Friction and wear properties of carbon nanotube/alumina composites under water lubricated conditions

An alumina composite reinforced with 10 vol.% acid-treated multi-walled carbon nanotubes (MWCNTs) is prepared by a simple mechanical mixing method followed by a spark plasma sintering. The effects of friction conditions such as contact pressure and sliding velocity on tribological properties of the composite in a water lubricated environment are investgated using a ball-on-disk technique against an alumina ball. In addition, the Stribeck curves, showing boundary, mixed and hydrodynamic lubrication, are presented for the composite and monolithic alumina. It has been demonstrated that the composite shows lower coefficient of friction (COF) (μ ∼; 0.1) compared with the MWCNT-free monolithic alumina (μ ∼; 0.2) in the mixed regime (the initial contact pressure and sliding verosity are 0.6 GPa and 0.10 m/s, respectively). This may be attributable to the formation of the water film which is expected to be formed by hydrogen bonds with the functional group introduced into MWCNTs by the acid treatment. The COFs of the composite and monolithic alumina decrease with the increasing Stribeck parameter, and the COF at the initial contact pressure of 0.4 GPa and sliding verosity of 0.16 m/s was approximately 0.01, suggesting that the lubrication regime may be in hydrodynamic lubrication. The specific wear rates of the counterparts are typically in the range of 1.9×10^-8-2.8×10^-8 mm³/Nm, and these values are decreased with the increasing Stribeck parameter. These results suggest that the COF and specific wear rate vary with the operating conditions under water lubrication, which reflects the transition of the lubrication mode of the composites.