## 3 Results and Discussion
### Results and Analysis
#### 3.2.5 Analysis of the Surface Morphology and the Mechanism of Skidding Damage  
Figure 10a,b shows the surface morphology of the roller and inner ring before the test, and Figure 10c,d shows the surface morphology of the roller and inner ring after skid damage. LEXT OLS4000 is used to obtain the surface topography and roughness data. The test conditions are as follows: the inner ring speed is 2250 r/min; the radial load is 120 N; and the lubricating oil quantity is 10 mL/min. The test results show that the slip rate is 0.9306 at the initial time of the skidding damage.  
According to the comparison of the surface morphology of the inner ring before and after the test, the surface of the inner ring has obvious adhesion wear, and the damaged area has cracks and scratches. The reason for this phenomenon may be a large slip between the roller and the inner ring. When the oil film breaks, the direct contact between the roller and the inner ring causes the material to transfer to each other. The surface roughness of the inner ring is 0.378 \(\upmu\)m before the test, and increases to 0.54 \(\upmu\)m after the test.  
Figure 9: Temperature and slip rate of the contact area under different lubricating oil cleanliness. (**a**) Temperature curve (clean oil); (**b**) Slip rate curve (clean oil); (**c**) Temperature curve (contaminated oil); (**d**) Slip rate curve (contaminated oil).  
According to the comparison of the surface morphology of the roller before and after the test, it can be seen that there are burning points with different sizes and cracks on the edge of the damaged area. In addition, the temperature increased significantly at the beginning of the test, indicating that the skid damage caused a high temperature instantaneously, and the high temperature caused a burning pit on the surface of the material. Thus, skid damage may tend to be thermal failure. The surface roughness of the roller was 0.446 \(\upmu\)m before the test, and increased to 0.692 \(\upmu\)m after the test.  
Figure 10(a),b shows the surface morphology of the roller and inner ring before the test, and Figure 10(c),d shows the surface morphology of the roller and inner ring after skid damage. The test conditions are as follows: the inner ring speed is 4350 r/min; the radial load is 100 N; and the lubricating oil quantity is 1 mL/min. The test results show that the slip rate is 0.9878 at the initial time of skidding damage.  
It can be seen from Figure 10(c) that there are peeling pits on the roller surface after skidding damage, which indicates that skidding damage can lead to material migration. As can be seen from Figure 10(d), the surface of the inner ring has the material adhesive, and some areas of the surface are covered with black burning points. Therefore, there may be a thermal failure phenomenon in the skidding damage of roller bearings.  
Figure 10: Surface morphology of the roller and inner ring. (a) Roller morphology before the test; (b) Inner ring morphology before the test [20]; (c) Roller morphology after the test; (d) Inner ring morphology after the test [20].  
The surface roughness of the inner ring was 0.123 \(\upmu\)m before the test, which increased to 0.629 \(\upmu\)m after the test. The surface roughness of the roller was 1.127 \(\upmu\)m before the test, which increased to 1.763 \(\upmu\)m after the test. It also can be inferred that the larger slip rate can lead to more obvious surface roughness changes before and after the test.  
The slip rate has a significant effect on the skidding damage of a rolling bearing. By comparing the skidding damage cases, it can be inferred that the greater the slip rate, the more serious the surface damage. The test results show that the skidding damage always occurs under larger slip rates, especially when the slip rate is bigger than 0.9. Combining with the surface morphology and the time-varying slip and temperature curve, it can be concluded that the skid damage under extremely light load is the comprehensive effect of adhesive wear and thermal failure.