Development and performance evaluation of bioluricant enhanced with nanoparticles for sustainable machining application

Abstract

Vegetable oil-based biolubricants usage for lubrication and cooling in the industry has been encouraged due to their pleasant attributes of being environmentally benign, lower volatility, biodegradable, good lubricity, and high-quality viscosity index, amongst others. However, failure of biolubricants at elevated temperatures which is attributed to the thermal degradation hinders their efficient performance. The performance limitations may be ameliorated using nano-additives to enhance the oxidation and thermal stability as well as improve the thermophysical and anti-wear properties of biolubricants. However, thermal stability evaluation of vegetable oil-based nano-enhanced biolubricants has not been reported in any literature. The research aimed to formulate and evaluate performance characteristics of nano-enhanced biolubricants dispersed with exfoliated graphene nanoplatelets and maghemite nanoparticles at varying volume concentrations of 0.1%, 0.2% and 0.3% in coconut oil as base biolubricants; denoted by XGO1, XGO2 and XGO3 for graphene enhanced and MGO1, MGO2 and MGO3 for maghemite enhanced nanolubricants. The developed nanolubricants and the base biolubricants were evaluated for thermal stability, thermophysical and tribological properties tests. The volume fraction 0.2% of the XGO and the MGO concentrations indicated best performance in terms of tribological properties. End milling machining experiment was performed using the best concentrations from the XGO and MGO nanolubricants (0.2% volume fractions) based on the outcome of tribological properties evaluation and compared with the base lubricant. From the results, thermogravimetric analysis (TGA) reveals that the addition of nanoparticles improves the oxidation onset temperature for all concentrations, which implies improved thermal stability. The oxidation onset temperature in the presence of nano-additives can be delayed by 36.3 °C and 14.5 °C for XGO1 and MGO1, respectively, at 0.1% volume concentration. An improvement of thermal conductivity for all concentration levels was observed with a maximum enhancement ratio of 15.5% and 7.9% at 0.3% volume concentration, respectively, for the graphene and maghemite enhanced nanolubricants. The tribological studies revealed a significant reduction in coefficient of friction and wear scar diameter. For graphene enhanced nanolubricants, maximum friction and wear reduction were 17.6% and 7.55%, while for maghemite enhanced nanolubricants were 7.39% and 6.25%, respectively. Machinability performance was performed on machining titanium alloy (Ti-6Al-4V) under the minimum quantity lubrication (MQL) technique to supply lubricants. The results reveal that the enhanced biolubricants indicated superior performance over pure biolubricants in cutting force, tool life, and surface roughness. The graphene nanoplatelets enhanced lubricant was better over the maghemite enhanced lubricant. The research has shown a comprehensive understanding of oxidation stability and thermal degradation process of enhanced nano-biolubricants which are rarely investigated. Thermophysical and tribological properties evaluated indicated significant improvement. Furthermore, the developed non-linear correlation will avail researchers and industry operators the opportunity of selecting potential nanoparticles for nano-enhanced biolubricants formulation. This will reduce downtime and save resources as blends of nanoparticle and vegetable oils can be formulated without performance testing, thus promoting the United Nations Sustainable Development Goals (SDGs) to ensure sustainable consumption and production pattern.