Abstract
Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers ($R0$) bears more similarities to graphene while that with antiparallel orientation ($R180$) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.
Highlights
Layered materials have recently attracted great interest for their wide range of applications, including tribology, where they constitute an important class of solid lubricants [1]
Its thickness and electrostatic properties vary according to the individual composition of each Transition-metal dichalcogenides (TMDs): for molybdenum dichalcogenides, the thickness increases going from MoS2 to MoSe2 and MoTe2, while the polarity decreases along the same sequence, following the electronegativity decrease from sulfur to tellurium
While most investigations on TMDs have dealt with the R180 structure, examples of 3R polytypes presenting the R0 orientation were found in both natural and synthetic crystals [25,42]
Summary
Layered materials have recently attracted great interest for their wide range of applications, including tribology, where they constitute an important class of solid lubricants [1]. Solid lubricants play a decisive role in applications where liquid lubricants are not effective, such as, e.g., nanoscale devices, the continuous experimental [2,3,4,5,6] and theoretical advances [7,8,9,10,11,12,13] in determining tribological performances of atomically thin sheets Powder lubricants such as graphite, boron nitride, and molybdenum disulfide share the common structure of stiff, strongly bound planes held together by weak interlayer forces, which is what favors their sliding capability. It was experimentally observed that TMDs can display lower friction coefficients when an external load is applied, at difference to most systems [19,20,21]
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