Solid Lubrication with MoS2: A Review

Abstract

Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by highlighting its structure, synthesis, applications and the fundamental mechanisms underlying its lubricative properties, together with a discussion of their environmental and temperature dependence. An effort is made to cover the main theoretical and experimental studies that constitute milestones in our scientific understanding. The review also includes an extensive overview of the structure and tribological properties of doped MoS2, followed by a discussion of potential future research directions.MoS2 belongs to the family of layered two-dimensional transitional metal dichalcogenides (TMDs). Like graphite and hexagonal boron nitride, its crystal structure consists of covalently bonded sheets, which form stacks that are held together only by weak Van der Waals interactions [16]. It occurs naturally as the mineral molybdenite, an important Mo ore. Due to the strong bonding inplane and weak bonding out-of-plane, mechanical and other properties are highly anisotropic [17], and the stacks can be easily sheared. Single-layer or few-layer (i.e. 2D) MoS2 (analogous to graphene) can be produced and studied individually [18].MoS2 has several different possible structures (polytypes), depending on the bonding within the sheets and the stacks of the sheets. While graphite has a single plane of atoms per sheet, in MoS2 each sheet consists a plane of Mo atoms sandwiched between two planes of S atoms. The single-sheet structure can feature trigonal prismatic coordination around Mo, which is the semiconducting 1H ("hexagonal") structure, or octahedral bonding, which is the metallic 1T ("trigonal") phase. 1H and the ideal 1T each have 3 atoms (MoS2) per unit cell ( Figure 1); however, theoretical calculations with density-functional theory (DFT)[19] and X-ray diffraction (XRD) experiments [20] have shown that in fact 1T is unstable with respect to distortions. The most commonly reported structure is a 3× 3 distortion that triples the unit cell, known as the 1T′ phase [21].Each single-sheet structure can be stacked into a crystal where the next sheet is exactly above the preceding one (AA stacking), producing the 1H, 1T, and 1T′ polytypes. The naturally occurring polytypes in molybdenite, however, are only based on the 1H sheet, and show more complicated stacking [9]. The more common is the 2H structure with 2 sheets per cell in AB stacking, where an S atom in one layer is above an Mo atom in the layer below [22,23]. The less common 3R ("rhombohedral") structure has 3 sheets per cell with ABC stacking [22,24]. These structures are shown in Figure 1. Mo-S bond lengths are around 2.4 Å for all polytypes [25]. DFT calculations show only very small energy differences between the 2H and 3R phases [25]. This insensitivity to stacking, and the low surface energy for 2H-MoS2 of 47 mJ/m 2 =...

Identification of a Material–Lubricant Pairing and Operating Conditions That Lead to the Failure of Porous Journal Bearing Systems

“…For the bronze bearings (Br, brown color series in Fig. 6), we found that the addition of graphite (BrGr) slightly improved tribological performance, whereas significant CoF reduction was achieved by adding molybdenum disulfide (MoS 2 -BrMo), as expected [25][26][27]. The two BrMo systems consistently exhibited remarkable running-in behavior, with a drop in the CoF at 20 rpm from 0.35 to below 0.1 within 8 h of operation in the case of BrMo-low.…”

Molybdenum Disulphide Heterointerfaces as Potential Materials for Solar Cells, Energy Storage, and Hydrogen Evolution

“…Its main attractive features are the nonzero bandgap, layer‐dependent bandgap tunability, easy and versatile functionalization, tailor‐made morphologies, etc. MoS 2 history is quite deep rooted with society; its scientific study and technical uses started since the ancient times . These developments are shown in Figure .…”

Tribological Performance of Nylon Composites with Nanoadditives for Self-Lubrication Purposes

“…Due to its lamellar shape made of individual atomically thin planes that can easily slide against each other [ 36 ], molybdenum disulphide (MoS 2 ) has been successfully used as a solid lubricant. MoS 2 can work, alone or with other additives, in different environments, such as: polymeric composites based on PEEK [ 37 , 38 ], ABS [ 39 ], PI [ 40 ] and PA [ 19 , 41 , 42 , 43 , 44 ]; liquid lubricants [ 45 , 46 , 47 ] or coatings [ 48 , 49 , 50 ], showing promising results in terms of improving friction coefficient and wear rate.…”