Comparative analysis of frictional behavior and mechanism of molybdenum ditelluride with different structures

Two-dimensional(2D)transition metal dichalcogenides(TMDCs)have layered structures with excellent tribological properties.Since the energy difference between hexagonal-molybdenum ditelluride(2H-MoTe_(2))and distorted octahedral-molybdenum ditelluride(1T′-MoTe_(2))is very small among the transition metal dichalcogenides(TMDCs),MoTe_(2) becomes one of the most promising candidates for phase engineering.In our experiment,we found that the friction force and friction coefficient(COF)of 2H-MoTe_(2) were an order of magnitude smaller than those of 1T′-MoTe_(2) by the atomic force microscope(AFM)experiments.The friction difference between 1T′-MoTe_(2) and 2H-MoTe_(2) was further verified in molecular dynamics(MD)simulations.The density functional theory(DFT)calculations suggest that the friction contrast is related to the difference in sliding energy barrier of the potential energy surface(PES)for a tip sliding across the surface.The PES obtained from the DFT calculation indicates that the maximum energy barrier and the minimum energy path(MEP)energy barrier of 2H-MoTe_(2) are both smaller than those of 1T′-MoTe_(2),which means that less energy needs to be dissipated during the sliding process.The difference in energy barrier of the PES could be ascribed to its larger interlayer spacing and weaker Mo–Te interatomic interactions within the layers of 2H-MoTe_(2) than those of 1T′-MoTe_(2).The obvious friction difference between 1T′-MoTe_(2) and 2H-MoTe_(2) not only provides a new non-destructive means to detect the phase transition by the AFM,but also provides a possibility to tune friction by controlling the phase transition,which has the potential to be applied in extreme environments such as space lubrication.

Superlubricity induced by partially oxidized black phosphorus on engineering steel

Macroscale superlubricity has attracted increasing attention owing to its high significance in engineering and economics.We report the superlubricity of engineering materials by the addition of partially oxidized black phosphorus(oBP)in an oleic acid(OA)oil environment.The phosphorus oxides produced by active oxidation exhibit lower friction and quick deposition performance compared to BP particles.The H-bond(–COOH…O–P,or–COOH…O=P)formed between P–O bond(or P=O)and OA molecule could benefit the lubricating state and decrease the possibility of direct contact between rough peaks.The analysis of the worn surface indicates that a three-layer tribofilm consisting of amorphous carbon,BP crystal,and phosphorus oxide forms during the friction,which replaces the shear interface from the steel/steel to carbon–oBP/carbon–oBP layer and enables macroscale superlubricity.

Interlayer friction behavior of molybdenum ditelluride with different structures

The interlayer friction behavior of two-dimensional transition metal dichalcogenides(TMDCs)as crucial solid lubricants has attracted extensive attention in the field of tribology.In this study,the interlayer friction is measured by laterally pushing the MoTe_(2)powder on the MoTe_(2)substrate with the atomic force microscope(AFM)tip,and density functional theory(DFT)simulations are used to rationalize the experimental results.The experimental results indicate that the friction coefficient of the 1T'-MoTe_(2)/1T'-MoTe_(2)interface is 2.025×10^(−4),which is lower than that of the 2H-MoTe_(2)/2H-MoTe_(2)interface(3.086×10^(−4)),while the friction coefficient of the 1T'-MoTe_(2)/2H-MoTe_(2)interface is the lowest at 6.875×10^(−5).The lower interfacial friction of 1T'-MoTe_(2)/1T'-MoTe_(2)compared to 2H-MoTe_(2)/2H-MoTe_(2)interface can be explained by considering the relative magnitudes of the ideal average shear strengths and maximum shear strengths based on the interlayer potential energy.Additionally,the smallest interlayer friction observed at the 1T'-MoTe_(2)/2H-MoTe_(2)heterojunction is attributed to the weak interlayer electrostatic interaction and reduction in potential energy corrugation caused by the incommensurate contact.This work suggests that MoTe_(2)has comparable interlayer friction properties to MoS_(2)and is expected to reduce interlayer friction in the future by inducing the 2H-1T'phase transition.