The Research Of Bulk Particle Cubic Boron Nitride Single Crystal Synthesis Process

With the high hardness, good thermal stability, good chemical inertness and high oxidation resistance, the cubic boron nitride (cBN) has been rapidly developed and applied since the first synthesis. As one of the most important applied materials in the 21th century, it is recognized to bring the new revolution for the grinding and cutting technology. At present although our country has became a major production country, compared with the technological country like Britain and the United States, the cBN synthesis technology is still backward, which only can product low-grade cBN and the high-grade cBN abrasive mainly rely on import. The research for the synthesis process of bulk particle cBN single crystal is conducive to the application and development of cBN material in abrasive industry and other fields, which can improve the competence of the cBN material synthesis technology in our country.In this paper, the cBN single crystals are synthesized with the hexagonal boron nitride (hBN) as raw material, and Li3N, Ca3N2 as the catalysts respectively under the condition of high temperature and high pressure (HPHT), and the optimal process conditions is obtained by changing the parameters of pressure, power and time in the synthesis process. And referring to the method of diamond synthesis process, the bulk particle cBN single crystals are synthesized by adding seed crystals. The main results of this paper are as follows:(1) In the experiment of synthesis cBN single crystal with catalyst method under HTHP: the optimized process curve is obtained by comparing the effect of synthesizing cBN single crystal in different synthesis process conditions, which is the constant power-slow elevating pressure synthesis process curve.(2) In the system of Li3N-hBN, with the synthesis process of constant power-slow elevating pressure, the optimal process parameters of synthesizing cBN single crystal is the synthesis pressure of 5.0 GPa, synthesis power of 5000 W, synthesis time of 15 minutes. In this process, the yield of synthesizing cBN single crystals is over 120 ct, the yield proportion of single crystals (>60 mesh) is over 16%, and the cBN single crystal has the feature of regular shape, flat surface and less defects. The static-pressure strength of cBN single crystals (50/60mesh) is 47N, which is higher than national standards (GB/T6408-2003,50/60 mesh: 38N).(3) In the system of Ca3N2-hBN, with the synthesis process of constant power-slow elevating pressure, the optimal process parameters of synthesizing cBN single crystal is the synthesis pressure of 4.8 GPa, synthesis power of 4700 W, synthesis time of 15 minutes. In this process, the yield of synthesizing cBN single crystals is over 110.0 ct, the yield proportion of single crystals (>80 mesh) is over 5.5%, and the cBN single crystal has flat surface and less defects. The static-pressure strength of cBN single crystals (50/60mesh) is 43N, which is higher than national standards. Comparing with the system of Li3N-hBN, the yield and conversion rate of synthesizing cBN single crystal is lower, the particle size is smaller, but price of Ca3N2 catalyst is cheaper than Li3N catalyst, which can greatly reduce the synthesis cost.(4) In the system of Li3N-hBN+seed crystal, the yield of cBN single crystal with adding the seed crystal (100/120mesh) of 4wt% is 99.6ct, the conversion rate is 33.2%, and the proportion of yield of bulk particle cBN (30/50mesh) is 56.8%. The static-pressure strength of cBN single crystals (50/60mesh) is 48N, which is higher than national standards. The cBN single crystal has amber color and high transparent, and partly grain size is about 0.5mm, which has the typical octahedral crystal shape, the crystal plane is (111) and (110) with regular crystal shape and less defects. This process is the optimum process on the present experimental research which is suitable to produce the high-grade cBN single crystal with large size in industry.(5) In the system of Ca3N2-hBN+seed crystal, the yield of synthesizing cBN crystal with adding the seed crystal(100/120mesh) of 4wt% is 67.1ct, the conversion rate is 22.4%, and the proportion of yield of bulk particle cBN (30/50mesh) is 14.4%. The static-pressure strength of cBN single crystals (50/60mesh) is 45N, which is higher than national standards. The cBN single crystal has light yellow and high transparent, and partly grain size is about 0.5mm, which has the shape of truncated octahedron, the crystal plane is (111) and (110) with regular crystal shape and less defects. But comparing with the system of Li3N-hBN+seed crystal, the cBN single crystals synthesis effect is little worse.