Synergistic effect of graphene and carbon nanotubes on wear behaviour of alumina-zirconia nanocomposites

Abstract

In this study, Al2O3-based nanocomposites reinforced with multiple nano-materials, namely zirconia (ZrO2), graphene (GN) and carbon nanotubes (CNTs) were processed to investigate the synergistic effect of the multi-materials on the microstructural evolution, mechanical properties and tribological behaviour, including the mechanism of wear. The monolithic Al2O3 and Al2O3-based nanocomposites reinforced with 10wt%ZrO2, 0.5wt%GN and 2wt%CNTs were prepared using a colloidal mixing and sintered at a temperature of 1600°C. Benchmarked against the monolithic Al2O3, a single addition of 10wt%ZrO2 gave about 25% decrease in matrix grain size, whilst a remarkable reduction in both the matrix and ZrO2 grains, up to 80% and 46% respectively, was achieved with the multiple combination of ZrO2, GN and CNTs. This was due to the effective parallel pinning action of the matrix grains by the homogeneously dispersed multi-materials to the monolithic Al2O3. Vickers hardness also increased to 48% with combined additions of GN and CNTs, as opposed to the 25% decrease with a single addition of ZrO2 relative to the monolithic Al2O3 material, which was attributed to the refined grain structures and effective load transfer capabilities of the combined multi-phase materials. In addition, the fabricated nanocomposite incorporated with only 10wt%ZrO2 resulted in 88% decrease in wear rate, whilst the multiple additions of GN and CNTs exhibited better wear resistance, with up to 93% wear rate reduction as compared to the monolithic Al2O3. The improved wear resistance of the nanocomposites with the multiple additions of GN and CNTs was due to the micro-crack bridging and restriction to intergranular fracture that leads to pull-out of the matrix grains under sliding wear. Also, the combined crack pinning by the finely dispersed ZrO2 particles, and bridging action of the GN and CNTs suggest a synergistic response of these multi-materials to the wear performance of the fabricated multi-material nanocomposites.