# Friction and Wear Performances of Materials for Wind Turbine Sliding Bearing Bushes
## 4 Wind Turbine Sliding Bearing Bush Friction and Wear Properties Analysis
### Comparative Analysis of Friction and Wear Properties of Two Groups of Bearing Bush Materials  
The experimental data on the friction and wear of the two groups of coated surfaces under the same load were compared and analyzed to identify a bearing material with superior friction and wear properties, as illustrated in Figure 10 and Table 5. The standard deviation of the friction coefficient reflects the degree of fluctuation in the friction coefficient of the coating during the stable wear stage [35]. A smaller standard deviation value indicates a more stable friction coefficient, as given by Formula (4):  
\[S=\sqrt{\frac{\sum\limits_{i=1}^{n}(f_{i}-\overline{f})}{n}} \tag{4}\]  
where \(f_{i}\) is the size of the friction coefficient of each sample, \(\overline{f}\) is the average friction coefficient, and \(n\) is the amount of data in the sample. As depicted in Figure 10a-c, under a 30 N load, the variation trend of the friction coefficients for the two groups is nearly identical, and the difference in average friction coefficients is less than 1%. Thus, the frictional properties of the two groups of coating surfaces are comparable under a 30 N load. Under 40 N and 50 N loads, the friction coefficient of the CoCrFeNiMo high-entropy alloy coating is lower than that of the Ni-Cr-Mo-Si alloy coating. The average friction coefficient is lower by 0.0579 and 0.0599, respectively, indicating a small difference. Additionally, the data in Table 5 indicate that the standard deviation of the friction coefficient for the CoCrFeNiMo high-entropy alloy coating is large under 40 N and 50 N loads, and the friction curve during the stable wear stage is highly volatile, resulting in an unstable friction coefficient. Therefore, relying solely on the friction coefficient is inadequate for material selection. As illustrated in Figure 10d, the wear characteristics and wear rates of the CoCrFeNiMo high-entropy alloy coating surpass those of the Ni-Cr-Mo-Si alloy coating under identical load conditions. Specifically, the wear mass of the CoCrFeNiMo high-entropy alloy coating is at its minimum under a 40 N load, measuring 2 mg. In contrast, the lowest wear mass for the Ni-Cr-Mo-Si alloy coating is 0.7 mg, achieved under a 50 N load. Table 5 further highlights that, under equivalent load conditions, the wear rate of the CoCrFeNiMo high-entropy alloy coating significantly exceeds that of the Ni-Cr-Mo-Si alloy coating, reaching up to six times. This phenomenon can be attributed to laser cladding, which ensures even distribution of powder alloy elements with high wear resistance in the coating. These elements fully fuse with the hard strengthening phase embedded in the wear-resistant cast iron, resulting in a substantial enhancement of the coating's wear performance. It is worth noting that the change in the coefficient of friction of the CoCrFeNiMo high-entropy alloy coating and the Ni-Cr-Mo-Si alloy coating is almost the same under an applied load of 30 N, but the wear quality is different. This is because the surface of the Ni-Cr-Mo-Si alloy coating is flatter compared to the CoCrFeNiMo high-entropy alloy coating, as shown in Figure 7. In the very short time at the beginning of the friction and wear experiment, the friction between the relatively rough CoCrFeNiMo high-entropy alloy coating and the counter-abrasive ball occurs more violently, with more broken micro-convex bodies and dropped abrasive chips on the coating surface. After this, the wear depth of both groups of materials gradually reaches below the coating surface, with similar friction coefficients and comparable wear quality. Therefore, in the whole frictional wear process, the wear quantity on the surface of CoCrFeNiMo high-entropy alloy coating is larger. The friction and wear properties of the coatings from both material groups were compared and optimized based on the results of the PSI (Preference Selection Index) method [36; 37].