Grain growth analysis of multiwalled carbon nanotube-reinforced bulk Ni composites

Abstract This work focuses on the influence of multiwalled carbon nanotube (MWCNT) concentration in the thermal evolution of the microstructure of MWCNT/Ni composites produced by pressureless sintering. The composites were prepared by a colloidal mixing process using 1.0, 2.0, 3.0 and 5.0 wt.%. The grain growth is analysed by X-ray diffraction (XRD) and the final microstructure is studied by electron backscattered diffraction (EBSD). The grain growth is observed by analysing the thermal evolution of the width of the XRD reflections. The grain growth rates are influenced by the high thermal conductivity of the nanotubes up to 3.0 wt.%, where the agglomeration of CNTs during sintering affects the growth kinetics. The distribution of grain sizes correlates the MWCNTs concentration in the matrix to the Zener pinning effect. The mean grain sizes were 47.6, 22.4, 4.9, 4.9 and 5.3 μm for pure Ni, 1.0, 2.0, 3.0 and 5.0 wt.%, respectively. An empirical reinforcement limit is settled at 2.0 wt.% beyond which, no further microstructural refinement is observed. At higher concentrations, agglomeration occurs and densification is hindered, resulting in large voids filled with MWCNT clusters. Our results highlight the use of blending processes for the manufacturing of metal/CNT composites according to empirical reinforcing limits.