In the 90 years since the first patent was issued for molybdenum disulfide (MoS2) as a friction and wear reducing additive, great strides have been made in understanding its remarkable lubricity. However, much remains to be understood about the mechanisms of friction at the molecular scale. We begin with results of investigations into the temperature- dependent friction behavior of pure MoS2. In the range -150 to 250°C, we report dramatic deviations from previous literature, as well as the existence of transitions between thermal and athermal behavior as a function of temperature. Evidence of deviations from classical Arrhenius behavior is presented, and the implications of these findings discussed in the context of a thermally-activated friction model at the molecular scale. Friction is shown to be strongly dependent on substrate material composition and stress, implying a connection between stress and microstructure evolution in the films and substrates that is discussed. We discuss how these findings lead to a practical predictive friction model for MoS2 that includes temperature, stress, substrate effects, defect density and commensurability as their foundation. Defect sites, epseically in the form of undercoordinated Mo on the edges of larger flakes, provide locations for chemical reactions with environmental species, such as oxygen and water, which can significantly degrade performance. We present the results of detailed simulations that study the effcts of of these species on pure and defective MoS2, including the potential for enhanced reactivity of MoS2 under applied stresses, and the ultimate effects on the tribological properties.

Thermodynamic basis of friction in MoS2

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.