Amorphous Carbon Films Research Articles

Controlling titanium incorporation in hydrogenated amorphous carbon films via closed-loop feedback in reactive high power impulse magnetron sputtering

A closed-loop feedback approach has been developed to control titanium incorporation in hydrogenated amorphous carbon (a-C:H) films during reactive high-power impulse magnetron sputtering (R-HiPIMS). The average discharge current measured at the magnetron target is used as the primary feedback signal to regulate the target coverage state. Hence, the titanium concentration in the films can be controlled. Significant changes were observed in the film microstructure and properties as the target state evolved with increasing target coverage. This causes the film transition from metallic titanium to a-C:H films with decreasing titanium concentration. For example, the XRD and Raman analyses indicated a microstructural change from hexagonal titanium to cubic titanium carbide and finally to amorphous carbon. The change in microstructure aligned with the density decreasing from 4.7 g∙cm‒3 to 1.6 g∙cm‒3 measured by XRR technique. In addition, a decrease in the Ti/C atomic ratio, from 1.53 to 0.03, clearly demonstrates that the titanium content can precisely be controlled. A simplified model was proposed to explain the relationship between the average HiPIMS current and the carbon coverage fraction on the target surface. The suggested relationship clarifies how adjusting the average discharge current effectively regulates the target coverage state and the consequent titanium concentration. The approach not only enhances process stability, but also offers an alternative to traditional control techniques during the deposition process.

Solid/liquid hybrid lubrication behaviors of amorphous carbon film coupling with nonpolar and polar base oils

The amorphous carbon film (a-C) has aroused intensive interest in solid-oil hybrid lubrication design due to its low friction, high wear resistance, and good chemical inertness. However, the solid/liquid hybrid lubrication behaviors and mechanisms of a-C films when coupled with nonpolar and polar oils of similar viscosity are still insufficiently understood and ambiguous to date. In this study, we compared the lubrication properties of nonpolar paraffin oil and polar polyether oils with different molecular structures when combined with a-C film. The results demonstrated that the lubrication performance of polyether oils coupled with a-C film surpassed that of paraffin oil across various lubrication regimes. Notably, the stable superlubricity (friction coefficient<0.01) was achieved under mixed lubrication regime on the a-C film when coupled with polyethylene glycol (PEG) oil after a prolonged sliding duration. The tribo-induced formation of carbon quantum dots (CQDs) at the steel/a-C friction interface lubricated by PEG oils was firstly observed. The hydroxylation of the counter ball surface and the fabrication of CQDs might jointly contribute to the achievement of the robust superlubricity of the PEG oil/a-C film hybrid lubrication system.

Current-carrying low friction of hydrogenated amorphous carbon film

Hydrogenated amorphous carbon (a-C:H) films, which equip excellent low friction performance but insulated, are naturally excluded from sliding electrical contact applications. In this work, we achieved current-carrying low friction of a-C:H film by utilizing a triboelectric triggered ignition effect. Protective resistors were introduced in the circuit during run-in for effectively alleviating undesired arc ablation. A T-shape crystalline structure of graphite was observed in the wear track by transmission electron microscopy. The parallel-aligned graphite at the top surface induced low friction, while the vertical-aligned graphite in the interior formed efficient electron transmission channels. This study expands the selection range of current-carrying friction films and provides a novel insight for the design and regulation of current-carrying friction films.

A-C/Au Film with Low Humidity Sensitivity of Friction by Forming Au Transfer Film.

