Amorphous Carbon Films Research Articles

The robust superlubricity behaviors of a-C:H films on different roughness interfaces at surface contact scale

Hydrogenated amorphous carbon (a-C:H) film has demonstrated superlubricity in previous studies, but its effectiveness may be limited when applied to engineering components with surface contact and non-ideal smoothness. In this work, four kinds of plane contact friction pairs with different surface roughness (roughness of Sa from 21 nm to 544 nm) were designed and coated with a-C:H film with 43% hydrogen content by ionized beam deposition method. The results demonstrate that the film exhibits exceptional lubrication performance across various roughness interfaces, with the minimum friction coefficient reaching 0.005. The graphitization process and the actual contact state are discussed and calculated. These findings offer valuable insights into the potential application of highly-hydrogenated a-C:H film for moving parts in industrial equipment.

Micron-scale Al particulates to improve the piezoresistive performance of amorphous carbon films

Amorphous carbon (a-C) has been extensively studied as a promising candidate for microelectromechanical system (MEMS) piezoresistive sensors owing to its excellent mechanical properties and sensitivity. However, the semiconductor features of a-C films complicate achieving a high gauge factor (GF) and low temperature coefficient of resistance (TCR). In this study, we introduced micron-scale Al particulates (2.4–5.2 μm average size) by dewetting before a-C deposition, which simultaneously improved the GF and reduced the TCR of the composite structure. An atomic bond analysis revealed that applying the Al particulates favored the formation of sp2, thus reducing the localized surface potential of the a-C film. When the density of the Al particulates reached 15,000/mm2, the GF (36.9) of the a-C film significantly increased by more than 350 %, whereas the TCR decreased as the average size of the Al particulates increased. Furthermore, the composite exhibited typical Schottky contact characteristics and formed a Schottky barrier of 0.43 eV at the Al/a-C contact interface. Carrier transport between the Schottky barriers was achieved by the holes (primary carriers) via tunnelling. These significant piezoresistive properties can be explained by the variation in the Schottky barrier height.

Tribological properties of graphene quantum dot hybrid polyethylene glycol lubricated molybdenum disulfide films

Herein, molybdenum disulfide (MoS2) films were obtained by magnetron sputtering, and graphene quantum dots (GQDs) prepared by the hydrothermal method were used as additives for polyethylene glycol (PEG) lubricants to investigate the tribological mechanisms under solid-liquid composite lubrication. The friction coefficient of GQDs hybridized PEG lubricated MoS2 solid-liquid composite films was reduced by 69.23%, and the wear rate was reduced by 79.31% as compared to solid lubrication (MoS2 film). This solid-liquid complex lubrication occurs mainly as abrasive wear. GQDs form a uniform and dense amorphous carbon film on the corresponding balls to protect the wear between the friction pair. This work demonstrates the great potential of zero-dimensional and two-dimensional materials for solid-liquid composite lubrication.

Probing the tribological behaviors of a-C films in CO2 environment based on ReaxFF molecular dynamics simulation

In this paper, ReaxFF simulation is used to study the friction characteristics of amorphous carbon (a-C) films in CO2 environment by considering the influences of CO2 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 CO2 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 CO2 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 CO2 molecules, graphitization has little effect on the friction behaviors of a-C films in CO2 environment.

Effect of amorphous carbon film on secondary electron emission of metal

Amorphous carbon films have attracted much attention in the field of abnormal discharge of vacuum microwave devices and equipment due to their extremely low secondary electron yields (SEYs). However, the dynamic process and microscopic mechanism of the effect of amorphous carbon film on secondary electron emission are still poorly understood. In this work, a numerical simulation model of the secondary electron emission of amorphous carbon film on copper surface is developed by the Monte Carlo method, which can accurately simulate the dynamic processes of electron scattering and emission of the film and the substrate. The results show that the maximum SEY decreases by about 20% when the film thickness increases from 0 to 1.5 nm. Further increasing the thickness, the SEY no longer decreases. However, when the film is thicker than 0.9 nm, the SEY curve exhibits a double-hump form, but with the thickness increasing to 3 nm, the second peak gradually weakens or even disappears. The electron scattering trajectories and energy distribution of secondary electrons indicate that this double-hump phenomenon is caused by electron scattering in two different materials. Compared with previous models, the proposed model takes into account the change of work function and the effect of interfacial barrier on electron scattering path. Our model can explain the formation of the double-hump of SEY curve and provides theoretical predictions for suppressing the SEY by amorphous carbon film.

