The tribological behavior and tribo-layers of AISI 1045 steel sliding against 52100 steel were investigated in the case of supplying MoS2, Fe2O3, and their mixtures onto the sliding interface. When nanoparticles were supplied, tribo-layers were formed on the worn surfaces. The tribological behavior of the sliding pair depended on the characteristics of tribo-layers, which were decided by different nanoparticles. As the additives—especially the ones containing MoS2—were supplied onto the sliding interface, the wear rates and friction coefficients of both 1045 steel and 52100 steel were markedly decreased to extremely low values, approaching zero and marginally undulated with the increase in load. Single-component Fe2O3 nanoparticles markedly reduced the wear rate of 1045 steel with slightly increased friction coefficient, but its decreased extent was merely half of that of the additives containing MoS2. The improvement of the tribological performance of steels was attributed to the formation of protective tribo-layers. The addition of pure Fe2O3 resulted in the formation of insert-type tribo-layers, while cover-type tribo-layers were formed by the addition of the mixture additives of Fe2O3+MoS2 and pure MoS2. The cover-type tribo-layers provided more protective and lubricative functions than that of the insert-types.
