Mazna A. Development of hexagonal boron nitride-based composite ceramics under conditions of reaction sintering that yields mullite and sialon.

To obtain composite materials based on hexagonal boron nitride and new phases as formed during the reaction of hot pressing, a systematic study of physical/chemical processes that occur in the BN-Al2O3 -SiO2 and BN-Al2O3-SiO2-Si3N4 systems has been conducted. It was found in the work that the BN-Al2O3 -SiO2 system forms mullite in its aluminosilicates, and the BN-Al2O3-SiO2-Si3N4 system yields some phases of sialon (X-phase, SiAlON, z=1,31 - 3,6), formation of which occurs through the stage of mullite formationю. Reaction hot pressing of a composite mixture was shown to provide the opportunity of a 200 to 300 C temperature reduction and a 10 to 20 MPa pressure lowering when hot pressing of composites due to the presence of liquid phases as compared to the hot pressing process of hexagonal boron nitride. As assessment of strength and mechanical fracture of boron nitride based composite ceramics in relation to temperature has shown, within the 20 to 1200 C range, with temperature increase above 1200 C, the composite strength is determined by the ductile fracture behavior due to matrix phases. Compositions and energy saving conditions of reaction hot sintering composites based on boron nitride (temperature 1700-1800 C, pressure 15-20 MPa) have been implemented into the pilot production. The bearings were producted from the developed composites. They were tested and used in the cathode-ray installations of the scientific&industrial centre of the Paton electro-welding institute of the NASU.