Nanoparticles as lubricant additives

Introduction

Lubrication refers to interposition of solid, liquid or gaseous material in between surfaces of two material in contact to minimize friction and wear. All commercially available lubricants have additives in their formulation which enhance certain properties of oil like friction reduction, wear reduction, corrosion or rust inhabitation, pressure resistance and viscosity improving. Nanoparticles are widely used as additives.

With the onset of Nanotechnology, use of Nanoparticles as additives in lubricant for achieving better tribological properties has gained much attention in recent times.

What is a nanoparticle?

A nanoparticle is a small particle that ranges between 1 to 100 nanometres in size (100nm is 25 times smaller than ~25 micrometer – diameter of a human hair). Undetectable by the human eye, nanoparticles can exhibit significantly different physical and chemical properties to their larger material counterparts.

How a nanoparticle can be made?

Types of nanoparticles used as additives in lubricant

Lubrication mechanism

Ball Bearing Effect: The spherical nanoparticles act as nanometric ball- bearings, which roll between the shearing surfaces, changing the nature of the friction phenomenon and modifying it from a sliding process to a mix of sliding and rolling friction.

Protective film formation: The nanoparticles form a thin protecting film on the surface. There are a number of studies which show friction as well as wear reduction by this mechanism of protective film formation.

Mending effect or self-healing effect: In this mechanism, nanoparticles deposit on the interacting surfaces and compensate for the loss of mass. Nanoparticles in the lubricating oil have the ability to fill scars and grooves of the friction surface. Majority of studies reporting the mending effect, have used EDX analysis to confirm the deposition of nanoparticles on the rubbing surface.

Polishing effect: The polishing effect is believed to be reported when roughness of the lubricating surface is reduced by nanoparticle-assisted abrasion. Polishing effect is also termed as smoothing effect in various studies. A reduction in surface roughness would contribute to less friction and thus less wear.

Effect of nanoparticle shape, size, surface, concentration and type of tribotest

Shape of Nanoparticle: Nanopaticle can be of any shape, it can be like platelets, spherical solids, spherical multilayered particles and nanoparticles with hollow core. The shape of nanoparticle can be selected on the basis of contact pressures between interacting surfaces. Spherical nanoparticles represents a much higher participation in most of the studies related to use of nanolubricants.

Size of nanoparticle: The size of nanoparticles determines their intrinsic mechanical and physicochemical properties that, which in turn influences their tribological properties. In certain regimes, as the size of nanoparticle increases, the hardness increases. When nanoparticles that are harder than shearing surfaces, are placed between these shearing surfaces, they indent and scratch the surfaces. Thus, the size induced variations in hardness of nanoparticles must be kept in mind while designing nanoparticle based lubrication systems.

While considering the suitable nanoparticle size selection, the ratio of root mean square roughness of lubricated surface to the radius of nanoparticle is an important parameter. If the nanoparticles size is too big as compared to gap between asperities, they will not deposit on the contact zone which lead to poor lubrication.

Surface functionalization: Surface functionalization plays two different roles in the context of nanoparticle-based lubricant formulations; it regulates colloidal stability of nanoparticle dispersion and enhances the lubricity of the nanoparticles’ outer most layers.

Nanoparticle concentration: Concentration of nanoparticles within base-oil strongly influences the tribological properties. Optimum concentration of nanoparticle is required for reducing friction and wear.

Nature of tribo-testing: Unlike dissolved additives, for nanoparticles the rubbing surfaces enhancement is also strongly dependent on the mechanics of the rubbing contact and the presence of separating oil films. Tribological effectiveness of nanolubricants is likely to be very system-specific and as a result, very different performance is likely to be observed in different test conditions.

References

1. Twi-global.com. What are Nanoparticles? Definition, Size, Uses and Properties. [online] Available at: <https://www.twi-global.com/technical-knowledge/faqs/what-are-nanoparticles> [Accessed 9 February 2022].
2. Peña-Parás L., Maldonado-Cortés D., Taha-Tijerina J. (2019) Eco-friendly Nanoparticle Additives for Lubricants and Their Tribological Characterization. In: Martínez L., Kharissova O., Kharisov B. (eds) Handbook of Ecomaterials. Springer, Cham. https://doi.org/10.1007/978-3-319-68255-6_72
3. Akbulut M (2012) Nanoparticle-Based Lubrication Systems. J Powder Metall Min 1:e101. doi:10.4172/2168-9806.1000e101
4. Technological improvements in lubricant development due to recent advances in nano additives – By Dr. Raj Shah, Mr. Blerim Gashi, Mr. Avishek Mojumdar, Dr. Steve Nitodas Jan 26, 2022. (Nanowerk Spotlight)
5. Yan, Tianhao & Ingrassia, Lorenzo & Kumar, Ravi & Turos, Mugurel & Canestrari, Francesco & Lu, Xiaohu & Marasteanu, Mihai. (2020). Evaluation of Graphite Nanoplatelets Influence on the Lubrication Properties of Asphalt Binders. Materials. 13. 772. 10.3390/ma13030772.
6. Gulzar, Mubashir & Masjuki, H.H. & Kalam, M. A. & Varman, Mahendra & Mohd Zulkifli, Nurin Wahidah & Mufti, Riaz & Zahid, Rehan. (2016). Tribological performance of nanoparticles as lubricating oil additives. Journal of Nanoparticle Research. 18. 223. 10.1007/s11051-016-3537-4.

Images

All images have been drawn using Microsoft Word and Microsoft Paint. The images reflect the scenarios and are similar to images present in above mentioned references. These are not screenshots from literature mentioned in references.

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