‘Water’ being an abundant natural resource, can certainly be considered as a primary fluid in lubrication. As of now, most of the lubricants are petroleum-based because of their superior functionality compared to water-based lubricants(WBLs). Global concern of sustainability and the use of ecofriendly resources in addition to the prediction that the world oil reserve might be exhaust in the coming 50 years led a new path to research on WBLs. Though they are poor in friction reduction, high in corrosiveness and have low viscosity, they exhibit excellent cooling capabilities and expected to overcome their drawbacks when combined with a range of external additives.
Researchers have found that additives can enhance the properties of WBL at atomic level making them functionally competitive with their petroleum counterparts. Helping recognize potential use of eco-friendly, reusable fluid as a primary component in lubrication.
Additives act as antioxidants, anti-foaming agents, corrosion inhibitors, detergents, friction modifiers, wear improvers, metal deactivators and viscosity index improvers. They can be briefly divided into two categories i) Water-soluble organic compounds and ii) Solid particles. Water soluble organic compounds work well with polar lubricants. As, organic compound molecules interact with the polarity or electrostatic attraction of surface materials and form a thin film. Major water-soluble organic compounds include ionic liquids and bio-based oils.
Ionic liquid additives are made up of salts of cations and anions with melting points lower than 100°C. Popularly used cations are imidazolium, pyridinium, ammonium and phosphonium. Though we have anions which are thousands to choose from, tetrafluoroborate and hexafluorophosphate are seen commonly. Ionic fluids have the capacity to handle severe conditions like under extreme pressure and temperature. They are thermally stable, non-flammable, highly conductive, with negligible vapor pressure and are miscible with water and other organic solvents.
Bio-degradable lubricants are made using oils found naturally in our environment. These oils are extracted from various biological sources, such as soyabeans, sunflowers, coconuts and other plants. The process used to extract these lubricants is called pyrolysis.
Fig1. Pyrolysis process (Taken from )
The potential application for water-based bio-lubricant is in automobile industry. In bio-lubricants, vegetable oils are observed to perform in par with mineral oils and in few cases even better than the latter. Their performance can be enhanced by adding Carbon or Boron. The sole disadvantage of these oils with respect to mineral oils is that they lack oxidative stability for lubrication.
Water-Based Lubricants with solid nanoparticles not only act as lubricants but also as coolants. They provide longer tool life, reduced thermal deformation and low friction. But they have low viscosity (but even worse, lower pressure-viscosity coefficient) and corrosive properties. Typical nano-particle additives used for WBL’s include Titanium and Graphene. Titanium based nano-additives help lower power consumption, improve the load carrying ability of water, reduce friction and wear. Moving to the advantages of Graphene based additives, if we add a mere 4 mg/mL of Graphene quantum dots into water the friction and wear rate will decrease 58.5% and 42.5% compared to pure water solution. This is considerable amount of enhancement. Other relative Graphene based additives exhibit anti-corrosion effect. Recent researches have found that inclusion of graphene as an additive decreases wear by four orders in magnitude and friction co-efficient by six. Silicon dioxide also has been drawing considerable amount of interest from researchers. Being a low-cost additive to water, it certainly has its economical advantage. Additionally, SiO2 adds rolling effect between surfaces of interest decreasing frictional value.
Consideration of nano-particles as additives has become significant in recent years. They are considered to be harmless as they are relatively small in size and are also chemically stable. They are the most ideal pick due to their eco-friendliness and overall simplicity. Main advantage while using nano-particles is that, we can combine different compositions and make new combinations of WBL’s. The application of WBL’s based on nano-particles can be widely observed in industrial scale hot steel rolling process.
Fig2. Demonstration of TiO2 as and additive in a lubricant (Taken from )
Currently the focus of using nano-scale additives is on achieving superlubricity. Recent research on WBLs with sulfuric acid proves that this is possible. During this process a silica layer is formed allowing the surfaces to glide over each other.
Properties effected by adding additives in WBL:
Viscosity of water is considered to be a major drawback in WBLs. Use of additives improve its value allowing better cavity formation and film thickness. The frictional value decreases and thermal behavior improves based on this film formation. Additionally, a reduced strain rate curve is also observed when viscous modifiers are used.
Density has a positive effect by addition of additives in WBLs. With the increase of density (by the addition of solid nano-particles) the thickness of film formations will also increase decreasing friction.
Wettability is the ability of the liquid to maintain contact with surface with either adhesive or cohesive process. Increased wettability is associated with better film formation. Addition of nano-particles will increase the wettability of WBLs. Graphene and graphene oxides exhibit reduction in the friction co-efficient by improving the lubricant’s wettability.
Solubility is the ability of a substance to dissolve in a solvent. This is one of the most important properties to be considered as the solubility of additives with respect to water will largely affect its behavior as a lubricant. This property is highly dependent on the type of additive used.
In conclusion, the referred review paper published by a group of researchers from USA and India provided in-depth information of the types of Water based lubricants used, their properties and performances. This understanding helps us identify drawbacks and inadequacies in WBLs and what to expect in coming years from this field of research.
 Md Hafizur Rahman, Haley Warneke, Haley Webbert, Joaquin Rodriguez, Ethan Austin, Keli Tokunaga and Dipen Kumar Rajak, Water-Based Lubricants: Development, Properties and Performances, https://doi.org/10.3390/lubricants9080073