## 3 Results and Discussion
### Friction and Wear Behavior  
Figure 6 shows the wear scar morphology of various samples. It can be seen that the width of the wear scar increases for the coated samples as compared to the uncoated substrate. It is because that during the wear test, the soft FeS coating is first deformed, and the broken FeS particles adhere to the friction balls and the sample surface, thereby making the wear scars appear wider. As shown in Figure 6a, the wear scar on the surface of the uncoated sample is composed of adhesive wear products and a large number of furrows, indicating that the adhesive wear occurs primarily because of localized high temperature heat generated by friction. From Figure 6b-e, it can be seen clearly although the FeS coatings have been damaged into a discontinuous gray black block after wear test, there is no obvious scratch and adhesion phenomenon on the sample surface. Moreover, the wear scars of the S50 and S70 sample are narrower than that of the S30 and S40 samples, indicating better wear resistance. This is because that the thicker FeS coatings provide better lubrication during friction and wear testing, and improve the wear resistance. For the S90 sample, a large number of furrows and some large exfoliation pits are found on the wear scar (Figure 6f), indicating a decrease in the wear resistance of the coating. It is because that although the S90 sample coating is the thickest, the coating has poor compactness and high surface roughness because of the existence of a large number of pits on the coating after the CBD treatment at 90 \({}^{\circ}\)C. It can be concluded that the FeS coating has excellent lubricity and its introduction can increase the wear resistance of the steel. The steel treated at low temperature has a thin coating, while the coating treated at high temperature has a poor density. In comparison, the steel treated at a medium temperature (about 50 \({}^{\circ}\)C in this study) has a moderate coating thickness and a coating density, corresponding to the best wear resistance.  
The friction coefficients plotted as a function of friction time for the various samples are shown in Figure 7, which further confirms the above conclusions. It has been pointed out that in the initial stage of friction (<1 min), the friction coefficients are affected significantly by the surface quality (e.g., roughness, oxidation, and contamination), and the curve ranging from 1 to 10 min is probably the real friction coefficient [5]. As can be seen, the S30, S40, and S50 samples have a lower coefficient of friction, while the S70 and S90 samples have a higher coefficient of friction, especially for the S90 sample, which has a higher coefficient of friction than the uncoated steel. As discussed above, when the CBD temperature is high, the coating has poor compactness because of the existence of a large number of pits on the coating (see Figures 2e and 5d). Moreover, there are some Fe\({}_{2}\)O\({}_{3}\) exiting in the S90 sample(see Figure 1). These factors make the initial roughness of the S90 sample significantly higher than the uncoated sample, resulting in a higher friction coefficient during the wear test. Generally, the smaller the coefficient of friction is, the better the lubrication performance and the wear resistance will be. The friction coefficients of the three samples S30, S40, and 50 are all low, but the friction coefficients change differently with increasing friction time. As the friction time increases, the friction coefficients of the S30 and S40 samples decrease significantly from 1 to 4 min and then increase after 4 min, while the wear coefficient of the S50 sample decreases sharply from 1 to 4 min and decreases slowly after more than 4 min. Obviously, it indicates that the S50 sample has a better lubrication effect, especially during prolonged wear testing.