Nanolubricants

Nanotechnology has gifted the mankind with particles which are much smaller yet more powerful as they exhibit superior mechanical, electrical, optical properties as compared to the micro particles. It has been reported that the bulk properties of materials can be enhanced with the inclusion of theses nano sized particles. Wear, being an inevitable mechanism in all mechanical systems, needs to be controlled effectively. Lubricantion is the most common way of reducing wear. A lubricant consists of a base oil and additives, each of the additives plays a major role in enhancing the quality of a lubricant. Among these additives, friction modifiers play a major role in controlling friction. Sulphur, phosphorus, graphite, molybdenum di sulphide, zinc, etc. were the most commonly used friction modifiers but they were used as micro size additives. With the advent of nanotechnology, the nano particles attracted the lubrication specialists and significant enhancement of anti-wear properties have been reported. Nano-additives both carbonaceous and non-carbonaceous particles help in reducing the friction coefficient either by acting as rollers between the mating surfaces, forming a protective layer , filling up the gaps on the mating pairs and even polishing the mating pairs [1-4]. Figure 1 shows the protective layer formation by nano friction modifiers preventing the contact between mating pairs.

Figure 1. Possible lubrication mechanisms by the application of nano-oik between frictional surfaces [1].

Wu et al. [75] studied the lubrication performance of TiO₂ nanoparticles-based lubricants using a ball-on-disc tribometer. The balls were of E52100 (9.5mm, Ra 0.02µm, 780HV) and the discs (40mm in diameter, 80 mm thickness, 90HV) were of low carbon micro alloy steel. TiO₂ was added as additive to a water-based lubricant in different mass fractions (0.2 to 0.8 wt.%). It was reported that the coefficient of friction decreased by 49.5% and 97.8% respectively as compared to the results of dry conditions. Further, it was reported that TiO₂ performed better at lower fractions. The best results was reported at 0.8% TiO₂ and the optimal concentration of TiO₂ was found to be at 0.8% beyond which the tribo properties of the lubricant decreased. The mechanisms involved in the formation of the tribo films were rolling and mending. Asnida et al. [6] dispersed copper (II) oxide nanoparticles (0.005 vol% to 0.01 vol %) in SAE 10W-30 in order to reduce the wear in piston-liner contact. They conducted the tests using a piston skirt-liner tester (speed: 200 rpm, 250 rpm, 300 rpm and load: 2N, 5.5N and 9N). The tests were conducted using aluminium 6061. CuO nanoparticles were seen to improve the tribological properties of SAE 10W-30 at 0.008% concentration.

Battez et al. [7] studied the tribological properties of zinc oxide nano particles dispersed in polyalphaolefin oil using a four ball tribometer. They also reported the dispersibility of zinc oxide nano particles using dispersants which were esters (OL100 and OL 300). They reported that zinc oxide did not perform satisfactory as an anti-wear additive but they were good extreme pressure additives. The enhancement of the extreme pressure property is due to the diffusion of zinc oxide nano particles on the wear tracks. Furthermore, both the esters also proved to be better anti-wear agents. In the recent years, nano-lubricants are being used in various mechanical applications. Sharma et al. [8] investigated the influence of alumina-graphene as additive in cutting fluids. The nano-lubricant was in various volumetric concentrations (0.25, 0.75 and 1.25 vol %). When using the nano-lubricant, the tool flank wear and nodal temperature reduced by 12.29% and 5.79% as compared to the alumina-based lubricant. Cornelio et al. [9] used functionalized single and multi-wall carbon nano tubes as additives in oil-based and water-based solutions. The functionalized multi wall carbon nano tubes (0.01% and 0.05% of MWCNTs and SWCNTs) were dispersed in the solution by mechanical stirring (30minutes) and ultrasonic agitation (1 minute) and the tribology tests were performed using a twin disc machine in order to simulate the wheel-rail contact (as in railway systems). They reported the enhancement in the tribological properties of oil with the addition of 0.01% multi-wall carbon nano-tubes and with 0.05% multi-wall carbon nano-tubes in water. The improvement in the tribological properties was reported due to the formation of carbon layer from carbon nano tubes. In other words, the formation of a tribo-film and the mending effect were the most plausible mechanisms.

Khalil et al. [10, 11] dispersed multi-wall carbon nano tubes (0.1, 0.5, 1.0, 2.0% wt.) in paraffinic mineral oil and Mobil gear 627. They reported that the wear decreased by 68% and 39% when carbon nano tubes were added to the mineral oil as compared to the base lubricants (paraffinic mineral oil and Mobil gear 627). The weld load of 1 wt.% multi wall carbon nano tube added mineral oil was found to be at 400kgf and 125 kgf as compared to the Mobil 627 gear oil (200kgf) and paraffinic gear oil (100 kgf). Bhaumik et al [12] dispersed multiple nano-friction modifiers in castor oil and reported a decrease of 45-50% in coefficient of friction and a significant 87.5% decrease in the wear scar diameter as compared to a commercial mineral oil containing Sulphur and phosphorus.

From the above discussions it can be seen that the nano-friction modifiers exhibit better tribological properties than the micro-particles added lubricants. The nano-friction modifiers can be plausible replacement of conventional lubricants. The usage of these nano-lubricants in gear boxes will reduce the problems of lubricant change due to lesser wear during operation which will indirectly reduce the cost of maintenance of the machinery.