Tribological evaluation of environmentally friendly MoS2 based solid film lubricants for gas turbine engines

Solid lubricants such as MoS2 have been widely used in the aerospace industry with the primary purpose to lower the friction and wear of the tribological interfaces. Pure MoS2 generally performs better (i.e., in terms of low friction and low wear) in vacuum conditions compared to humid environments as the premature failure of the lubricant can be attributed to undesirable interfacial processes as well as the formation of oxides such as MoO3. To overcome these limitations as well as increase the endurance life of these lubricants in ambient conditions, they are generally doped with different elements or compounds. For instance, Pb based and Sb2O3 compounds have been used as dopants with MoS2 to improve the tribological performance of the lubricant. However, due to the recent environmental concerns and targets to eliminate non-green lubricants, there is a strong desire to eliminate Pb based compounds due to its toxic nature. In addition, bonded MoS2 based solid lubricants contain a binder system and various carriers to bind and deposit the lubricating solid (i.e., pigment) and the dopant with the substrate. Currently, the carriers are mainly composed of the organic solvents and thus, are responsible for the release of volatile organic compounds (VOCs) during the curing process. The release of the VOCs can be harmful for the ecosystem due to its toxicity and carcinogenicity. Hence, to eliminate these organic solvents, alternatives are preferred which are generally aqueous solvents. The main purpose of this research is to provide a better understanding of the influence of the toxic elements on the interfacial processes of MoS2 based solid lubricants. This thesis is composed of two research studies where we critically evaluate the tribological performance of ‘non-green’ and ‘green lubricants’ in order to fully capture the influence of the various elements on the interfacial phenomenon. In the first study, the baseline ‘non-green lubricant’ was doped with Pb based compound and Sb2O3 and in the ‘green lubricant’, the Pb based compound was removed and was replaced by higher percentage of Sb2O3. In the second study, the bonded MoS2 based solid lubricants were doped with Sb2O3, with one of the lubricants containing solvent-based carrier and the other lubricant with water-based carrier. In both the studies, the tribological evaluation was performed using ball-on-disc tribometer with various normal loads. Subsequently, the ex-situ analysis was done using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Micro-Raman Spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) to understand the chemical composition and to analyze the interfacial processes at different normal loads. It was observed that the non-green lubricant performed better in terms of tribological behavior (i.e., low friction and low wear) when compared with the green lubricant. The improved tribological properties were attributed due to the formation of dense and uniform tribofilm and transfer film with the basal planes oriented in the parallel direction. Furthermore, it was observed that the solvent-based lubricant had performed better tribologically (i.e., low friction and low wear). The increased coefficient of friction and reduced wear resistance of the water-based lubricant was attributed to the increased cohesiveness within the lubricant. The findings of the interfacial phenomenon have provided vital information for developing green alternatives that may have the ability to replace toxic compounds in the future for sustainable ecosystem