Low energy proton irradiation tolerance of molybdenum disulfide lubricants

Abstract

As the world’s space agencies look to long duration spaceflight beyond low earth orbit, engineering materials in deep space flight will endure galactic cosmic ray irradiation, of which 85% is protons. Characterization of proton irradiation damage near the threshold displacement energy is required to understand the mechanisms behind performance degradation and the fluence dependent effects of a thermalized spectrum on surface sensitive tribological characteristics. Here, tribological degradation of a space-grade molybdenum disulfide lubricant exposed to low energy (500 eV) protons is investigated as a function of fluence. A 21% increased coefficient of friction and 86% increase in wear rate is measured at 1020H/m2, and 170% increased CoF and 465% increased wear rate at 3·1023H/m2. Associated softening and decreased stiffness is measured by nanoindentation. X-ray photoelectron spectroscopy and Raman spectroscopy identify the mechanism of tribological degradation as being the preferential removal of sulfur leading to a disordered and reactive non-stoichiometric MoS2-x surface with compromised tribological properties. Missions beyond low-earth orbit will be required to withstand GCR radiation, and it is not suggested that the density of radiation damage sustained will be acutely detrimental to MoS2 lubricants. Still, research missions should plan to characterize the magnitude of radiation damage effects.