Extreme pressure (EP) additives are widely used in many industries such as engine and gear systems for vehicles [1, 2] and cold working of metals . EP additives such as molybdenum dithiocarbamate (MoDTC)  and zinc dialkyldithiophosphate (ZDDP) [5, 6] have been added to engine oil in recent years. MoDTC reduces friction by forming an MoS2 tribofilm on the contact area [7–10] whereas ZDDP prevents wear by forming an amorphous tribofilm with multi-layered structures [5, 6, 11]. The role of metallic and non-metallic substrates in the tribochemical reaction of MoDTC [12–14] and ZDDP [15–17] has been the focus of recent studies.
Chlorine-containing EP additives are widely used in the cold working process of steel because they are relatively cost effective and exhibit high lubricity even for stainless steel with high corrosion resistance . However, its use is restricted owing to environmental and public health hazards, such as carcinogenicity. Much effort has been made to develop chlorine-free lubricating oils. However, the use of chlorine-free additives is still limited in the cold reduction of metal working for two reasons. First, for high-alloy steels used in applications that require a high degree of corrosion resistance, such as the energy-related industry, chlorine-based additives are still utilized during cold working processes to prevent scuffing. Second, there is a concern that specific elements in additives may invade the steel and affect its mechanical properties if the heat treatment process is initiated with the retention of the lubricating oil during processing. Additionally, phosphorus-containing additives such as ZDDP cannot be used to cause embrittlement of steel. Sulfur-containing EP additives such as mono-, di-, and poly-sulfides, which are less toxic to the environment than chlorine-containing additives, are expected to be used as an alternative to chlorine-containing EP additives in the cold working of steel.
The structures of tribofilms, formed as a result of chemical reactions between steel and sulfur-containing EP additives, have been extensively investigated [3, 18–27]. Sulfur-containing EP additives react with the steel surface to form inorganic tribofilms, such as iron sulfide, with a relatively low shearing strength that prevents scuffing. The FeS crystal, which has a hexagonal crystal structure, exhibits a low-shear strength[28, 29]. Wheeler et al.  reported that FeS or FeS2 tribofilms with FeSO4 contaminants are formed with sulfur-containing EP additives. Lara et al.  reported that dimethyl disulfide thermally decomposes on iron surfaces via half-order kinetics to yield a film comprising only FeS at temperatures as low as 523 K. However, Miyajima et al. [22, 23] reported that polysulfide chemically reacts with carbon steel during the rubbing process to yield a film composed of FeS2 and graphite structure. These results indicate that changes in the chemical composition of the tribofilms ultimately depend on the applied load, speed, temperature, and chemical structure of the EP additives used for the various types of tribo-testers. However, many studies have employed carbon steel for experimentation.
Few studies have reported the chemical reactions between stainless steel and sulfur-containing EP additives [3, 18, 21, 26]. The structures of the tribofilms formed from chlorine- and sulfur-containing EP additives on stainless steel were investigated using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) . Matsumoto et al.  investigated tribofilms formed from polysulfide on carbon, stainless steel, other high alloys, Ni, and Cr using XPS. They reported that materials containing a large amount of Fe formed thick tribofilms. They also found that materials with films indicating a high hardness exhibited low friction. However, the reason why materials with a large amount of Fe form thick tribofilms is unclear. They did not investigate the chemical and crystal structures of the tribofilms over the entire thickness range because they only conducted XPS. To prevent scuffing and to replace chlorine-containing additives with sulfur-containing ones, a detailed investigation of the structure of the tribofilm on the steel type is necessary.
In this study, the structures of tribofilms formed from polysulfide on carbon steel and stainless steel were investigated using four ex situ techniques: XPS [7, 10, 16, 21, 30], Raman spectroscopy [8, 9, 12–14, 22, 23, 31–34], X-ray diffraction (XRD) analysis [12, 13, 35–40], and transmission electron microscopy (TEM) [7, 11, 14–16] to understand the causes of scuffing in stainless steel.