A theoretical quantification of the possible improvement in the mechanical properties of carbon nanotube bundles by carbon ion irradiation

Improvement of single wall carbon nanotube (CNT) bundle mechanical properties through carbon ion irradiation is investigated using molecular dynamics. Increased inter-tube shear and toughness properties through formation of inter-tube cross-links is balanced against decreased tensile strength from induced defects. Bundles irradiated with carbon ions with energy 50–300 eV/ion, and fluence between 4 × 1013 cm−2 and 2 × 1014 cm−2, are mechanically tested. We find that with careful control of irradiation parameters, shear and toughness parameters increase by an order of magnitude, while tensile properties reduce by only 30–40%; in real CNT fibres with discontinuous CNT filaments the reduction would be much less. The nano-scale interface response resembles that of micro-scale composites, in which interstitial C atoms play a key role. This makes C ion deposition an attractive option over irradiation by electrons or other types of ions, since the extra C atoms can provide the required interstitial atoms. Within a certain cross-link density range, the interface shear modulus, shear stress at bonding onset, and frictional sliding stress after debonding are all linearly related to cross-link density making controlled design of fibre shear properties feasible. A possible post-treatment with very low energy irradiation is proposed for healing holes and partially restoring tensile strength.