Molecular dynamics investigation of the lubrication mechanism of carbon nano-onions

Abstract

Carbon nano-onions have been shown to provide exceptional friction and wear reduction through a rolling and sliding mechanism of the nano-onions at the interface, but the influence of this behavior on the their tribological properties is not well documented. Understanding the lubrication mechanisms of such nanostructured materials aids in determining their frictional properties and promotes the use of these materials in tribological applications. Here, we characterize the mechanisms of rolling and sliding through which carbon nano-onions provide low coefficients of friction by performing atomistic molecular dynamics simulations of carbon nano-onions sliding between diamond-like carbon substrates. The results indicate that the ability of the nano-onions to roll is inhibited both by increased contact pressure and the presence of a diamond core within the nanoparticles that enhances the formation of interfacial bonds during friction. The transition from rolling to sliding behavior is accompanied by a significant increase in the coefficient of friction.

Introduction

Tribological interest in various inorganic and carbon fullerenes and nanotubes has grown continually since the discovery of carbon fullerenes in 1985 [1] and the discovery of carbon nanotubes (CNTs) in 1991 [2]. Another material of great potential importance for lubrication is the carbon nano-onion (CO), which was discovered by Ugarte [3] and may be thought of as a spherical nested carbon structure. Other more common solid lubricants such as graphite and metal dichalcogenides, MX₂ (M = Mo, W; X = S, Se) are lamellar materials characterized by strong covalent intralayer interactions and weak van der Waals (vdW) interlayer interactions that allow for easy shear of the individual layers resulting in a significant reduction in friction [4], [5], [6]. The desirable tribological performance of nanomaterials, similarly, stems from their vdW interactions with surrounding materials. Also contributing to their tribological performance are the chemical inertness and structural stability provided by the elimination of dangling bonds through the formation of the nanomaterials [3], [6], [7]. Additionally, the spherical morphology of fullerenes and COs along with the columnar morphology of nanotubes allow for the potential of rolling at the frictional interface, opening up the possibility of additional interesting properties [6], [8].

As was noted by Hirano and Shinjo [9], controlling friction is one of the most critical goals in the area of tribology; the first step to controlling friction is to develop an understanding of the mechanisms involved in the lubrication process [10]. With this in mind, a number of computational and experimental studies have been performed in recent years in an attempt to characterize and understand the frictional behavior and lubrication mechanism of many different lubricants including nanomaterials [11], [12], [13], [14], [15], [16]. For instance, analyses performed on single-wall CNTs using Raman spectroscopy inside the contact area during friction processes have shown that they undergo amorphization during the tribological tests leading to the reduction of friction [15], [17]. Similarly, images of inorganic fullerene-like (IF) materials (MoS₂ and WS₂) before and after friction using high resolution transmission electron microscopy, as well as in situ Raman spectroscopy analyses during friction, have shown that the exceptional frictional properties of the IFs under severe conditions are due to the exfoliation of individual lamellar sheets of lubricant at the friction interface [15], [18]. It has been further suggested that the spherical morphology of IFs could result in a lubrication mechanism derived from rolling and/or sliding of the individual IF nanoparticles at the tribological interface at low contact pressures [6], [8].

There are inherent structural similarities between COs and IFs in that both are spherical nested nanostructures. It is therefore reasonable to assume that the lubrication mechanism of COs will be similar to that of IFs, with graphitic exfoliation occurring at the sliding interface when subjected to compressive and frictional forces, especially at higher contact pressures. A recent publication by Joly-Pottuz et al. [19], however, demonstrated through a combination of experimental and computational methods that no evidence is predicted or observed for the exfoliation of graphene sheets from COs at the interface during friction. Rather, their results indicate that the COs provide friction reduction through the rolling and sliding of the individual nano-onions at the tribological interface.

Here, we use classical molecular dynamics (MD) simulations to investigate the rolling/sliding mechanism of COs as they are subjected to friction between coupled, amorphous, hydrogen-terminated diamond-like carbon surfaces in a perfect ultra-high vacuum environment. Through detailed analysis we quantify the relative importance of rolling and sliding on the frictional properties of COs as a function of the structure and conditions of these tribological simulations.