Friction Mechanism\nof Sulfur-Containing Lubricant\nAdditives Confined between Fe(100) Substrates

Sulfur-containing\nlubricant additives can chemically\nreact with\nmetal surfaces under extreme conditions, such as high temperature\nand high pressure, forming protective films on the surfaces. However,\nthe formation mechanisms and the friction-reducing and antiwear properties\nof these films remain unclear. In this study, we investigated the\nfriction process of sulfur-containing additives confined between two\niron surfaces using reactive molecular dynamics simulations. Our research\nrevealed that in systems with a higher S/C ratio, an iron sulfide\nlayer formed on the iron surfaces with Fe–S–Fe bridging\nbonds at the interface, resulting in relatively smaller friction and\nwear even under higher loads. However, in systems with lower S/C ratios,\nthe presence of numerous interfacial Fe–C<sub><i>n</i></sub>–Fe bridging bonds, caused by the hydrocarbon radicals\nreleased from the additives, led to the formation of thick amorphous\nshearing bands at the interface between the two substrates. In this\ncase, the distributed sulfur atoms also exhibited some effect in reducing\nthe shear resistance of the amorphous shearing bands due to the weak\nstrength of S–Fe bonds compared to the strength of C–Fe\nbonds. These atomic-level insights help understand the antiwear characteristics\nof sulfur-containing lubricant additives confined between iron substrates.