Amorphous carbon is recognized as an excellent lubricating material; however, its tribological properties are significantly influenced by humidity. To elucidate the mechanism underlying this humidity dependence and to propose a novel enhancement method, we investigated and compared the tribological properties of hydrogenated amorphous carbon (a-C:H) and amorphous carbon/gold (a-C/Au) composite films. First, the friction coefficient of these carbon films under different humidity conditions was tested using a rotational ball-on-disk tribometer. Subsequently, we analyzed the morphology and structure of the sliding interface employing optical microscopy (OM), Raman spectroscopy, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Finally, first-principle calculations were carried out to calculate the adsorption energy of water molecules on different surfaces. The results indicate that the friction coefficient of a-C:H film and the area of transfer film increase with the increase of humidity. This phenomenon can be attributed to the fact that water molecules enhance the interaction between the a-C:H film and steel counterfaces. Notably, in contrast, the friction coefficient of a-C/Au film demonstrates low sensitivity to humidity due to the formation of an Au transfer film that exhibits weak interaction with water molecules. These findings provide a promising strategy for developing environment-adaptive amorphous carbon films and play an important role in promoting the development of intelligent lubricating film.

Effects of carbon nanoparticle insertion on stress reduction in hydrogenated amorphous carbon films using plasma chemical vapor deposition

Stress reduction in hydrogenated amorphous carbon (a-C:H) films is of considerable interest for improving their performance as protective films. Herein, we reduced the compressive stress of the films by inserting carbon nanoparticles (CNPs) between two a-C:H layers to form an a-C:H/CNP/a-C:H sandwich structure using a system of plasma chemical vapor deposition (CVD). We measured the deposition features of the CNPs via transmission electron microscopy (TEM) and assessed the dependence of the stress on the thickness of the second a-C:H layer as a parameter of surface coverage, Cp. We represented the stress reduction as a function of Cp; mean size of large CNPs, and thickness of the first and second a-C:H layers. In addition, the surface and film morphologies were investigated using atomic force microscopy (AFM) and cross-sectional scanning electron microscopy (SEM). Compared with films without CNPs, those containing CNPs with 8.9 % Cp showed 36 % lower stress when the thickness of the second a-C:H layer was similar to that of the first (154 nm). The deposited CNPs exhibited two size groups, with mean sizes of the small and large CNPs of 5 and 16 nm, respectively. Stress reduction was strongly correlated with the distance between large CNPs. The insertion of CNPs between a-C:H films affects the surface morphology of the second layer, and the stress decreases when the growth stage of the film starts transitioning from three-dimensional (3D) to two-dimensional (2D).

Effect of inserting a-C layers on anticorrosion behavior of Ni-NiCr-NiCrAlSi composite coating on copper through magnetron sputtering for marine applications

Amorphous carbon film by magnetron sputtering was inserted in the NiCrAlSi layer to form NiCrAlSi/a-C multilayer with modulation period in a range of 131.6 nm to 240.2 nm in the Ni-NiCr-NiCrAlSi coating for a further improvement to copper on corrosion resistance aiming at application as a heat exchanger for mariculture in sea water. After structure characterization, the corrosion behavior of the samples with variant modulation periods of the NiCrAlSi/a-C multilayer keeping the same total thickness(1 μm) was compared using potentiodynamic polarization and electrochemical impedance spectroscopy. The polarization curves show that the corrosion potentials of the coatings with modulation periods of 240.2 nm, 198.4 nm, 158.4 nm and bilayer number 4, 5, 6 for NiCrAlSi/a-C layer are more negative compared to that with modulation period of 131.6nm and bilayer number 7, which indicates that the smallest modulation period results in the highest corrosion resistance. The amorphous a-C sublayer in the multilayer coatings plays an active role in the improvement, for an introduction of the more interfaces between the NiCrAlSi and the a-C sublayers, as well as the compact structure and the chemical inertness to sea water (Cl−). In addition, the NiCrAlSi sublayer can be regarded as an interlayer to release stress in both the a-C sublayer and the NiCrAlSi sublayer, mainly from corrosion products, which also plays an important role for the improved corrosion resistance.

