Amorphous Carbon Films Research Articles

Plasma diagnosis of tetrahedral amorphous carbon films by filtered cathodic vacuum arc deposition

Filtered cathodic vacuum arc (FCVA) deposition is regarded as an important technique for the synthesis of tetrahedral amorphous carbon (ta-C) films due to its high ionization rate, high deposition rate and effective filtration of macroparticles. Probing the plasma characteristics of arc discharge contributes to understanding the deposition mechanism of ta-C films on a microscopic level. This work focuses on the plasma diagnosis of an FCVA discharge using a Langmuir dual-probe system with a discrete Fourier transform smoothing method. During the ta-C film deposition, the arc current of graphite cathodes and deposition pressure vary from 30 to 90 A and from 0.3 to 0.9 Pa, respectively. The plasma density increases with arc current but decreases with pressure. The carbon plasma density generated by the arc discharge is around the order of 1010 cm−3. The electron temperature varies in the range of 2‒3.5 eV. As the number of cathodic arc sources and the current of the focused magnetic coil increase, the plasma density increases. The ratio of the intensity of the D-Raman peak and G-Raman peak (I D/I G) of the ta-C films increases with increasing plasma density, resulting in a decrease in film hardness. It is indicated that the mechanical properties of ta-C films depend not only on the ion energy but also on the carbon plasma density.

Metastable hybridized structure transformation in amorphous carbon films during friction—A study combining experiments and MD simulation

Amorphous carbon films have attracted substantial interest due to their exceptional mechanical and tribological properties. Previous studies revealed that the amorphous carbon films exhibited lower coefficient of friction (COF) because of the transformation in bond structure from sp3-C to sp2-C during friction processes. However, the mechanism for such a transformation during friction is not well understood. This study is conducted to get an insight into the metastable transformation in amorphous carbon film during friction by means of experiments and molecular dynamics (MD) simulation. Relevant wear tests showed that wear of the film changed from an abrasive wear mode to a mixture of abrasion and adhesive wear, resulting in a decrease in growth rate of the wear rate after the running-in stage. It is worth noting that the sp3-C atoms were increased during the running-in stage when the films contained lower sp3/sp2 ratios. However, the formed sp3-C atoms could only be short-lived and gradually transformed to sp2-C atoms with the graphitization generated on the wearing surface of the films. The radial distribution function and translational order parameter indicated that the films’ high sp3/sp2 ratio led to an increased sp2-C proportion on the wear scar after friction, which caused an increased structural ordering.

Characterization of structural and mechanical properties of DLC films deposited on the surface of minute-scale 3D objects: Comparison of PECVD and FCVA deposition technique

Diamond-like carbon (DLC) films have received considerable attention owing to its excellent properties such as high hardness, low friction coefficient, high wear resistance, and chemical inertness. Because DLC film is considered an effective coating to improve the surface properties of materials, these films are used in various applications such as parts for automobile engines, hard disk surfaces, cutting tools and dies, and so on. The structure of the DLC film consists of a mixture of sp2- and sp3-bonded carbon atoms. Among them, the tetrahedral amorphous carbon (ta-C) film is known to be the hardest film owing to its diamond-like structure composed mainly of sp3-bonded carbon atoms. The most common deposition method for synthesizing ta-C is the filtered cathodic vacuum arc (FCVA) method. This method can remove the droplets that are non-ionic neutral species, and a high-quality film can be synthesized. However, the mechanical parts that require ta-C coating have a three-dimensional (3-D) shape, and it is difficult to coat the ta-C film on their entire surface conformity by the FCVA process. In this study, we deposited ta-C on three-dimensional parts by the FCVA method and evaluated the microstructure and surface morphology of the film in comparison with the plasma enhanced chemical vapor deposition (PECVD) method. Films were successfully coated over the entire surface of both millimeter- and nanometer-sized trench patterns. The film properties were investigated by Raman spectroscopy, SEM, nanoindentation tester and transmission electron microscopy.

