Correlation between structural features and mechanical properties of boron nitride fibres derived from alkylaminoborazines

The microstructure and texture of a series of four boron nitride fibres prepared from three alkylaminoborazines 2,4,6-[(CH3)2N]3B3N3H3 (1), 2,4-[(CH3)2N]2-6-(CH3HN)B3N3H3 (2) and 2-[(CH3)2N]-4,6-(CH3HN)2B3N3H3 (3) have been studied by means of transmission electronic microscopy (TEM) and polarized light microscopy (PLM) on the basis of relevant X-ray diffraction (XRD) and Raman spectroscopy data. TEM results indicate that the general structure of the fibres consists in three concentric parts with a gradual increase in the crystallites size from the outer region to the core of the fibres. The grains appear to be randomly distributed in the fibre prepared from 1 whereas a pronounced orientation of crystallites is found in the fibres derived from 2 and 3. This local feature was extended to a larger scale by the use of PLM, which showed that the orientation of crystallites with respect to the fibre axis is improved within the fibres prepared, respectively, from 1, 2 and 3. The comparison of these results evidenced a correlation between microstructural and textural properties and the mechanical performances of the studied boron nitride fibres. Thus, this work exemplified that mechanical properties of BN fibres are closely associated with both a good crystallization state, e.g. large crystallites and a preferred mean alignment of BN crystallites following their (0 0 2) planes (i.e. planes of hexagons) in a direction parallel to the fibre axis.