# Studying the Tribological Properties of Coffee Oil-Loaded Water-Based Green Lubricant
## 3 Results and Discussions
### Friction and Wear Reduction Ability  
The energy required to keep surfaces in relative motion is directly proportional to frictional losses. The mean coefficients of friction observed in these experiments are presented in Figure 7. It was expected that lubrication with additive-free water would result in an extremely high coefficient of friction, e.g., exceeding 0.35. However, the friction was significantly reduced when introducing coffee oil and protic ionic liquid additives. Interestingly, the amounts of the particular additive had a marginal effect on the mean coefficient of friction. Generally, introducing investigated additives reduced friction by 6.6 times; this reached the value of 0.051 in the case of W + CO10 + PIL1. Figure 6: Corroded filter paper observed after cast iron chip corrosion test when several CO and PIL concentrations were investigated. The variations of the coefficient of friction during the tribo-tests are presented in Figure 8. With an increased amount of coffee oil, the coefficient of friction became more stable. Moreover, samples with 2.5 and 5% of coffee oil possess running-in periods at the onset of the test. The duration of this period varied from 10 to 20 min. An increased amount of coffee oil, up to 10%, resulted in less varied friction. Moreover, the running-in period was not discerned in these cases. The explanation for this could be the higher viscosity of the lubricating sample. Lubricating samples containing 10% coffee oil have considerably higher kinematic viscosity (Figure 3). The formation of surfaces separating layers is more likely to occur when higher-viscosity lubricants are used. Therefore, friction was lower and more stable when more coffee oil was blended with water. The second additive counterpart, protic ionic liquid, also influences friction. In the cases of a higher amount of coffee oil, a higher amount of protic ionic liquid resulted in  
Figure 8: Variation of the coefficient of friction recorded during the tribo-test when lubricating samples having 2.5% (**a**), 5% (**b**), and 10% (**c**) of coffee oil where investigated. Figure 7: Mean values of the coefficient of friction observed in tribo-test. lower and more stable friction. The higher amount of protic ionic liquid increased viscosity, thus increasing lubricity of the lubricating samples. On the other hand, a higher amount of protic ionic liquid resulted in better oil dispersion. The homogenous dispersion provides better lubrication. The higher protic ionic liquid will also result in a better-adsorbed layer. In our previous study, we found that an increased amount of protic ionic liquid could increase friction and wear [29]. The tribo-corrosion was assigned responsibility for this. In the present case, the higher amount of protic ionic liquid combined with 2.5% of the coffee oil resulted in higher, unstable friction. Fortunately, the coefficient of friction stabilised after the running-in period. The longevity of lubricated mechanisms depends on the lubricants' ability to reduce wear. Similar to friction, wear reduction is achieved by separating interacting surfaces. The surface separation in the boundary lubrication regime could be implemented by forming adsorption or chemisorption layers, metal soaps, and friction polymers [30; 31; 32]. At higher sliding speeds, mixed lubrication takes place. In this regime, the viscosity of the lubricant determines surface separation. In the case of reciprocation, boundary lubrication occurs at the ends of the wear trace, while a mixed lubrication regime could be expected in the middle of the wear trace. The lubrication ability of investigated lubricating samples, expressed as wear, is presented in Figure 9. Figure 9a represents the wear scar diameter on the ball, while Figure 9b represents the wear volume. The WDS is easy to measure value, but it could not evaluate the wear on the plate. Therefore, wear volume is more comprehensive and representative of lubricity. In the present case, both inspected indicators showed a positive effect from investigated additives.