## 3 Experimental Section
### Tribological Tests on the Ball-on-Disc Tribometer  
A tribometer designed by the Laboratory for Machine Elements and Tribology (MuT) at the Hamburg University of Applied Sciences (HAW-Hamburg, Germany), as shown in Figure 2, was used to determine the friction coefficient values of tribologically stressed material combinations. The test set-up was composed of a rotatory plate and a fixed ball. In the tribometer, the steel plate was lubricated with a grease sample and stressed by applying a normal force. By means of a load cell, the friction force was measured during the test, thereby allowing the friction coefficient to be determined.  
In the MuT laboratory of HAW-Hamburg, all tests were carried out in rotational mode with a relative speed of 0.129 m\(\cdot\)s\({}^{-1}\), using a steel ball (100 Cr6) 12.7 mm in diameter and steel plates (S235)R (St37), Zentrale Laborwerkstatt at the HAW-Hamburg, Hamburg, Germany). In order to achieve comparable results, frictional and wear behaviors of the model greases were investigated under the same test parameters and conditions for the tribometer. All the steel plates were metallographically grinded and polished with a soft finish diamond paste of particle size 3 \(\upmu\)m in order to significantly reduce surface roughness. Grease layers of the same thickness were applied to the steel plates, as shown in Figure 2. The normal load was kept constant at 12.62 N, with a resulting Hertzian stress value of 930.76 MPa and a test duration of 5 min. Each test was replicated ten times on the same track at an ambient temperature of \(22\pm 1\)\({}^{\circ}\)C in order to get a representative average friction coefficient value.  
To quantify the wear of the solids, the wear marks on the steel plates created after the ten repetitions of the test on the same track of the tribometer were investigated using white light interferometry with a Zygo Nexview apparatus (Middlefield, OH, USA). Wear depths were quantified by means of the Zygo Mx software.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Grease Sample (Code)** & **Base Oil** & **Substance of Content** \\ \hline R1 & Synthetic ester & Lithium/calcium soap \\ \hline R2 & HOSO 1 & Lithium-12-hydroxysterate \\ \hline
1 & HOSO and castor oil & Beeswax, glyceryl monostearate, and celyl alcohol \\ \hline
2 & HOSO and castor oil & Glyceryl monostearate, celyl alcohol, and sorbitan monostearate \\ \hline
3 & HOSO and glycerol & Cellulose ether \\ \hline
4 & HOSO & Isoprene derivative \\ \hline
5 & HOSO & Lignosulfonate \\ \hline
6 & HOSO & Natural cellulose fibers, 18 \(\upmu\)m \\ \hline
7 & HOSO & Cornocgh grits, 80–120 \(\upmu\)m \\ \hline
8 & HOSO & Natural cellulose, 20–40 \(\upmu\)m \\ \hline
9 & HOSO & Natural wood pulp from softwood, 70–150 \(\upmu\)m \\ \hline
10 & HOSO & Natural cellulose fibers, 120 \(\upmu\)m \\ \hline
11 & HOSO and castor oil & Ethyl cellulose, carnauba wax, and celyl alcohol \\ \hline
12 &
\begin{tabular}{c} Castor oil, HOSO and MCT oil \\ (triglyceride of CS/C10 fatty acids) \\ \end{tabular} & Polyhydroxybutyric acid and ethyl cellulose \\ \hline
13 & Glycerol and sorbitan monooleate & Chitosan, glyceryl monostearate, and calcium phosphates \\ \hline S1 & Castor oil & Lignin/PEGICE 2 (weight ratio of 1/0.25) \\ \hline S2 & Castor oil & Lignin/PEGICE (weight ratio of 1/1) \\ \hline S3 & Castor oil & Lignin/HMDI 3 (weight ratio of 1/2) \\ \hline S4 & Castor oil & Lignin/HMDI (weight ratio of 1/1) \\ \hline \hline \end{tabular} \({}^{1}\)HOSO:high-oleic sunflower oil, 2 PEGDGE: polyethylene glycol diglycidyl ether, 3 HMDI: hexamethylene diisocyanate.  
\end{table}
Table 2: Compositions of greases.