Nano-additives show promise in reducing friction and wear

Dr. Raj Shah, Dr. Steve Nitodas and Isaac Kim

Nanoparticles, which range from metals and metal oxides to carbon nanomaterials, have started to play an important role as lubricant additives because of their exceptional thermal stability at elevated temperatures.¹

The size of nanoparticles enables them to adhere to and smooth out irregularities in the contact surfaces.  Furthermore, their high surface-to-volume ratio facilitates interaction with their environment.

Nanoparticle additives or nano-additives have a particle size of less than 100 nanometer (nm) in at least one dimension. In comparison, a human hair is approximately 80,000- 100,000 nm wide.

Nano-additives have demonstrated their ability to reduce friction and wear of lubricants even at concentrations of less than 1 wt%. This has been made possible by several methods:²

1. The ball-bearing effect, where nanoparticles effectively act as ball bearings between two surfaces, thus reducing friction and wear;    
2. Protective film effect, where the nanoparticles form an anti-wear film;    
3. Mending effect, where nanoparticles deposit onto a surface and compensate for the loss of mass;    
4. Polishing effect, where nanoparticles act as an abrasive, reducing surface roughness.

With the growth of nanotechnology, it is not surprising that nanomaterials constitute one of the most revolutionary technologies in the field of tribology. Due to the myriad of materials, extensive research has been conducted to find the nano-additives with the optimum performance.

One factor that has been extensively considered is the high surface energy of nanoparticles that promotes aggregation, especially when the temperature or pressure rises. Chemical modification or functionalisation of the nanoparticle surface is the key to the uniform dispersion of nanoparticles in the base lubricant and the synthesis of high-performance lubricants.³ The surface coating with the functional groups prevents material transfer and cold-welding of contact surfaces. The hybrid structure means these nanoparticles are soft outside and rigid inside, enabling good lubricant functionality with excellent load-bearing capability.

Carbon nanotubes (CNTs) exhibit a unique combination of exceptional properties such as high hardness, high strength, and excellent thermal conductivity. Owing to their very high thermal conductivity (2000 W/m.K compared to thermal conductivity of Ag 419 W/m.K), CNTs have become one of the most promising nano-additives in lubricating oils. Among their excellent properties are good thermal conductivity, enhancing the efficiency of heat dissipation for the engine, and improving the overall performance efficiency of the engine.⁴

With respect to carbon nanomaterials, Bhaumik et al. studied the effects of multi-walled carbon nanotubes (MWCNT) and graphite in industrial gear mineral oil with a viscosity of 250 cSt utilising a four-ball test and a pin-on-disk test.⁵ A previous study by Curasu et al. found that the optimal concentration of single wall carbon nanotubes (SWCNT) was 0.5 wt%, where the smallest value of friction coefficient was observed.⁶      

Bhaumik et al. created three oil formulations containing MWCNTs with concentrations of 0.1 wt%, 0.5 wt%, and 0.6 wt% to test that theory. A total of five oil formulations were created. Three MWCNT oil formulations, one oil formulation with 0.5 wt% graphite, and one oil formulation without additives (reference sample). The coefficient of friction values from the four-ball test are summarised in Figure 1. The 0.5 wt% MWCNT oil formulation was shown to reduce the coefficient of friction immensely. In terms of wear, the 0.5 wt% MWCNT oil formulation also performed better than the other four formulations. The graphite formulation (41 µm), and the mineral oil formulation (56 µm) came in last.

­­­The reduced performance of MWCNTs beyond 0.5 wt% can be attributed to the agglomeration and precipitation of carbon nanotubes, due to strong electrical forces between the carbon atoms, known as Van der Waals interactions, thus reducing the potential for friction and wear reduction with higher concentrations of CNTs. Van der Waals interactions occur when adjacent atoms come into close proximity, such that their outer electron clouds somewhat overlap. This induces a fluctuation in charge that causes the atoms to feel an attractive force with each other leading to the aforementioned agglomeration/precipitation. Researchers have found ways to improve the dispersion of carbon nanotubes with several methods, such as modifying them with ionic liquids based on imidazolium cations⁷, as well as the use of surfactants to stabilise the nanoparticles in the lubricant.⁸ Metal nano-additives have also shown promise with regard to reducing friction and wear.      

