(538b) Environmental Effects on the Tribological Properties of MoS2

Molybdenum disulfide (MoS₂) is a lamellar solid with applications in many areas including semiconductor devices, catalysis, and lubrication. The layers of MoS₂ interact via weak van der Waals forces, enabling the layers to slide with a coefficient of friction as low as in dry or vacuum environments. This makes MoS₂ an attractive solid lubricant for use in satellites and other aerospace applications. However, exposure to environmental species, e.g. oxygen and water, has been shown to degrade the lubricating properties of MoS₂. The precise mechanisms of the interactions between MoS₂ and water and how these interactions affect its tribological properties remain poorly understood.

We will report the results of density functional theory, Monte Carlo, and molecular dynamics simulations of water adsorption on MoS₂ under realistic atmospheric conditions. We will discuss the interaction of water and oxygen with common defects in MoS2, including sulfur vacancies and oxidized sites. We have additionally computed isotherms for water adsorption between MoS₂ lamella for a range of realistic temperatures, relative humidities, and defect densities, to determine the chemical environment between MoS₂ layers in representative application conditions. While water does not typically strongly wet MoS2 (contact angle ≥ 70^o), we show that the presence of defects greatly increases the adsorption of water between lamella. We will also discuss how the presence of water affects the tribological properties of MoS₂.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND Number: SAND2020-3912 A