Effect of graphene and zirconia on microstructure and tribological behaviour of alumina matrix nanocomposites

Abstract

The inherent brittleness of monolithic alumina (Al2O3) has consistently limited its potential for several high-performance structural applications such as engine turbine parts and high temperature space materials. The fracture toughness and wear properties of alumina can be improved by the uniform dispersion of sub-micron and nano-phase particles in the alumina parent matrix. In the current study, alumina-based nanocomposites were fabricated by homogeneous dispersion of zirconia (4 wt% and 10 wt%) and graphene (0.5 wt%) using colloidal mixing followed by hot-pressing at 1600 °C for 1 h. The microstructure of the processed composite was extensively observed under high-resolution electron microscopes. The effects of zirconia and graphene on the mechanical and tribological properties were also investigated. The final microstructures depict extensive grain-refinement by the uniform dispersion of 0.5 wt%Gn and 10 wt%ZrO2 in the parent alumina matrix. A coarse-grain microstructure was observed with the addition of the 4 wt%ZrO2 due to insufficient inhibition of alumina grain growth during sintering. The coefficient of friction results from the wear test using a UMT Tribolab equipment with constant normal load of 25 N, 100 mm/s sliding velocity for 1000s, under ASTM G133-05 illustrates a steep increase for the 0.4 wt% ZrO2 addition but a significant decrease for the 10 wt%ZrO2 addition. Consequently, the addition of 10 wt% ZrO2 resulted in ∼98% decrease in the wear rate which was attributed to its refined microstructure and effective load-bearing activities of the uniformly dispersed ZrO2 precipitates. Meanwhile, the addition of 0.5 wt%Gn revealed a fine-grained microstructure and consequent lower wear rate (∼90.5%), but there was a relative increase in the coefficient of friction, which suggested that there is a direct relationship between the microstructure and wear behaviour than the coefficient of friction.