Tribological properties of MoS2 and CaF2 particles as grease additives on the performance of block-on-ring surface contact

Abstract

Recently, the performance effects of lubricant additives on greases have been actively investigated. In order to improve their properties of anti-friction, load-carrying capacity, wear resistance, corrosion resistance, and temperature-enhancing, some additives are added to greases. Determining the appropriate additive ratios is one of the most important research topics in tribology. In this study, friction, wear, and contact temperature were analyzed by adding MoS2 and CaF2 additives to the lithium soap grease. The experiments were carried out at different revolutions (200, 500 and 800 rpm), additive ratios (0.25, 0.5, 1, 1.5 and 2 wt%) and contact loads (72, 108 and 144 N) using a block-on ring tester. The structure and morphology of additives analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of additives on surface morphology and roughness were also characterized with the scanning electron microscopy (SEM) and profilometer. In addition, energy dispersive spectrometry (EDS) was used to detect the elements present on the wear scar surfaces. Results reveal that the MoS2 and CaF2 additives can improve the wear resistance of lithium grease and reduce the friction coefficient. In this experimental study, suitable additive ratios that are anti-wear and friction-reducer were found to be in the range of 1–1.5 wt% for both additives. Besides, both additives continued their friction-reducing characters at over the room temperatures. While the MoS2 additive oil gives the lowest friction and wear values, the same ratio of CaF2 additive oil shows a more stable operation.