Interfacial sliding in carbon nanotube/diamond matrix composites

Carbon matrix–carbon nanotube (CNT) composites have a broad range of applications because of the exceptional mechanical properties of both matrix and fibers. Since interfacial sliding plays a key role in determining the strength and toughness of ceramic composites, here interface behavior during nanotube pull-out is studied using molecular dynamics models. The degree of interfacial coupling/adhesion between a diamond matrix and a carbon nanotube is captured through interstitial carbon atoms located in the interface, which can form bonds with both the matrix and CNT atoms. Bonding is accurately captured using the modified REBO potential of Pastewka et al. that introduces an environmental screening coefficient to better capture covalent bond breaking and reforming. Pull-out tests reveal that, after an initial transient, the pull-out force becomes constant, mimicking frictional sliding. The pull-out force is directly proportional to the number of interstitial atoms per unit area in the interface, showing that “friction” is generated by the energy dissipated during breaking and reforming of bonds involving the interstitial atoms. The effective friction stresses are quite high (several GPa) for interstitial areal densities of 0.72–2.18 nm−2 and the energy dissipated during pull-out can thus be substantial. No differences were found in the pull-out of single wall nanotubes and double wall nanotubes having interwall sp3 bonding. These results demonstrate that “friction-like” behavior can emerge from non-smooth interfaces and that chemical control of interfacial bonding in CNT can yield substantial sliding resistance and high potential toughening in nanoceramic composites.