Metallic nanoparticles have unique chemical and physical properties that render them ideal as lubricant additives.⁹ For example, copper (Cu) exhibits low shear stress and is softer than ceramics (metal oxides). The lubrication mechanisms of metallic nanoparticles are:

1. Formation of a tribofilm or an adsorption film. These films could change the surface properties and separate two contacting surfaces, resulting in promising tribological performance;
2. Compacting of the nanoparticles on the wear track due to heat and pressure generated during the friction process. This phenomenon is known as sintering or repair effect.¹  

Padgurskas et al. studied the utilisation of iron (Fe), copper, and cobalt (Co) as nanoparticles in an SAE 10 mineral base oil, to compare metal nanoparticles. Each mixture utilised 0.5g of nanoparticles to 100 mL of base oil, with a four-ball tribotester.¹⁰ The results of the tribotest clearly show the improvement of the SAE 10 antifriction performance, with the introduction of metal nanoparticles in the oil formulation. Compared to the base SAE 10 oil, Co, Fe, and Cu reduced the coefficient of friction by approximately 20%, 39%, and 49%, respectively. The reduction in friction was attributed to the nanoparticle’s ability to fill micro-asperities. The addition of Fe or Cu nanoparticle additives allowed the new oil formulation to decrease its coefficient of friction. According to surface analysis, these trends can be attributed to the formation of a protective film. Additional studies on metal nano-additives have confirmed the findings of Padgurskas et al. that mineral oil formulations with copper nanoparticles performed best, in terms of friction and wear reduction.^11,12    

The future of the lubricant industry is currently being driven by the incessant need to reduce its environmental footprint. While research has shown a multitude of promising renewable alternatives to petroleum products, it is not realistically possible to simply switch from one to the other. Thus, it is important to improve upon current technology during the transition period. Nano-additives provide a pathway for the lubricants industry during this transition and hold seemingly limitless potential. The chemical composition of nanoparticles is important to consider for improving anti-wear performance, whereas the morphology of nanoparticles is mostly critical for friction reduction. The size of nanoparticles, which is governed by specific operating conditions and lubrication regimes, can also be adjusted to optimise friction and wear.

References

¹ Dai, W., Kheireddin, B., Gao, H., Liang, H., “Roles of nanoparticles in oil lubrication,” Tribology International, Volume 102, pages 88-98, 2016.

² Lee, K., Hwang, Y., Cheong, S. et al. “Understanding the Role of Nanoparticles in Nano-oil Lubrication,”      Tribology Letters 35,127–131,2009.

³ Chen, Y., Renner, P., Liang. H., “Dispersion of Nanoparticles in Lubricating Oil: A Critical Review, Lubricants,” 7, 7, 2019.

⁴ Hong, N.M., Bui, T.H., Hong. P.N, Hong, T.H., Khoi, P.H., Minh, P.N., “Carbon nanotubes based lubricating oils for UAZ 31512 engines,” Micro & Nano Letters, 11, p. 636-639, 2016.

⁵ Bhaumik, S., Prabhu, S. and Singh, K.J., “Analysis of tribological behavior of carbon nanotube based industrial mineral gear oil 250 cSt viscosity,” Advances in Tribology, Article ID 341365, Volume 2014.

⁶ D. L. Curasu, C. Andronescu, C. Pirva, and R. Ripeanu, “The efficiency of Co-based single-wall carbon nanotubes (SWNTs) as an AW/EP additive for mineral base oils,” Wear, vol. 290-291, pp. 133–139, 2012. ISSN 0043-1648.

⁷ Yonghui Liu, Li Yu, Shaohua Zhang, Jie Yuan, Lijuan Shi, Liqiang Zheng, “Dispersion of multiwalled carbon nanotubes by ionic liquid-type Gemini imidazolium surfactants in aqueous solution,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 359, Issues 1-3, 2010, Pages 66-70, ISSN 0927-7757.

⁸ Richa Rastogi, Rahul Kaushal, S.K. Tripathi, Amit L. Sharma, Inderpreet Kaur, Lalit M. Bharadwaj, “Comparative study of carbon nanotube dispersion using surfactants,” Journal of Colloid and Interface Science, Volume 328, Issue 2, 2008, Pages 421-428, ISSN 0021-9797.

⁹ Yang, G., Zhang, Z., Zhang. S., Yu, L., Zhang, P., “Synthesis and characterization of highly stable dispersions of copper nanoparticles by a novel one-pot method,” Materials Research Bulletin, 48, 1716, 2013.

¹⁰ Juozas Padgurskas, Raimundas Rukuiza, Igoris Prosyčevas, Raimondas Kreivaitis, “Tribological properties of lubricant additives of Fe, Cu and Co nanoparticles,” Tribology International, Volume 60, 2013, Pages 224-232, ISSN 0301-679X.

¹¹ S. Tarasov, A Kolubaev, S. Belyaev, M Lerner, F Tepper, “Study of friction reduction by nanocopper additives to motor oil,” Wear, Volume 252, Issues 1–2, 2002, Pages 63-69, ISSN 0043-1648.

¹² Y.Y. Wu, W.C. Tsui, T.C. Liu, “Experimental analysis of tribological properties of lubricating oils with nanoparticle additives,” Wear, Volume 262, Issues 7–8, 2007, Pages 819-825, ISSN 0043-1648.