Monolayer NbSe2 Favors Ultralow Friction and Super Wear Resistance

Self-Lubricating Tribo-Catalytic Activity of 2D High Entropy Alloy Nanoflakes

High Entropy Alloys (HEAs) have garnered attention due to their remarkable tribological attributes. Predominantly, failure mechanisms in HEAs emanate from stress-induced dislocations, culminating in crack propagation and film delamination. In this study, we report on the synthesis of 2D HEA of (MoWNbTaV)0.2S2 which facilitates shear-induced energy dissipation at sliding interfaces. The ball-on-disk tribological investigations demonstrate unprecedentedly low average coefficients of friction (0.076) and wear rates (10-9 mm3 (N∙m)-1) under high contact pressures (0.936 GPa) within ambient conditions. Employing multi-scale characterizations alongside molecular dynamic simulations, we elucidate that the presence of the HEA triggers tribocatalytic activity under high contact pressures emerging as a pivotal factor in extending lubricant lifespan during tribological tests. The resilient lubriciousness coupled with the facile spray coating methodology of (MoWNbTaV)0.2S2 in ambient environments paves the way for the development of a new class of solid lubricants based on 2D HEA.

Macro-Superlubricity Induced by Tribocatalysis of High-Entropy Ceramics

Macroscale superlubricity has attracted considerable attention as a promising strategy to minimize frictional energy dissipation and achieve near-zero wear. However, realizing macroscale superlubricity with prolonged durability remains an immense challenge, particularly on engineering steels. Current superlubricants render steel surfaces susceptible to corrosion, causing severe wear and superlubrication failure. Herein, high-entropy ceramics (HEC) with catalytic properties are innovatively introduced to prevent corrosion of engineering steels and achieve macro-superlubricity through tribo-catalytic effect. Furthermore, this catalytically induced superlubricity system exhibits an ultra-low friction coefficient of 0.0037 under contact pressure up to 1.47 GPa, an ultra-long cycle lifetime of 1.25 × 106 cycles (corresponding sliding distance up to 5 km), and an extremely low wear rate of 3.032 × 10-10 mm3·N-1·m-1 on the HEC surface. Based on the experimental analysis and theoretical simulation, the in situ formed HEC nanocrystals reduce the Gibbs free energy of hydrolysis of PA molecules into inositol and phosphoric acid molecules in the lubricant. Notably, the hydrolysis products favorably contributed to the reduction of shear force in the lubrication system, which is essential for achieving macroscale superlubricity over a long time. This study provides a new perspective for designing robust superlubricity systems by harnessing the tribocatalytic effect of high-entropy materials.

Observing and Modeling the Wear Process of Heterogeneous Interface

Understanding and controlling the wear process of heterogeneous interfaces between soft and hard phases is crucial for designing and fabricating materials, such as improving the wear resistance of particle reinforced metal matrix composites and the accuracy and efficiency of chemical mechanical polishing. However, the wear process can be hardly observed, as interfaces are buried under the surface. Here, we proposed a nanowear test method by combining focused ion beam cutting to expose interfaces, atomic force microscopy to rub against interfaces, and scanning electron microscope to characterize the interface damage. Using this method, three typical wear forms had been observed in Al/SiC composite, i.e., merely matrix wear, particle fracture, and particle pullout. A theoretical model was proposed that revealed that the increasing interfacial friction would induce particle fracture or pullout, depending on the particle edge angle and tip edge angle. This work sheds light on wear control in composites and nanofabrication.

Observation of Anisotropic Second Harmonic Generation in Two-Dimensional Niobium Diselenide

The intrinsic anisotropy of NbSe2 provides a favorable prerequisite of second harmonic generation (SHG) and rich possibilities for tailoring its nonlinear optical (NLO) properties. Here we report the highly efficient SHG of mechanically exfoliated NbSe2 flakes. The nonlinear optical response changes with excitation wavelengths, layer thicknesses, and polarizations of the excitation laser. The anisotropic SHG response further exhibits the intrinsic non-centrosymmetric crystal structure and could effectively assign the crystalline orientation of NbSe2 flakes. Interestingly, although NbSe2 flakes with tens of nanometers thickness experience attenuations in SHG performance, more efficient SHG anisotropy ratios were obtained, which are around 4 times higher than that of the 5-layer counterpart. The determined second-order nonlinearities of NbSe2 flakes (monolayer: ∼1.0 × 103 pm/V; 3-layer: ∼73 pm/V) are comparable to those of the commonly reported two-dimensional materials (e.g., MoS2, WSe2, graphene) with the same number of layers and much higher than those of commercial nonlinear optical crystals.

Fundamental Limits of Few-Layer NbSe2 Microbolometers at Terahertz Frequencies

The rapid development of infrared spectroscopy, observational astronomy, and scanning near-field microscopy has been enabled by the emergence of sensitive mid- and far-infrared photodetectors. Superconducting hot-electron bolometers (HEBs), known for their exceptional signal-to-noise ratio and fast photoresponse, play a crucial role in these applications. While superconducting HEBs are traditionally crafted from sputtered thin films such as NbN, the potential of layered van der Waals (vdW) superconductors is untapped at THz frequencies. Here, we introduce superconducting HEBs made from few-layer NbSe2 microwires. By improving the interface between NbSe2 and metal leads, we overcome impedance mismatch with RF readout, enabling large responsivity THz detection (0.13 to 2.5 THz) with a minimal noise equivalent power of 7 pW/ H z and nanosecond-range response time. Our work highlights NbSe2 as a promising platform for HEB technology and presents a reliable vdW assembly protocol for custom bolometer production.