Mechanical properties of MWNT/Ni bulk composites: Influence of the microstructural refinement on the hardness

The grain growth stagnation produced by the addition of multiwalled carbon nanotubes to bulk nickel composites manufactured by hot uniaxial pressing is investigated. A hardness improvement up to 27% compared to pure nickel was achieved, mainly based on the microstructural refinement. However, this enhancement is only observable up to concentrations of 3 wt%. Beyond this point, no significant improvement is observed, thus setting an empirical reinforcing limit. The main strengthening mechanism identified was the Hall-Petch effect, which hinders the dislocation movement and is strongly dependent on the final grain size of the composites. Finally, a model for the prediction of the final grain size is proposed based upon the Zener pinning equation for the particular system studied in this work. This model is intended to provide a basic tool for a further tailoring of the mechanical properties, adjusted for a certain application.