## 2 Materials and Methods
### Abrasive Wear Testing System and Procedure  
An abrasive wear testing system was developed to study the anti-abrasive wear performance of test samples. With the purpose of evaluating the abrasive wear resistance performance of soil-engaging components of agricultural machinery, in this study, a rotational abrasive wear testing system that was dedicated to the abrasive wear test of working parts was designed. As shown in Figure 3, the testing system mainly included an electric motor, an abrasive storage box, a sample fixture, a speed control device and a data acquisition device. The motor was a three-phase asynchronous motor, the rated power was 2.2 kw and the rated speed was 2840 r/min. The spindle inside the abrasive material bin was powered by chain drive. The size of the abrasive material bin was 600 \(\times\) 600 \(\times\) 600 mm; the spindle had a length of 750 mm, with a diameter of 20 mm, and was located in the center of the material storage box. The sample fixture was installed in the central position of the spindle and rotates with the spindle simultaneously. The test sample fully contacted the abrasive material during the test. Four of the test samples could be installed on the fixture at the same time. Two leakproof flanges were installed inside the contact position between the spindle and the abrasive material bin to prevent the leakage of the abrasive material. The electric motor speed was controlled by the frequency converter and RC41B. The RC41B digital display tachometer was equipped with a Hall sensor. The speed range was ranged between 3.8 and 10,000 r/min, and the error range was \(\pm\)0.5%, which met the test requirements. Figure 3: Abrasive wear testing system. Figure 2: Five types of abrasive wear experimental sample (the direction of abrasive material is from left to right). The particle size of abrasive particles has a certain effect on the wear of the sample surface during abrasive wear. The abrasion loss generally increases with the abrasive particle size [40]. When the abrasive particle reaches a critical size (70 to 100 \(\upmu\)m), the abrasive particle size continues to increase and the wear amount tends to be stable [41]. In this study, the mixture of 96.5% quartz sand and 3.5% Bentonite soil was selected as the abrasive of the abrasive wear test. The moisture content of the abrasive was measured by a water sensor, and the moisture content was controlled between 3% and 5% (mass fraction) [42]. The #40 mesh sieve filter screens were used to filter large particles, and #60 mesh sieve filter screens were used to filter small particles, so as to ensure that the abrasive particle size range was between 260 and 420 \(\upmu\)m.  
The material used to prepare test sample was polylactic acid (PLA). PLA has weak abrasive strength, which aids in speeding up the abrasive wear procedure and shortening the testing time. In addition, PLA has excellent thermal stability [43]. With the increase in friction time, considerable heat could be accumulated, which will raise the temperature of the friction surface of the friction sample. During the test, the temperature of the test sample was checked by an infrared thermometer (GM321, Biaozhi Science and Technology Co., Ltd., Shenzhen, China). It was found that when the test duration was kept within 1 h and the speed of the motor was controlled at 200 r/min, the temperature of the sample surface remained below 50 \({}^{\circ}\)C. Therefore, the continuous testing time duration was set at 1 h. After each continuous experiment for 1 h, the motor was stopped. The mass loss of the sample before and after the tests was measured with an electronic balance (Excellence II Excellence, Shanghai Analytic Ping Scientific Instrument Co., Ltd., Shanghai, China). After the test samples were cooled for 0.5 h, the samples were reinstalled to the fixture, the motor was restarted, and the tests continued. The cumulative time of abrasive wear test was 80 h. After the tests were finished, the wear distance was calculated to be 6.09 \(\times\) 10\({}^{5}\) m in total.