Zhibin Lu

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.

Controlled water adhesion and electrowetting of conducting hydrophobic graphene/carbon nanotubes composite films on engineering materials

Based on the microbumps of graphene nanosheets and the nanostructure of carbon nanotubes (CNTs), novel graphene/CNTs composite films with hierarchical micro- and nanoscale surface roughness were successfully fabricated by simply spraying the mixed acetone dispersion of graphene nanosheets and CNTs onto stainless steel substrates. The as-prepared composite films exhibited controlled surface hydrophobic, adhesive and electrowetting properties via altering the film surface structure and surface energy. Among them the composite film with a 1:5 mass ratio of graphene to CNTs showed high hydrophobicity and conductivity, low water adhesion and contact angle sensitivity to the external electric field, which would help to resolve the surface electrostatic problems and unstable hydrophobicity under applied potential that exist in many conventional insulating hydrophobic materials, and could be useful in some application fields.

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...

Probing the intrinsic failure mechanism of fluorinated amorphous carbon film based on the first-principles calculations

Fluorinated amorphous carbon films exhibit superlow friction under vacuum, but are prone to catastrophic failure. Thus far, the intrinsic failure mechanism remains unclear. A prevailing view is that the failure of amorphous carbon film results from the plastic deformation of substrates or strong adhesion between two contacted surfaces. In this paper, using first-principles and molecular dynamics methodology, combining with compressive stress-strain relation, we firstly demonstrate that the plastic deformation induces graphitization resulting in strong adhesion between two contacted surfaces under vacuum, which directly corresponds to the cause of the failure of the films. In addition, sliding contact experiments are conducted to study tribological properties of iron and fluorinated amorphous carbon surfaces under vacuum. The results show that the failure of the film is directly attributed to strong adhesion resulting from high degree of graphitization of the film, which are consistent with the calculated results.

Study of tribological mechanisms of carbon-based coatings in antiwear additive containing lubricants under high temperature

With the development of low-emission and low-consumption engines, diamond-like carbon (DLC) coatings have been considered as promising surface coatings of engine parts in terms of friction and wear performance. However, the performance of DLC-coated parts depends on the compatibility and interaction between the coating and lubricant additives. To obtain high-performance coatings in the engine environment, the tribological behaviors of non-doped and metal-doped carbon-based coatings (a-C and a-C/WC, respectively) were studied systematically in poly-alpha-olefin (PAO) oil at 180 °C (operating temperature of engine oil) with and without antiwear additive zinc dialkyldithiophosphate (ZDDP) under various applied loads. Energy dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed on a friction surface to understand tribological mechanisms. Results showed that ZDDP-derived tribofilms were able to suppress the surface graphitization of carbon-based coatings. The combination of ZDDP-derived tribofilms and tribochemical products WS2/WO2, which formed under the joint actions of heat and contract pressure, led to improved tribological behavior of a-C/WC coating in PAO with ZDDP additive.

Evaluation of flow units and free volumes in metallic glasses

We investigate the changes of the fractions of flow units and free volumes in two typical metallic glasses by variation of their preparation cooling rates. It is found that the fractions of the flow units and the free volumes show a similar dependence on the cooling rate, which is suggestive of the existence of a strong connection between them. The microstructural correlation between flow units and free volumes of the metallic glasses is discussed, which could be helpful for understanding the formation and structural features of the flow units as well as their effects on mechanical properties and relaxation behaviors in metallic glasses.

Planar Hall effect and magnetoresistance in spin valve multilayers

Magnetoresistance (MR) and planar Hall effect (PHE) in spin valves were simultaneously measured in fields applied at different angles (α) in the film plane with respect to the easy axis of the free layer. MR curve measurements showed that the MR response to the field was linear in the α angle range from ±60° to ±120°. However, it was found that when the applied field was near perpendicular to the easy axis of the free layer, PHE curves were nonsymmetrical on both sides of the axis along the sample height. The Boltzmann transport equation was used to simulate the MR and PHE curves and determine magnetization orientation of the free layer. Results showed that the nonsymmetrical PHE curves originated from the interaction of the interlayer exchange coupling field between the pinned and free layers. Consequently, the magnetization reversal was a coherent rotation when the applied field angle α>90° and incoherent when α<90°.

Influence of Load on the Tribological Behavior of a-C Films: Experiment and Calculation Coupling

Although the tribological performances of various non-hydrogenated amorphous carbon films (a-C) have been investigated for decades, most of previous studies are reported with the experimental point, and the individual mechanism still lacks reasonable coupling of experimental and physical description. In this paper, influence of load on the tribological behaviors of a-C films was systemically investigated based on the correlation of tribological tests and physical calculations. Results shows that coefficient of friction (COF) and wear rate of a-C films both decreased with the increase of applied load during wear tests. It is observed that the coverage of easy-shear transfer film on the surface of counterpart almost keeps same for different applied load, indicating that the decreased COF is mainly attributed to the higher graphitization level. Besides, according to the energy dissipation theory, both the decreased COF and increased heat dissipation cause the decrease of the wear rate. Furthermore, with the increased applied load, entropy decreases and keeps well consistent with wear rate. All the analyzed curves based on the physical calculations possess high regression coefficient with the experimental data, which would deepen our understanding of the physical mechanism of a-C films with various contact pressure.

Preparation and properties of DLC/MoS2 multilayer coatings for high humidity tribology

The DLC/MoS2 multilayer coatings with different modulus ratios were deposited by magnetron sputtering in this study. The morphology, structure, composition, mechanical properties and tribological properties were investigated using several analytical techniques (FESEM, AFM, TEM, AES, XPS, nanoindentation and high humidity tribological test). The results showed that the well-defined multilayer coatings were composed of densely packed particles in which many nanocrystallines with some kinds of defects were distributed in matrix. The incorporation of oxygen into the lattice led to the degraded chemical stability. The coatings hardness and elastic modulus were almost in the same range. Moderate improvement on the high humidity tribological properties were obtained, which was important for the extension of the service life of MoS2 in humid air.

Facile fabrication of boron nitride nanosheets–amorphous carbon hybrid film for optoelectronic applications

A novel boron nitride nanosheets (BNNSs)–amorphous carbon (a-C) hybrid film has been deposited successfully on silicon substrates by simultaneous electrochemical deposition, and showed a good integrity of this B–C–N composite film by the interfacial bonding. This synthesis can potentially provide the facile control of the B–C–N composite film for the potential optoelectronic devices.