Effect of milling time, MWCNT content, and annealing temperature on microstructure and hardness of Fe/MWCNT nanocomposites synthesized by high-energy ball milling

Abstract

Abstract Nanocrystalline pure Fe and Fe/MWCNT nanocomposites powders with 0.25, 0.5, 1, and 10\xa0wt% MWCNT contents were synthesized by high-energy ball milling (HEBM). The as-milled powders were cold-compacted and annealed at 400\xa0°C and 600\xa0°C for 1\xa0h in Ar atmosphere. The effect of ball milling on pristine MWCNT and Fe/MWCNT composite powders was also investigated as a function of milling time up to 20\xa0h. The physical properties of MWCNT were imaged by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after HEBM. The structural damage of MWCNT as a function of milling time and MWCNT content was studied using Raman spectroscopy. The structural characterization of MWCNT and Fe/MWCNT composites was conducted by X-ray diffraction (XRD) as a function of milling time, MWCNT content, and annealing temperature. The chemical properties of the synthesized composite powders were investigated using X-ray photoelectron spectroscopy (XPS). The microhardness test was performed to assess the effect of milling time, annealing temperature, and MWCNT content on the mechanical properties. The results indicated that after the ball milling process, the structure of MWCNT was destroyed, and the formation of the amorphous carbon phase was observed, which was confirmed by XRD and TEM analyses. In addition, decreased defect and carbon intensity ratios (ID/IG) were calculated from the Raman results with longer ball milling processes, which is attributed to the destruction of carbon bonds. The XPS results confirmed the presence of Fe C bonds as a result of the formation of carbide phases. A fine dispersion of precipitated carbides determined by TEM is found to promote the grain size stability below 100\xa0nm in the nanocrystalline Fe matrix. The results from the micro-hardness tests showed that Orowan particle strengthening resulting from the carbide formation, as well as grain size hardening, is an important contributor to strengthening in Fe/MWCNT composites.