## 2 Test Equipment and Specimens
### Test Specimens  
The conventional materials of frictional pairs were M120D (graphite) and SSiC (silicon carbide) [16]. However, at low rotational speeds, the friction pair formed by the rotary ring and the stationary ring was in contact for a long period, so the stationary ring (M120D) wore out significantly, leading to premature failure of the mechanical seal. In this study, two pairing schemes of the friction pairs were designed, that is, M120D-SSiC (graphite and silicon carbide) and SSiC-SSiC (silicon carbide and silicon carbide). The M120D was only used as the upper specimen. The SSiC could be used as the upper or lower specimen. The structural dimensions of the upper and lower specimens are shown in Figure 1. The physical parameters of the M120D and the SSiC are shown in Table 1.  
Considering the effects of the surface characteristics of the friction pairs on the tribological performance, three widely used texture patterns--triangular dimples [17], elliptical dimples [18], and conventional spiral grooves--were engraved on the SSiC surface. The specific geometric parameters of the texture patterns are shown in Table 2. Fiber optic processing technology was used to create these three textures. A galvanometer scanner and numerical control system were used to drive the laser beam to pass through the matrices [19; 20; 21; 22]. A pulsed solid-state YAG laser with a pulse width in the range of 30 fs to 30 ps was used. The depth and aspect ratio were controlled by the laser pulse duration, wavelength, and power. The engraving process was as follows:  
1. The laser power was set to 80%, and the engraving was performed repeatedly at this power to obtain the required depth of the texture.
2. The laser power was set to 50%. The surface was scanned, and finishing was performed on the texture areas that met the depth requirement. The uneven bottoms of the texture areas were repaired, and the roughness was improved.
3. The laser power was reduced to 20%. The burr on the edge of the pattern was improved.
4. Fine polishing was performed on the engraved specimens to further improve the quality of the bottoms of the texture.
5. Ultrasonic cleaning was performed on the specimen, after which it was placed in a drying oven to remove surface moisture.  
The textured surface profile of the specimen was scanned, and a white-light interference three-dimensional profiler was used, as shown in Figure 2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Material Parameter** & **M120D** & **SSiC** \\ \hline Elastic modulus (MPa) & 25 & 410 \\ Poisson ratio & 0.15 & 0.15 \\ Density (kg/m\({}^{3}\)) & \(1.8\times 10^{3}\) & 3.21 \\ Thermal conductivity (W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)) & 96.3 & 110 \\ Hardness (MPa) & 30 & 2840 \\ Roughness (\(\mu\)m) & 0.026 & 0.02 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material properties of M120D and SSiC.  
Figure 2: _Cont_.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Texture Parameter** & **Spiral Groove** & **Triangle** & **Ellipse** \\ \hline Outer diameter (mm) & 34 & 34 & 34 \\ Root diameter (mm) & 25.4 & / & / \\ Inner diameter (mm) & 20 & 20 & 20 \\ Deflection angle (\({}^{\circ}\)) & / & / & \(\pm 9\) \\ Spiral angle (\({}^{\circ}\)) & 30 & / \\ Radial number & 1 & 3 & 5 \\ Circumferential number & 15 & 52 & 96 \\ Side length (mm) & / & 1 & / \\ Half of the long axis (mm) & / & / & 0.25 \\ Half of the short axis (mm) & / & / & 0.125 \\ Groove depth (mm) & 0.04 & 0.04 & 0.04 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Geometry parameters of triangle, ellipse, and conventional spiral groove texture patterns.