Lubricants are any substances that are introduced between surfaces or components that move over each other, with the purpose of reducing the friction between them and thus the wear on them. A natural example of a lubricant is synovial fluid which allows bones to slide smoothly over one another at the highly mobile synovial joints in the human body.
The earliest artificial lubricant may have been animal fat, but today almost all are petroleum derivatives. The years have seen the emergence of better lubricants designed for different applications from powerful industrial machinery to transport vehicles and aircraft.
What are the General Types of Lubricants?
Lubrication may be classified as fluid-film, boundary and solid. In the first type, a film of fluid interposes between the sliding surfaces. The film must have a pressure that can counteract the load being applied between the contact parts, and this may be supplied from outside or because of the way the surfaces are designed.
Boundary lubrication is typically observed during the starting or stopping of machines, and depends on the non-viscous characteristics of the lubricant, while fluid-film lubrication is dependent on the viscosity. With solid lubrication, such as graphite, the resistance to load and to harsh temperatures is higher than is possible with either of the other two.
Modern lubrication is associated with the work of Osborne Reynolds at the beginning of the 20th century. He observed that with a shaft rotating with bearings and cases, the applied lubricant was drawn in wedge shape between the shaft and the bearing by the rotation of the shaft. The greater the rotational speed, the higher the rate at which the lubricant flowed between the surfaces.
The rate of lubricant flow was dependent on the viscosity of the liquid, which was responsible for the liquid pressure in the wedge of lubricant. This pressure actually maintained the working separation between the contact surfaces. With ideal lubricant flow, the pressure would constantly avoid friction between the sliding parts.
With boundary lubrication, flow is essential because this helps maintain the sliding contact without much resistance to shear, and good tolerance to temperature rises.
Engine Oil as an Example of a Lubricant
One of the most commonly used lubricants is engine oil lubricants, which keeps moving parts in the engine operating properly despite variations in the temperature, speed and pressure at which they have to work.
One required condition is that the lubricant must flow well so as to keep the oil between the moving parts even when the temperature is low. Similarly, with high temperatures, the parts must not be allowed to wear down. An ideal lubricant reduces friction and wear, while acting as a heat sink.
One measure of lubricant flow is pour point testing, that is, the lowest temperature at which gravity-dependent flow occurs. If the oil is viscous, it stops flowing once the temperature drops, due to the excessive viscosity.
For machines which often have to start from the cold, highly viscous lubricants cannot circulate or be supplied to all the vital parts of the engine. This prevents rapid starting, increases the wear and may even endanger operation.
In this respect, nanoparticles added to lubricant oil have been under the spotlight. The most commonly used nanoparticles include metals like gold, silver, copper and metal oxides like dioxides of silicon, titanium or zirconium. Their lubricant action is attributed to:
- Direct effect of the protective nanoparticle film
- Ball bearing effect
- Surface enhancement by nanoparticle deposition which artificially smooths out the surface
Some important characteristics of lubricants based on nanoparticle additives include:
The size of the nanoparticle determines its mechanical, physical and chemical properties, and hence its tribology. Below a grain size of 10 nm, the hardness reduces, but above 100 nm, it increases. Harder particles may cause damage to the surfaces.
The melting point of nanoparticles also drops steeply below 50 nm; thus the lubricant must be designed to fit the operational temperature profile of the system.
Nanoparticle size must be maintained within a range such that they can adhere to and smooth out irregularities in the contact surfaces.
Lastly, the particle must disperse homogeneously. When the nanoparticle size is reduced ten-fold, sedimentation rate reduces a hundred-fold.
Nanoparticle Shape and Structure
At any load, sheet-form nanoparticles experience less pressure and are tribologically less efficient than nanospheres. Again, limited internal atomic cavities enhance mechanical strength, but additional defects reduce Young’s modulus in a nanotube.
Layered compounds composed of polyhedral fullerene-like inorganic nanoparticles (such as MoS2 and WS2) roll and slide between the surfaces in shearing contact, while the layered structure allows the breakdown to cover the surfaces with thin films that lower friction significantly.
Functionalization of Nanoparticles
The high surface energy of nanoparticles of all sizes promotes aggregation, especially when the temperature or pressure rises. Adding functionalized surfaces such as surfactants or polymers to nanoparticles allows steric stabilization, limiting van der Waal interactions and limiting aggregation in the base lubricant.
Secondly, these coatings prevent material transfer and cold-welding of contact surfaces. The hybrid structure means these nanoparticles are hard inside and soft outside, making them both slippery and rigid. This enables high lubricant functionality coupled with excellent load-bearing capability.
Lubricant viscosity is observed to decrease in proportion to the concentration of nanoparticles with increasing temperatures, until a certain point. After this, viscosity increases with temperature.
Issues with Nanolubricants and Possible Solutions
The ideal nanolubricant will show even dispersion of the nanoparticles in the base liquid irrespective of the load applied.
Various metal nanoparticles using soft metals have disadvantages including failure of self-repair under high sliding loads, poor heat transfer, inadequate relubrication and requirement for sophisticated application procedures. Nanodiamonds may increase wear and tear over the long term, but could be useful in improving the surface finish in the running-in stage.
Surfactants inhibit agglomeration but do not ensure even dispersion of the nanoparticles throughout the base liquid. They introduce contaminants into the base fluid, may cause foam formation with elevated temperatures, and could increase corrosion. These are avoided with the use of capping agents, but these are more difficult to use. Polymer brushes used as coatings for inorganic nanoparticles present a unique tunable combination which can produce stable dispersion and excellent wear resistance by cushioning the asperities on the shearing surfaces.
Nanolubricants show the potential to improve lubricant flow and function, but the development of marketable formulations is not yet in sight.