Study On Synthesis Process And Mechanism Of High Quality Coarse-grained Cubic Boron Nitride Single Crystals

The hardness of cubic boron nitride(c-BN)single crystal is second only to that of diamond single crystal.It has good thermal stability and oxidation resistance,and has excellent machinability in refractory materials such as ferrous metals,superalloys and chilled cast iron.The application value of high-quality coarse-grained c-BN single crystal is higher.However,due to incomplete research on the synthesis process and unclear synthesis mechanism,it is still difficult to prepare high-quality coarse-grained c-BN single crystal with a size of≥50 mesh.The method to synthesize c-BN crystal in industry is high temperature and high pressure(HTHP)method.The raw material used is hexagonal boron nitride(h-BN).It is of great significance to study the HTHP reaction mechanism of the interface phase between c-BN single crystal and catalyst layer.In this paper,the HTHP synthesis experiment of c-BN single crystal was systematically carried out on the domestic cubic press.The high-quality coarse-grained c-BN single crystals≥50 mesh were obtained in batches.The mechanism of the synthesis of high quality coarse-grained c-BN single crystal was physically characterized and theoretically calculated based on the Li3N-BN system,which laid a solid application foundation for industrial production.The morphology of c-BN single crystal/catalyst layer was observed by scanning electron microscope(SEM)and atomic force microscope(AFM).The phase structure of the interface of c-BN catalyst layer was characterized by X-ray diffraction(XRD).The morphology and phase of the catalyst layer were analyzed by high resolution transmission electron microscope(HRTEM).The variationon B and N electron structures in the interface of c-BN single crystal/catalyst layer was qualitatively and quantitatively analyzed by Auger electron spectroscopy(AES).The coexistence point of h-BN→c-BN and the phase transition point of Li3BN2 were calculated by first principles,and meanwhile,the surface energy of each phase was calculated.The synthesis mechanism of c-BN single crystal at HTHP was analyzed by combining the physical characterization and theoretical calculation results.Based on the previous process experiments and mechanism research,the specifications of raw materials were further optimized,the catalyst microstructure underHTHP was effectively controlled,and the highquality coarse-grained c-BN single crystal with≥50 mesh was synthesized by the new optimized synthesis process.System comparison experiment results of different catalyst systems show that the conversion rate of c-BN single crystal synthesized by Li3N+h-BN(hexagonal boron nitrid)system is the highest,the size of single crystal particles is the largest,and the crystal shape is relatively best.Therefore,the Li3N+h-BN system is most suitable for the synthesis of highquality coarse-grained c-BN single crystals.The c-BN single crystal synthesized based on the Ca3N2+h-BN system has low yield and conversion rate,finer grain size,and crystal defects on the crystal surface.The c-BN single crystal synthesized by the Mg3N2+h-BN system has the lowest yield and conversion rate,the finest particle size,and imperfect crystal growth.In this paper,the Li3N+h-BN system is selected as the key research object through the optimization and comparative experimental research,and the further optimization researchis carried out for the synthesis process of high quality coarse-grained c-BN single crystal in this system.Through the systematic comprehensive experiment of Li3N+h-BN system,the optimized synthesis process is determined as follows:Use Li3N with a particle size of 140/200 mesh as a catalyst(the addition amount of Li3N is 10wt%),use 100/120 mesh c-BN micropowder as a seed crystal(the addition amount of the seed crystal is 4wt%),and the rest is h-BN;use the HTHP synthesis process curve of slow pressure rise,stable power distribution and 15min heating time(the synthesis power is 4890 W,the synthesis pressure is 95 MPa),the synthesized 30/50 mesh c-BN single crystal accounts for 56.8%.The c-BN single crystal has high crystal integrity,flat crystal face and high crystal quality.The strength test results of the synthesized highquality coarse-grained c-BN single crystal showed that its static compressive strength reached 48 N,its impact toughness reached 49%,and its strength and toughness index exceeded national standards.According to the results of characterization and analysis of quenched c-BN single crystal and its catalyst layer,it can be found that,the exposed surface of high-quality coarse-grained c-BN single crystal is mainly(110)crystal planes,and some single crystal surfaces have defects such as impurities,holes,lamellar and large step structure.These results indicate that there are many ways in the growth of c-BN single crystal,such as two-dimensional nucleation growth and screw dislocation growth.The main phase structure of the catalyst layer is h-BN,c-BN,Li3BN2,no Li3N is found.It is found that there are different BN structures in different catalyst layers,such as h-BN structure with high degree of order,h-BN structure with different degree of defects,disordered BN structure and cubic phase structure observed in this disordered BN structure.Comparing the AES(Auger Electron Spectroscopy)spectra of each layer of the catalyst layer,it is found that there are sp2 and sp3 hybrid Auger electron spectra peaks of B and N atoms in the AES spectra of each catalyst layer.The peak of sp3 hybrid states of B and N atoms in the catalyst layer gradually increases from outside to inside,while the peak strength of sp2 hybrid states of B and N atoms decreases from outside to inside.This indicates that the B and N atoms in the catalyst layer gradually changed from the hexagonal structure of sp2 hybrid state to the cubic structure of sp3 hybrid state from the outside to the inner.The first-principles calculation results of the c-BN single crystal/catalyst layer interface phase show that,the temperature and pressure at the coexistence point of the phase transition of h-BN to c-BN are lower than that of Li3BN2 from the perspective of thermodynamics.This indicates that Li3BN2 can exist stably when h-BN transforms to c-BN.The(110)crystal surface of c-BN has the lowest surface energy,so the exposed surface of high quality coarse-grained c-BN crystal should be(110)surface as the main surface.This is also consistent with the results of morphology observation.The(100)plane of Li3BN2 has the highest surface energy and can be used as the base for the aggregation of micro-elements of other phases in the system.Combining physical characterization and theoretical calculation results,it can provide important experimental and theoretical basis for analyzing the growth mechanism of highquality coarse-grained c-BN single crystals and the theoretical model that Li3BN2 promotes the transformation of h-BN to c-BN under HTHP.The(BN2)3-and Li+in molten Li3BN2 will destroy the van der Waals bond between the layers of h-BN,which will decompose h-BN into smaller BN clusters with low degree of aggregation.Li+can attract the outer electrons corresponding to N atoms,and then transfer to the empty orbit of the outer electron layer of B atoms,so as to realize the electron transfer between B and N atoms,and obtain the cubic c-BN growth unit with sp3 state The h-BN in contact with Li3BN2 is transformed into c-BN growth unit,which are deposited on the surface of the seed crystal to make the crystal grow continuously.The(100)plane of Li3BN2 can assist the accumulation of c-BN growth units and accelerate the accumulation of growth units on the surface of the seed crystal.Based on the analysis of the synthesis mechanism of c-BN single crystal,h-BN with a finer particle size has a larger contact area with the catalyst and is decomposed into BN clusters faster,which is more suitable for the synthesis of high-quality coarse-grained c-BN single crystals.The particle size of raw material h-BN was refined,and the median diameter D50 was adjusted from 7～9 μm to 2～4 μm.According to the further optimized synthesis process,the HTHP synthesis experiment of c-BN single crystal was carried out,and the coarse-grained c-BN single crystal with better strength index was obtained in batch.The static pressure strength of the c-BN single crystal reaches 50 N and the impact toughness reaches 51.5%.The results show that the structure of tubular catalyst in the periphery of c-BN single crystal is basically the same as that in the previous experiment.The effective control of catalyst structure and new optimized synthesis technology are realized,which provides important experimental basis and theoretical basis for the industrial production of high-quality coarse-grained c-BN single crystal.