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

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 h-BN fibres under thermal stretching, amorphous BN fibres were heated at 2000 °C under tensions 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 grain and pore, a fact substantiated by the radial cracking of the fibres upon extending the 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.