Revealing the structure-property relationships of amorphous carbon tribofilms on platinum-gold surfaces

Nanocrystalline metal alloys have shown great promise as electrical contact materials, given their mechanical and tribological properties. In particular, platinum-gold (Pt–Au) nanocrystalline alloys have demonstrated coefficients of friction as low as 0.01 and specific wear rates on the order of 10−9 mm3 N−1 m−1, largely due to the formation of carbon-based tribofilms at the sliding interfaces. In this study, we advance our understanding of the Pt–Au tribofilm structure-property relations and growth mechanisms via high-throughput and high-resolution measurements as a function of Pt–Au composition. As the solute content increased from 0 at. % to 10 at. % Au, cross-sectional and plan-view transmission electron microscopy demonstrated a decrease in average grain size d and an accompanied increase in grain boundary (GB) segregation. The decrease in d and increase in GB solute segregation translated to a decrease in modulus Er and an increase in hardness H as determined via nanoindentation; the Er trend was mainly described using a rule-of-mixtures approximation, whereas the H trend was ascribed to solid solution strengthening and GB stabilization. The steady state-friction μ and wear rate decreased with the addition of Au; low Au-content films showed substrate wear, while high Au-content films showed stable tribofilm growth in both macroscale and nanoscale friction tests. The carbon bonding configuration of the tribofilms was investigated by near-edge X-ray absorption fine structure spectroscopic analyses and found to be similar to that of hydrogenated amorphous carbon films. Altogether, the study provided insight into the mechanistic origins of the tribofilms, thus opening the door to tunable properties ranging from mitigation for electrical contacts to the creation of self-healing films for solid lubricants.

Evolution of the Microstructure, Hybridization, and Internal Stress of Al-Doped Diamond-Like Carbon Coatings: A Molecular Dynamics Simulation.

This work modified some parameters related to the bond order in REBO-II of the C-C interaction and simulated the ta-C:Al film deposition using the large-scale atomic/molecular massively parallel simulator especially focused on the effect of the Al-doping content on the microstructural and mechanical properties of tetrahedral amorphous carbon films. According to the Al existence state, the Al content in the films can be divided into three ranges: range I─under 5 at % Al, a single Al atom or a small cluster with 2-3 Al atoms disperses separately in the matrix; range II─at 5-20 at. % Al, the number and incorporating Al atoms of the clusters increase with the Al content; range III─above 20 at. % Al, only a solid network of aluminum atoms forms, which becomes thickened and densified with Al content increment. The existence states of Al atoms play an essential role in determining mechanical and structural properties. With Al content increasing in the films, the isolated small cluster of atoms evolved into a whole network of aluminum inter-crossing with the C-network. With the evolution of Al existence states, the sp3C fraction decreases monotonically, while the sp2C fraction increases. In range III, the network of aluminum promotes the growth of sp1C sites. The residual compressive stress in the film decreased rapidly with the Al content increasing in range I and II, but it reached a low-level constant value in range III.

Study on the Adsorption and Thermal Diffusion of D-4OH Lubricant on Amorphous Carbon Film by Reactive MD Simulation for HAMR

Heat-assisted magnetic recording (HAMR) is an innovative type of storage technology. The PFPE lubricants can improve the friction properties of the HAMR head-disk interface (HDI) in the writing and reading processes. The adsorption and thermal diffusion processes of three D-4OH lubricants with polar functional end groups on amorphous carbon (a-C) film are investigated by reactive molecular dynamics simulation. The potential energy, atomic distribution, radial distribution function (RDF), bonds’ number, atomic migration vector, and atomic shear strain are analyzed both in the relaxation and heating processes. The H atoms of polar functional end groups of D-4OH are bonded with a-C unsaturated carbon atoms via the secondary bonds. The main chains of D-4OH lubricants are adsorbed by a-C film via van der Waals forces. Laser heating helps to enhance the adsorption performance of lubricant on an a-C surface at a low temperature. The thermal diffusion of the lubricant makes the lubricant form a more stable conformation on the surface of the a-C film. Increasing the amount of −OH can improve the adsorption of PFPE lubricant on the a-C film.

