Guangan Zhang

The preparation and mechanical properties of Al-containing a-C : H thin films

Al-containing hydrogenated amorphous carbon (Al?C?:?H) films were deposited on silicon substrates using a mid frequency magnetron sputtering Al target in an argon and methane gas mixture. The composition, surface morphology, hardness and friction coefficient of the films were characterized using x-ray photoelectron spectroscopy, atomic force microscopy, nanoindentation and tribological tester. The Al?C?:?H films deposited at low CH4 content show high surface roughness, low hardness and high friction coefficient, similar to metallic Al films; in contrast, the Al?C?:?H films prepared under high CH4 content indicate low surface roughness, high hardness and low friction coefficient, close to that of hard a-C?:?H films as wear-resistance films.

Interface Architecture for Superthick Carbon-Based Films toward Low Internal Stress and Ultrahigh Load-Bearing Capacity

Superthick diamond-like carbon (DLC) films [(Six-DLC/Siy-DLC)n/DLC] were deposited on 304 stainless steel substrates by using a plane hollow cathode plasma-enhanced chemical vapor deposition method. The structure was investigated by scanning electron microscopy and transmission electron microscopy. Chemical bonding was examined by Raman, Auger electron, and X-ray photoelectron spectroscopy techniques. Mechanical and tribological properties were evaluated using nanoindentation, scratch, interferometry, and reciprocating-sliding friction testing. The results showed that implantation of a silicon ion into the substrate and the architecture of the tensile stress/compressive stress structure decreased the residual stress to almost 0, resulting in deposition of (Six-DLC/Siy-DLC)n/DLC films with a thickness of more than 50 μm. The hardness of the film ranged from 9 to 23 GPa, and the adhesion strength ranged from 4.6 to 57 N depending on the thickness of the film. Friction coefficients were determined in three tested environments, namely, air, water, and oil. Friction coefficients were typically below 0.24 and as low as 0.02 in a water environment. The as-prepared superthick films also showed an ultrahigh load-bearing capacity, and no failure was detected in the reciprocating wear test with contact pressure higher than 3.2 GPa. Reasons for the ultrahigh load-bearing capacity are proposed in combination with the finite-element method.

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...

Corrosion and tribocorrosion performance of multilayer diamond-like carbon film in NaCl solution

The anticorrosion and tribocorrosion properties of a multilayer diamond-like carbon (DLC) film were systematically investigated in NaCl solution. Electrochemical measurements suggest that the corrosion performance of the multilayer DLC film is superior to those of the substrate and single layer DLC film in NaCl solution, which is attributed to the successively multilayered structure with a well-bonded interface and the formation of Si oxides. An extremely high Warburg impedance value, higher than 107 Ω cm2, of the multilayer DLC film has been observed. Tribocorrosion tests show that the multilayer DLC film presents lower wear rate in NaCl solution, with the substrate and single layer DLC film as comparisons. We demonstrate that the multilayer DLC film is an excellent protective material for improving both corrosion and wear performance of the substrate.

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.

The corrosion behaviors of multilayer diamond-like carbon coatings: influence of deposition periods and corrosive medium

Electrochemical measurements, salt spray tests and immersion tests were employed to investigate the influence of deposition periods and corrosive medium (NaCl, H2SO4, HCl, NaOH) on the corrosion behaviors of silicon doped multilayer diamond-like carbon (DLC) coatings. The results showed that the corrosion resistance of the multilayer DLC coatings was significantly improved with the increase of deposition periods. Interestingly, the coating with the highest deposition periods provided good corrosion protection in neutral and acidic solutions while poor corrosion protection in alkaline and acidic chloride solutions.

The role of tribo-pairs in modifying the tribological behavior of the MoS2/Ti composite coating

Mechanical contact conditions and the shear strength of the interface are the two independent factors that control the friction behavior of the MoS2 coating. Although ZrO2 and GCr15 had similar mechanical contact conditions, they showed rather different friction behaviors. Therefore, the shear strength of the interface was prominent in determining the friction behavior of the MoS2/Ti composite coating, particularly in the initial stage of friction. The strong interfacial shear strength between GCr15, WC and MoS2/Ti resulted in the high friction coefficient in air and short wear life in vacuum. Furthermore, we found that the interfacial shear strength was affected by the Ti content in the MoS2/Ti coatings, and the adhesive behavior of Ti and GCr15 was responsible for the high friction coefficient and high wear rate. The shear strength between the two sliding materials was amplified with the increase in the vacuum degree. These results benefit us to tailor or select the MoS2-based composite coatings and the corresponding tribo-pairs for vacuum operating conditions.

Synthesis and characterization of low-friction Al-DLC films with high hardness and low stress

Al-diamond-like carbon films were successfully deposited by the reactive magnetron sputtering of Al target (>99.9%) in the argon and methane gas mixture atmosphere under selected substrate negative bias with high pulse cycle duty (50%). The microstructure, mechanical and tribological properties of the as-prepared Al-diamond-like carbon films were investigated. Results showed that these films were dominated by the typical amorphous structure, and the internal stress of films dramatically decreased while the hardness remained a high level (∼20 GPa) with increasing substrate pulse negative bias. Especially, the Al-diamond-like carbon film fabricated at substrate bias of −500 V displayed a longer wear life and lower friction coefficient due to both itself superior mechanical properties and the formation of continuous and compact graphitized transfer layer, making it a good candidate for solid lubricating film in engineering applications.

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.