We cut the spindle to produce specimens that were convenient for microscopic observation and then conducted SEM to characterize the changes in the surface and cross section of the hook teeth during field work. We also analyzed the reasons for the wear failure of the hook teeth. Lastly, we described the wear failure process of the spindle hook teeth by extracting the wear parameters of the hook teeth.
4.1. Surface Morphology of the Hook Tooth
A scanning electron microscope was used to characterize the wear changes and microscopic morphology of the surface of the hook teeth during field work. The results are presented in Figure 3a–e. As shown in Figure 3a, the surface of a hook tooth of the new spindle was completely covered with chromium coating. However, many microcracks occurred on the surface of the coating, as observed in the microscopic morphology of the hook tooth surface shown in Figure 3(a1). The analysis indicated that the existence of these microcracks was related to the inevitable internal stress generated during the electroplating and machining processes [25,26,27,28,29]. After completing 133.33 ha of spindle picking in the field, the coating on the tooth tip and tooth edge exhibited a slight abrasion, as displayed in Figure 3b. Nevertheless, the coating on the tooth edge of the hook tooth was not yet completely destroyed. Noticeable scratches and holes appeared on the surface of the hook tooth’s chrome coating, as shown in Figure 3(b1); this abrasive wear phenomenon was caused by hard particles in the field sliding on the surface of the coating [19,20,21,22,23]. The holes in the chromium coating were related to the precipitation of hydrogen on the cathode during the electroplating process [30,31]. When the picking work area reaches 266.66 ha, the coating on the tooth edge of the picking hook tooth was completely destroyed. Meanwhile, the coating on the surface of the spindle hook tooth presented a trapezoidal wear area that exposed the substrate material, as shown in Figure 3c. The number of scratches on the surface of the hook tooth’s chrome coating increased and became more complex, as shown in Figure 3(c1). When the field picking work area reached 400 ha, the trapezoidal wear area on the surface of the spindle hook tooth was evidently enlarged, as illustrated in Figure 3d. At this moment, the phenomenon of peeling of the coating on the surface of the hook tooth was observed, as shown in Figure 3(d1). From the analysis, the spindle was assumed to collide with harder materials, such as cotton stalks and bell shells, during the field picking process. Through the continuous collision with such hard materials, the fatigue life of the chrome coating was reduced, and fatigue peeling was likely to occur [21,32,33,34,35,36,37], accelerating the wear rate of the coating on the surface of the spindle. When the picking work area reached 533.33 ha, the trapezoidal wear area on the surface coating of the hook teeth increased significantly, as shown in Figure 3e. The fatigue peeling phenomenon of the coating was further aggravated, as depicted in Figure 3(e1). The hardness of the spindle substrate was considerably lower than that of the coating, and thus, the hook tooth substrate without the protection of the coating underwent rapid wear and lost its original structure, eventually leading to spindle failure [12].
Figure 3. Wear changes of hook teeth during field work: (a) hook tooth of the new spindle, (a1) surface morphology of the new hook tooth, (b) hook tooth of the field picking area with 133.33 ha, (b1) surface morphology of the hook tooth of the field picking area with 133.33 ha, (c) hook tooth of the field picking area with 266.66 ha, (c1) surface morphology of the hook tooth of the field picking area with 266.66 ha, (d) hook tooth of the field picking area with 400 ha, (d1) surface morphology of the hook tooth of the field picking area with 400 ha, (e) hook tooth of the field picking area with 533.33 ha, (e1) surface morphology of the hook tooth of the field picking area with 533.33 ha, (f) EDS analysis image at Point 1, and (g) EDS analysis image at Point 2.
An energy spectrometer is an important auxiliary instrument of a scanning electron microscope. It can complete the qualitative and quantitative analyses of the elements in the microscopic area of a material within a short period. In the current study, an X-ray spectrometer was used to study the distribution of elements on the wear surface of the spindle hook tooth. Point scanning was performed on Points 1 and 2 of the worn surface of the spindle. The scanning position is shown in Figure 3e. The scanning results are presented in Figure 3f,g. The element composition at Points 1 and 2 is mostly oxygen and iron, with a small amount of sodium, potassium, and chlorine, indicating that the substance at Points 1 and 2 is an iron oxidation product [23,38,39,40]. After the coating on the surface of the hook tooth wears and fails, the substrate is exposed to air, and the surface of the substrate exhibits oxidized wear, accelerating the wear failure of the hook tooth substrate [41,42,43].
