Development of Al and Cu-Based Hybrid Nanocomposites using Graphite Nanoplatelets and Multiwalled Carbon Nanotubes as Nanoreinforcements

Abstract

Nanocomposites are a new class of material that contain a relatively small amount of nano-sized particles having a high specific surface area as reinforcement. Metal matrix nanocomposites (MMnCs) are an emerging class of nanocomposites that have excellent physical and mechanical properties and have potential applications in a wide range of areas. These reinforcements have higher interaction with the metal matrix leading to superior physical, structural, thermal, and mechanical properties of the nanocomposites. MMnCs reinforced with carbonaceous nanofillers like multiwalled carbon nanotube (MWCNT) and graphene are novel materials showing excellent mechanical and wear performance even under adverse environmental conditions with respect to that exhibited by monolithic conventional materials. Hybrid nanocomposites are a recent advancement in the area of nanocomposites that can show far better properties as compared to conventional monolithic materials and monocomposites. Hybrid composites can have a combination of different materials or two or more forms of reinforcements like fibers, particulates, whiskers, and nanotubes. The focus of the present study is mainly on the fabrication of Al and Cu-based MMnCs by reinforcing them with two carbon allotropes, namely, MWCNTs and exfoliated graphite nanoplatelets (xGnPs), a derivative of graphene. Apart from the MWCNT-xGnP hybrid nanofiller, MWCNT and xGnP monofillers were also used to develop Al and Cu-based nanocomposites. The MMnCs were fabricated through the powder metallurgy (PM) route, and sintering was carried out by both conventional sintering and spark plasma sintering (SPS). The microstructure, mechanical properties, and wear performance of the Al and Cu-based nanocomposites were investigated in the present study. The characterization of the various powder mixtures and the sintered MMnCs along with the sintered pure Al and Cu samples has been carried out through various analytical tools like x-ray diffraction (XRD), scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. Density, Vickers microhardness, compressive strength, and strain to failure of the various sintered samples were also determined. Five different MWCNT-xGnP hybrids having MWCNT-xGnP in weight ratios of 1:9, 3:7, 1:1, 7:3, and 9:1 were prepared by ultrasonicating the MWCNT and xGnP in appropriate weight ratios. Along with pure Al and Cu samples, 1, 2, 3, and 5 wt.% of each of the five MWCNT-xGnP hybrids were added to Al and Cu to develop the various Al-MWCNT-xGnP and Cu-MWCNT-xGnP hybrid nanocomposites. MWCNT and xGnP were also added as monofillers to develop Al-MWCNT, Al-xGnP, Cu-MWCNT, and Cu-xGnP nanocomposites. From the characterization and mechanical properties analysis, it has been observed that conventionally sintered and SPSed Al and Cu-based hybrid nanocomposites exhibited superior mechanical properties and wear behaviour as compared to the similarly developed monoreinforced Al and Cu-based nanocomposites and pure Al and Cu samples. Among the various MWCNT-xGnP hybrid MMnCs, the MMnCs reinforced with hybrid having MWCNT:xGnP weight ratio of 1:1 exhibited the best mechanical and wear performance. In the case of monofiller reinforced nanocomposites, the addition of xGnP gave better results as compared to MWCNT. SPSed nanocomposites show a very high relative density of ~98.90% for Al-1 wt.% MWCNT50xGnP50 nanocomposite and ~98.23% for Cu-2 wt.% MWCNT50xGnP50 nanocomposite, which is ~3.35% and ~6.12% higher than that of similarly developed pure Al and Cu samples. A significant improvement in hardness was observed for both the Al and Cu-based hybrid nanocomposites and a hardness of ~654.2 MPa was achieved for Al-5 wt.% MWCNT50xGnP50 nanocomposite and ~1.35 GPa for Cu-2 wt.% MWCNT50xGnP50 nanocomposite developed by SPS which is ~45.57% and ~29.31% higher than that of similarly developed pure Al and Cu samples. Orowan strengthening, grain refinement, and dislocation strengthening are the major reasons for the enhanced mechanical properties. The compressive strength of SPSed Al-1 wt.% MWCNT50xGnP50 nanocomposite was found to be ~700 MPa and SPSed Cu-3 wt.% MWCNT50xGnP50 nanocomposite was found to be ~720 MPa which is higher than that of similarly developed pure Al and Cu samples by ~95.8% and ~73.9% respectively. The wear characteristics of both the Al and Cu-based nanocomposites were also improved significantly, up to a loading level of 2 wt.% MWCNT50xGnP50 hybrid reinforcement. Further addition of the hybrid nanofiller led to a deterioration in the wear properties due to agglomerations of the nanofiller in the metal matrix.