Tuning of solid-to-solid structural transitions in amorphous carbon films by optical pumping and chemical modification

Amorphous carbon (a-C) attracts great attention in tribology research and thin film technologies due to its versatile properties. However, high temperatures and mechanical stresses may cause significant changes in the structural ordering of the a-C network. We present an optical method to initiate structural ordering and to probe solid-to-solid structural transitions of element modified a-C films. A pulsed pump laser introduces heat into the film in a controlled manner, while a second laser probes confocally the first- and second-order Raman scattering signatures of the a-C network. For low pump power, the number of defects and non-sixfold aromatic rings is reduced. A further increase in the laser power leads to sharply evolved changes in the Raman scattering features, indicating a transition from a-C to defected graphite and an effusion of hydrogen. Moreover, graphite-dominant defect relaxation and an enhancement in hexagonal lattice areas occur and, in turn, activate second-order Raman scattering lines. A rising laser power subsequently results in progressive graphitization. Chemical modification of the films with Si or Cu enhances their thermal stability and even shifts the upper thermal limit of the film ablation, while the a-C:W film demonstrates a more efficient enrichment of nanocrystalline graphitic clusters.

Combining the good tribological properties with the high adhesion strength of the amorphous carbon films in situ grown on PI

Polyimide (PI) is widely utilized in the modern industry because of its excellent heat resistance, dielectric properties, radiation resistance and chemical stability. However, PI will appear wear failure due to different working environments in the application of various friction components. In order to ensure the normal function of Pi-based parts and reduce the wear of PI materials, it is necessary to deposit amorphous carbon film with high adhesion strength and wear resistance on PI substrate. In this experiment, we treated polyimide surfaces with carbon plasma to prepare the amorphous carbon films with in situ transition layers. The microstructure, mechanical properties and tribological properties of the amorphous carbon films were studied. The results showed that the hardness and wear resistance of the PI surface were greatly improved by the amorphous carbon film with in situ transition layer. More importantly, the in situ transition endowed the film high adhesion strength on the PI substrate. The roles of the carbon plasma in the deposition process of the amorphous carbon, namely, the deposition effect, and the induction effect, referring to the effect on the top layer of PI substrate, were systematically analyzed. This work realizes the purpose of protecting the PI surface with the amorphous carbon film with high adhesion strength, and also provides a new idea for improving the adhesion between hard coating and flexible substrate.

Quantitative depth‐dependent analysis using the inelastic scattering backgrounds from X‐ray photoelectron spectroscopy and hard X‐ray photoelectron spectroscopy

The inelastic scattering contribution to an X‐ray photoelectron spectrum (XPS) reflects the distribution of elements within the sample depth. Varying the energy of the incident photons changes the inelastic scattering contribution. We present a standardless, automated method that exploits this change to enable compositional analysis within the XPS information depth (also known as amount of substance, AOS3λ). In this method, the photoelectron intensities observed using two or more X‐ray photon energies are normalised using correction factors. These correction factors are derived to determine the AOS3λ from the inelastic scattering signals collected from different samples. The AOS3λ are then compared with the ratio of the elastic XPS peak intensities calculated for different sample depths to find the layer thickness. The method has been applied to analyse spectra collected from thin amorphous carbon films to yield quantitative, standardless and automated XPS analysis.

Highly Stable Electrochemical Supercapacitor Performance of Self-Assembled Ferromagnetic Q-Carbon.

