## 1 Introduction  
Gear transmission is one of the most important transmission methods in modern machinery because of its many attractive properties like high transmission efficiency, accurate and stable, high transmission power, and wide applicable speed range [1; 2; 3]. With the progress of technology and the development of gear transmission, the gear is developed toward high speed, heavy load, complex service environment, and high reliability [4]. The development of transmission systems has made gear materials to be upgraded in the direction of light weight, good wear resistance, good corrosion resistance, and economic efficiency [5].  
The primary failure mode of gears is wear, which is a gradual loss of surface materials because of the friction between the two contact gears [5; 6]. The friction on the gear surfaces greatly affects the power loss and service life of the transmission system [7]. Unfavorable working environment, such as poor lubricity, can lead to rapid failure of the gears, seriously undermining the reliability of the transmission system [8]. According to incomplete statistics, about 1/3-1/2 of primary energy is consumed by friction, and about 70% of equipment failures are related to the wear failure of gears [9]. Therefore, how to improve the gear service performance, reduce material loss and energy consumption, and avoid safety accident caused by friction and wear are of great significance.  
Improving the wear resistance of gear materials mainly starts from two aspects: one is to improve the surface hardness of the gear material, and the other is to introduce a lubricating medium. For the first, surface strengthening methods such as carburizing [10; 11], nitriding [12; 13], carbonitriding [14; 15], thermal chromizing [16; 17; 18], electron beam surface treatment [4; 16], electro brush-plating [5], double glow plasma surface alloying [19; 20], or deformation induced martensite [21] are commonly used to improve the performance of the steel surface. However, the hard surface accelerates the wear of its counterpart and is ineffective in preventing strain fatigue of the substrate, which results in scuffing failure, especially under boundary lubrication of rubbing-pairs [22]. Generally, the lubricity of the materials can be improved by introducing liquid or solid lubrication medium, designing shallow holes for storing lubricating oil on the surface of the material [23], introducing nanoparticles as lubricant additives [24], replacing metal-metal friction pairs by metal-polymer ones [25], surface texturing treatment [26; 27; 28], and surface geometry modification [29]. Depositing a solid lubricating coating on the gear surface is considered to be a simple and very effective approach to improve the wear resistance of materials. FeS is considered as an excellent solid lubricant because of its hexagonal structure, low shear strength, and high melting point [22; 30; 31; 32; 33; 34]. Moreover, the cost is very low, only about 1% that of MoS\({}_{2}\) coating which is also considered as a good solid lubricant for the gears [35; 36]. Recently, it has been confirmed that the FeS solid lubrication coatings deposited on steels by ion sulfurizing [22; 37; 38; 39], plasma spraying [22; 40], plasma source ion sulfurizing [41], can significantly improve the lubrication property of steels. In this study, the FeS coatings were prepared on commercial AISI 5140 steel by a chemical bath deposition (CBD) coating technique at various temperatures. The influence of reaction temperature on the microstructure, microcracks, and lubrication properties were investigated.