The Synthesis Of PCBN Under High Temperature-high Pressure

Diamond as the hardest material in nature is widely used in various fields,such as machinery,building and precision machining.But diamond does not have enough thermal stability and is easily oxidized in great heat.In addition,diamond has affinity with the ferrous metals,so it can only be used for the cutting of non-ferrous metals and non-metallic materials.The cubic boron nitride(CBN)was firstly synthesized with the high-temperature and high-pressure apparatus by the American GE Company in 1957.CBN rank second only to diamond as the hardest material and it is widely used in many fields.Due to the characteristics of high chemic inertia, the high thermal stability and affinity with the ferrous metal of CBN,it can be used for the cutting of ferrous metals.Like the diamond,CBN is also not all-purpose.For example,the single crystal CBN is too Small to cut.The advent to PCBN changed the circumstances.PCBN does not only have high rigidity but also has high abrasive resistance.Above all,it is also compatible for the cutting of ferrous metals.PCBN is the CBN polymer-caking agent and CBN powder,the main row material,which is synthesized under high-temperature and high-pressure.The rigidity of PCBN mostly comes from CBN.Next,the characteristics of CBN will be introduced as follow:The structure of CBN is blende which is as same as the diamond.The lattice constants of the CBN and the diamond are similar too.There is a B atom at the center of the N atom's regular tetrahedron.And also,there is a N atom at the center of the B atom's regular tetrahedron.The B atom and the N atom form the covalent bond at the hybridized orbital.The picture 1 shows the structure of CBN.CBN has a high rigidity as a sort of super hard material.The rigidity of the diamond is 43,but the CBN is 26.Though the rigidity of CBN is less than the diamond,compare with the ceramic and the cemented carbide,CBN is worthy the name of super hare material.At the same time,CBN has better cleavability than the diamond.It make CBN has wider range of applications than the diamond.CBN has high heat conductivity,so the quantity of heat can leave from the cutting-tool rapidly and avoided the high temperature.In the meantime,CBN has high thermal stability.It's one of the characteristics of CBN which is better than the diamond.In the air and the oxygen,when the temperature rises to 1570K,CBN can create B₂O₃,and stop the oxygenation.The B atom and the N atom in the CBN crystal can form saturation ova lent bond,which can help CBN to resist oxidation.CBN can support the temperature of 1300～1500℃,so it has good performance in cutting.PCBN is a sort of polymer which is made under high pressure and high temperature.Compare with the single crystal CBN,PCBN has the following characteristics:1.the grain form disorderly,no cleavage surface,the rigidity is uniform 2. can be directly processed into specific shapes required.3.can compose PCBN which has different characteristics in order to fit different purpose.The main performance of PCBN is high rigidity,resistance,thermo stability,heat conductivity,chemical stability and low friction factor.The technique we use to make PCBN is that once agglutinate:at HPHT make the CBN and the binder together.Stick the CBN the binder with the WC.There are three kinds of binder:the metallic binder,the metallic ceramic binder and the nonmetallic binder. Co,Ni,Ti,Fe,Al,Cr,Cu are the normal metallic binders.The PCBN which used the metallic binder has high tenacity,and good resistance to impact.But it will be soften in great heat.The ceramic binder can solve this problem,but it has no high resistance or good resistance to impact at room temperature.The factors can affect the quality of PCBN:1.The content of CBN.2.The size of CBN grain.3.The temperature and the time for composing.We made PCBN at the SPD 6^*600MN six faces pressure machine made in China.The temperature and the pressure has been demarcated.First of all,We chose the Ni as the binder.Mix the Ni with two granularity of CBN(200m 400m).There are three mass ratios for CBN(40%, 60%,80%).Fixed the pressure to 5.3Gpa,the temperature ranged 1200～1400℃,the time ranged 3-15 minutes.All of the above,the samples' surface topography are in good condition in good condition.There were no phenomenons of losing parts or delamination. But the rigidity and the abrasion performance of the samples were not good enough.In order to improve the rigidity of PCBN,we increased content of CBN to 90%.1.When the content of Ni declined to 10%,most of the samples appeared the phenomenon of losing parts or delamination.Only a few samples' surface topography was in good condition. 2.The samples made of 200m CBN performed serious phenomenon of losing parts or delamination.The samples made of 400m CBN performed better.3.When the time was more than 14 minutes,the surface topography of samples of 400m CBN are in good condition.But the same samples which were made in 10 minutes were not so good.4.Though some samples(from 400m CBN)made in a long time not at a high temperature also showed the phenomenon of delamination.The sample made in great heat but in the short time seemed better than the former one.After that we chose 270m CBN as the raw material to compare with the 200m CBN.The result showed that these samples' surface topography were better than the 200m CBN samples,but worse than the 400m CBN samples.At last we chose Al as the binder to make PCBN.The content of CBN were 80%and 90%,the pressure fixed to 5.3Gpa,temperature ranged 1200～1400℃,the time ranged 3-15 minutes.But all of the samples manifested the phenomenon of delamination.Maybe it's because the expansivity between Al and WC was different,or the temperature was not so high to product the mesophase.We selected the following samples to analysis XRD. According to the XRD,only the sample5 and the sample6 appeared the diffraction peak of CBN and Ni.But the other four samples appeared new diffraction peak.Maybe it's because of the metastable phase at HPHT.We can't sure whether the new metastable phase caused the losing parts and the delamination.Under the SEM,we know that the diffusion of Ni uniformer in the samples made of 400m CBN was better than the 270m CBN.And the granule of CBN was not obvious in the sample which was made in 15 minutes.The Ni has diffused in the CBN.In the experiment of energy spectrum,we find that there were some Co at the surface of different samples and the longer the time is,the more Co oozed. That may be the reason for the perfect surface of the sample made for a long time.