Preparation of cBN-Diamond Composite Sintered Compact by Reaction Sintering

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An objective of the present research is to prepare a composite sintered compact in the cBN-diamond system in which fine diamond particles are dispersed homogeneously in the reaction sintered cBN matrix.1. Optimum reaction sintering conditions : The starting powder of hBN and diamond (0-90 wt%) which were mixed with various kinds of volatile catalysts was treated under high pressure and temperature conditions (6-7.5 Gpa, 1300-1700ﾟC, 0-30 min). The optimum sintering conditions were found to be 7 GPa, 1700ﾟC and 15-30 min.2. Microstructure and properties of the sintered compacts : Conversion from hBN to cBN increased with decreasing diamond grain size and increasing diamond content. The microhardness increased as the diamond content increased, but the toughness decreased because of a weak particle bonding between cBN and diamond grains. A sintered compact with the composition of 70 wt% cBN - 30 wt% diamond showed the conversion of 99 %, the relative density of 99 % and Vickers microhardness of 5100 kg/mm^2.3. Effects of volatile catalysts : The effects of volatile catalysts were investigated by adding 0-30 wt% NH_4NO_3, NH_4Cl and NH_2NH_2 under the above optimum sintering conditions. An excess addition of NH_4Cl deteriorated the properties of sintered compact because of increased amount of non-ammonium components. An activation energy required for the conversion from hBN to cBN was 200-230 kJ/mol. NH_X components in these catalysts which decomposed under high pressure and temperature conditions, would play an important role in the conversion from hBN to cBN.