Analysis of the synergetic effect of process parameters of hydrogenated amorphous carbon deposition in plasma-enhanced chemical vapor deposition using machine learning

The synergetic effects of process parameters on hydrogenated amorphous carbon (a-C:H) films properties were quantitatively analyzed in plasma-enhanced chemical vapor deposition. Predictive models, created from experimental datasets and machine learning, indicate a synergetic effect between the H2 ratio and ion energy impacting on the substrate. At a H2 ratio of 75 %, etch rates of 200 nm/min were predicted, regardless of radio frequency (RF) bias. These rates decreased to 120–140 nm/min at lower H2 ratios, primarily depending on RF bias. Quadrupole mass spectrometry and Raman scattering were used to investigate the underlying mechanisms. High H2 ratios led to a greater presence of low-mass hydrogen-rich molecules in the plasma. The C2H3 radical intensity at a 75 % H2 ratio was 15 times greater than that at 0 % at an RF bias of 50 W, and similar trends were observed for other low-mass neutrals. The synergetic effects of H2 ratio and RF bias decreased the film's hydrogen content. Machine learning and those diagnostics revealed that ion bombardment induces dehydrogenation of a-C:H, influenced by hydrogen-rich species. This study demonstrates that machine learning can uncover complex plasma processes and optimize material synthesis.

Cathodic arc deposited tetrahedral amorphous carbon as thin film contact pressure sensing material

Tetrahedral amorphous carbon (ta-C) film was deposited using the filtered pulsed cathodic arc deposition method. The ta-C structures deposited on high velocity oxygen fuel thermally sprayed steel beam substrates were investigated for piezoresistive properties under direct contact pressing. A load of up to 1.4 GPa was applied with point contact, with no detrimental influence on the material. The linear response for the strain applied homogeneously on the whole film surface and on the local portion of the film surface under a cylindrical contact was observed. A high gauge factor of the measured system of 305 was measured, showing the potential of the material as a good candidate for sensing applications on direct contact measurement devices.

Pre-planting amorphous carbon films based on Ir composite substrates for diamond nucleation

A tunable locally ordered amorphous carbon layer was pre-implanted on the iridium (Ir) composite substrate using a multi-excitation source plasma coating system. The nucleation interface was mainly studied by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results show that by designing and modulating the content and ordering of sp2/sp3 hybridized carbon bonds in the amorphous carbon on the surface of Ir thin films, an overall ordered diamond nucleation layer was obtained. When the pulse frequency of the carbon source was regulated to 9 Hz, the (100) diamond grains were uniformly aligned without the appearance of twins after 4 h of growth, and the nucleation density was 7.5 × 109 cm−2, which was subsequently expected to obtain single-crystal diamond by grain boundary annihilation. Based on the Ir-amorphous carbon pre-growth layer, it can accelerate the dissolution-precipitation process of carbon ions into the Ir film to form a supersaturated solid solution during the bias nucleation, and increase the nucleation sites, which is of great significance for improving the nucleation density of large-size single-crystal diamond heterogeneous epitaxy.

Effects of ultraviolet sterilization exposure on the structural and surface properties of graphite-/polymer-like carbon films

Existing research on the biological responses of amorphous carbon films has not adequately explored the influences of exposure to ultraviolet (UV) sterilization at 253.7 nm before cell culture. Thus, in this study, two types of amorphous carbon films, namely hydrogen-rich graphite-like carbon (GLC) and hydrogen-free polymer-like carbon (PLC) films, were deposited on Si substrates. The influences of different doses of UV sterilization exposure (at 253.7 nm) on the structures and surface properties of the amorphous carbon films were investigated. Spectroscopic ellipsometry and Raman analysis showed that there is no significant change in the optical constants and Raman spectra of GLC and PLC films. The surface properties of the amorphous carbon films, characterized by water contact angle measurements and X-ray photoelectron spectroscopy analysis, indicated that both types of films exhibited hydrophilicity and surface oxidation in response to UV sterilization. However, the effects of UV sterilization on PLC were more notable than those on GLC. Our findings highlight the need for standardization and optimization of UV sterilization conditions for specific films to promote the application of amorphous carbon film coatings in the medical domain.