Frame-by-frame observations of structure fluctuations in single mass-selected Au clusters using aberration-corrected electron microscopy.

The multi-dimensional potential energy surface (PES) of a nanoparticle, such as a bare cluster of metal atoms, controls both the structure and dynamic behaviour of the particle. These properties are the subject of numerous theoretical simulations. However, quantitative experimental measurements of critical PES parameters are needed to regulate the models employed in the theoretical work. Experimental measurements of parameters are currently few in number, while model parameters taken from bulk systems may not be suitable for nanosystems. Here we describe a new measurement methodology, in which the isomer structures of a single deposited nanocluster are obtained frame-by-frame in an aberration-corrected scanning transmission electron microscope (ac-STEM) in high angle annular dark field (HAADF) mode. Several gold clusters containing 309 ± 15 atoms were analysed individually after deposition from a mass-selected cluster source onto an amorphous carbon film. The main isomers identified are icosahedral (Ih), decahedral (Dh) and face-centred-cubic (fcc) (the bulk structure), alongside many amorphous (glassy) structures. The results, which are broadly consistent with static ac-STEM measurements of an ensemble of such clusters, open the way to dynamic measurements of many different nanoparticles of diverse sizes, shapes and compositions.

Enhanced anti-tribocorrosion property of a-C film under high hydrostatic pressure by high power pulsed magnetron sputter (HiPIMS)

In this study, the amorphous carbon (a-C) films were deposited using HiPIMS and DCMS at same average power. The composition, microstructure and the anti-tribocorrosion property at hydrostatic pressure environment were investigated in detail. The results showed the HiPIMS:a-C film exhibited a higher fraction of sp3 bonding (59 %) compared to DCMS. The increased sp3 fraction led to improved mechanical properties and significantly enhanced the film's capacity loading during sliding. Furthermore, due to the denser structure, the HiPIMS:a-C film demonstrated enhanced localized corrosion resistance at 30 MPa, which can be attributed to its lower defect density. These findings highlight the substantial advantage of HiPIMS in producing high-performance a-C films suitable for harsh deep-sea environments.

Ti-inducing sp2 nanocrystallites into amorphous carbon film for low-friction

We reported a low-friction performance of Ti-doped a-C film, which is Ti-inducing sp2 nanocrystallites embedded in a-C structure. Mechanical and tribological tests showed that Ti-inducing sp2 nanocrystallites softened the film hardness from 22.5 to 18.8 GPa and lowered the friction coefficient from 0.15 to 0.05. Friction interface characterization indicated that abundant sp2 nanocrystallites lay down on the contact surfaces. The low-friction was attributed to the interaction between sp2 C layers, which originated from original Ti-inducing and new Ti-promoting sp2 nanocrystallites. The Ti-promoting effect was speculated that Ti atoms with Ti-C bond reacted with O atoms to form TiO bond (TiO2), then some released C atoms turned to sp2 nanocrystallites at the friction interface.

Ultrathin quasi-2D amorphous carbon dielectric prepared from solution precursor for nanoelectronics

Materials keeping thickness in atomic scale but extending primarily in lateral dimensions offer properties attractive for many emerging applications. However, compared to crystalline counterparts, synthesis of atomically thin films in the highly disordered amorphous form, which avoids nonuniformity and defects associated with grain boundaries, is challenging due to their metastable nature. Here we present a scalable and solution-based strategy to prepare large-area, freestanding quasi-2D amorphous carbon nanomembranes with predominant sp2 bonding and thickness down to 1–2 atomic layers, from coal-derived carbon dots as precursors. These atomically thin amorphous carbon films are mechanically strong with modulus of 400 ± 100 GPa and demonstrate robust dielectric properties with high dielectric strength above 20 MV cm−1 and low leakage current density below 10−4 A cm−2 through a scaled thickness of three-atomic layers. They can be implemented as solution-deposited ultrathin gate dielectrics in transistors or ion-transport media in memristors, enabling exceptional device performance and spatiotemporal uniformity.