Novel phase Q-carbon thin films exhibit some intriguing features and have been explored for various potential applications. Herein, we report the growth of different Q-carbon structures (i.e., filaments, clusters, and microdots) by varying the laser energy density from 0.5 to 1.0 J/cm2 during pulsed laser annealing of amorphous diamond-like carbon films with different sp3-sp2 carbon compositions. These unique nano- and microstructures of Q-carbon demonstrate exceptionally stable electrochemical performance by cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy for energy applications. The temperature-dependent magnetic studies (magnetization vs magnetic field and temperature) reveal the ferromagnetic nature of the Q-carbon microdots. The saturation magnetization and coercive field values decrease from 132 to 14 emu/cc and 155 to 92 Oe by increasing the temperature from 2 to 300 K, respectively. The electrochemical performances of Q-carbon filament, cluster, and microdot thin-film supercapacitors were investigated by two-electrode configurations, and the highest areal specific capacitance of ∼156 mF/cm2 was observed at a current density of 0.15 mA/cm2 in the Q-carbon microdot thin film. The Q-carbon microdot electrodes demonstrate an exceptional capacitance retention performance of ∼97.2% and Coulombic efficiency of ∼96.5% after 3000 cycles due to their expectational reversibility in the charging-discharging process. The kinetic feature of the ion diffusion associated with the charge storage property is also investigated, and small changes in equivalent series resistance of ∼9.5% and contact resistance of ∼9.1% confirm outstanding stability with active charge kinetics during the stability test. A high areal power density of ∼5.84 W/cm2 was obtained at an areal energy density of ∼0.058 W h/cm2 for the Q-carbon microdot structure. The theoretical quantum capacitance was obtained at ∼400 mF/cm2 by density functional theory calculation, which gives an idea about the overall capacitance value. The obtained areal specific capacitance, power density, and impressive long-term cyclic stability of Q-carbon thin-film microdot electrodes endorse substantial promise in high-performance supercapacitor applications.

Wrinkled and cracked amorphous carbon film for high-performance flexible strain sensors

Wrinkled and cracked amorphous carbon film for high-performance flexible strain sensors

X-ray photoemission electron microscopy observation of wear tracks on hydrogenated amorphous carbon films after ball-on-disk tests

X-ray photoemission electron microscopy observation of wear tracks on hydrogenated amorphous carbon films after ball-on-disk tests

Effect of Size on the Formation of Solid Solutions in Ag-Cu Nanoparticles.

Modern technologies stimulate the quest for multicomponent nanosized materials with improved properties, which are ultimately defined by the atomic arrangement and interphase interactions in the nanomaterial. Here, we present the results of the experimental study of the formation of solid solutions in Ag-Cu nanoparticles in a wide size and temperature range using in situ TEM techniques. The Ag-Cu nanoparticles with a eutectic ratio of components were formed on an amorphous carbon film by the physical vapor deposition technique. Electron diffraction, HAADF-STEM imaging, energy-dispersive X-ray spectroscopy, chemical element mapping, and electron energy loss spectral imaging were used for the characterization of mixing patterns and composition of phases in AgCu nanoparticles down to the atomic level. As a result, we constructed the solid-state part of the Ag-Cu phase diagram for nanoparticles with a size down to 5 nm. We found a highly asymmetric behavior of the solvus lines. Thus, the content of Cu in Ag gradually increased with a size reduction and reached the ultimate value for our configuration of 27 wt % Cu at a nanoparticle size below ∼8 nm. At the same time, no Cu-rich solid solution was found in two-phase AgCu nanoparticles, irrespective of the size and temperature. Moreover, a quasi-homogeneous solid solution was revealed in AgCu nanoparticles with a size smaller than 8 nm already at room temperature. A size dependence of the terminal temperature T term, which limits the existence of AgCu alloy nanoparticles in a vacuum, was constructed. Evaporation of the AgCu phase with the composition of 86 wt % Ag was observed at temperatures above T term. We show the crucial role of the mutual solubility of components on the type of atomic mixing pattern in AgCu nanoparticles. A gradual transition from a Janus-like to a homogeneous mixing pattern was observed in Ag-Cu nanoparticles (28 wt % Cu) with a decrease in their size.

