# Studying the Tribological Properties of Coffee Oil-Loaded Water-Based Green Lubricant
###### Abstract  
Lubrication is the primary solution to reduce friction and wear. However, conventional lubricants cause pollution when not properly disposed of or due to accidental leaks. Therefore, environmentally friendly lubricating fluids are welcome in any application where they can meet the performance requirements. This study suggests using coffee oil produced from spent coffee grounds to improve the lubricity of water-based lubricating fluid. Bis(2-hydroxyethyl)ammonium oleate protic ionic liquid facilitates the dispersion of coffee oil in water. Kinematic viscosity, wettability, corrosion prevention, and lubricity tests were performed to evaluate the tribological properties provided by these additives. It was observed that a higher amount of coffee oil could be dispersed with the introduction of a higher amount of prolic ionic liquid. In this study, ten wt.% of coffee oil was successfully dispersed using one wt.% of protein ionic liquid. Introducing additives increased dispersions' viscosity, improved wettability, provided protection against corrosion, and reduced wear and friction. It was proposed that polar molecules of protic ionic liquid were responsible for most of the improvement, while coffee oil contributed by increasing viscosity. Further studies could be directed toward determining rational concentration to meet each particular application's requirements.  
coffee oil; protic ionic liquid; additive; lubricant; friction; wear; environmentally friendly lubricating fluid; water; solid =0  
These mixtures are further modified using various additives to improve their properties. For example, [PERSON] et al. [8] used protic ionic liquids as additives in glycerol aqueous solution. The additive-loaded samples showed enhanced lubricity. This was ascribed to its adsorption layer and tribo-film formation. [PERSON] et al. [11] reported nanodiamonds which, in combination with ammonium palmitate ionic liquid, enhanced the lubricity of water. It was observed that these additives could reduce wear rates by more than two orders of magnitude with respect to additive-free water.  
Vegetable oils can also be blended in water. An oil-in-water lubricating mixture is typical for producing water-based lubricating fluids used in metalworking applications. However, vegetable oils are not soluble in water, and a certain amount of emulsifier is required for dispersion [5; 12]. Usually, one to twenty wt.% of oil in water is applied. [PERSON] et al. [5] formulated green cutting fluid using a six wt.% of raw coconut oil in water. Moreover, emulsifiers, corrosion inhibitors, and other additives were applied. The investigated formulation outperformed commercial alternatives in the lubrication of a tool-workpiece friction pair. However, the authors note that different applications may require individual formulation.  
Coffee grounds are coffee brewing waste. About 25 million tons of coffee grounds are generated annually. Coffee grounds contain various organic compounds such as oil, fatty acids, lignocellulosic, and polysaccharides. This oil is unsuitable for food. Therefore, it could be an attractive raw material for producing lubricants and fuels [13; 14]. [PERSON] et al. [15] investigated coffee oil as a base fluid and used two phosphonium ionic liquids to enhance its tribological properties. It was observed that ionic liquids could improve friction and wear reduction ability.  
To our knowledge, coffee oil has never been tested as an additive in water-based lubricating fluids. Therefore, this study investigates the tribological properties of water-based green lubricating fluid formulated using coffee oil and protic ionic liquid. The wettability, corrosion prevention ability, and lubricity employing friction and wear were investigated. The proposed lubricating fluid contains more than 99 wt.% of renewable raw materials. Therefore, it is expected to be readily biodegradable.