Background and purpose of the article: Hexagonal boron nitride (h-BN) is a ceramic material that is difficult to densify due to its special lamellar structure and low self-diffusion coefficient. To prepare h-BN ceramics, pressureless sintering (PLS), hot pressing sintering (HP) and spark plasma sintering (SPS) are usually used. This article aims to compare the effects of these three sintering methods on the microstructure, relative density and mechanical properties of h-BN ceramics, and to explore the effects of the initial density of the green body on the density and mechanical properties of the final prepared h-BN ceramics.
Methods and results: The researchers used submicron h-BN powder as raw material, without adding any sintering aids, and prepared h-BN ceramics by PLS, HP and SPS. The phase composition and microstructure of the sintered samples were tested and observed by X-ray diffraction and scanning electron microscopy. The results show that pressureless sintering cannot achieve h-BN ceramic sintering densification and has poor mechanical properties; while hot pressing and spark plasma sintering methods can both obtain h-BN ceramics with dense structure and good mechanical properties. In particular, h-BN ceramics prepared by SPS method have higher density and better mechanical properties, more uniform and fine grains, and the sintering temperature can be reduced by more than 200℃. In addition, increasing the initial density of the green body can significantly improve the flexural strength and fracture toughness of h-BN ceramics, but has little effect on the densification of h-BN ceramics prepared by hot pressing and spark plasma sintering.
Conclusion:
1. h-BN ceramics with basically isotropically arranged h-BN grains can be prepared by PLS, HP and SPS methods, but the h-BN ceramics prepared by SPS method have finer and more uniform grains, higher densification, and the sintering temperature can be reduced by more than 200℃.
2. Compared with the traditional PLS and HP methods, h-BN ceramics prepared by SPS method have better mechanical properties, and the flexural strength and fracture toughness can be increased by about 450%, 44.2% and 175%, 16.3%, respectively.
3. Increasing the initial density of the green body can significantly improve the flexural strength and fracture toughness of h-BN ceramics, but has little effect on the density of h-BN ceramics prepared by HP and SPS sintering.
4. SPS is a relatively good sintering method for preparing high-density h-BN ceramics. Single-phase h-BN ceramics with a relative density of about 95% can be obtained under the condition of 1600℃ for 5 minutes.
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