Liping Wang

Highly hydrophobic and adhesive performance of graphene films

Graphene films with high hydrophobic and adhesive performance were fabricated via two simple steps: chemical exfoliation of natural flake graphite following redox, and film formation by suction filtration without any chemical modification. Irregularly stacked multilayer graphene nanosheets comprised the microstructure, whereas folding and agglomeration of graphene nanoflakes with few layers comprised the nanostructure. The films also showed remarkable surface wettability and reversible transition from hydrophobicity to hydrophilicity via periodic alternation of ultraviolet irradiation and air storage. Based on Wenzels theory and adsorption dynamics, an optimum mechanism is proposed for the surface wettability behavior. On the one hand, the film microstructure and nanostructure enhance the graphene surface hydrophobicity. On the other hand, the capillary force is maximized by the nanostructure such that water fills the grooves of the rough solid surface. This result is a strong interaction between water and the film surface giving highly adhesive property to the films. The highly hydrophobic and adhesive performance of the graphene films could be useful in the device and biomaterials application.

High-performance lubricant additives based on modified graphene oxide by ionic liquids

Graphene oxide (GO) is a layered material bearing a variety of oxygen-containing functional groups on its basal planes and edges, which allow it as a substrate to conduct a variety of chemical transformations. Here modified graphene oxide (MGO) was prepared using alkyl imidazolium ionic liquids (ILs) (1-butyl-3-methylimidazolium tetrafluoroborate (LB104), 1-butyl-3-methyl imidazolium hexafluorophosphate (LP104) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide (LF106)) via epoxide ring-opening reaction, cation-π stacking or van der Waals interactions, with LB104 modified graphene (MG) exfoliated from graphite rod by a moderate electrochemical method as a comparison. The stability and tribological properties of MGO and MG as multialkylated cyclopentanes (MACs) additives were investigated in detail. The results show that GO is converted into graphene through the chemical modification using ILs, and MGO with good dispersion and stability in MACs significantly improves the tribological performance (friction and wear were reduced about 27% and 74% with pure MACs as a comparison, respectively). The excellent tribological properties are attributed to the formation of an ILs-containing graphene-rich tribofilm on the sliding surfaces, which as the third body can prevent the sliding surfaces from straight asperity contact and improve friction reducing and anti-wear behaviors.

Tailoring microstructure and phase segregation for low friction carbon-based nanocomposite coatings

Friction has a direct relation with the energy efficiency and environmental cleanliness in all moving mechanical systems. To develop low friction coatings is extremely beneficial for preserving not only our limited energy resources but also the earth’s environment. This study proposes a new design for low friction carbon-based nanocomposite coatings by tailoring the microstructure and phase segregation, and thereby it contributes to better controlling the mechanical and tribological properties. Experimental findings and theoretical calculations reveal that high-hardness (18.2 GPa), high-adhesion strength (28 N) as well as low-internal stress (−0.8 GPa) can be achieved by a nanocrystallite/amorphous microstructure architecture for the nc-WC/a-C(Al) carbon-based nanocomposite coating; in particular low friction (∼0.05) can be acquired by creating a strong thermodynamic driving force to promote phase segregation of graphitic carbon from the a-C structure so as to form a low shear strength graphitic tribo-layer on the friction contact surfaces. This design concept is general and has been successfully employed to fabricate a wide class of low friction carbon-based nanocomposite coatings.

Design and Fabrication of Nanopillar Patterned Au Textures for Improving Nanotribological Performance

Fast development of micro/nanoelectromechanical systems (MEMS/NEMS) and high-density storage technology (HDT) have stimulated the development of new materials that require hydrophobic surfaces with low adhesion and friction. Micro/nanohierarchical structures and chemical modification are two useful methods for improving nanotribological properties of mechanical components. In this study, Au surfaces with micro/nanohierarchical structures were prepared by replication of micropatterened silicon surfaces using PDMS and self-assembly of alkanethiol [CH(3)(CH(2))(9)SH] to create hydrophobic micro/nanohierarchical structures and to improve nanotribological properties of MEMS/NEMS. The effects of nanoscaled roughness (including pillar height and pillar fractional surface coverage) and chemical modification on the wetting and nanotribological properties of surfaces were systemically investigated. Results show that with the increasing of nanoscale roughness and lowering of surface energy, the surface becomes more hydrophobic, and the adhesive force and friction force are reduced greatly.

Graded composition and structure in nanocrystalline Ni?Co alloys for decreasing internal stress and improving tribological properties

In this paper, nanocrystalline Ni?Co alloys with continuously graded composition and structure were produced by the electrodeposition method. The internal stress of the graded Ni?Co nanocrystalline alloys generated during the electrocrystallization and the tribological properties were investigated and compared with Ni?Co alloys with a uniform structure. The results show that with continuous changes in composition and structure, the internal stress generated during the electrodeposition process was decreased to approximately the minimum level. Additionally, the graded Ni?Co nanocrystalline alloys exhibited a remarkably improved wear resistance and a much lower friction coefficient compared with the Ni?Co alloys with a uniform structure under the dry sliding wear conditions. And the graded Ni?Co nanocrystalline alloys retain perfect friction and wear properties as the annealing temperature increases up to 400?C.