Diamond-like films of tetrahedral amorphous carbon deposited by anodic arc evaporation of graphite

A physical vapor deposition process using anodic arc evaporation in combination with a hollow cathode arc discharge was applied to the evaporation of graphite for deposition of hydrogen-free carbon layers. The diamond-like carbon (DLC) films deposited on 100Cr6 steel substrates were investigated by nanoindentation, Raman spectrometry, FE-SEM, AFM and spectroscopic ellipsometry. The relationships between the process parameters and the coating properties are discussed. Coatings deposited without bias voltage at substrate temperatures <200 °C are very hard (61–75 GPa) with also very high Young's modulus (588–685 GPa). The evaluation of the Raman spectra indicated a high proportion of tetrahedral sp3 bonds in the range of 70–88 %. Obviously, the vapor particle energies are high enough to achieve such high hardness values even without application of a bias voltage. The coatings proved to be completely droplet-free and have a very low surface roughness as confirmed by FE-SEM and AFM. The deposition rates in the range of 4–18 nm/s are exceptionally high for such tetrahedral amorphous carbon (ta-C) coatings, which is a good prerequisite for industrial applications. The ta-C layers with very high hardness and smooth surface are well suited as wear resistant layers.

Control of the tribological temperature dependence of hydrogenated amorphous carbon films by doping with W

While numerous strategies have been developed to enhance the high-temperature performance of amorphous carbon films, challenges persist regarding high coefficients of friction and wear rates within the mid-temperature range (200–300 °C). In this study, we developed a series of a-C:H(W) films with varying tungsten concentrations and systematically investigated their wear and friction characteristics using Al2O3 ceramic balls at different temperatures. The results reveal that the a-C:H(W) film (W3) with a WC target current of 0.3 A exhibits friction coefficients of 0.10 and 0.06 at 300 °C and 500 °C, respectively. The addition of hydrogen to the film enables stable lubrication in the intermediate temperature range. The presence of carbide facilitates the formation of a continuous protective layer during friction at 500 °C. Additionally, the inclusion of tungsten trioxide effectively reduces friction during high-temperature lubrication. Overall, this a-C:H(W) film shows promise in addressing the challenges of high-temperature wear in mechanical engineering applications under harsh operating conditions.

Cu doping and friction-induced self-migration in a-C:H films to improve anti-wear life in vacuum

Hydrogenated amorphous carbon (a-C:H) films can achieve superlubricity, but the problem of rapid failure in vacuum remains a challenge for their applications. In this study, a novel solution route is demonstrated by friction-induced Cu self-migration to improve the anti-wear life of Cu/a-C:H films. The results show that Cu doping promotes the increase of sp2C, reduces the internal stress and improves the adhesion. Additionally, the doped Cu atoms mainly exist in a metallic state, which is benefit for the diffusion of Cu atoms in the matrix under friction effect. Further results imply that the anti-wear life of the Cu/a-C:H films with different Cu content are improved, especially, the most significant increase in anti-wear life occurring at a Cu content of 1.15 at%, which is related to the Cu-rich transfer films that form a Cu/C friction interface. Furthermore, the distinct Cu migration channels consisting of crystal clusters aggregated by the Cu atoms dispersed in the matrix are observed in the cross-section HRTEM and EDS characterizations of the wear track, which verifies the self-migration behavior of Cu atoms. The formed Cu/C friction interface eliminates the strong covalent interactions and maintains a stable lubricity state, thus prolonging the anti-wear life.

FORMATION OF TRACHYTES FROM WANGTIAN’E VOLCANIC NECK OF THE CHANGBAISHAN AREA (NORTHEAST CHINA) AS A RESULT OF THOLEIITIC MAGMA CRYSTAL FRACTIONATION

Tholeiitic basalts and trachytes of Wangtian’e bimodal association are studied. It is shown that trachytes were formed under the leading role of crystal fractionation of the initial tholeiitic magma. Based on the method of melt and fluid inclusion studies, it was determined that hedenbergite phenocrysts from Wangtian’e volcanic neck crystallized in the temperature range of 1080–1100°С, and plagioclase phenocrysts crystallized at temperatures of 1050–1060°С. After thermometric experiments with melt inclusions in plagioclase from trachytes, in some cases relics of carbonates and carbon dioxide were observed in the inclusions. Trachyte phenocrysts contain hydrous Fe-rich silicate globules and CO2 inclusions containing carbonate phases. Fe-rich silicate globules are often intergrown with titanomagnetite and covered with films of amorphous carbon. A model the appearance of hydrous Fe-rich globules and CO2 inclusions with carbonate relics is proposed. It results due to the silicate-silicate and silicate-carbonate liquid immiscibility processes, that are caused by the evolution of the initial basaltic melt. As the trachyte melt has been risen to the surface, the ferrocarbonate liquid decomposed into magnetite, carbon, and carbon dioxide.

