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In this work, the effects of inserting an interlayer on the anti-corrosion performance of amorphous carbon (a-C) films in a simulated Proton Exchange Membrane Fuel Cell (PEMFC) environment have been systematically studied. Four a-C films with different interlayers, namely C Cr, C-Cr-N, CrN and Cr, were prepared on 316L stainless steel substrates using unbalanced magnetron sputtering. The surface/fractural morphologies, carbon bonding structure and the anti-corrosion performance of the 4 films were observed/characterized in detail. The nanostructure and composition evolution from the film surface down to the film-substrate interface before and after electrochemical tests were also analysed via high resolution transmission electron microscopy (HRTEM). Results indicate that a-C films with C Cr and C-Cr-N interlayers display superior corrosion resistance compared to undoped samples and those lacking an interlayer, with the lowest self-corrosion current density (i.e. 2.1 × 10−7 A/cm2, C-Cr-N interlayer) being nearly two orders of magnitude lower than that of the substrate (i.e. 1.1 × 10−5 A/cm2). The doped C-Cr-N interlayer plays a vital role in the corrosion resistance of the samples by accelerating oxidation of surface defect locations, inhibiting the continuous penetration of electrolyte, and improving corrosion resistance. Additionally, a-C films with a C-Cr-N interlayer exhibited an outstanding interface contact resistance (ICR) value (~3.5 mΩ/cm2) than that with a CrN interlayer (~30 mΩ/cm2) or the 316L substrate (~100 mΩ/cm2) at a typical contact pressure of 1.5 MPa. All 4 films demonstrate obviously improved hydrophobicity (contact angles against distilled water under room temperature up to 94.2°) compared to the substrate (75.8° under identical condition). In a word, this study proves that the strategy of inserting an appropriately designed interlayer (with the ternary C-Cr-N being most promising) to a-C films could alter and enhance the anti-corrosion, ICR and hydrophobic properties of 316L, proposing valuable solutions for high-performance bipolar plates (BPs) in the PEMFC industry. [ABSTRACT FROM AUTHOR]

[Display omitted] In this paper, ReaxFF simulation is used to study the friction characteristics of amorphous carbon (a-C) films in CO 2 environment by considering the influences of CO 2 molecular number and the normal load. The results manifest that the decrease of friction at the beginning of sliding stage is mainly due to the physical adsorption of CO 2 molecules, while in the stable stage, the low and stable friction is caused by the combination of chemical adsorption and physical adsorption. The large amount of CO 2 molecules greatly reduce the friction force by generating more C-O terminal groups, which are not easily damaged by the increased normal load. In addition, due to the presence of CO 2 molecules, graphitization has little effect on the friction behaviors of a-C films in CO 2 environment. [ABSTRACT FROM AUTHOR]

Atomic perspective on the tribological mechanism of a-C films in different gas atmospheres. [Display omitted] • In H 2 , N 2 and O 2 atmosphere, the friction force is the largest in H 2 atmosphere. • Graphitization and passivation are not the key factors to reduce friction force. • The interfacial mechanical mixing caused by C–C bonding increases the friction force. • Gas-dynamic lubrication generated in the O 2 atmosphere greatly reduces friction force. In this work, the friction mechanism of amorphous carbon (a-C) films in hydrogen (H 2), nitrogen (N 2), and oxygen (O 2) atmospheres are investigated by using reactive molecular dynamics simulations. It is found that the a-C film has the highest friction in the H 2 atmosphere and ultra-low friction in the O 2 atmosphere. Further studies show that a-C films are difficult to passivated by H atoms. The violent diffusion of H atoms to a-C film under the action of high friction temperature leads to the increase of C atom density at the friction interface, resulting in high friction. In the O 2 atmosphere, O atoms almost do not diffuse to the a-C film, mainly exist at the friction interface, weaken the density of C atoms, and form gas-dynamic lubrication to produce ultra-low friction. In addition, graphitization and passivation are not the key factors determining the low friction in the gas atmosphere. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Si- and Cr-containing C films were deposited by magnetron sputtering combined with CVD onto silicon wafers. The composition and chemical structure were characterized by X-ray Photoelectron Spectroscopy (XPS) and nanomechanical properties by depth-sensing hardness and scratch techniques. The incorporated Si and Cr are preferentially bonded to carbon, in accordance with simplified thermodynamic calculations and as manifested by the XPS chemical shifts. At relatively high Cr- and low Si-content silicides (Cr x Si) may also form as indicated by X-ray induced Auger electron spectroscopy. The chromium content in the C–Si–Cr films varied between 1 and 55at% while the silicon content in the same films between 25 and 0at%. For comparison two-component films of Si–C and Cr–C were also deposited with Si-content up to 42at% and Cr-content up to 55at% by varying the input power of the magnetrons. The nanohardness (H) and reduced modulus (E) were higher for all the films than that of the silicon substrate being 10GPa, 127GPa, respectively. Interestingly, the H and E of the three-component CrSiC films were almost invariant of the changes of the components'' concentration within the indicated range and varied between 13–16GPa and 120–140GPa. H and E values for the two-component Cr–C films were much higher, reaching about 22GPa and 170GPa, respectively. [Copyright &y& Elsevier]

Abstract: A simple thermal chemical vapor deposition technique is employed for the pyrolysis of a natural precursor “camphor” and deposition of carbon films on alumina substrate at higher temperatures (600–900°C). X-ray diffraction measurement reveals the amorphous structure of these films. The carbon films properties are found to significantly vary with the deposition temperatures. At higher deposition temperature, films have shown predominately sp2-bonded carbon and therefore, higher conductivity and lower optical band gap (Tauc gap). These amorphous carbon (a-C) films are also characterized with Raman and X-ray photoelectron spectroscopy. In addition, electrical and optical properties are measured. The thermoelectric measurement shows these as-grown a-C films are p-type in nature. [Copyright &y& Elsevier]

