We have carried out a comparative x-ray analysis of boron nitride powders with different degrees of ordering and sintered specimens of boron carbonitride based on such powders. We have established that an indicator of sintering activity for the boron carbonitride specimens is the rate of formation of the r-BN phase, the content of which depends on the degree of ordering of the starting BN powder.
The dependence of the phase composition and properties of a boron carbonitride (BCN) composite on hot-pressing temperature is studied. It is established that the composite forms with the synthesis of new reinforcing phases in reaction sintering under pressure. The optimum conditions to obtain the BCN composite are determined. The BCN composite has high thermal strength, oxidation resistance, and electrical insulating properties.According to [1, 2], a boron carbide and nitride (boron carbonitride) material produced by reaction sintering in nitrogen has high resistivity, resistance to thermal shocks and to different corrosive media, and good machinability. It is used in high-temperature technology as an insulating and refractory material. Boron carbonitride is disadvantageous in that it has relatively high porosity that decreases mechanical strength and oxidation resistance.The objective of the paper is to produce a hot-pressed boron carbonitride composite with satisfactory strength and with the above properties.We consider the use of molybdenum silicide MoSi 2 as an addition to boron carbonitride to improve the mechanical strength and oxidation resistance. Molybdenum silicide is a high-temperature (T melt = 2020°C) and chemically stable compound with oxidation resistance in air up to 1500°C and a low thermal expansion coefficient (5.1 ⋅ 10 -6 deg -1 at 20-1480°C) [3].The starting materials were powders of boron carbonitride (conventionally BNC) and molybdenum silicide (MoSi 2 ). To produce boron carbonitride, billets were pressed from a mixture of boron nitride and carbide powders (in the weight ratio 1 : 1) and sintered at 1800-1900°C in nitrogen. In the reaction sintering, B 4 C is nitrided to form secondary BN with rhombohedral crystal lattice. The ratio between BN and B 4 C changes toward higher BN content to 75-80 wt.%. This is because B 4 C is thermodynamically unstable in nitrogen and interacts with it by reaction: B 4 C + 2N 2 = 4BN + C [1, 2]. The boron carbonitride powder was produced by grinding the reaction-sintered billets to obtain particles 5-20 μm in size.
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