Performance and structure of Ti-doped amorphous carbon/CrN/Ti multilayer coating deposited on 316L stainless steel for use as bipolar plate in proton exchange membrane fuel cell

In order to improve the corrosion resistance and conductivity of stainless steel bipolar plates, three-layer Ti-doped amorphous carbon/CrN/Ti films were deposited on 316L stainless steel used for bipolar plates in PEMFCs. Titanium as the doped element added into amorphous carbon. Magnetron sputtering technology was used to deposit the films and control the titanium doped content by different titanium target currents. The structure transformation law of Ti content in amorphous carbon coating was studied. The application principle of corrosion resistance and conductivity of bipolar plates during the transformation of the coating structure from amorphous carbon to carbide was analyzed. With increasing Ti-doped content, the sp2/sp3 ratio increased, and the amorphous carbon film gradually transformed to titanium carbide matrix film. Characterization of the electrochemical corrosion confirmed that the anti-corrosion property of the films was closely related to the Ti-C phase and sp2/sp3 ratio. Ti-doped amorphous carbon/CrN/Ti film, which is built at 1A Ti target current, has the best corrosion current density. The interfacial contact resistance (ICR) first decreased and then increased with increasing Ti content. When excessive doping of titanium atoms makes the coating become a TiC matrix, the corrosion resistance and ICR property of the coating worsen. The film’s state of chemical bonds after electrochemical corrosion was tested by XPS to study the ICR increase mechanism. The Ti-doped amorphous carbon/CrN/Ti multilayer film can be used as a modified coating of 316L stainless steel bipolar plates for PEMFCs.

TEMAS: A Flexible Non‐AI Algorithm for Metrology of Single‐Core and Core‐Shell Nanoparticles from TEM Images

AbstractAn essential application of electron microscopy is to provide feedback to tune the fabrication of nanoparticles (NPs). Real samples tend to follow a size distribution commonly linked to the synthesis process used and in turn to their functional properties. This study presents an algorithm for measuring particle size distributions in electron microscopy images. State‐of‐the‐art methods based on Artificial Intelligence (e.g., Deep Learning) require extensive datasets of labeled images similar to those expected to be analyzed, and extensive supervised re‐training is often required for cross‐domain application. In contrast, the non‐AI algorithm described in this study is accurate and can be quickly set up for measuring new experimental images in different domains. The accuracy of the method is validated quantitatively and comparing graphical and descriptive statistics. Different size distributions are measured on images of platinum and gold nanocatalysts supported on carbon black, amorphous carbon, and titanium dioxide crystals. Also, images of platinum‐iron core‐shell NPs supported on thin amorphous carbon film are successfully analyzed. The limitation of evaluating different algorithms for NPs metrology is the lack of standards that different researchers can use as ground truth. In order to overcome this limitation, the images and the ground truth measurements presented here are shared as an open dataset.

Influence of air exposure on structural isomers of silver nanoparticles

Up to date, the influence of ambient air exposure on the energetics and stability of silver clusters has rarely been investigated and compared to clusters in vacuum. Silver clusters up to 3000 atoms in size, on an amorphous carbon film, have been exposed to ambient air and investigated by atomic-resolution imaging in the aberration-corrected Scanning Transmission Electron Microscope. Ordered structures comprise more than half the population, the rest are amorphous. Here, we show that the most common ordered isomer structures is the icosahedron. These results contrast with the published behaviour of silver clusters protected from atmospheric exposure, where the predominant ordered isomer is face-centred cubic. We propose that the formation of surface oxide or sulphide species resulting from air exposure can account for this deviation in stable isomer. This interpretation is consistent with density functional theory calculations based on silver nanoclusters, in the size range 147-201 atoms, on which methanethiol molecules are adsorbed. An understanding of the effects of ambient exposure on the atomic structure and therefore functional properties of nanoparticles is highly relevant to their real-world performance and applications.

