Publication: Investigation on the physical, thermal and rheological properties of graphene and functionalised multi-walled carbon nanotube lubricant
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Date
2022-09-01
Authors
Mohammed, Aws Sadoon
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Abstract
The purpose of this study is to determine the effects of graphene (GR) and functionalised multi-walled carbon nanotube (FMWCNT) as mono and hybrid nanoparticles in SUNISO SL68 refrigerant lubricant in terms of dispersion, stability, thermal conductivity (TC), rheological, and tribological properties. This study is divided into three stages, finding the volume per cent (vol%) of the nanoparticles in the nanolubricant that represent higher stability, finding the surfactant ratio with nanolubricant to give the highest stability, and finding ratio combination between these nanoparticles and surfactant that gives higher viscosity index (VI) and less coefficient of friction (COF) and wear scar diameter (WSD) values. The dispersion and the stability parts have been examined through visual observation, Zeta potential value measurements, and UV-vis spectrum intensity. The TC part has been measured at different temperatures and vol%. The rheological properties have been concluded by viscosity values at different temperatures and shear rates, and by conducting ASTM 2270 standard for viscosity index measurement. The tribological properties have been concluded by ASTM D4172 and ASTM D2783 standards for both wear and extreme pressure analysis. The outcome of the first stage revealed a vol% between 0.025 vol% to 0.100 vol% is suitable for the study of both GR and FMWCNT in mono and hybrid system. The value of 0.100 vol% was the best for both GR and FMWCNT. The outcomes highlight higher stability and TC for the samples with higher vol%. The TC improvement recorded 9.5% TC higher than pure lubricant. The outcome of the second stage revealed better dispersion and stability for Cetrimonium bromide (CTAB) surfactant than Sodium Dodecylbenzene Sulfonate (SDBS) and Sorbitan monooleate 80 (SPAN-80) surfactants for both GR and FMWCNT in mono nanoparticle system. GR's best ratio between GR:CTAB is 1:1, and FMWCNT's best ratio between FMWCNT:CTAB is 1:8. UV-vis spectrum recorded over 800% higher absorbance between samples with best CTAB ratio and pure oil after 14 days. In hybrid nanolubricant, the CTAB surfactant gains stability over non-surfactant samples even after 30 days. The outcome of the last stage highlights the shear-thinning flow
behaviour of the nanolubricant. FMWCNT nanolubricant samples show an intensive level of shear-thinning flow behaviour than GR nanolubricant samples. The viscosity index (VI) tends to increase with CTAB samples, low vol% GR nanolubricant samples, and high vol% FMWCNT nanolubricant samples. The VI increased as much as 6% for FMWCNT100 sample. Tribological outcomes indicate GR nanoparticles tend to reduce the coefficient of friction (COF) and increase wear scar diameter (WSD), as they act as a nano ball bearing. On the other hand, FMWCNT nanoparticles tend to increase the COF and reduce the WSD, as they act as fillers. However, hybrid nanolubricant samples aided with CTAB show a higher trend in COF and WSD compared to samples without CTAB.