Nanoparticles in lubricants
In the industry, most of the surfaces in mutual contact (i.e., bearings, seals and gears) are lubricated with specific oils in order to control friction and wear. Many scientific researches have been published on the tribological properties of nanoparticles-based lubricants and the phenomena regarding any mechanical system in relation to friction and wear, strongly depends upon the characteristics of the nanoparticles, such as shape, size and concentration within the lubricant.
Even a small concentration of nanoparticles (a few tenths of a percent by weight) could be sufficient to improve the tribological properties of the system. When the load between the sliding parts is small (low load conditions), friction reduction is mainly ascribable to the bearing-like behavior of nanoparticles, that roll between the contact surfaces, keeping their shape intact; for high load conditions, a coating, induced by the presence of nanoparticles, is deposited on the crests of surface roughness and it can reduce direct contact between the asperities, thus, minimize wear.
Because of the nature (inorganic and refractory) of the nanoparticles generally used as filler, the optimal performances achieved by the nano-lubricant can also be maintained in the working conditions at high temperature, thus avoiding the typical degradation of the traditional organic additives.
A major challenge to face, in order to scale up the use of nanoparticles as filler for lubricants, is related to their dispersion within fluids, that is often not uniform. Their small size, in fact, causes the attractive forces to rule over the other types of forces. This phenomenon generally causes aggregation and precipitation of nanoparticles. This issue shows that nanoparticles in lubricants need to be dispersed with other methods in order to optimize their stability, most likely they need to be surface-functionalized with organic treatments.