Hardness of Multi Wall Carbon Nanotubes reinforced aluminium matrix composites

•Among the highest hardness has been measured for Al-MWCNT composites.•Up to 9 wt% CNT could be dispersed by high energy milling.•TEM observations show that CNTs sustained the milling process.•Hall-Petch strengthening explains partly the mechanical properties.•A load transfer mechanism between the Al-matrix and the CNT is evidenced.

The macro hardness (HV20) was measured for aluminium and 1–9 wt% Multi Wall Carbon Nanotubes (MW-CNTs) composites that were milled and hot compressed. The hardness increased with increasing fraction of MW-CNTs up to 6 wt% (HV20 = 151) and then remained constant. The content of MW-CNTs was significantly higher than reported for similar materials and measurements. The composites were analysed by Raman, TEM and XRD. The Raman and TEM showed MW-CNTs were still present after milling and hot pressing. The XRD was used to determine the Al crystallite size which was used to determine Hall–Petch contribution to the composite hardness.