Abstract: Friction and wear behaviors of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) films sliding against Si3N4 balls were investigated in different testing environments. The result showed that two films with extreme chemical disparity (one hydrogenated, and the other hydrogen free) showed distinct different friction and wear behaviors, and the friction and wear behaviors of the both films were strongly dependent on the environment. For a-C:H films, much low friction coefficient and wear rate were obtain in dry N2. In the water and/or oxygen containing environments, the friction coefficient and wear rate of a-C:H films were obviously increased. On the contrary, a-C films only provided low friction coefficient and wear rate in the presence of water and/or oxygen in the test chamber. In dry N2, the highest friction coefficient and wear rate were observed for a-C films. By investigating the worn surfaces of the films using XPS, it was proposed that the environment dependence of the friction and wear behaviors of the films was closely related with the friction-induced chemical interactions between the films and water and/or oxygen molecules. The specific roles of hydrogen, water and oxygen molecules and their tribochemical effects on the friction and wear mechanism of the films are discussed. [Copyright &y& Elsevier]

Abstract: Amorphous carbon (a-C) films have many potential applications due to their good mechanical properties. However, poor adhesion on substrates such as tool steels limits its applications. In the present work, a-C films were deposited on W-implanted (20 kV, 3×1017 ions-cm−2) steel substrates by means of plasma immersion ion implantation and deposition (PIII&D). Compared to the films deposited on untreated steel substrates, the films deposited on the W-implanted steel substrates exhibit improved hardness, surface morphology, and adhesion strength. The modified layer with graded mechanical properties not only provides a structural continuity from the substrate to the film but also acts as the interlayer to mitigate stress concentration under applied loads. [Copyright &y& Elsevier]

Currently, there is a strong drive to make micro-electro-mechanical system (MEMS) devices from higher performance materials such as diamond-like carbon or amorphous carbon (a-C) films, due to their excellent tribological properties, low-stiction (hydrophobic) surfaces, chemical inertness and high elastic modulus, compared to that of Si. The hydrogen free a-C films prepared, by Nanyang Technological University’s (NTUs) patented filtered cathodic vacuum arc (FCVA) technique, at 100 eV exhibits high fraction of tetrahedral (sp3 bonded) carbon atoms. These films exhibit relatively high hardness, stiffness and wear resistance in addition to low friction and stiction behaviour. However, the primary problem lies in the large intrinsic compressive stress induced during the deposition process. By making use of high substrate pulse bias, we have successfully produced low stress, thick a-C films. The films were then characterised using different equipments to evaluate the stress, microstructure and morphological roughness. Large area a-C membranes, of 2 mm×2 mm in size, have also been fabricated using the low stress, thick film deposited by the above method. [Copyright &y& Elsevier]

The primary problem of large intrinsic compressive stress induced during the deposition of amorphous carbon (a-C) films prepared by Filtered Cathodic Vacuum Arc (FCVA) technique has been overcome by preparing the films in conjunction with high substrate pulse biasing. However, it has been observed that the stress reduction is achieved by sacrificing the mechanical properties such as hardness and Young's modulus of the films. Hence the mechanical properties of the films were studied as a function of substrate bias voltage. In addition, to demonstrate these low stress films, one-micron thick, a-C membranes were fabricated by photolithography technique together with deep reactive ion etching (deep RIE) and wet KOH etching. [ABSTRACT FROM AUTHOR]

• MD was used to simulate deposition process of a-C films on γ-Fe substrate by HiPIMS. • Hybridization percentage of a-C films can be adjusted by substrate bias voltage. • Internal stress of C films can be adjusted by substrate bias voltage. • Internal stress in intrinsic zone is increased with increase of sp3-C atoms percentage. • Internal stress in pseudo diffusion zone is decreased with increase of C and Fe atoms mixing degree. In this paper, the deposition process of amorphous carbon (a-C) films on γ-Fe substrate was simulated by molecular dynamics (MD). The effects of substrate bias voltage (abbreviated as bias) on structure and internal stress of a-C films was investigated. The results showed that the growth process of a-C films on γ-Fe substrate can be divided into five stages, namely, the diffusion stage of C atoms, island-like nucleation stage of a-C films, formation stage of the pseudo diffusion zone, diffusion stage of Fe atoms and stable growth stage of a-C films. The structure of a-C films after deposition can be divided into four zones, namely, the substrate zone, pseudo diffusion zone, intrinsic zone and surface zone. The intrinsic zone is the main part of a-C films, and its structure is mainly composed of sp3-C and sp2-C atoms. The percentage of sp3-C atoms is fluctuated with the increase of bias. When bias is 25 V, the percentage of sp3-C atoms is the largest, which is 26.19%. When bias is 75 V, the percentage of sp3-C atoms is the smallest, which is 23.92%. The percentage of sp2-C atoms is increased with the increase of bias. When bias is 75 V, the percentage of sp2-C atoms is the largest, which is 73.69%. With the increase of bias, the internal stress of the intrinsic zone is firstly increased. When bias is 25 V, the internal stress is the highest, which is 6.94 GPa and then is decreased gradually. The pseudo diffusion zone is a transition zone between a-C films and γ-Fe substrate, and is a mixed zone composed of C and Fe atoms. With the increase of bias, the internal stress in the pseudo diffusion zone is decreased gradually. When bias is 75 V, the internal stress is the lowest, which is 0.40 GPa. [ABSTRACT FROM AUTHOR]