Development of Alumina and Silica Based Nanocomposites using Graphite Nanoplatelets and Multiwalled Carbon Nanotubes as Reinforcement

Abstract

The present research provides a comprehensive investigation in understanding the effects of the addition of exfoliated graphite nanoplatelets (xGnPs) and multiwalled carbon nanotubes (MWCNTs) on the microstructure and mechanical properties of two types of ceramic matrices, namely, polycrystalline alumina (Al2O3) matrix and amorphous silica (SiO2) matrix. The main objective of this work is to understand the difference in the properties of ceramic matrix nanocomposites (CMNCs) reinforced with xGnPs and MWCNTs, due to the different characteristics of both nanofillers. xGnPs and MWCNTs when used as nano-reinforcements, can extensively improve the mechanical properties like hardness, fracture toughness and wear resistance of the brittle Al2O3 and SiO2-based materials, which can be further used for various mechanical and structural engineering applications. For the fabrication of xGnP and MWCNT reinforced Al2O3 and SiO2-based nanocomposites, efforts have been made on three major aspects, namely (i) production of good quality xGnPs and MWCNTs, (ii) homogeneous dispersion of xGnPs and MWCNTs in the ceramic matrices, and (iii) preservation of the graphitic structure of both nanofillers during high temperature processing. xGnPs were synthesised from a graphite intercalation compound (GIC) by rapid evaporation of the intercalant at an elevated temperature and its subsequent ultrasonication in acetone. The synthesis of MWCNTs was done by low pressure chemical vapour deposition (LPCVD) method. In order to solve the problem of xGnP/MWCNT agglomeration at higher loading levels in the ceramic matrices, powder metallurgy route was adopted and processing techniques like ultrasonication and surface modification were employed. Al2O3-xGnP, Al2O3-MWCNT, SiO2-xGnP and SiO2-MWCNT powder mixtures were prepared by blending the monolithic powders of pure Al2O3 and pure SiO2 with desired volume fractions of xGnPs and MWCNTs by ball milling. To avoid thermal degradation of the carbonaceous nanofillers during high temperature processing, sintering of the nanocomposites was carried out at optimised parameters. Al2O3-0.2, 0.5, 0.8, 3, 5 vol.% xGnP, Al2O3-0.2, 0.5, 0.8, 3, 5 vol.% MWCNT, SiO2-0.5, 1, 3, 5 vol.% xGnP and SiO2-0.5, 1, 3, 5 vol.% MWCNT nanocomposites were fabricated and analyzed. Conventional sintering of nanocomposites was carried out in vacuum whereas spark plasma sintering (SPS) was done in Ar atmosphere. In the case of conventionally sintered nanocomposites, green compacts of Al2O3-xGnP and Al2O3-MWCNT nanocomposites were prepared under uniaxial load of ~390 MPa and later sintered at 1650oC for the period of 2, 3, 4 h and 1, 2, 3 h respectively, whereas for SiO2-xGnP and SiO2-MWCNT nanocomposites the cold compaction was achieved at ~310 MPa and the nanocomposites were sintered at 1350oC for a period of 2 h and 4 h. For the SPSed samples, Al2O3-xGnP and Al2O3-MWCNT nanocomposites were prepared at 1450oC under 50 MPa pressure for the duration of 5 min and 10 min respectively. For the development of various SiO2-xGnP and SiO2-MWCNT nanocomposites, SPS was carried out at 1350oC under 40 MPa pressure for the duration of 10 min. Near full densification was achieved for all the SPSed Al2O3-xGnP, Al2O3-MWCNT, SiO2-xGnP and SiO2-MWCNT nanocomposites. The microstructural characterization and analysis of mechanical properties of developed nanocomposites was carried out. A significant improvement in the mechanical properties like relative density, hardness, wear resistance and indentation toughness was observed for various Al2O3 and SiO2 based CMNCs. The loading level of nanofillers play a vital role in effecting the properties of the nanocomposites. A remarkable improvement in the mechanical properties of various conventionally sintered and SPSed nanocomposites, upto an optimum loading level of both nanofillers was observed. The indentation toughness of various Al2O3-xGnP and Al2O3-MWCNT nanocomposites showed an improvement only upto the addition of 0.8 vol. % of xGnPs and MWCNT, whereas for the SiO2-xGnP and SiO2-MWCNTs nanocomposites, an improvement in the fracture toughness value was observed up to the loading level of 3 vol. % of both the nanofillers. The toughening mechanisms such as crack bridging, crack deflection and crack branching attributed to the improvement of the fracture toughness of the nanocomposites. An increase in the content of xGnPs and MWCNTS up to 5 vol. % deteriorated the mechanical properties of various CMNCs due to the agglomeration of both the nanofillers. The wear mechanism in the various nanocomposites was found to involve abrasion, galling and pullout of the nanofillers. Results suggest that the Al2O3-xGnP and SiO2-xGnP nanocomposites possess better mechanical attributes as compared to Al2O3-MWCNT and SiO2-MWCNT nanocomposites when prepared under similar conditions. Furthermore, SPSed nanocomposites were found to possess superior mechanical properties as compared to the conventionally sintered nanocomposites.