Friction dependence on processing priority for graphitization/passivation coupled amorphous carbon films

In this study, amorphous carbon (a-C) films were modified using different process sequences—H passivation followed by graphitization (a-C@H@G2000K) and graphitization followed by H passivation (a-C@G2000K@H). The friction dependence on the surface H content and the processing priority was comparatively investigated at the atomic scale, with a focus on the coupling mechanism for achieving low friction. The results indicated that the friction properties closely depended on the H content of the contacted a-C surfaces. An appropriate H content significantly improved the friction property through the coupling effect of the lubrication between surface graphitized structures and the repulsion between H atoms, resulting in a rapid decrease in the friction coefficient; however, the graphitization mechanism remained dominant. Excessive H reduced the repulsion between the contacted graphitized structures and hindered the sliding of these structures (shear susceptible), resulting in a slow increase in the friction coefficient. Most importantly, compared with the a-C@H@G2000K systems, the a-C@G2000K@H system exhibited higher effectiveness in reducing the friction coefficient, achieving a lower friction coefficient under the same surface H content; this was attributed not only to the small surface roughness and the low fraction of unsaturated bonds but also to the well stress distribution of the surface H atoms.

Integrated Amorphous Carbon Film Temperature Sensor with Silicon Accelerometer into MEMS Sensor.

Amorphous carbon (a-C) has promising potential for temperature sensing due to its outstanding properties. In this work, an a-C thin film temperature sensor integrated with the MEMS silicon accelerometer was proposed, and a-C film was deposited on the fixed frame of the accelerometer chip. The a-C film was deposited by DC magnetron sputtering and linear ion beam, respectively. The nanostructures of two types of films were observed by SEM and TEM. The cluster size of sp2 was analyzed by Raman, and the content of sp2 and sp3 of the carbon film was analyzed by XPS. It showed that the DC-sputtered amorphous carbon film, which had a higher sp2 content, had better temperature-sensitive properties. Then, an integrated sensor chip was designed, and the structure of the accelerometer was simulated and optimized to determine the final sizes. The temperature sensor module had a sensitivity of 1.62 mV/°C at the input voltage of 5 V with a linearity of 0.9958 in the temperature range of 20~150 °C. The sensitivity of the sensor is slightly higher than that of traditional metal film temperature sensors. The accelerometer module had a sensitivity of 1.4 mV/g/5 V, a nonlinearity of 0.38%, a repeatability of 1.56%, a total thermomechanical noise of 509 μg over the range of 1 to 20 Hz, and an average thermomechanical noise density of 116 µg/√Hz, which is smaller than the input acceleration amplitude for testing sensitivity. Under different temperatures, the performance of the accelerometer was tested. This research provided significant insights into the convenient procedure to develop a high-performance, economical temperature-accelerometer-integrated MEMS sensor.

Corrosion-Resistant Ultrathin Cu Film Deposited on N-Doped Amorphous Carbon Film Substrate and Its Use for Crumpleable Circuit Board.

Copper (Cu) is widely used as an industrial electrode due to its high electrical conductivity, mechanical properties, and cost-effectiveness. However, Cu is susceptible to corrosion, which degrades device performance over time. Although various methods (alloying, physical passivation, surface treatment, etc.) are introduced to address the corrosion issue, they can cause decreased conductivity or vertical insulation. Here, using the nitrogen-doped amorphous carbon (a-C:N) thin film is proposed as a substrate on which Cu is directly deposited. This simple method significantly inhibits corrosion of ultrathin Cu (<20nm) films in humid conditions, enabling the fabrication of ultrathin electronic circuit boards without corrosion under ambient conditions. This study investigates the origin of corrosion resistance through comprehensive microscopic/spectroscopic characterizations and density-functional theory (DFT) calculations: i) diffusion of Cu atoms into the a-C:N driven by stable C-Cu-N bond formation, ii) diffusion of N atoms from the a-C:N to the Cu layer heading the top surface, which is the thermodynamically preferred location for N, and iii) the doped N atoms in Cu layer suppress the inclusion of O into the Cu lattice. By leveraging the ultrathinness and deformability of the circuit board, a transparent electrode and a crumpleable LED lighting device are demonstrated.

