Nanotribology and lubrication mechanisms of inorganic fullerene-like MoS2 nanoparticles investigated using lateral force microscopy (LFM)

Inorganic fullerene-like (IF) MoS2 nanoparticles were produced by arc discharge in water, and their tribological properties were investigated using a lateral force microscope in dry nitrogen and humid air. Two types of tips - Si and Si3N4 tips were used in this work. The sharp Si tip produced a much higher contact stress than the blunt Si3N4 tip. The measurement of lateral forces using a Si3N4 tip resulted in almost no wear, while the measurement made using a Si tip resulted in MoS2 transfer due to the high contact stress. For comparison, measurements were also made on MoS2 films grown by pulsed laser deposition (PLD). The experimental results demonstrated that IF-MoS2 nanoparticles had significantly lower friction than the MoS2 films prepared by PLD. Variation of the test environment from dry to wet did not affect the tribological performance of the IF material as much as it did PLD films due to the chemical inert structure of the IF-MoS2 nanoparticles. The multi-wall-encapsulated structure of inorganic fullerenes has a nearly isotropic geometry. They can supply a slippery surface in all orientations, though only the basal planes of 2H-MoS2 crystals are optimum for lubrication. Therefore, the inorganic fullerenes do not have to be oriented by rubbing as does most layer-structured solid lubricants. However, the lack of reactive edge planes impedes bonding of the lubricant to the surface. The lubrication mechanisms of IF-MoS2 nanoparticles are discussed in detail.