Tribological Performance of Dichalcogenides Formed by the Decomposition of Water-Based Tetraammonium Thiometallates Lubricant Additives

Dichalcogenides such as MoS2 and WS2 are anistropic layered materials which find extensive use as photoactive materials, catalysts for hydro treating applications and solid lubricants. Among many methods to prepare MoS2 and WS2 films and nanoparticles for tribological applications, they can also be prepared from the decomposition of water-soluble tetraalkylammonium molybdates and tungstates, respectively. In the present work we report the tribological performance of tetraammonium molybdate and tungstate as water-soluble lubricant additives in steel-steel and steel-aluminum contacts. Aqueous solutions of tetraammonium molybdate (NH4)2MoS4 and tungstate (NH4)2WS4 were prepared and their tribological properties were tested using a ball on disc tribometer. Tests were performed at the same conditions of load, entrainment speed, sliding distance, temperature and concentration in order to compare the lubrication effect of both thiometallates. Although there is a reduction of friction and wear compare to pure water, a significant difference in friction coefficient and wear rate is observed depending on the mating surfaces and the thiometallates used. The wear scars were analyzed by scanning electron microscopy (SEM), X-ray photo electron specectroscopy (XPS) and Raman spectroscopy. These analyses revealed the in-situ presence of a solid lubricant reach in molybdenum, tungsten and sulfur indicating the in-situ formation of MoS2 and WS2 films depending on the thiosalt used as lubricant additive.