Effect of CNTs concentration on the microstructure and properties of B, Ni, CNTs co-doped diamond like carbon films

B, Ni, and CNTs co-doped diamond-like carbon (B, Ni, CNTs co-doped DLC) films were synthesized on the surface of AZ91D magnesium alloy by electrodeposition with low cost. The effect of CNTs concentration on the microstructure, microhardness, tribological properties and corrosion behavior of B, Ni, CNTs co-doped DLC films were evaluated. Results revealed that the B, Ni, CNTs co-doped DLC films were hydrogenated amorphous carbon films. Raman spectra showed that the D and G peaks of DLC films appear near 1330 cm−1 near 1580 cm−1, respectively. When the concentration of CNTs was 0.25 g/L, the CNTs were evenly distributed, making the structure of co-doped DLC films uniform, dense and flat. At this time, since the combined synergistic effect of reticulation of CNTs and the network-like structure of DLC films, the microhardness of the co-doped DLC films reached the maximum of 248.3 HV. The tubular structure of CNT supports and lubricates the substrate, which weakens the frictional resistance between the friction partners and reduces the friction coefficient to the lowest value of 0.128. The wear loss was the minimum with 0.1 × 10−5 kg/m, which is the result of CNTs exerting their self-lubricating properties to improve the wear resistance of the DLC film so that less wear occurs during the repeated friction process. Meanwhile, the positive corrosion potentials of B, Ni, CNTs co-doped DLC films indicate that the three elements co-doped DLC films can improve the corrosion resistance of magnesium alloy substrates.

Molecular Dynamics Investigation of Metastable Structure and Hybridization Bond Evolution in High-Entropy CoCrFeNi Heterostructured Amorphous Carbon Films.

Heterogeneous element doping in amorphous carbon films can reduce residual stresses and improve plastic deformation. Nevertheless, the effects of dopant content and size on the metastable transition mechanism between sp2-C and sp3-C atoms during the deformation process are unclear and difficult to be in situ observed and researched, experimentally. In this work, the mechanical properties and the structural evolution during the nanoindentation of amorphous CoCrFeNi sphere-doped carbon heterostructured films with different radii were simulated. The results indicate that the hardness H and elastic modulus E of the films decreased with the increase of the dopant addition. H decreases from 50.69 to 28.94 GPa, and E decreases from 664.39 to 448.62 GPa. The decrease in the elastic recovery and the enlargement of the shear transition zones indicate that the presence of the amorphous CoCrFeNi dopant can significantly improve the plastic deformation capacity of the films. During the nanoindentation process, the spherical dopants reduce the stress and shear strain of the regions under the indenter in a-C films. The reduction of compressive and shear stresses in the film can inhibit the C atom metastable transition from sp2-C to sp3-C. This can provide a theoretical basis for the development and design of heavy-load and high-deformation-rate a-C films.

Atomic scale smoothing of nanoscale quartz mold using amorphous carbon films

Abstract Surface roughness control of end products is increasingly becoming significant, especially with the miniaturization trends in the semiconductor industry. Ultra-thin amorphous carbon (a-C) films offer a prime solution to optimize surface roughness due to their outstanding characteristics. In this study, hydrogenated a-C films are deposited on two-dimensional quartz plates and three-dimensional quartz molds to evaluate the growth mechanisms and changes in the surface roughness, which is supported by molecular dynamics simulations. Results reveal that surface roughness encounters multiple variations until it reaches stable values. These fluctuations are categorized into four different stages which provide a concrete understanding of various growing mechanisms at each stage. Different behavior of the atoms in the top layers is recorded in the cases of normal and grazing incidents of carbon atoms. Lower surface roughness values are obtained at low-angle deposition. Interestingly, surface smoothing is attained on the sidewalls of the nanotrench mold where the deposition occurs with high incident ion angles.

