Compressibility of nanofibre-grafted alumina fabric and yarns: Aligned carbon nanotube forests

Growth of nanowires, particularly aligned carbon nanotubes (CNTs) on fibre surfaces, is an additive approach to micro/nanoscale reinforcement for realising hierarchical (or nanoengineered) fibre reinforced composites (FRCs). While eliminating dispersion issues and allowing morphology control, the aligned growth could pose challenges when adapting traditional composite manufacturing methods. To quantify the effect of CNT morphology on manufacturing, the compressibility of CNT-grafted alumina fibres (woven fabric and yarn) is studied experimentally. Radially-aligned CNTs are grown on fibres to lengths of 20 μm, with CNT weight fractions up to 10%. To understand compression as a key aspect of composite processing, pressure vs. thickness measurements are made for tows, single plies, and stacked plies through consecutive compression cycles. The results show substantial decreases in compressibility of the fibrous assemblies in the presence of CNT grafting, consistent with earlier observations of random CNT forests morphologies on carbon fibres. The pressure needed to reach a typical-use fibre volume fraction of 40% increases from 176 kPa for non-grafted fabric to 1390 kPa for a fabric grafted with 5 wt.% aligned CNTs, and 2794 kPa for 10 wt.% of aligned CNTs. Compared to the random CNT morphology, the aligned CNT forest-grafted textiles are less compressible, offering higher compression resistance and higher resilience for the same weight or volume fraction of CNTs: at 0.1 MPa pressure, the fabric thickness is increased (or fibre volume fraction is decreased) by a factor of 1.7…2.5 for 5…10 wt.% of the aligned CNT grafting (against a factor of 1.3…1.5 for the same amount of random grafting). The aligned-CNT morphology exhibits significant compression hysteresis that was not observed for the case of random CNT forests. Analysis of the compression curves reveals that pressures needed to achieve typical laminate microfibre packing fractions are at the limit of standard composite processing pressures.