Temperature-structure-induced metastable structural transformation mechanism of the amorphous carbon film during friction

Temperature has a significant impact on the tribological behavior of amorphous carbon films. However, the fundamental question of "the effect of temperature-structure-induced on metastable structural transformation mechanism of amorphous carbon films " is not well understood. This study is conducted to get an insight into the temperature-structure-induced structural transformation of amorphous carbon films. The relevant studies show that the effect of high temperature within a certain threshold on the "sp3-C to sp2-C" and the structural ordering degree changes from inhibition to promotion with amorphous carbon films’ density increases, and the structural transformation of high-density amorphous carbon films responds more significantly to temperature changes. The structural ordering degree of amorphous carbon films is promoted when the temperature exceeds a certain threshold.

Enhancing tribological performance of graphene sheet-embedded carbon films on steel substrates via texture engineering and chromium doping

Amorphous carbon (a-C) film is a promising solid lubricant coating due to its unique structure and excellent properties. However, its poor adhesion and long running-in period hinder the application in metal components. Graphene sheet-embedded carbon (GSEC) film has a significantly shorter running-in period but suffers from inferior tribological properties due to its rough surface and weak mechanical characteristics. In this study, we utilized an electron cyclotron resonance (ECR) plasma nano-surface manufacturing device to deposit GSEC films onto steel substrate surfaces. By manipulating the surface/interface texture and incorporating chromium (Cr) doping, the tribological properties of the GSEC films were improved both chemically and physically. The results demonstrated that the textured carbon film exhibited superior adhesion to the steel substrate and a wear life that was three orders of magnitude longer than the untextured GSEC film, under low normal load conditions while sliding against the GCr15-bearing steel ball. After doping the Cr into textured carbon films, there was a significant reduction in the friction coefficients, and the wear type of carbon film transformed from adhesive wear to abrasive wear. Moreover, the doped carbon films demonstrated prolonged wear lives under high normal loads. This improvement can be attributed to the formation of hard CrC nanocrystalline, which effectively reduced surface roughness and enhanced the mechanical properties of the carbon films. Notably, even under a contact pressure of 0.83 GPa, the doped carbon film maintained favorable tribological performances. These findings have extended the potential applications of carbon films on metal substrates.

Formation of Trachytes from Wangtian’e Volcanic Neck of the Changbaishan Area (Northeast China) as a Result of Tholeiitic Magma Crystal Fractionation

Tholeiitic basalts and trachytes of the bimodal association of Wangtian’e volcano are studied. It is shown that trachytes formed with the leading role of crystal fractionation of the parental tholeiitic magma. It has been identified from the melt inclusion study that hedenbergite and plagioclase phenocrysts of trachytes from Wangtian'e volcanic neck crystallized in the temperature range of 1080–1100 and 1050–1060°C, respectively. After heating experiments with melt inclusions in plagioclase of trachytes, in some cases relics of carbonates and CO2 were observed in the inclusions. The trachyte phenocrysts host water-bearing Fe-rich globules and CO2 inclusions with carbonate phases. The Fe-rich silicate globules are often intergrown with titanomagnetite and are covered by amorphous carbon films. A model is suggested explaining the formation of water-bearing Fe-rich globules and CO2 inclusions with carbonate relics as a result of silicate–silicate and silicate–carbonate liquid immiscibility, that was caused by the evolution of the parental basaltic melt. Upon the trachytic melt ascending to the surface, the ferrocarbonate liquid was decomposed on magnetite, carbon, and carbon dioxide.

Surface sulfurization of amorphous carbon films in the chemistry of oxygen plasma added with SO2 or OCS for high-aspect-ratio etching

Etching of oxygen-based plasmas with sulfur dioxide (SO2) or carbonyl sulfide (OCS) can form high-aspect-ratio (HAR) features of amorphous carbon films as carbon hard masks (CHM). The etched profiles showing shapes such as bowing or tapering are essentially dependent on the partial pressures of SO2 or OCS in the O2 plasma. The surface treated after the OCS-added plasma exhibited strong sulfurization by S2 and CS species in S 2p of the X-ray photoelectron spectroscopy (XPS). The gas-phase interactions in the sulfur-oxygen-carbon system generated atoms and molecules, such as O, O+, and O2+, which etched at the bottom and, conversely, SO, CO, CS, CS2, and S2, which inhibited isotropic etching at the sidewalls of the HAR features. The chemical reactions of the CS sulfurizing precursors in the gas phase were monitored by comparing their optical emission intensities at a wavelength of 257 nm with those of SO2 at approximately 320 nm. The optimization of the HAR profiles of the CHM can be controlled by sidewall sulfurization of the CHM to obtain desirable profile shapes for the HAR features.

