High Pressure Sintering of Super-hard Boron Suboxide and Evaluation of Sintered Compacts

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Hexaboron oxide (B_6O) is one of the super-hard boron compounds which has a microhardness comparable to diamond or cubic boron nitride, and is expected to an actual application to cutting tools or nozzle parts. However, B_60 is so difficult to sinter and to densify the compact that its mechanical or thermal property is less known so far. In the present paper, we prepared B_60 sintered compact in full density by high pressure and high temperature treatment of sinterable B_60 powder which was synthesized in our laboratory. Then, we investigated the mechanical and thermal properties of the sintered compacts and evaluated the possibility of application to various tools.1.Synthesis and characterization of B_60 powderB_60 powder was synthesized by heat treatment (1350ﾟC, 4 h) of the mixture of amorphous B_20_3 and boron (1 : 16 in mol ratio) in an pure argon stream. The synthesized B_60 powder was observed by SEM to comprise crystalline and dispersible particles with 1.2-1.4 mu m diameters.2 … More .High pressure sintering of B_6O powder and the mechanical or thermal properties of the sintered compactAn optimum sintering condition of the vacuum treated B_60 powder was investigated using the girdle-type high pressure apparatus. A single phase B_6O sintered compact was prepared by the high pressure and temperature treatment at 5 GPa, 1700ﾟC and 20 min. The relative density and Vickers microhardness were 97% and 3400 kg/mm^2, respectively. TG-DTA analyses exhibited that B_60 sintered compact had an oxidation resistance higher than B4C, and a mild oxidation proceeded up to 1000ﾟC accompanied by a formation of protective layer of liquid B_2O_33.Preparation of B_60-based sintered composites and their possibility for application to toolsSintered composites in the B_60-B_4C system or B_60-cBN system were prepared in order to increase the toughness of B_60 sintered compact. High pressure and temperature treatments of the vacuum treated mixed powder of B_60 and B_4C/cBN were carried out under the conditions of 3-8 GPa, 150O-1800ﾟC and 15-30 min. Maximum values of microhardness and fracture toughness in both systems were 4600 kg/mm^2 and 1-1.8MNm^<-3/2>, respectively. Less