by SEM,and the mechanical properties of PcBN such as bending strength,density,and hardness were also tested and analyzed.The effects of PSN and PSN+Al composite bond on the properties of PCBN materials which have been sintered by high-temperature and high-pressure sintering were investigated.The results are shown as followers:（1）High energy ball milling is an effective method for refining cBN grains.With the milling time of high energy ball increasing,the particle size of cBN gradually decreased.Meanwhile,WC impurity and the oxygen content of cBN powder gradually increased.After 120 minutes of high energy ball milling process,the cBN particles with the original sizes of 1-2μm and 3-6μm were refined to approximately303 nm and 680 nm,respectively.（2）When the amorphous SiCN formed by pyrolysis of PSN was sintered under high-temperature and high-pressure,nano SiC and Si₃N₄ can be formed directly on the surface of cBN particles.Such nanocrystals can effectively improve the strength of PcBN.In addition,AlN was generated around the cBN particles when PSN+Al composite bond was sintered by high-temperature and high-pressure.The formation of AlN can not only improve the densification of PcBN,but also promote the formation of SiC and Si₃N₄ nanocrystals.Therefore,the PcBN prepared from the PSN+Al composite bond has higher density,hardness and bending strength than the PcBN prepared using the single PSN bond.PcBN with 60wt.%cBN+30wt.%PSN+10wt.%Al,which has been sintered by high-temperature and high-pressure at1450℃,has a maximum density of 98.4%,maximum hardness of 23.00±0.75 GPa and maximum bending strength of 398.33±12.11 MPa.（3）The cBN particles refined by high energy ball milling can promote the nucleation and growth of SiC and Si₃N₄ nanocrystals.When cBN particles milled by high energy ball for 120 min were used as raw materials and PSN+Al were used as composite bond to prepare PcBN materials,the relative density of PcBN reached99.7%.Its hardness and bending strength are also significantly higher than those of PcBN without high energy ball milling process,and the values are 25.17±0.80 GPa and 602.50±12.08 MPa,respectively.（4）Dense PcBN materials can be successfully prepared by Spark Plasma Sintering（SPS）at 1600℃ under a pressure of 30 MPa.SPS can promote the crystallization process of amorphous SiCN and generate nanoSi C whiskers on the surface of cBN particles.However,compared with the PcBNsintered by high temperature and high pressure,the hardness and bending strength of SPS-sintered PcBN were significantly lower due to the formation of hBN phase.The rapid sintering technology of spark plasma sintering has opened a new window for the preparation of PcBN materials,and controlling the transformation of cBN to hBN is a direction for future studies.