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Friction and wear reduction is a challenge to the modern transportation industry due to high energy consumption and maintenance costs[1]. A high friction coefficient will degrade the durability and reliability of a tribosystem. Therefore, reducing the friction coefficient has to be considered during practical applications[2]. High-quality lubrication is very important for operation under harsh working conditions such as high temperatures and extreme pressures. Solid lubricants lower the friction coefficient with efficient heat transfer.
Single-Walled Carbon nanotubes (SWCNT) as additives for lubricants offer large load carrying capacity, high thermal conductivity, and excellent mechanical stability. SWCNTs have the ability to trap free radicals and function as an anti-knock additive (inhibiting pre-ignition). Carbon nanotubes also act as a burning rate catalyst, promote clean-burning and suppress smoking[3]. The functionalization of SWCNT improves the dispersive rate of SWCNT in oil and water. The cage-like tubular morphology of SWCNT helps to slide and roll between the sliding surface, resulting in low friction and wear[4].
J.L. Margrave et.al studied that chemical treatment of HiPCO® SWCNT surface shows ultra-low friction properties and is found to be applicable as a good solid lubricant. The friction coefficient of fluorinated-SWCNT is measured to be in the range 0.002–0.3, which lies below the graphite value (most commonly used solid lubricant) and similar to the best-known lubricants, such as Diamond-like carbon (DLC: 0.05–0.15) and Polytetrafluoroethylene (PTFE:0.03–0.1), or even outperform the latter. The fluorination of SWCNT causes the creation of reactive edge sites enhancing the binding of lubricant and adjacent surfaces. Also, the fluorination of the SWCNT surface helps to separate the tubes within bundles[4].
Humic acid(HA)-coated HiPCO® SWCNT showcases promising lubricant additive properties as a water-based additive. The Addition of SWCNT changes the friction forces between two mica surfaces of the lubricant from adhesion controlled (which causes wear/damage of the surfaces) to load controlled friction causing lateral sliding. No wear was observed by K. Kristiansen et.al. during continuous shearing experiments even under high loads after the inclusion of SWCNT. The high aspect ratio of SWCNT creates a high degree of disorder in the SWCNTs trapped between the contact region. The steric repulsion between the SWCNTs enables an additional repulsive force between mica surfaces. The trapped SWCNTs act as spacers between the mica surfaces and thus reduce the van der Waals attractive force, resulting in weaker adhesion compared to pure HA solution[5].
Gholamreza et. al studied the Viscosity of SWCNTs/oil suspension as a function of temperature at various concentrations. The results showed that the viscosity of the lubricant increased with decreasing temperature and increasing the SWCNT concentration. The viscosity index of oil-based SWCNT lubricant is increased by up to 32.94% at a weight fraction of 0.2%. The viscosity increases, because the amount of free volume in the internal structure decreases due to the occupation of SWCNT, causing high flow resistance. The lubricant tends to clump and become less volatile, therefore, the molecules move less freely and the internal friction forces increase[6].
Such novel properties of this wonder material are now made available on an industrial scale by NoPo. Further steps are being taken to scale-up the production of HiPCO® SWCNT to cater to the growing demand from innovators across the globe. Every day, we take one step towards the same.
References:
[1] J. A. C. Cornelio, P. A. Cuervo, L. M. Hoyos-Palacio, J. Lara-Romero, and A. Toro, “Tribological properties of carbon nanotubes as lubricant additive in oil and water for a wheel–rail system,” J. Mater. Res. Technol., vol. 5, no. 1, pp. 68–76, Jan. 2016.
[2] G. Yamamoto, T. Hashida, K. Adachi, and T. Takagi, “Tribological Properties of Single-Walled Carbon Nanotube Solids,” May-2008. [Online]. Available: https://www.ingentaconnect.com/content/asp/jnn/2008/00000008/00000005/art00075. [Accessed: 04-Nov-2019].
[3] D. Moy, C. Niu, H. Tennent, and R. Hoch, “Lubricants containing carbon nanotubes,” US6828282B2, 07-Dec-2004.
[4] R. L. Vander Wal et al., “Friction properties of surface-fluorinated carbon nanotubes,” Wear, vol. 259, no. 1, pp. 738–743, Jul. 2005.
[5] K. Kristiansen, H. Zeng, P. Wang, and J. N. Israelachvili, “Microtribology of Aqueous Carbon Nanotube Dispersions,” Adv. Funct. Mater., vol. 21, no. 23, pp. 4555–4564, Dec. 2011.
[6] G. Vakili-Nezhaad and A. Dorany, “Effect of Single-Walled Carbon Nanotube on the Viscosity of Lubricants,” Energy Procedia, vol. 14, pp. 512–517, Jan. 2012.
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