Development of Cu and Al-Based Metal Matrix Composites using Graphite Nanoplatelets and Multiwalled Carbon Nanotubes as Reinforcement

Abstract

In the present study, Cu and Al-based metal matrix composites (MMCs) have been developed by powder metallurgy (PM) route using exfoliated graphite nanoplatelets (xGnP) and multiwalled carbon nanotubes (MWCNTs) as nanofillers and their microstructure, mechanical properties, sliding wear behaviour and crystallographic texture were investigated. The Cu and Al-based MMCs were developed by using as-received Cu and Al powder as well as nanostructured Cu and Al powder developed by mechanical milling (MM). Exfoliated graphite nanoplatelets (xGnP) have been synthesized from the graphite intercalation compound (GIC) by rapid evaporation of the intercalant at an elevated temperature and subsequent ultrasonication in acetone for a period of 5 h. The MWCNTs were synthesized by low pressure chemical vapour deposition (LPCVD) technique and was later acid functionalized. Cu-1, 2, 3, 5 wt. % xGnP, Cu-1, 2, 3 wt. % MWCNT, Al-1, 2, 3, 5 wt. % xGnP and Al-1, 2, 3 wt. % MWCNT composites were developed by powder metallurgy route. Cu-xGnP/MWCNT and Al-xGnP/MWCNT composites were developed by blending the elemental metal powders of Cu and Al with xGnP/MWCNTs in the desired weight fraction. The blended powders were later compacted under a uniaxial load of 565 MPa and sintered at 550oC for the Al-xGnP/MWCNT composites and 850oC for the Cu-xGnP/MWCNT composites for a period of 2 h in argon (Ar) atmosphere. A significant improvement in both relative density and hardness of the Cu-xGnP/MWCNTs upto addition of 2 wt. % of the nanofiller was observed. Al-xGnP composites showed an improvement in relative density and hardness upto the addition of 3 wt. % xGnP and in the case of Al-MWCNT composites these properties showed an improvement upto the addition of 2 wt. % MWCNT. An increase in the content of the nanofiller in these composites led to the deterioration of these properties in these composites due to the agglomeration of the nanofiller in the Cu and Al matrix. The wear beahviour of the various composites also showed a similar trend with the addition of the nanofillers. A significant improvement in the wear resistance of the Cu-xGnP/MWCNTs upto addition of 2 wt. % of the nanofiller was observed. Al-xGnP composites showed improvement in the wear behaviour upto the addition of 3 wt. % xGnP and in the case of Al-MWCNT composites the wear behaviour showed an improvement upto the addition of 2 wt. % MWCNT. The wear mechanism in the various composites was found to involve a combination of abrasion, ploughing, delamination, microcracks, deep grooves and pullout of nanofillers. The tensile strength and strain to failure of the various Cu-xGnP composites show an improvement upto the addition of 2 wt. % xGnP. However, the tensile strength of the various Cu-MWCNT composites continuously decreases with the addition of MWCNTs. The decrease in tensile strength can be attributed to the increase in the number of sites of interconnected MWCNTs with increasing MWCNTs content, which causes premature failure of the composite. On the other hand, the tensile strength of the Al-xGnP/MWCNT composites continuously decreased with the addition of xGnP/MWCNTs due to the formation of aluminium carbide (Al4C3) particles at the interface of the Al and xGnP/MWCNTs which has a detrimental effect on the mechanical properties of composites. The nature of fracture in the sintered pure Cu and Al, as well as the various Cu-xGnP/MWCNT and Al-xGnP/MWCNT composites, was found to be ductile. The residual stress in the various composites was found to be compressive in nature, and a similar trend as those of hardness and wear behaviour with the addition of xGnP/MWCNTs was observed. The addition of xGnP/MWCNTs also altered the crystallographic texture of the composites. A dominant <011> fiber was observed in the samples with/without the addition of xGnP/MWCNTs. However, the intensity of such fiber decreased with the addition of xGnP/MWCNTs. The volume fraction of the <011> fiber was found to decrease with the addition of upto 2 wt. % xGnP/MWCNTs in both the Cu and Al-based MMCs and then showed an increase with the further addition of the xGnP/MWCNTs. The Cu and Al-based composites were also developed using nanostructured Cu and Al powder synthesized by milling of elemental powders of Cu and Al for 25 h in a high energy planetary ball mill. In this case, higher values of hardness, as well as better wear resistance of the composites as compared to those achieved for Cu and Al-based composites developed using unmilled Cu and Al powder, was observed.