Evaluation of the tribological and thermo-physical properties of coconut oil added with MoS2 nanoparticles at elevated temperatures

Abstract

Superior tribological properties of nano-lubricants in boundary/thin film lubrication are well established for mineral oils. However, in view of the environmental aspects, nano-lubricants developed from vegetable oils are more preferable and studies are to be carried out to evaluate the performance of vegetable oils when nanoparticles are added to them. In this paper, tribological properties of nano-lubricants formulated from vegetable (coconut) oil and a mineral oil (500N base-oil) are compared using a modified pin-on-disc tribometer and a four-ball tester, under boundary/thin film lubrication regime. Molybdenum disulfide (MoS2) nanoparticles (unmodified and surfactant-modified) are added separately to the base-oils and ultrasonic shaking is carried out to obtain a uniform dispersion of the nanoparticles in the nano-lubricant. The friction-reduction and anti-wear properties of the nano-lubricants have been experimentally studied at ambient (30°C) and at elevated temperatures upto 120°C, for various concentrations of nanoparticles. The experimental data are used to formulate a response surface methodology (RSM) model in ANOVA using central composite design. The simulation results are used to optimize the concentration of nanoparticles for the best tribological properties. The optimum concentration of MoS2 nanoparticles in coconut oil and 500N base-oil are estimated to be 0.53 and 0.58 weight percentage respectively. Detailed wear scar analyses of the balls from the four-ball tester have also been carried out using FESEM. Enhancement of thermo-physical properties such as viscosity and flash and fire-points of the base-oils and the nano-lubricants at various temperatures have been evaluated for different nanoparticle concentrations. Dispersion analyses of the nano-lubricants have been carried out by UV visible spectroscopy and the results show that the nano-lubricants formulated using surfactant-modified nanoparticles are more stable and are more suitable for stationary long term applications than the nano-lubricants prepared using unmodified nanoparticles. Surface topography analyses of the worn surfaces using AFM and FESEM show that the average roughness of the pin surface decreases when nano-lubricants are used at the sliding interface rather than the base lubricant. The results from the current experimental studies envisage the prominence of vegetable oil based nano-lubricants in the foreseeable renewable-based economy and also the prominence of the use of nanoparticles as additives in the base lubricants.