Manipulation of etch selectivity of silicon nitride over silicon dioxide to a-carbon by controlling substrate temperature with a CF4/H2 plasma

The effects of substrate temperature (Ts) on the etch rate (ER) of the PECVD-prepared SiN, SiO2 and amorphous carbon (a-C) films, and their selectivity were investigated with a CF4/H2 plasma. The ERs for the SiN at all Ts were higher than that for the SiO2 films. As Ts was decreased from 50 to −20 °C, the ER for the SiN decreased. Contrarily, the ER of the SiO2 films increased. The etching selectivity of SiN over SiO2 reached to near unity when the Ts was −20 °C. At the same time, the ER of for the a-C films was found to be around 0.1 nm/s and irrespective of Ts. The fluorocarbon (FC) thickness was greater for the SiO2 films than that of the SiN. The lower ER for the SiO2 was therefore attributed to the thicker FC layer and resultant etching mechanism. As the Ts was decreased, the FC thickness on the SiO2 films decreased, which led to the ER decrease. The decrease of ER for the SiN etching at the low temperature was likely due to the higher stability of the surface N–H modification layer, compared with that processed at 20 °C, which was confirmed by the in situ FTIR.

Friction-induced reconstruction of sliding interface and low friction mechanism of WC/a-C films

Tungsten carbide reinforced amorphous carbon (WC/a-C) films exhibit significantly lower coefficient of friction (COF) than typical a-C films in the atmospheric environment; thus, they are promising candidates for use as solid lubricants. However, the low friction mechanism of WC/a-C films remains poorly understood. Here, we revealed that the friction-induced formation of WO3-rich transfer film changed the real sliding interface between the initial friction pair materials into an interface between a WO3-rich transfer film and carbon-rich worn film surface (WO3/C interface). First-principles calculations indicated that the WO3/C interface showed weaker interactions than the C/C interface of the a-C film under the same conditions, particularly when the dangling carbon bonds at the sliding interface were not completely passivated. This resulted in a reduced COF of the WO3/C interface. Thus, friction-induced reconstruction of the sliding interface to an intrinsically weak-interacting interface is proposed as a new path to further improve the tribological performance of a-C-based films.

Self-lubrication of single-phase high-entropy ceramic enabled by tribo-induced amorphous carbon

Designing single-phase ceramics with favorable self-lubricity is a substantial challenge due to the strong chemical bonds and difficulty to shearing. Most conventional lubricating ceramics only benefit from adding solid lubricants, which largely limits the mechanical properties of the resulting materials. Here, we report a single-phase ceramic inspired by the novel high-entropy design concept and tribo-induced effect, and which can achieve an outstanding self-lubricity (friction coefficient is as low as 0.1) at 200–400 °C in a vacuum environment. The self-lubricity is attributed to two factors, namely the decomposition of the (Hf-Mo-Nb-Ta-Ti)C ceramic leading to formation of the amorphous carbon film at the sliding interfaces, and the diffusion of carbon atom under the effect of high-temperature tribo-induced enabling easy shear contact. Based on the experimental and theoretical findings, we formulate a novel design strategy for single-phase self-lubricating ceramics by in-situ forming lubricating phase.

Theoretical superlubricity and its friction stability of amorphous carbon film induced by simple surface graphitization

Amorphous carbon (a-C) film as one of the most promising superlubricious materials arouses much attention from technical and industrial fields. However, the superlubricious state of a-C film strongly depends on the formation of special nanostructures or components, increasing the difficulty and complexity of the preparation process. Reducing the friction coefficient of a-C film to superlubricity is still a challenge for industrial scale-up. Herein, a simple and effective approach is proposed to achieve superlubricity by in-situ formation of graphitized structure at a-C surface via fast annealing. Results reveal that the friction property closely depends on the graphitized degree of mated a-C surfaces and the friction coefficient below 0.01 or even 0.001 can be obtained for the highly graphitized system. In particular, under extreme working condition, it also demonstrates excellent stability and reliability of superlubricious behavior. This is not only mainly attributed to the decrease of spacing distance between mated graphitized structures following the high repulsive force, but also related to the tight anchoring of graphitized structure to the intrinsic a-C. In addition, compared to the widely applied surface passivation approach, the graphitization modification of a-C surface exhibits higher effectiveness in superlubricity without deteriorating the intrinsic mechanical properties of a-C film.