## 2 Experimental Setup
### Tribological Experiment  
The tribological experiment was conducted on a thrust collar tribotester designed by the authors, as shown in Figure 2. The thrust of SK2 steel with a hardness of 60 HRC was fixed and slid against an iron plate made of S50C steel with a hardness of 254 HV. The outer diameter, inner diameter, and the length of thrust were 25 mm, 10 mm, and 49.5 mm, respectively. The dimensions of the flat plate were 50 mm \(\times\) 50 mm, 6 mm thick. The chemical properties of the thrust and collar are listed in Table 4.  
The surface roughness of the specimen was measured by a Mitutoyo Surftest SJ-400 apparatus (Mitutoyo Corporation, Kanagawa, Japan). The average roughness value (\(Ra\)) of the thrust and the collar were from 0.024 \(\mu\)m to 0.048 \(\mu\)m. When the friction test was completed, the collar was removed and cleaned in an ultrasonic bath for further SEM and EDS analysis.  
The normal force was applied by a dead weight and the friction force between the thrust and the collar was measured via a load cell every 0.5 s. The weight of the collar was measured using a SI-304 precision electronic balance (Denver Instrument, Bohemia, NY, USA) before and after the experiment was run to calculate the wear rate of the collar.  
The coefficient of friction \(\mu\) is given by equation [22]:  
\[\mu=\frac{3M\big{(}R_{1}^{2}-R_{2}^{2}\big{)}}{2F\big{(}R_{1}^{3}-R_{2}^{3} \big{)}}, \tag{1}\]  
where \(M\) (N\(\cdot\)mm) is the friction moment, \(F\) (N) is the applied load, and \(R_{1}\) (mm) and \(R_{2}\) (mm) are the outer radius and inner radius of thrust, respectively.  
The wear rate is calculated as:  
\[w=\frac{\Delta W}{\rho FL}\left(\frac{\text{cm}^{3}}{\text{N}\cdot\text{m}} \right), \tag{2}\]  
where \(w\) (cm\({}^{3}\)/N\(\cdot\)m) is the wear rate, \(\Delta W\) (g) is the mass loss, \(\rho\) (g/cm\({}^{3}\)) is the density of steel, \(F\) (N) is the applied load, and \(L\) (m) is the sliding distance.  
Each experiment was performed for 120 min at a rotational speed of 600 rpm, applied load of 90 N, 27 \({}^{\circ}\)C, and the humidity was between 45%-50%.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multicolumn{6}{c}{**Chemical Elements (wt \%)**} \\ \cline{2-7}  & **C** & **Si** & **Mn** & **S** & **P** & **Fe** \\ \hline Thrust SK2 & 1.15–1.25 & 0.15–0.35 & 0.15–0.50 & \(\leq\)0.030 & \(\leq\)0.030 & Balance \\ Collar S50C & 0.47–0.53 & 0.15–0.35 & 0.60–0.90 & \(\leq\)0.035 & \(\leq\)0.030 & Balance \\ \hline \hline \end{tabular}
\end{table}
Table 4: The chemical composition of specimens.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Morphology**} & **Spherical** \\ \hline Purity (\%) & 99.9 \\ Color & Black \\ Average particle size (nm) & 50 \\ Specific surface area (m\({}^{2}\)/g) & 20 \\ Bulk density (g/cm\({}^{3}\)) & 0.23 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Properties of aluminum nanoparticles.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Chemical Formula**} & \multicolumn{1}{c}{**NaC\({}_{12}\)H\({}_{25}\)SO\({}_{4}\)**} \\ \hline Purity (\%) & 99 \\ Relative molecular mass (g/mol) & 288.372 \\ Appearance & White solid \\ Density (g/cm\({}^{3}\)) & 1.01 \\ Melting point (\({}^{\circ}\)C) & 206 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Properties of sodium dodecyl sulfate.