RESUMO
This study investigates the temperature-dependent frictional behavior of MoS2 in humid environments within the context of a long-standing debate over increased friction due to oxidation processes or molecular adsorption. By combining sliding friction experiments and density functional theory (DFT)-based first-principles simulations, it aims to clarify the role of water molecule adsorption in influencing frictional properties of MoS2, addressing the challenge of identifying interfacial bonding behavior responsible for friction in such conditions. Sliding experiments revealed that magnetron-sputtered MoS2 exhibits a reduction in the coefficient of friction (COF) with an increase in temperature from 25 to 100 °C under 20 and 40% relative humidity. This change in the COF obeys the Arrhenius law, presenting an energy barrier of 0.165 eV, indicative of the temperature-dependent nature of these frictional changes and suggests a consistent frictional mechanism. DFT simulations showed that H2O molecules are adsorbed at MoS2 vacancy defects with adsorption energies ranging from -0.56 to -0.17 eV, which align with the experimentally determined energy barrier. Adsorptive interactions, particularly the formation of stable H···S interfacial hydrogen bonds at defect sites, increase the interlayer adhesion and impede layer shearing. TEM analysis confirms that although MoS2 layers align parallel to the sliding direction in humid conditions, the COF remains at 0.12, as opposed to approximately 0.02 in dry air. This demonstrates that parallel layer alignment does not notably decrease the COF, underscoring humidity's significant role in MoS2's COF values, a result also supported by the Arrhenius analysis. The reversibility of the physisorption process, demonstrated by the recovery of the COF in high-temperature humid environments, suggests its dynamic nature. This study yields fundamental insights into MoS2 interfaces for environments with variable humidity and temperature, crucial for demanding tribological applications.
RESUMO
Two-dimensional (2D) molybdenum disulfide exhibits a variety of intriguing behaviors depending on its orientation layers. Therefore, developing a template-free atomic layer orientation controllable growth approach is of great importance. Here, we demonstrate scalable, template-free, well-ordered vertically-oriented MoS2 nanowire arrays (VO-MoS2 NWAs) embedded in an Ag-MoS2 matrix, directly grown on various substrates (Si, Al, and stainless steel) via one-step sputtering. In the meta-structured film, vertically-standing few-layered MoS2 NWAs of almost micron length (â¼720 nm) throughout the entire film bulk. While near the surface, MoS2 lamellae are oriented in parallel, which are beneficial for caging the bonds dangling from the basal planes. Owing to the unique T-type topological characteristics, chemically inert Ag@MoS2 nano-scrolls (NSCs) and nano-crystalline Ag (nc-Ag) nanoparticles (NPs) are in situ formed under the sliding shear force. Thus, incommensurate contact between (002) basal planes and nc-Ag NPs is observed. As a result, robust superlubricity (friction coefficient µ = 0.0039) under humid ambient conditions is reached. This study offers an unprecedented strategy for controlling the basal plane orientation of 2D transition metal dichalcogenides (TMDCs) via substrate independence, using a one-step solution-free easily scalable process without the need for a template, which promotes the potential applications of 2D TMDCs in solid superlubricity.
RESUMO
Sliding contact experiments and first-principles calculations were performed to elucidate the roles of environmental molecules containing -OH functional groups on the friction behavior and structural evolution of hexagonal boron nitride (h-BN). A significant decrease in the friction coefficient (COF) is established by the physisorption and dissociative adsorption of molecules containing -OH functional groups on h-BN, compared with that in a H2 or N2 atmosphere. A key finding is the existence of two friction mechanisms to reconstruct the sliding interface for h-BN crystallites in humid air and carbon contaminant (CH3OH and C2H5OH) atmospheres, which is verified by the friction behavior and morphologies of the wear track. There is a running-in period in the friction process to induce the formation of defects in h-BN in humid air, which facilitates dissociative adsorption of water molecules on h-BN. The formation of nanostructured water at defect sites will promote lamellar slip of h-BN crystal materials for friction reduction. In carbon contaminant environments, both molecules exhibit strong adsorption on the h-BN surface regardless of the presence of defects, thereby weakening the structural damage rate and enhancing the bearing capacity. C2H5OH molecules are more likely to dissociate and bind onto defect sites, endowing h-BN with high in-plane stress to form a coiled structure. h-BN in the annular or tubular form would exhibit a self-protective effect, facilitate incommensurate contact, and reduce the contact area to enhance lubrication. Our work may establish the fundamental basis for future applications of h-BN in new energy vehicles.
RESUMO
Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (E b ) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (E B ) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.
