Impact of Microstructure on MoS<sub>2</sub> Oxidation and Friction

Curry, John F.; Wilson, M.; Luftman, H. S.; Strandwitz, Nicholas C.; Argibay, Nicolas; Chandross, Michael; Sidebottom, Mark A.; Krick, Brandon A.

doi:10.1021/acsami.7b06917

Cited by 94 publications

(86 citation statements)

References 47 publications

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“…Annealing is also expected to increase the coverage of oxygen along the basal plane of MoS2 in addition to at the edges/defect sites. [11]…”

Section: Resultsmentioning

confidence: 99%

“…Though both of the materials consist of weakly interacting layered structures (2D-MoS2 and graphene), they display a contrary dependence on the humidity at the microscale, where the coefficient of friction (COF) is reported to drop for graphite in the presence of water, yet increases for microscale-MoS2 coatings. [7][8][9][10] In the case of MoS2, the widely accepted mechanism for the increase in COF is the oxidation of microscale-MoS2 to the less lubricious MoO3 from exposure to humidity [11] .…”

Section: Introductionmentioning

confidence: 99%

“…[13,14] Oxidation thickness of microscale-MoS2 is reported to be thinner and on the surface with less oxygen penetration within the coating when MoS2 layers are highly ordered as compared to amorphous. [11] While there is some understanding of mechanisms (localized interlayer shearing, puckering effect, electron phonon coupling etc.) at the nanoscale associated with water influencing friction for graphene, graphene oxide (GO) and graphite edges [15][16][17][18][19][20][21][22][23][24] , there is only limited understanding for MoS2 and oxidized-MoS2 at the nano/molecular scale.…”

Section: Introductionmentioning

confidence: 99%

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Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)

Arif

Yadav

Colas

et al. 2019

Adv Materials Inter

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In this work, the tribological behavior of ultrathin-MoS2 was investigated to understand the independent roles of water and oxidation. Water adsorption was identified as the primary interfacial mechanism for both SiO2/pristine-MoS2 and SiO2/graphene interfaces, however, tribological behavior of pristine-MoS2 was observed to be more sensitive to presence of water due to stronger MoS2-water interaction.Comparison of pristine-MoS2 and oxidized-MoS2 revealed that the oxidation of MoS2 significantly increased its friction and sensitivity to water by play a more detrimental role. The specific effect of oxygen on friction via chemical interactions was studied in isolation through density functional theory (DFT) simulations of a tip sliding on MoS2 basal planes and over edges before and after oxidation. The maximum change in energy, or energy barrier correlating with friction, as the tip moved across the surface, increased after oxidation by up to 66% for the basal plane and by 25% at the edge. Charge density analysis suggests that the more localized and non-uniform interfacial charge distribution on oxygen rich surfaces, as compared to pristine surfaces, leads to higher resistance to sliding. This confirms that oxygen presence alone increases friction and when coupled with the presence of water, both effects are additive in increasing friction.

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“…Annealing is also expected to increase the coverage of oxygen along the basal plane of MoS2 in addition to at the edges/defect sites. [11]…”

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)

Arif

Yadav

Colas

et al. 2019

Adv Materials Inter

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“…Highly ordered MoS2 films with surface-parallel basal orientation exhibited higher resistance to oxidation compared to amorphous MoS2 films. Moreover, X-ray photoelectron spectroscopy (XPS), and high-sensitivity low-energy ion scattering (HS-LEIS) analysis revealed that highly oriented MoS2 lamellae restricted the oxidation occurrence to the first top few layers, which led to shorter run-in period compared to amorphous MoS2 in which the further penetration of oxygen into the surface resulted in a longer run-in period [116]. Therefore, deposition techniques that result in a lower density of edge sites or highly oriented lamellae of MoS2 can reduce the possibility of oxidation, thereby minimizing the degradation of tribological performance under ambient conditions.…”

Section: Environmental Dependencementioning

confidence: 99%

Solid Lubrication with MoS2: A Review

et al. 2019

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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 =...

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“…MoS 2 based coatings are used as solid lubricants in aerospace/space applications such as actuators and slip rings, among many others [5][6][7]. In ambient conditions, the tribological performance of MoS 2 degrades in the presence of moisture and oxygen due to reactions occurring with edge sites of crystallites that inhibit basal plane shearing and thereby increase the friction coefficient and wear rate [8,9]. To improve the tribological performance of MoS 2 in ambient conditions, researchers developed various technologies that can be broadly classified into two categories.…”

Section: Introductionmentioning

confidence: 99%

Rolling Contact Performance of a Ti-Containing MoS2 Coating Operating Under Ambient, Vacuum, and Oil-Lubricated Conditions

2019

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Solid lubricant molybdenum disulfide (MoS2) coatings have been frequently used to lubricate mechanisms operating in environments where oil and grease lubrication are ineffective. This work evaluated the rolling contact performance of a Titanium-containing MoS2 coating under humid ambient, vacuum, and oil-lubricated conditions. Weibull analyses of L50 lifetimes of AISI 52100 steel balls coated with a Ti-MoS2 coating paired with uncoated M50 steel rods were determined to be 3.7, 14.5, and 158.6 million cycles in ambient, vacuum, and oil-lubricated environments, respectively. In the ambient and vacuum tests, failures were determined to be associated with the onset of abrasive wear rather than fatigue or spalling. The L50 lifetimes of tests performed in those environments were found to depend upon the wear rate of the coatings on the balls. That is, the Ti-MoS2 functioned as a barrier to the onset of abrasive wear between the steel alloys until the coating was sufficiently worn away. Under oil-lubricated (boundary lubrication) conditions, L50 was found to depend on the durability and composition of tribofilms formed in-situ on the surfaces of the uncoated M50 rods. The tribofilms were comprised of mixtures of MoS2 crystallites and amorphous hydrocarbon (a-C:H). The crystalline MoS2 in the tribofilm originated from the amorphous Ti-MoS2 coating and likely underwent a thermodynamic phase transition as a result of the applied Hertz stress and frictional heating in the contact. The a-C:H in the tribofilm probably originated from a catalytic scission of the polyalphaolefin (PAO) molecules caused by the d-band character of the Mo or Ti in the coating. Overall, the Ti-MoS2-coated balls were effective at extending the operational lifetimes of M50 rods under ambient, vacuum, and oil-lubricated conditions by an order of magnitude.

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Impact of Microstructure on MoS₂ Oxidation and Friction

Cited by 94 publications

References 47 publications

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)

Solid Lubrication with MoS2: A Review

Rolling Contact Performance of a Ti-Containing MoS2 Coating Operating Under Ambient, Vacuum, and Oil-Lubricated Conditions

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Impact of Microstructure on MoS2 Oxidation and Friction

Cited by 94 publications

References 47 publications

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)

Solid Lubrication with MoS2: A Review

Rolling Contact Performance of a Ti-Containing MoS2 Coating Operating Under Ambient, Vacuum, and Oil-Lubricated Conditions

Contact Info

Product

Resources

About

Impact of Microstructure on MoS₂ Oxidation and Friction

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS₂)