Metal-Containing Nanomaterials As Lubricant Additives: State-Of-The-Art And Future Development
Recently, the use of nanomaterials as lubricant additives (also known as nanolubricants) has become an important research area. The nanolubricant approach is used to overcome the drawbacks of conventional anti-wear and anti-friction additives associated with the need for chemical reactions with substrates, and hence the induction period for obtaining a tribo-film on the friction surface.
The main advantages of nanoparticles (NPs) are their size in the nanometer range, which is well adapted for the ideal filling of the friction interface, allowing the combination of several properties, including anti-wear (AW) and extreme pressure (EP) additives, as well as friction modifiers (FM). Owing to their low melting point and high chemical reactivity, NPs can deposit on micro defects of friction surfaces and, to some extent, play the role of “self-repairing”.
In addition, NPs have higher thermal conductivity than the base fluid, which facilitates the release of the heat generated by friction and contributes to the stability of the tribo-pairs. An essential advantage of nanolubricants is that they do not require triboactive elements such as phosphorus and sulfur to improve the tribological properties of the base oil, exhibiting excellent friction, and wear reduction characteristics. NPs are of considerable interest for improving the properties of biodegradable lubricants.
Finally, most NPs are environmentally friendly, as they minimize the use of hazardous materials and additives, which is useful for environmental and economic sustainability. In addition, eco-friendly NPs may also facilitate the reduction of energy consumption in production processes, thus leading to a reduction of the carbon footprint.
Nanolubricants meet the requirements of green tribology, which is a new area for a large number of tribologists. To date, a large number of nanomaterials used as additives to lubricating oils, in particular, carbon materials, carbon nanotubes, graphene oxide, boron nitrides, and silicon oxide, have been obtained. However, the most extensive studies have been carried out with metal-containing nanomaterials, whose NPs contain, e.g., metals, their oxides, and sulfides. Importantly, some of the nanomaterials studied are commercial products, but most studies are concerned with self-made NPs.
Various chemical and physical methods are used to obtain NPs, and they continue to improve. An interesting example is the use of self-propagating high-temperature synthesis for the production of various tribological nanomaterials. It is well known that lubrication can be divided into three different regimes: boundary lubrication, mixed lubrication, and elastohydrodynamic/hydrodynamic lubrication. Among them, friction and wear are particularly high in boundary and mixed lubrication, which leads to high machine wear and energy loss. Consequently, lubricant additives are highly important in boundary lubrication owing to the higher coefficient of friction (COF).
Actual problems of reducing friction and wear require an adaptable lubricant for various operating conditions. Accordingly, a large amount of research has focused on the concept of nanolubrication in internal combustion engines as the main strategy for reducing COF and the wear of contact surfaces, which ultimately leads to improved tribological characteristics.
In most of the studies carried out, it is noted that the addition of NPs to the lubricant can increase its tribological characteristics, which largely depend on the composition of the lubricant. According to the available data, metal-containing nanomaterials account for 72% of the nanolubricants studied. Metallic NPs have unique chemical and physical properties as lubricant additives. Nano-metals with low shear stress, high extension, and low melting point have been used as FMs owing to their excellent friction-reducing, anti-wear, and self-repairing ability.
Among metallic NPs, Cu-containing nanolubricants have received particular attention owing to their remarkable properties. Copper NPs usually have small particle size, low melting point, and the desired ductility; therefore, they are well perceived as an excellent AW and EP agent in comparison with similar products [72, 73]. Copper NPs as an additive can significantly improve the tribological properties of lubricants, which allows the necessary lubrication of equipment.
A typical example is the use of two commercially available base oils with synthetic engine oil SAE 5W40 grades dispersed with 0.2 wt% Cu NPs. A significant reduction in friction and wear on the order of less than 13% was observed, and so was tribological performance of base oils. Cu nanolubricants form boundary films on friction surfaces, thus increasing tribo-efficiency by reducing friction and wear.
In another interesting example, the tribological properties of nanolubricants based on Fe, Cu, and Co NPs, which were added individually and in pairs into mineral oil, were estimated. Cu-containing nanolubricants significantly reduced friction and wear compared to other NPs when added individually. In particular, the presence of Cu, Fe, and Co NPs reduced friction by 49%, 39%, and 20%, respectively.
[Uflyand, I.E., Zhinzhilo, V.A. & Burlakova, V.E. Metal-containing nanomaterials as lubricant additives: State-of-the-art and future development. Friction 7, 93–116 (2019). https://doi.org/10.1007/s40544-019-0261-y]