*2.5. Wear Tests*

Due to the obvious directionality of the friction between the brake drum and the brake pad, in order to better simulate the friction conditions during the actual operation of the drum brake, the matching wear test was carried out on the self-made linear reciprocating wear test machine, and the structural diagram of the test machine is shown in Figure 6. The wear testing machine is composed of a controller, a counter, a servo motor, a linkage driving mechanism, and a wear testing area. The rotation of the servo motor drives the connecting rod to create reciprocating motion. The rotation speed of the servo motor controls the relative motion speed between the friction pairs. The slideway is coated with lubricating oil to ensure smooth reciprocating motion and maintain the level. The slider is equipped with a metal bracket to adjust the load by adding different weights to the bracket. The size specifications of the friction pairs used in this test were as follows: bionic sample was 120 × 30 × 10 mm<sup>3</sup> rectangular, the ground cast iron sample was 100 × 20 × 10 mm<sup>3</sup> rectangular, and the edge was chamfered by 1 mm. The brake pad sample was placed under the wear test area and kept stationary, and the cast iron sample was placed in the fixture on the sliding block. The bionic friction surface processed on the sample was opposite to the brake pad. After loading, it makes contact with the brake pad and follows the guide rod for reciprocating motion. In the experiment, by adjusting the weight on the metal plate and adjusting the speed of the motor with the controller, the sliding friction process under different loads and different friction speeds was created. All the experiments were sliding dry friction experiments conducted at room temperature. The test load was 100 N, and the motor speed was 70 r/min. The stroke of the connecting rod was 0.07 m, and the wear time was set to 40 h for each rotation of the motor. After the wear experiment, the matching cast iron samples were cleaned with an ultrasonic cleaning instrument, and the wear debris on the surface of the brake pad samples was cleaned and removed. All samples were weighed by an electronic balance with an accuracy of 0.001 g. Each experiment was repeated three times, and the average weight loss of the three experiments was taken as the final experimental result.

**Figure 6.** Linear reciprocating sliding friction and wear tester.

### *2.6. Microstructure Observation and Wear Morphology Observation*

A DK77 Electrical discharge machining was used to cut the laser-melted sample along its cross-section. Processing traces and oil stains on the cross-section were polished with different sandpaper grades. The samples were etched with a 4% nitric acid/alcohol solution to prepare them for metallographic observation. The morphology of biomimetic cells was observed under an optical microscope, and a cell without cracks and pores was selected for microstructure observation. A JEOL JSM-5600lv (SEM, Zeiss, Evo18, Oberkochen, Germany) scanning electron microscope (SEM) was used to observe the cross-section structure of the bionic unit bodies. In addition, after the unit body sample and brake pad samples were worn, their surfaces were also observed under SEM and their wear morphology was recorded. In this process, the samples did not require polishing and corrosion.

To characterize the wear performance, in addition to measuring the mass change before and after wear, the wear morphology of different biomimetic unit models and different brake pads were compared and analyzed using three-dimensional laser confocal microscopy (LEXT-OLS 3000 Olympus).
