Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)

Abstract

In this work, the tribological behavior of ultrathin-MoS2 was investigated to understand the independent roles of water and oxidation. Water adsorption was identified as the primary interfacial mechanism for both SiO2/pristine-MoS2 and SiO2/graphene interfaces, however, tribological behavior of pristine-MoS2 was observed to be more sensitive to presence of water due to stronger MoS2-water interaction.Comparison of pristine-MoS2 and oxidized-MoS2 revealed that the oxidation of MoS2 significantly increased its friction and sensitivity to water by play a more detrimental role. The specific effect of oxygen on friction via chemical interactions was studied in isolation through density functional theory (DFT) simulations of a tip sliding on MoS2 basal planes and over edges before and after oxidation. The maximum change in energy, or energy barrier correlating with friction, as the tip moved across the surface, increased after oxidation by up to 66% for the basal plane and by 25% at the edge. Charge density analysis suggests that the more localized and non-uniform interfacial charge distribution on oxygen rich surfaces, as compared to pristine surfaces, leads to higher resistance to sliding. This confirms that oxygen presence alone increases friction and when coupled with the presence of water, both effects are additive in increasing friction.

High Temperature Microtribological Studies of MoS2 Lubrication for Low Earth Orbit

“…The average coefficient of friction as a function of temperature for the steady-state friction cycles (cycles 100-1024) is shown in Figure 2a. For the tests in air, the friction is found to decrease with respect to temperature until 110 • C and begin to increase again at 120 • C. This effect is characteristic of MoS 2 tribological contacts at elevated temperatures when ambient humidity is present [28,29]; the ambient humidity allows water intercalation between 2D MoS 2 sheets [30,31] which hinders the van der Waals sliding and increases the interfacial shear strength. As temperature increases, the desorption of water from the contact increases and the MoS 2 has reduced resistance to shear.…”

Structurally Driven Environmental Degradation of Friction in MoS2 Films

“…This is not likely a concern when comparing to simulation results, as wear during the first few cycles of sliding has been shown to remove the oxidized surface film, eventually leading to a recovery of low friction by the formation of ordered films and transfer films on the countersurface [45,63]. This behavior is different from edges terminated by sulfur atoms, as oxides are also more likely to attract environmental contaminants like water [24,25,40].…”

“…Interactions between water molecules and unterminated edges of MoS 2 lamellae have also been suggested as an impediment to low-strength shear [33]. In macroscale contacts, edges of MoS 2 lamellae at the surface are likely oxidized prior to testing, further enhancing interactions with water [15,24,[38][39][40]. Previous work by our group showed that the surface microstructure (i.e., crystallite or lamellae size and thickness of the oriented layer) has a significant impact on friction behavior and its evolution in the presence of various vapor species, including water and molecular oxygen.…”

Hydrogen bonding sewing interface

“…[11][12][13][14] Within the interfacial region, the hydrogen bonding network tends to form dynamically and further glues the surroundings together, which will renormalize the local interfacial interaction and further mediate the entire mechanical performance. [15][16][17][18] As one of carbon-based engineering structures, carbon bers with ultrahigh surface-to-volume ratios are usually bundled together to reinforce other materials. 19,20 The entire mechanical performance is mainly restricted by the interfacial bonding.…”