Abstract
Solid lubricants have received substantial attention due to their excellent frictional properties. Among others, molybdenum disulfide (MoS2) is one of the most studied lubricants. Humidity results in a deterioration of the frictional properties of MoS2. The actual mechanism at the nanoscale is still under debate, although there are indications that chemical reactions are not likely to occur in defect-free structures. In this study, we performed nonequilibrium molecular dynamics simulations to study the frictional properties of multilayer MoS2 during sliding in the presence of water. Moreover, we also investigated the effect of sliding speed and normal load. We confirmed earlier results that a thin layer of water organizes as a solidified, ice-like network of hydrogen bonds as a result of being confined in a two-dimensional fashion between MoS2. Moreover, we found that there exists an energy-driven, rotational dependence of the water network atop/beneath MoS2. This orientational anisotropy is directly related to the dissipative character of MoS2 during sliding. Finally, three distinct frictional regimes were identified, two for a thin layer of water and one for bulk water. In the case of a thin layer and low coverage, water represents a solid-like contaminant, causing high energy dissipation. For a thin layer and high coverage, water starts to act as a solid-like lubricant, reducing dissipation during sliding. Finally, a regime where water acts as a liquid lubricant, characterized by a clear velocity dependence was found.
Highlights
The presence of friction and wear has been linked to a loss of almost a quarter of the world’s total energy consumption (Holmberg and Erdemir, 2017)
At the value of 8.5% water coverage, there is a transition toward a new type of dynamics, where we found that MoS2 starts to divide into two blocks, one above and one beneath the water molecules
We investigated the frictional properties of bulk commensurate MoS2 in the presence of water
Summary
The presence of friction and wear has been linked to a loss of almost a quarter of the world’s total energy consumption (Holmberg and Erdemir, 2017). As a result of this unique binding character, easy shearing between the layers is accommodated, creating an enormous potential for achieving low Nanoscale Lubrication Mechanisms in MoS2/H2O coefficients of friction (COF) (Martin et al, 1993; Scharf and Prasad, 2013). This is confirmed by numerous studies on their remarkable tribological characteristics (Martin et al, 1993; Watanabe et al, 2004; Cho et al, 2006; Evaristo et al, 2008; Pimentel et al, 2011; Mutafov et al, 2015). This, in combination with their low toxicity (Teo et al, 2014; Chng and Pumera, 2015), versatile chemistry (Chhowalla et al, 2013), and peculiar electronic properties, has resulted both in macroscale applications, such as the automotive, aerospace, and space industries (Cho et al, 2006; Nian et al, 2017; Vazirisereshk et al, 2019) and nanoscale applications, such as photovoltaic and optoelectronic devices, transistors, and solar cells (Li and Zhu, 2015; Choi et al, 2017)
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