Tribological mechanism of hydrogenated amorphous carbon film against pairs: A physical description

The objective of the present study was to investigate the friction and wear mechanisms of hydrogenated amorphous carbon (a-C:H) films sliding against different counterparts. Friction tests were performed by a reciprocating ball-on-disk tribometer with an applied load of 5 N, amplitude of 5 mm, and frequency of 5 Hz, in ambient air at room temperature. The coefficient of friction (COF) was consistent with the varied tendency of the contact area of the counterparts on films and also coincided with the varied tendency of the coverage of transfer film on friction ball surface. It was important to point out that the coverage of the transfer film on the counterpart surface was inversely proportional to the contact area. Furthermore, COF of a-C:H films against different pairs was independent with the film graphitization level. Additionally, wear rate of a-C:H films against different friction pairs was discussed in details. Some indexes including hardness ratio of pair and film, elastic energy density of the fric...

An explanation for laser-induced spallation effect in a-C:H films: Altered phase evolution route caused by hydrogen doping

The laser-induced spalling effect has been recognized as a unique phenomenon for amorphous carbon (a-C) films during laser processing. In this work, the origin of spalling effect was investigated by ablating two different types of a-C film: hydrogenated a-C (a-C:H) and nonhydrogenated a-C with an Nd-yttrium aluminum garnet laser system. Comparisons of ablating results demonstrated that the spalling effect only occurred in a-C:H rather than nonhydrogenated a-C. Laser heating simulation indicated that the temperature distributions in both films after laser pulse are similar with a high temperature gradient in depth direction. Annealing test results, Raman spectra and nanoindentation show that with the increase in annealing temperature, a-C film transforms into grassy carbon directly, while a-C:H experiences two subprocess under heating: the hydrogen mobilization and rearrangement of Cue5f8C network at a relatively low temperature range resulting in a denser Cue5f8C network and raised film density; the graphitizatio...

Preparation and characterization of ultrathin dual-layer ionic liquid lubrication film assembled on silica surfaces

A novel ultrathin dual-layer film, which contained both bonded and mobile phases in ionic liquids (ILs) layer, was fabricated successfully on a silicon substrate modified by a self-assembled monolayer (SAM). The formation and surface properties of the films were analyzed using ellipsometer, water contact angle meter, attenuated total reflectance Fourier transform infrared spectroscopy, multi-functional X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to evaluate the microtribological performances of the films. Compared with the single-layer ILs film deposited directly on the silicon surface, the as-prepared dual-layer film shows the improved tribological properties, which is attributed to the special chemical structure and outstanding physical properties of the dual-layer film, i.e., the strong adhesion between bonded phase of ILs and silicon substrate via the chemical bonding with SAM, the interlinked hydrogen bonds among the molecules, and two-phase structure composed of steady bonded phase with load-carrying capacity and flowable mobile phase with self-replenishment property.

A near-wearless and extremely long lifetime amorphous carbon film under high vacuum

Prolonging wear life of amorphous carbon films under vacuum was an enormous challenge. In this work, we firstly reported that amorphous carbon film as a lubricant layer containing hydrogen, oxygen, fluorine and silicon (a-C:H:O:F:Si) exhibited low friction (~0.1), ultra-low wear rate (9.0u2009×u200910–13u2009mm3u2009N–1u2009mm–1) and ultra-long wear life (>2u2009×u2009106 cycles) under high vacuum. We systematically examined microstructure and composition of transfer film for understanding of the underlying frictional mechanism, which suggested that the extraordinarily excellent tribological properties were attributed to the thermodynamically and structurally stable FeF2 nanocrystallites corroborated using first-principles calculations, which were induced by the tribochemical reaction.

Tribological Performances of Graphite-Like Carbon Films Coupled to Different Ceramics in Ambient Air and Water

The tribological performance of graphite-like carbon (GLC) films coupled with different ceramics including Si3N4, SiC, WC, Al2O3, ZrO2, and SiO2 were comparatively studied in ambient air and water. The results showed that the GLC films could exhibit low friction and wear with small differences when sliding against engineering ceramics in both environments. Water played a different role in the tribological performance of GLC films, which was closely related to the nature and wear behavior of the coupled ceramics. Accordingly, three conditions of friction coefficient (FC) and wear rate (WR) of GLC films sliding against different ceramics in water compared to those in ambient air were found: similar FC with lower WR, lower FC with lower WR, and lower FC with higher WR. The SiO2 counterpart resulted in high friction and wear of GLC film in ambient air, but it only slightly affected the tribological performance of GLC film in water. The GLC film coupled to Si3N4 ceramic exhibited optimum tribological performance in both ambient air and water.