## 4 Results and Discussion  
### Apparent Friction Energy Density  
The average friction coefficient and the average volume of the wear tracks on the steel plate were obtained after performing the frictional tests in the tribometer; results from all the examined model greases are shown in Figure 5.  
According to the data in Figure 5, a linear correlation was found between the friction coefficient and the wear data for most of the examined model grease samples. In this sense, the biogenic grease samples that exhibited low friction coefficient values, such as grease samples R1, 1, 2, 5, 13, and S3, produced less wear on the steel plate. On the other hand, the use of some biogenic grease samples, such as 3, 12, S1, and S4, provided high friction coefficient values, which were reflected in the wear volume. Moreover, biogenic grease sample 11, which was based on a mixture of castor oil and HOSO as the base oil, stood out because of its low friction coefficient but exhibited the highest wear volume in comparison with model greases 1 and 2, which also contained castor oil and HOSO. Under appropriate assumptions, the microstructure and physical and chemical properties of the thickeners of biogenic grease 11 could lead to a negative influence in the present steel ball on the steel disc frictional contact. Biogenic grease 11, which was based on ethyl cellulose and carnauba wax as the thickener, which was a different composition to biogenic grease samples 1 and 2, presented a coarse and fragile structural appearance, probably due to its poor mechanical stability and low resistance to structural breakdown. This property can cause the grease microstructure to be easily destroyed upon tribological contact. Therefore, the grease can be gradually ejected from the contact, eventually yielding lubricant starvation. \begin{table}
\begin{tabular}{c c c c} \hline
**Load** & **Speed** & **Temperature** & **Time** \\ \hline
5 kN & 200 rpm & 50 \({}^{\circ}\)C & 120 h \\ \hline \end{tabular}
\end{table} 
Table 4: Operating conditions of the wear tests. Figure 4: Grease spread on (**a**) the rolling bearing house, lower raceway, and cage, and (**b**) on the upper raceway shaft. The effects of the different thickeners were also illustrated when comparing the results provided by biogenic greases 6, 8, and 10. Biogenic grease 10, which was prepared with a natural cellulose fiber of 120 \(\upmu\)m in length as the thickener, exhibited the least wear on the steel plate compared with model grease 6, which was based on a natural cellulose fiber 18 \(\upmu\)m in length, and grease 8, which was based on a natural cellulose fiber 20-40 \(\upmu\)m in length. This result indicated that the longer the natural cellulose fiber of the thickener was, the lower the wear volume on the steel plate (i.e., 22% lower for sample 10 compared to sample 6). These results showed that the different behaviors of different grease lubricant formulations were more explicitly observed in the wear results than in the frictional results in the tribological process. To illustrate the wear on the steel plates produced from the ten repetitions of the measurements on the same track in the tribometer, the wear marks were investigated using an optical 3D surface profilometer, as shown in Figure 6. The pictures allowed for the contacting surface on the steel plate to be observed. As seen in Figure 6, while the use of biogenic grease 3 and reference grease R2 caused wide and strong abrasive wear, biogenic grease 2 and reference grease R1 produced wide but less pronounced wear on the steel disc. Looking at the detail, the use of biogenic grease 3 and reference grease R2 caused higher wear depth values, which are described as \"Mean [\(\upmu\)m]\" in Figure 6, compared with biogenic grease 2 and reference grease R1. These data support the wear data shown in Figure 5.  
Figure 7 shows the apparent friction energy densities (\(e_{R}^{*}\)) plotted as a function of the wear intensities (\(I_{h}\)) for all the model greases.