## 2 Experimental
### Test Bench and Test Method  
Firstly, the friction force of the standard piston and the piston with a cylindrical pit array was measured by a friction force test bench (Figure 5) built in the laboratory. For the test method, we first applied grease evenly to the contact surface between the piston and the cylinder liner to facilitate adequate lubrication within the contact surfaces of the friction pair and the cylindrical pits of the textured piston. Then, the test piston was assembled on the pull rod. On the other side, a guiding piston was paired to create a sealed chamber between the two pistons. Subsequently, the sealed chamber was filled with mud, and the mud composition followed the relevant provisions of the national industry standard \"Reciprocating Mud Pump for Geological Drilling\" (DZ/T 0090-2017). The experimental mud possessed a density of 1.306 g/cm\({}^{3}\) and a sand content of 2.13%. The experimental pressure was controlled at 0.1 MPa, achieved by employing a pressurization device in conjunction with a pressure gauge. When the driving device was turned on, the pull rod drove the piston to reciprocate. The motor speed was set to 2.67 r/min. The experimental methods and conditions mentioned above were essentially consistent with the actual operating conditions of the BW-250 mud pump. The friction force of the test piston was measured by a pull-press sensor. The friction force experiments were conducted indoors at a temperature of approximately 20 \({}^{\circ}\)C. To ensure the accuracy of the experimental data, after each test, it was necessary to wait for the test bench and mud to cool down to room temperature before proceeding with the next test. Friction force tests were conducted for both the standard piston and the 27 groups of textured pistons. The friction force data for the tested pistons were collected after the test equipment had been running steadily for 30 min. Finally, the friction force data were exported using an AD conversion device, and the average friction force during the reciprocating stroke was calculated. Figure 5: Friction force test bench. According to the results of the friction test, the parameters of the optimized array of cylindrical pits were obtained. The life test, wear test and thermal imaging test of the optimized piston were carried out on the BW-250 mud pump test equipment (Figure 6). The BW-250 mud pump test equipment includes a drive motor, gearbox, pump body, pressure valve, pressure gauge, inlet pipe and outlet pipe. Our research object was the piston and cylinder liner in the pump body, as shown in Figure 7. Consistent with the friction test bench, the pull rod drove the piston to reciprocate in the cylinder liner under the drive of the motor. As the BW-250 mud pump is a three-cylinder pump, it can test three pistons at the same time. The test was divided into two groups, and each group was equipped with a standard piston.Pistons 4, 13 and S-1 # (standard piston) were in the first group. Pistons 22, 25 and S-2 # (standard piston) were in the second group. Wear and life tests were carried out in each group. The mud pump test gear was set to A2-B3. The test stroke was 200 strokes/min, the displacement was 250 L/min, and the pressure was 0.5 MPa. We calculated the wear rate \((w)\) of the piston using the formula  
\[w=\frac{m_{0}-m_{f}}{m_{0}}\times 100\% \tag{1}\]  
Figure 6: BW-250 mud pump test equipment. Figure 7: Pump body: (**a**) cylinder liner and (**b**) piston. where \(m_{0}\) is the mass before wear and \(m_{f}\) is the mass after wear. Calculate the percentage \((v)\) increase in the life of the piston using the formula  
\[v=\frac{L_{1}-L_{0}}{L_{0}}\times 100\% \tag{2}\]  
where \(L_{0}\) is the life of the standard piston and \(L_{1}\) is the life of the piston with cylindrical pit arrays. Temperature is an important factor affecting the friction and wear of rubber. Friction heat generation between the piston and cylinder liner has an important influence on piston wear.