## 2 Materials and Methods  
Commercial AISI 5140 steel with a chemical composition of Fe-0.40C-0.23 Si-0.7 Mn-0.8 Cr-0.03 Ni (wt%) was selected as the substrate material. The steel was cut into samples with a gauge dimension of \(25\times 15\times 3\) mm\({}^{3}\). A facile CBD method was carried out for preparing the FeS coatings on the steel. The CBD was originally used to prepare thin films or nanomaterials [42; 43; 44], and was recently used to deposit coatings [45]. The samples were sequentially ground, polished and ultrasonically cleaned, and finally immersed into a CBD solution bath. The solution formulation is as follows: 8.0 g Na\({}_{2}\)SO\({}_{3}\), (\(\geq\)97.0%), 2.5 g Na\({}_{2}\)S\({}_{2}\)O\({}_{3}\).5H\({}_{2}\)O (\(\geq\)99.0%), 3.0 g C\({}_{4}\)H\({}_{4}\)O\({}_{6}\) (\(\geq\)99.5%), 1.5 g FeSO\({}_{4}\).7H\({}_{2}\)O (\(\geq\)99.0%), and 100 mL de-ionized water.  
In the bath, the steel substrate (anode) is oxidized and dissolved because of its low potential and the loss of electrons. The reaction formula is as follows:  
\[\mathrm{Fe}\rightarrow\mathrm{Fe}^{2+}+2\mathrm{e}-\]  
For the cathode, the main chemical reaction is the decomposition reaction of S\({}_{2}\)O\({}_{3}\)\({}^{2-}\). The reaction formula is as follows:  
\[\mathrm{S}_{2}\mathrm{O}_{3}\mathrm{{}^{2-}}+2\mathrm{e}^{-}\rightarrow\mathrm{ S}\mathrm{O}_{3}\mathrm{{}^{2-}}+\mathrm{S}\mathrm{{}^{2-}}\]  
With the increase of S\({}^{2-}\) and Fe\({}^{2+}\) in the metal surface accessories, the coating formation reaction occurs when [S\({}^{2-}\)] [Fe\({}^{2+}\)] \(\geq\) [FeS]. The reaction formula is as follows:  
\[\mathrm{Fe}^{2+}+\mathrm{S}\mathrm{{}^{2-}}\rightarrow\mathrm{FeS}\]  
Finally, the FeS is deposited on the surface of the steel substrate. The CBD treatments were carried out for 1 h at 30, 40, 50, 70, and 90 \({}^{\circ}\)C (denoted as S30, S40, S50, etc., respectively). After the CBD treatment, the obtained black samples were rinsed with running water and ethyl alcohol to clean the residual solution. The samples were dried by air and then soaked in engine oil for 3 h at room temperature. Finally, the soaked samples were taken out and placed on the table for two days (atmospheric environment) to obtain an oil film on the sample surface, which brings the wear test conditions closer to the actual service environment.  
A field emission scanning electron microscope (FE-SEM, Zeiss \(\Sigma\)IGMA HDTM, Oberkochen, Germany) equipped with secondary electron imaging (SEI) and energy dispersive spectrometry (EDS) was utilized to characterize the microstructure and element distribution of the treated samples. X-ray diffraction technique (XRD, PANalytical Empyrean Series 2, Almelo, Nederlands) was employed to determine the phase components of the coatings. The wear property was determined by a reciprocating tribometer (MS-T3001, Lanzhou Huahui Instrument Technology Co., Ltd., Lanzhou, China) with the 5 mm diameter steel balls (AISI 52100) as friction pairs. The steel ball slides linearly back and forth on the sample surface. The testing parameters were as follows: Load 6.0 N, duration time 10 min, frequency 2.0 Hz, wear scar length 10 mm. At least two sets of data were tested for each sample to verify the repeatability of the test results.