Microstructural evolution of polymer-derived hexagonal boron nitride fibres under high-temperature stretching

• Published in: Journal of advanced ceramics Vol. 12; no. 10; pp. 1973 - 1988
• Main Authors: Wang, Zhiguang, Ge, Min, Yu, Shouquan, Sun, Xiaoming, Qi, Xueli, Zhang, Hao, Xiao, Wen, Zhang, Weigang
• Format: Journal Article
• Language: English
• Published: Tsinghua University Press 01-10-2023
• Subjects: boron nitride fibres; high-temperature stretching; microstructural evolution; polymer-derived ceramics
• ISSN: 2226-4108; 2227-8508
• DOI: 10.26599/JAC.2023.9220801

High-temperature stretching plays a crucial role in enhancing the performance of fibres, while a quantitative investigation into the impacts of tension and stretching duration on the microstructure and performance of hexagonal boron nitride (h-BN) fibres remains absent. In this study, to elucidate the microstructural evolution of the h-BN fibres under thermal stretching, amorphous BN fibres were heated at 2000 ℃ under tension of 30, 50, and 70 N for 1, 3, and 5 h in a nitrogen atmosphere. Subsequently, the grain size, pore structure, orientation degree, microscopic morphology, and mechanical properties were analysed at room temperature. The results show that high-temperature stretching enhances the orientation degree of the BN fibres, consequently elevating Young’s modulus. The maximum orientation degree of the BN fibres was 86%, aligning with a corresponding Young’s modulus of 206 GPa. Additionally, high-temperature stretching enlarged the sizes of grains and pores, a fact substantiated by the radial cracking of the fibres upon extending thermal stretching time. Owing to the expanded pore structure of the BN fibres and the inability to form a sufficiently strong "card structure" between shorter microfibre bundles, the tensile strength of the BN fibres did not increase continually, reaching a maximum of 1.0 GPa. Microstructural observations revealed that the BN fibres, composed of highly oriented lamellar h-BN grains, tend to form radial textures under high-tensile thermal stretching and onion-skin textures under prolonged thermal stretching. These findings offer a theoretical foundation for the preparation of high-performance h-BN fibres.