To provide a clear description of the wear change process of the hook tooth of the spindle, the wear width and wear area of the hook teeth were used as indicators to describe the wear degree of the hook teeth. The wear area refers to the wear area of the hook tooth section of the spindle, as shown in Figure 4a. It is represented by S The wear width refers to the closest distance between the worn part of the hook tooth edge and the unworn part of the coating, as depicted in Figure 4a. It is represented by W The wear area and wear width of the spindle hook teeth were extracted using an image processing method. Figure 4b shows the changes in the wear area of the spindle hook teeth under different field work area conditions. The wear area of the spindle hook teeth continued to increase with the increase in field work area. The change curve of the wear area and wear width is presented in Figure 4c,d. The wear area and wear width of the hook tooth of the new spindle were both 0. When the field picking area reached 133.33 ha, the wear area was 2.86 × 104 µm2. At this moment, no wear width matched the definition in the text, and the wear width was still 0 µm, because the wear on the surface coating of the hook tooth occurred on the tooth tip and tooth edge. At this moment, the wear area change rate was 214.51 µm2/ha. When the field picking area reached 266.66 ha, the wear area increased to 1.86 × 105 µm2, and the wear width was 259.92 µm. The wear area and wear width change rate were 697.52 µm2/ha and 0.97 µm/ha, respectively. When the field picking area reached 400 ha, the wear area was 5.32 × 105 µm2, and the wear width was 583.83 µm. The wear area and wear width increased significantly, and the change rates of the wear area and wear width were 1.33 × 103 µm2/ha and 1.46 µm/ha, respectively. When the field picking area reached 533.33 ha, the wear area was 1.24 × 106 µm2, and the wear width was 981.49 µm. The change rate of the wear area was 2.33 × 103 µm2/ha, and that of the wear width was 1.84 µm/ha. The preceding data indicate that the wear failure rate of the surface of the hook tooth of the spindle was accelerated with an increase in the field picking area. The reason for this finding was as follows. On the one hand, the fatigue peeling phenomenon of the coating on the surface of the hook tooth of the spindle is increased with an increase in the field picking area, and the wear failure rate of the coating is accelerated [21,32,33,34,35,36,37]. On the other hand, the substrate is exposed after the coating is worn out, and the appearance of oxidative wear on the surface of the substrate further accelerates the wear rate of the hook teeth [41,42,43]. As shown in Figure 4c,d, the wear area of the spindle hook tooth during the picking process exhibited an exponential trend. The wear width of the spindle hook tooth presented an approximately linear relationship with the area of field picking operation.
Figure 4. Wear change on the surface of the spindle: (a) schematic of wear area and wear width, (b) wear area extraction result, (c) change curve of wear area, and (d) change curve of wear width.
4.2. Cross Section Morphology of the Hook Tooth
The spindle hook teeth were cut, and SEM was performed to characterize the cross-section morphology of the coating of the spindle hook teeth. The results are presented in Figure 5a. As shown in the figure, microcracks and hole defects occurred in the chromium coating of the new spindle [22]. Through the analysis, the existence of microcracks in the chromium coating was assumed to be related to inevitable internal stress during the machining and electroplating processes [25,26,27,28,29]. The holes were caused by the precipitation of hydrogen at the cathode during the electroplating process, and hydrogen bubbles were formed on the surface of the coating [30,31].
Figure 5. EDS analysis of the coating and substrate: (a) cross-section morphology of the hook tooth of the spindle, (b) coating analysis results, and (c) substrate analysis result.
EDS was performed to scan the coating and substrate of the hook tooth, with a scanning area of 0.01 mm2, as shown in Figure 5a. Figure 5b,c present the EDS spectra of the coating and substrate of the hook tooth. Chromium and nickel were detected in the coating; while iron, manganese, and nickel were present in the substrate [12]. The content of each element in the coating and substrate is provided in Table 3. The major element in the coating was chromium [22], with a content of 95.60%. The content of nickel was less, i.e., 4.40%. The primary element in the substrate was iron, with a content of 91.48%. Meanwhile, the contents of manganese and nickel were less, namely, 6.08% and 2.44%, respectively.
Table 3. Element content in the coating and substrate.
The cutting surface was set as Position 1. The remaining four points were determined at intervals of approximately 0.3 mm, as shown in Figure 6b,c. The thickness of the coating on the surface of the hook tooth of the spindle was measured at each position, as illustrated in Figure 6a. Figure 6c presents the change in the thickness of the coating on the surface of the spindle hook teeth during field picking. The surface coating of the hook teeth of the new spindle was thick in the middle but thin on both sides. The thickness of the coatings at Positions 1–5 of the new teeth was 68.5, 71.2, 73.4, 74.4, and 66.1 µm, respectively. Under different picking area conditions, the thickness of the coating on the surface of the same hook tooth gradually decreased from Positions 1 to 5. The curve of the coating thickness changed more evidently as the area of field picking operations increased. In the same position, the thickness of the coating continued to decrease as the area of field picking operations increased [23]. The wear failure rate of the coating near the cutting surface was slower, and the wear rate near the tooth edge was faster, as shown in Figure 6d. The wear rates of the coating at Positions 1 (cutting surface), 2, 3, 4, and 5 (tooth edge) were 0.01, 0.05, 0.14, 0.19, and 0.25 µm/ha, respectively. The wear rate gradually decreased from Positions 1 to 5. The wear rate of Position 1 was the slowest, and that of Position 5 was the fastest.
Figure 6. Change in coating thickness of the hook tooth surface during field picking: (a) thickness of the coating on the surface of the spindle at each position, (b) coating thickness measurement specimen, (c) hook tooth profile of the spindle, and (d) change curve of coating thickness on the surface of the hook tooth of the spindle.