## 3 Results and Discussion
### Microstructure Evolution  
Figure 2 presents the SEI images from the vertical and cross-sectional views of the samples prepared at various temperatures. Figure 2a shows the microstructure of the S30 sample. It can be seen that when the reaction temperature is 30 \({}^{\circ}\)C, the coating thickness is very thin (about 4 \(\upmu\)m, see Figure 2f), the surface morphology of the coating is similar to the dry riverbed covered with network-like cracks (see Figure 2a), and the crystalline products do not cover the entire surface. When the temperature is increased to 40 \({}^{\circ}\)C, no significant increase in coating thickness is observed (see Figure 2g), but a large amount of crystalline product having a chrysanthemum shape is found on the surface of the coating (see Figure 2b). As the temperature rises to 50 \({}^{\circ}\)C, the coating is completely covered by CS particles (see Figures 2c and 3b). The EDS line scanning results show that the sulfur content of the CS coating is higher compared to the substrate (see Figure 3a). According to the EDS data analysis of points P1 and P2 (Figure 3c), it is known that the atomic percentage of Fe and S of P1 and P2 is almost equal. Combined with the XRD results, it is considered that the chrysanthemum-shaped (CS) particle is the FeS and a complete sulfide layer has been formed on the substrate, because the ratio Fe/S (see P1 and P2 in Figure 3c) on the treated substrate surface has been approaching to 1 [46]. High magnified micrograph (Figure 3b) clearly shows that the CS particles are composed of FeS crystals (each crystal is equivalent to a chrysanthemum petal). Additionally, there are loose pores between the CS particles and between the petals, and microcracks are found on some of the petals. It is considered that the pores can store lubricating oil and form oil film easily on the friction surface, which is beneficial to the synergistic lubrication effect of the fluid (lubricating oil) and the solid (FeS coating) [47]. Additionally, no transition zone is found between the coating and substrate, indicating that the sulfur element does not penetrate deep into the steel substrate at this temperature. When the temperature increases to 50 \({}^{\circ}\)C, the CS particles grow up to an average diameter of 26 \(\upmu\)m (see Figure 2c) and the coating thickness increases to 24 \(\upmu\)m (see Figure 2h). When the temperature further increases to 70 \({}^{\circ}\)C, no significant changes in CS particle size and coating thickness are observed (see Figure 2d,j). Interestingly, as the temperature rises to 90 \({}^{\circ}\)C, the CS particles disappear and are replaced  
Figure 3: Energy dispersive spectrometry (EDS) line scans (**a**) and point scans (**b**) showing element distribution and content. (**c**) Is the quantitative analysis result of the elements of the scanning points in (b). Figure 2: Secondary electron imaging (SEI) images showing FeS coating morphology evolution with increasing temperature: (**a**,**f**) 30 \({}^{\circ}\)C; (**b**,**g**) 40 \({}^{\circ}\)C; (**c**,**h**) 50 \({}^{\circ}\)C; (**d**,**j**) 70 \({}^{\circ}\)C; (**e**,**k**) 90 \({}^{\circ}\)C.  
by disordered thick petals (see Figure 2e). Moreover, the average size of the petals of the S90 sample is much larger than the size of the petals of the other samples. The coating thickness and the CS particle size (in diameter) are measured and shown in Figure 4. For each sample, at least 10 SEM photographs (500\(\times\)) were used to measure the average diameter. It should be noted that since no significant CS morphology was observed in the S90 sample, the CS particle size was not measured.