Boron nitride ceramics have a unique combination of chemical, electrical, mechanical, and thermal properties, making them suitable for various high-performance industrial applications. It has unique properties such as excellent heat resistance and thermal shock resistance, excellent electrical insulation, and easy cutting. Boron nitride is a covalently bonded compound and is difficult to sinter. It requires the addition of additives and then hot-pressing sintering. It can also be sintered by atmospheric pressure sintering, reaction sintering, etc. However, hot-pressing sintering is still the most effective process for preparing high-density, high-performance boron nitride rods. The following briefly introduces the three sintering methods of boron nitride.
1. Hot pressing sintering
The additives are mainly B2O3, SiO2, etc. Generally, 2%~5% B2O3 is added, and sintering is carried out at 1700~1900℃ and 25~40 MPa pressure. The holding time at the highest temperature ranges from more than 10 minutes to several hours. Ceramic products with a density of 2.0~2.2g/cm3 and a porosity of 0~1% can be obtained. B2O3 produces a liquid phase at high temperatures, thereby promoting the sintering of boron nitride, but the presence of B2O3 will cause BN products to absorb moisture, causing the electrical and thermal properties of the products to deteriorate. Therefore, when hot pressing BN products, it is necessary to consider the effects of these two aspects and select the appropriate amount of B2O3 added (Kanai, 1990).
2. Normal pressure sintering
The normal pressure sintering of BN ceramics has always attracted much attention. To improve the density, in addition to using suitable sintering aids, it is also necessary to improve the sintering activity of BN powder. Hagio et al. (1994) used hexagonal boron nitride powder treated with mechanochemical activation for pressureless sintering to obtain BN ceramics with a relative density of 70% and good performance. Using a similar method, Tsuyoshi et al. (1991) prepared a BN-sintered body with a density of 1.8g/cm3 and a bending strength of 50 MPa. Kurita et al. (1989) used AIN and amorphous B as additives, sintered at 1500℃ and N2 atmosphere, and obtained BN ceramics with a relative density of 75.8%. Lei Yucheng et al. (2005) introduced Al2O3, Y2O3 and B2O3 as additives, sintered at 1700~1850℃ under N2 atmosphere, and the strength and density of the sintered body were significantly improved, but there was still a porosity of 20%.
Some studies have shown that the card house structure is the main reason that hinders the densification of BN ceramics. This is because the card house structure formed by the cross-stacked flaky BN grains plays a supporting role, making it difficult for the green body to shrink. During normal pressure sintering, because there is no external pressure, although there is a certain liquid phase, it is difficult to eliminate this card house structure. When the external force applied during hot pressing sintering is large enough, this card house structure can be destroyed, so the density can be effectively improved.
3. Reaction sintering
Reaction sintering BN ceramics is to fully mix the fine powder of boron and its additives, and then heat the pressed green body in a flowing N2 atmosphere to obtain a sintered body of BN through the reaction 2B+N2→2BN. Kuznetsova et al. conducted research and reports earlier. They used H3BO3, NH3, and Ca3(PO4)2 as starting materials, and obtained hexagonal boron nitride with a density of 2.06-2.08g/cm3 under the optimal conditions of 1800℃ for 10min and pressure of 10MPa.
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