Se Nanopowder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions.

Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2 , even though selenides and tellurides have demonstrated superior tribological properties. Here, an innovative in operando conversion of Se nanopowders into lubricious 2D selenides, by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films, is described. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in the shear-induced synthesis of selenide monolayers from nanopowders. The use of Se nanopowder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nanopowder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in operando and exploit their friction- and wear-reducing impact.

Effect of vacancies and edges in promoting water chemisorption on titanium-based MXenes.

The functionality of two-dimensional (2D) transition metal carbides and nitrides (MXenes) in technological applications greatly depends on their wettability. For instance, MXenes' layer stability against degradative oxidation is notably reduced when stored in aqueous solutions, leading to the transformation into oxides. In this work, we study water adsorption on Ti-based MXenes by ab initio calculations. The energy gains for the molecular adsorption on Tin+1XnT2 is evaluated as a function of the termination (T = F, O, OH, mixture), the carbon/nitrogen ratio (X = C, N), the layer thickness (n) and water coverage. MXenes' hydrophilicity tends to increase due to the presence of defects as vacancies and flake edges. We demonstrate that physical adsorption occurs through hydrogen bonding on both defect-free layers and layers containing C/N or Ti atomic vacancies, with -OH terminations providing the strongest interactions (0.40-0.65 eV). In contrast, strong water chemisorption is observed on surfaces with a single termination vacancy (0.60-1.20 eV), edges (0.75-0.85 eV), and clusters of defects (1.00-1.80 eV). We verified that the presence of undercoordinated Ti atoms on the surface is the key factor in promoting H2O chemisorption, i.e., the degradative oxidation.

Nanoscale MXene Interlayer and Substrate Adhesion for Lubrication: A Density Functional Theory Study.

Understanding the interlayer interaction at the nanoscale in two-dimensional (2D) transition metal carbides and nitrides (MXenes) is important to improve their exfoliation/delamination process and application in (nano)-tribology. The layer-substrate interaction is also essential in (nano)-tribology as effective solid lubricants should be resistant against peeling-off during rubbing. Previous computational studies considered MXenes' interlayer coupling with oversimplified, homogeneous terminations while neglecting the interaction with underlying substrates. In our study, Ti-based MXenes with both homogeneous and mixed terminations are modeled using density functional theory (DFT). An ad hoc modified dispersion correction scheme is used, capable of reproducing the results obtained from a higher level of theory. The nature of the interlayer interactions, comprising van der Waals, dipole-dipole, and hydrogen bonding, is discussed along with the effects of MXene sheet's thickness and C/N ratio. Our results demonstrate that terminations play a major role in regulating MXenes' interlayer and substrate adhesion to iron and iron oxide and, therefore, lubrication, which is also affected by an external load. Using graphene and MoS2 as established references, we verify that MXenes' tribological performance as solid lubricants can be significantly improved by avoiding -OH and -F terminations, which can be done by controlling terminations via post-synthesis processing.