| Cubic boron nitride(cBN)is a new type of man-made superhard material,whose hardness is the second only to that of diamond.In addition to its superhardness,it shows higher degree of thermal stability and chemical inertness to iron group metals and their compounds relative to diamond.As a result,it has been widely used as machining materials to process ferrous metals or alloys.Meanwhile,the applications of the cBN have also extended into the multiple realms of high technology due to its excellent electrical,thermal and optical properties that are not owned by diamond.Thus,cBN is considered one of the most promising materials in the 21st century.A series of experiments to synthesize cBN single crystals were conducted under high temperature and high pressure condition using cubic press with the hBN and the Li3N as raw materials and catalysts,respectively.The work of this paper includes:optimization of the assembly structure of cBN synthetic unit;the effects of pressure loading methods and power loading schemes on the yield,particle size and particle size distribution of cBN single crystal.The technology of manufacturing cBN from Li3N-hBN system was further optimized.All the work provides a solid foundation for industrial production.The assembly structure for synthesis unit of cBN crystal was improved.By adding a insulation layer in the cBN synthesis unit,the productivity of cBN crystal synthesized at the periphery of the core column was significantly improved.The yield of cBN crystal in a core column increased from 18.2%to 22.5%.In addition,the cBN crystals were uniformly distributed in the core column.The density of core column was increased from 1.25 g/cm3 to 1.49 g/cm3 by increasing the granulation and compaction times.In the case of using the same synthetic temperature and pressure,the yield of cBN crystal was higher if a core column with a higher density was applied.In consideration that an increase in the granulation and compaction times would lead to an increase in manufacturing cost,the optimal processing route was summarized as follows:fine and uniform-sized particle was produced by four times of granulation followed by two times of compaction to form a dense cylinder core column.The synthesis technology of the cBN crystal was further upgraded.The pressure loading mode was modified to optimize the middle stage of the synthetic process.The yield of a core column was increased by 21%and the cBN particle size became more uniform by a rapid increase from the mid-plateau pressure to the final plateau pressure.In the present work,the optimal pressure loading spectrum was summarized as follows:the pressure was elevated from the mid-plateau to the final plateau in 5 minutes followed by pressure maintenance for 75 minutes.The total time of 80 minutes at the mid-stage kept unchanged.A new strategy was applied to optimize the last stage pressure loading scheme,i.e.,the pressure was raised gradually from the synthesis pressure to the target pressure.This method resulted in an increase in the yield of a core column by 21.8%and a promotion in an average cBN crystal size without any cost of the cBN crystal quality.However,a rapid increase from the synthesis pressure to the target pressure followed by a pressure maintenance was favorable of an increase of the average cBN crystal size at the cost of cBN crystal quality.An optimal power loading method was offered for the middle stage of the synthesis process,i.e.,a rapid increase to the target power followed by power maintenance was beneficial to the improvement in the quality of the synthesized cBN crystal without impairing its growth. |
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