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Graduate Student, Texas A&M University


Study the influence of surface roughness and chemistry on the frictional properties of MoS2

Reducing friction and wear in devices is important because of the potential cost savings from minimizing energy loss. It has been reported that 1.0% to 1.4% of a country’s GDP is lost because of friction and wear in transportation, industry, etc. Thus it is important to find better lubricants for the reduction of friction and the improvement of the lifetime of devices. With the development of nanodevices, solid lubricants such as molybdenum disulfide (MoS2) have been studied a lot nowadays on their frictional properties. MoS2 is a 2-dimensional semiconductor that can be exfoliated into a single layer with a thickness of ~0.6 nm. In single layer MoS2, a layer of molybdenum atoms is sandwiched with two layers of sulfur atoms. Adding a single layer of MoS2 (SLM) restores a crystal lattice to amorphous interfaces, leading to a reduction of friction and increased ease of sliding. In real applications, the substrates can have nanoscopic roughness. The nanoscopic roughness present at sliding interfaces poses a challenge to the implementation of SLM because its frictional properties depend on the interactions with the supporting substrate. In my study, to understand and control the mechanical behavior of SLM in sliding contacts, it is crucial to learn how the frictional response of SLM is influenced by both the SLM-substrate interaction strength and its conformity on rough surfaces.


Adhesion and Friction at Graphene/Self-Assembled Monolayer Interfaces Investigated by Atomic Force Microscopy

Abstract: The functional implementation of graphene as a solid boundary lubricant requires the ability to control its frictional response across a variety of interfaces. This is challenging, as being a single atomic layer thick, the nanotribological properties of graphene depend highly on the competing interaction strengths with the converse sides of the top and bottom contacts of the interfaces it is placed in between. One method to modulate these interactions is to tune the surface chemistry (of one or both counter-faces) with self-assembled monolayers (SAMs). To fully understand the effects on the graphene/SAM (G-SAM) composite interfaces formed, however, first necessitates a basic understanding of graphene–SAM interactions. To explore graphene–SAM adhesive and frictional interactions over a range of chemical functionalities, SAMs were used to functionalize atomic force microscopy (AFM) tips with varying terminal end-groups (−NH2, −CH3, and –phenyl, compared to unfunctionalized −OH terminated reference tips). AFM pull-off force measurements and thermal gravimetric analysis (TGA) were used to evaluate the work of adhesion (mJ/m2) and interaction energy (kcal/mol) of the functionalized tips with graphene. Friction force microscopy (FFM) measurements were performed with the same functionalized AFM tips to examine the graphene-molecule frictional response. Tip–graphene interaction strength was increased for hydrophobic and aromatic functional groups. The frictional response was found to depend on a balance of graphene-molecule adhesion and shear strain.

Pub.: 20 Feb '17, Pinned: 03 Jul '17

The evolving quality of frictional contact with graphene

Abstract: Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction1. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction2, 3, 4, 5, 6, 7. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits8, 9, 10, 11, 12, 13 unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate8. This layer-dependent transient phenomenon has not been captured by any simulations14, 15. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick–slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

Pub.: 23 Nov '16, Pinned: 30 Jun '17

The influence of nanoscale roughness and substrate chemistry on the frictional properties of single and few layer graphene.

Abstract: Nanoscale carbon lubricants such as graphene, have garnered increased interest as protective surface coatings for devices, but its tribological properties have been shown to depend on its interactions with the underlying substrate surface and its degree of surface conformity. This conformity is especially of interest as real interfaces exhibit roughness on the order of ∼10 nm that can dramatically impact the contact area between the graphene film and the substrate. To examine the combined effects of surface interaction strength and roughness on the frictional properties of graphene, a combination of Atomic Force Microscopy (AFM) and Raman microspectroscopy has been used to explore substrate interactions and the frictional properties of single and few-layer graphene as a coating on silica nanoparticle films, which yield surfaces that mimic the nanoscaled asperities found in realistic devices. The interactions between the graphene and the substrate have been controlled by comparing their binding to hydrophilic (silanol terminated) and hydrophobic (octadecyltrichlorosilane modified) silica surfaces. AFM measurements revealed that graphene only partially conforms to the rough surfaces, with decreasing conformity, as the number of layers increase. Under higher mechanical loading the graphene conformity could be reversibly increased, allowing for a local estimation of the out-of-plane bending modulus of the film. The frictional properties were also found to depend on the number of layers, with the largest friction observed on single layers, ultimately decreasing to that of bulk graphite. This trend however, was found to disappear, depending on the tip-sample contact area and interfacial shear strain of the graphene associated with its adhesion to the substrate.

Pub.: 23 Apr '15, Pinned: 30 Jun '17