## 5 Conclusions  
In this study, completely biogenic lubricating greases were tribologically investigated using a ball-on-disc tribometer and compared with two traditional reference greases. A linear relationship was observed between the friction coefficient and the wear data for most of the examined model grease samples. This result revealed that the lower the friction coefficient was, the lower the wear volume that occurred on the steel plate due to the friction process. Moreover, the results showed how the different compositions of model greases highly influenced the frictional and wear behaviors. Using [PERSON]'s energetic wear model, the apparent friction energy density (\(e_{R}^{*}\)), the intensity of wear (\(I_{h}\)), and the friction shear stress (\(\tau_{R}\)) for all the model greases were experimentally estimated and the relationship between these tribological parameters was calculated. Plotting these three associated tribological parameters on the Fleischer's graph of the basic energy equation of wear for the model greases allowed for the determination of the friction and wear states the experiments were conducted in, which was a combination of fluid friction and mixed friction for all the grease samples. In addition, the estimation of the apparent friction energy density revealed that the grease-lubricated contacts with high friction energy densities led to low wear intensities on the steel plate. In order to assess the tribological behavior of the biogenic greases in real machine elements, rolling bearing power loss and rolling bearing wear tests were carried out with two selected biogenic grease samples. Results from the power loss tests showed that model grease 2, which had a lower base-oil viscosity, provided smaller friction torque values than model grease 1 under the operating conditions of the experiment. Comparing the tested greases, model grease 2 showed a better overall performance, a slightly lower internal torque loss, and less wear particle generation even though both model greases were similar in composition, with sample 2 containing sorbitan monostearate instead of beeswax in the thickener blend. Regarding the wear tests, similar conclusions were found, i.e., higher friction torque was associated with greater wear. Model grease 1 exhibited higher wear indexes and showed worse performance than model grease 2. Moreover, both grease samples showed very dark coloration after the test, and a considerable number of wear particles were found, some with large dimensions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Identification** & **Power Loss Tests** & \multicolumn{2}{c}{**Wear Tests**} \\ \hline Sample & 1 & 2 & 1 & 2 \\ Bearing Load (kN) & 5 & 5 & 5 & 5 \\ Number of cycles & 250–300 k & 250–300 k & 1440 k & 1440 k \\
**Ferrometry** & & & & \\ Large Particles Index (DL) & 67.7 & 17.7 & 86.0 & 57.1 \\ Small Particles Index (DS) & 17.7 & 5.1 & 30.7 & 17.7 \\ Particle Concentration Index (CPUC) & 85.4 & 22.8 & 116.7 & 74.8 \\ Severity of Wear Index (ISUC) & 4270.0 & 287.3 & 6453.5 & 2947.1 \\
**Ferrograph** & & & & \\ Normal Wear & Strong & Medium & Strong & Strong \\ Severe Wear & Strong & Strong & Medium & Medium \\ Fatigue Wear Particles & Medium & Medium & Medium & Medium \\ Adhesion Wear Particles & Fair & Fair & Fair & Fair \\ Friction Polymers & Fair & Fair & Fair & Fair \\ Black Oxides & Medium & Fair & Strong & Strong \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Ferrometry and ferrography results of the power loss and wear tests. **Author Contributions:** Most of the experiments and writing were done by [PERSON], [PERSON], [PERSON], and [PERSON]. [PERSON] contributed by supervising the experimental work and writing the paper. Rolling bearing tests were performed by [PERSON] and [PERSON] at INEGI (Institute of Science and Innovation in Mechanical and Industrial Engineering). [PERSON] designed and synthetized the bio-thickeners of samples S1-S4 at Pro2 TecS (University of Huelva). All authors have read and agreed to the published version of the manuscript.