ICMCTF2019 Session E1-2-ThA: Friction, Wear, Lubrication Effects, and Modeling III

Low-dimensional materials have recently attracted immense interest due to their fascinating physical properties and potential for application in diverse fields such as (opto)electronics, energy harvesting and dry lubrication. Transition metal dichalcogenides (TMDs), of general form MX₂ (M = Mo, W; X = S, Se, Te), are posited as being some of the best solid-state lubricants currently available. They exhibit a lamellar structure in which covalently-bonded MX₂ layers are held together by weak van der Waals forces, which, together with very low ideal shear strengths (i.e., the maximum load applied parallel to the face of the material that can be resisted prior to the onset of sliding) render them suitable for use in the mitigation of friction. Our extensive density functional calculations highlight the dependence of important nanomechanical properties of TMDs on their chemical composition and bilayer orientation (sliding direction); in particular, our calculations underscore the intrinsic relationship between incommensurate layers and superlubricity.[1] Our latest calculations have focused on TMD-based van der Waals heterostructures (e.g. WS₂ sliding on MoS₂), with the aim of formalizing the relationship between fundamental quantum chemical parameters of the constituent elements and the nanomechanical properties of the material. Ultimately, we wish to improve the predictive capabilities of in silico methods during the material design process.

[1] B. Irving, P. Nicolini and T. Polcar, Nanoscale, 2017, 9, 5597-5607

Current research in medical device technology and information storage at the Center for Memory and Recording Research will be discussed with emphasis on mechanics and materials issues.

The tribological characteristics of molybdenum disulfide (MoS₂) as solid lubricants are well-described starting from the discovery of the super low friction behavior of MoS₂ by Martin et al. in 1993 [1]. However, despite numerous experimental and theoretical efforts, a fundamental understanding of the frictional mechanisms at the nanoscale is lacking.

In this contribution, we aim to elucidate the phenomena taking place at the nanoscale when several layers of MoS₂ slide atop another. In particular, by means of molecular dynamics simulations, we studied the effect of rotational sliding anisotropy [2] (i.e., the changing frictional behavior upon introducing a misfit angle between the layers or by varying the relative sliding angle) on the energy dissipation due to friction. We simulated different sliding conditions (varying e.g. normal load) in order to highlight their effect on the lubricating properties. These results will help on the one hand to identify the fundamental mechanisms that govern friction at an atomistic level, as well as providing guidelines for the design of novel layered materials with improved tribological properties.

[1] J.M. Martin et al., Phys. Rev. B, 48, 10583(R) (1993). [2] Onodera et al., J. Phys. Chem. B, 114, 15832 (2010).

We conducted systematic quantum mechanic ab initio simulations on prototypical layered MX2 (M = transition metal, X = chalcogen anion) TMDs with hexagonal structure in the presence of small molecules in the inter-layer region. We combined group theoretical analysis and phonon band structure calculations with the characterization of the electronic features using non-standard methods such us orbital polarization and the recently formulated bond covalency and cophonicity analyses. We will then study how the vibrational frequencies of the pristine material are shifted in the presence of the contaminant, by relating such shift to the nanoscale friction between adjacent MX2 layers. Finally, we will present guidelines on how to engineer intrinsic friction in TMDs at the atomic level.

Thanks to the generality of the used simulation protocol, the outcomes of the present study can be exploited to harness the macroscopic response of electro-dynamic entangled systems; the latter find application in fields beyond tribology based on physical phenomena like metal-insulator transition, second harmonic generation, and exciton effects (optoelectronics), among others.

Transition metal dichalcogenides (TMDs) have been shown to be a promising source of applications in multiple fields, such as nanoelectronics, photonics, sensing, energy storage, and opto-electronics.

We investigated the atomic scale tribological properties of TMDs, using ab-initio techniques. Such compounds are formed by triatomic layers with MX2 stoichiometry (M: transition metal cation, X: chalcogen anion) held together by van der Waals forces.

We considered 6 prototypical MX2 TMDs (M=Mo, W; X=S, Se, Te) with hexagonal P63/mmc symmetry, focusing on how specific phonon modes contribute to their intrinsic friction. Within the DFT framework, we described the exchange-correlation interaction energy by means of the PBE functional, including long range dispersion interactions in the Grimme formulation (DFT-D3 van der Waals).

We identified and disentangled the electro-structural features that determine the intra- and inter-layer motions affecting the intrinsic friction by means of electro-structural descriptors such as orbital polarization, bond covalency and cophonicity.¹

We show how the phonon modes affecting the intrinsic friction can be adjusted by means of an external electrostatic field. In this way, the electric field turns out to be a knob to control the intrinsic friction.

We also show the structural distortion induced by some various cation substitution.

The presented outcomes are a step forward in the development of layer exfoliation and manipulation methods, which are fundamental for the production of TMD-based optoelectronical devices and nanoelectromechanical systems.

[1] Cammarata, Antonio and Polcar, Tomas (2015) DOI 10.1039/C5RA24837J

In recent years, solid lubricants such as transition metal dichalcogenides have attracted attention in form of nanoparticle additives for conventional oil-based lubricants. Those nanoparticle additives entail several disadvantages, including the need to compose a stable dispersion. An in-situ formation of lubricating solid compounds via a tribochemical reaction path between a coating and conventional S containing extreme pressure additives could prove to be much more advantageous.

While MoS₂ can easily be formed on metallic Mo surfaces, the conditions in lubricated tungsten contacts are not severe enough to produce WS₂. A new generation of functional coatings is helping to overcome this energetic barrier, which is linked to the formation of intermediate tungsten oxide phases. WN and WC based coatings are able to facilitate this in-situ formation of WS₂ and combine excellent mechanical properties with an optimization of friction and wear via the formation of a low friction tribofilms.

The coatings were prepared by magnetron sputtering and their mechanical, structural and chemical properties were investigated with nanoindentation, energy dispersive x-ray spectroscopy, X-ray diffraction, and transmission electron microscopy.

The tribofilms, resulting from lubricated pin on disk tests, were studied in detail and consist of a fraction of amorphous WS₂ and C based oil derived debris, but also of nanocrystalline WS₂ domains along the surface. These lubricating WS₂ sheets lead to a low coefficient of friction around 0.05 and the tribofilm protects coating and counter body from any wear, even after 100 h of testing.

Due to the increasing demand to reduce emissions, friction reduction is an important topic in the automotive industry. Among the various possibilities to reduce mechanical friction, the usage of a low-viscosity lubricant in the engine is one option. Thus, continuously lower viscosity lubricants are being developed and offered on the market. Other approach is to adjust the properties of the components using surface technology in order to minimize friction losses and meet the more stringent environmental requirements. Coated components using thin film vacuum technology offers the possibility to reduce CO₂-emissions and fuel consumption of the vehicle by mandatory lightweight design, reducing friction losses in tribological contacts. In present study, uncoated and DLC (Diamond-like Carbon) coated roller finger followers were investigated for the first time using application oriented driven cylinder head tests with 0W20 low viscosity oil under relevant conditions. The results show a significant friction reduction in rolling contact with coated cam rollers compared to the uncoated refence.