Atomistic Insights into Interfacial Optimization Mechanism for Achieving Ultralow-Friction Amorphous Carbon Films under Solid-Liquid Composite Conditions.

The combination of fluid lubricants and textured amorphous carbon (a-C) can provide an ultralow friction state, which can improve the reliability and service life of dynamic machinery. However, the coupling effects of the contact pressure and oil content on the friction-reducing efficiency is still lack of study, and the corresponding friction mechanism is also not fully understood, which cannot be achieved by experiment due to the limitation of in situ characterization. In this study, using the reactive molecular dynamics simulation, the insight into the evolution of interfacial structures induced by both contact pressures and oil contents on a-C surface was systematically investigated to explore the fundamental mechanism. In particular, the friction difference between textured and untextured a-C films was evaluated comparatively. Results indicate that the tribological performance strongly depends on the interfacial lubrication state, which is jointly determined by the oil content and contact pressure; the best operating condition to achieve ultralow friction coefficient (0.002) is obtained, and the evolution of friction coefficient with oil content and contact pressure is highly dominated by the lubricant mobility, cross-linking between mating a-C surfaces, or competition/synergy of the H stress state from the lubricant with interfacial passivation. Furthermore, the difference in friction reduction between textured and untextured systems is unveiled; with the increase of contact pressure, the role of texturing a-C surface in antifriction changes from positive to negative effect, which is related to the transformation of interfacial hybridized structure and anomalous flow of lubricant. These results can significantly enhance the understanding of composite lubrication systems through computation and also provide a roadmap for the R&D of the advanced lubrication system according to the working conditions.

Nanoscale friction of tetrahedral amorphous diamond-like carbon film after thermal annealing

Diamond-like carbon (DLC) coatings have demonstrated significant potential as solid lubricants for a wide range of applications. However, thermal-induced structural changes lead to alterations in its mechanical and frictional properties. In this study, we investigate the friction properties of DLC films after high-temperature annealing, by conducting reciprocating single asperity scanning experiments of self-mated hydrogen-free DLC tribopair under ultra-high vacuum (UHV) conditions. As the annealing temperature increases, the coefficients of friction (COFs) first increase owing to the desorption of water molecules; while, with the further increase of temperature, friction shows a decreasing trend because of the high-temperature-induced sp3-to-sp2 rehybridization which lowers the interfacial shear strength. The insights gained from this study provide a better understanding of the frictional properties of DLC films.

Exploring the role of surface hydrogenation in anti-friction of circular-textured amorphous carbon film from the atomic level and its dependence on textured shape

Hydrogenating the amorphous carbon (a-C) surface can achieve the ultra-low friction performance. However, the effect of surface hydrogenation on anti-friction of textured a-C surface, especially its dependence on different textured shapes, is still lack of study; the information on the complicated friction interfacial also remains unclear due to the limitation of in-situ characterization in experiment. Here, the friction behavior of circular-textured a-C film was investigated by reactive molecular dynamics simulations, and its dependence on the hydrogenated degree of textured a-C surface was mainly focused, in which the effect from textured shape was also comparatively studied. Results show that the anti-friction efficiency of the circular-textured a-C film strongly depends on the surface hydrogen content; when the surface H content is 31.0 %, the friction coefficient decreases by 97.5 %, which is attributed to the passivation effect of H atoms on the a-C surface, inhibiting the generation and crosslinking of unstable dangling bonds. Moreover, the migration of H atoms in the friction process provides a strong repulsive force, which also significantly reduces the shearing strength of the interface and the friction coefficient. In particular, further study discloses that the effect of textured shapes of a-C surface on friction behavior is also related with the surface H content; with the increase of H content, the textured shape of a-C surface, which exhibits the low-friction performance, transforms from rectangular to circular-textured one, and the corresponding low-friction mechanism also changes from the passivation of friction interface to H-induced repulsive effect. These results provide an effective way to develop an efficient a-C anti-friction system.