High Temperature Microtribological Studies of MoS2 Lubrication for Low Earth Orbit

Molybdenum disulfide is one of the most common lubricant coatings for space systems but it displays enormous susceptibility to environmental conditions making it hard to predict performance throughout the entire lifetime. The majority of mechanisms for space operate in low Earth orbit where temperatures typically reach 120 degrees C along with exposure to highly reactive atomic oxygen which can be detrimental to lubricant performance. In the present study, a MoS2 lubricant coating is tested using friction force microscopy under different environmental conditions including air and dry nitrogen environments with temperatures ranging from 25 degrees C to 120 degrees C. The increased temperature was found to be beneficial for friction behaviour in air up to 100 degrees C as ambient humidity is removed from the contact, but higher temperatures become detrimental as increased reactivity leads to oxidation. These competing effects resulted in a minimum coefficient of friction at 110 degrees C in the air environment. The high temperature also increases the wear of the coatings as the intrinsic shear strength decreases with thermal energy which in turn disrupts tribofilm formation leading to increased friction. The run-in duration and magnitude are both found to decrease with temperature as the energy barrier to optimal reconfiguration is reduced. Finally, contextualization of the present findings for mechanisms operating in low earth orbit is discussed.