Effect of Sulfuric Acid Immersion on Electrical Insulation and Surface Composition of Amorphous Carbon Films

Sulfuric acid is a concern for contacts within electronic devices, and the application of amorphous carbon films as thin electrical insulating coatings for small coils requires full investigation of its effects. Five types of amorphous carbon films were fabricated on Si substrates under different deposition conditions using vacuum coating systems. Based on their optical constants (ISO 23216:2021(E)), the films were classified into three types: hydrogenated amorphous carbon (a-C:H), polymer-like carbon (PLC), and graphite-like carbon (GLC). The structure, surface composition, and electrical insulation properties of the films were evaluated before and after immersion in sulfuric acid. Although the PLC and a-C:H showed progression of surface oxidation due to sulfuric acid immersion, none showed obvious changes in their structure or DC dielectric breakdown field strength due to sulfuric acid immersion, proving their stability. Furthermore, the PLC and a-C:H, which had a relatively low extinction coefficient, exhibited excellent insulation properties. Our results suggest that amorphous carbon films can be useful as thin insulating films for small coils that may come in contact with sulfuric acid. Our study offers a valuable tool for general users in the industry to facilitate selection of electrical insulating amorphous carbon films based on optical constants, such as extinction coefficients.

Growth of thick amorphous carbon films on alumina for improved tribological properties

Alumina, as a pivotal ceramic material extensively utilized in contemporary applications, the surface modification and enhancement of tribological properties are crucial. In this study, reactive magnetron sputtering was employed to deposit Ti, Si co-doped hydrogen-free (TiSi/a-C), and hydrogenated amorphous carbon films (TiSi/a-C:H) with a thickness exceeding 7 μm onto the surface of alumina ceramic substrates. Utilizing the design of the film structure, the difficulty of film-substrate adhesion was resolved by controlling the deposition time to influence the growth rate of the film, thereby enabling the reliable fabrication of thick films. The adhesion of the TiSi/a-C film exhibits the highest value, with an Lc1 value of 36.7 N. The hardness of the TiSi/a-C:H reaches a maximum of 15.2 GPa, and the hardness is closely correlated with the reaction gas utilized during its depositional process. The wear rate of amorphous carbon film decreases by 34 times compared to alumina substrate. During the dissociation of CH4 at a high hydrogen-to-carbon ratio, more hydrogen atoms are involved in the chemical reaction. The introduction of hydrogen elevates the H/E and sp3/sp2 ratios within the film, thereby favoring the generation of low-friction surfaces and refining the tribological performance of alumina ceramics.

Graphene-reinforced amorphous carbon film for heat-assisted magnetic recording head: A molecular dynamics and density functional theory study

Heat-assisted magnetic recording (HAMR) is technology which can extend the area density to 10 Tb/in2. The high temperature at the head/disk interface (HDI) leads to lubricant degradation and transfer to the HAMR head. The friction characteristics and reliability of HAMR HDI are reduced. The graphene-reinforced amorphous carbon (a-C) film may be a potential coating for HAMR head. The molecular dynamics and density functional theory are used to investigate the adsorption and diffusion behaviors of lubricants on the graphene-reinforced a-C film. The thermal stability of graphene-reinforced a-C film is studied. As the temperature increases, D-4OH lubricants exhibit uniform sliding on the surface of graphene-reinforced a-C film. And the covalent bonds between the graphene and the a-C film are broken. The graphene-reinforced a-C film effectively prevents the decomposition of the lubricants. The graphene makes the surface of the a-C film electrically neutral, which improves the inertness and reduces the adsorption capacity of the D-4OH lubricant end groups. Compared to uncovered a-C film, the graphene-reinforced a-C film has better thermal stability. The graphene-reinforced a-C film with high thermal stability can minimize the adsorption and degradation of lubricants on the HAMR head, and improve the reliability of HAMR HDI.

Characterization of structural and mechanical properties of DLC films deposited on the surface of minute-scale 3D objects: Changes in the incident behavior of carbon ions

Diamond-like carbon (DLC) films have attracted great interest from the industry because they have beneficial mechanical properties such as low friction, high hardness, and wear resistance. DLC films have been used for the surface modification of various mechanical components, although many components that require surface treatments have complex three-dimensional (3D) surface structures. To date, 3D deposition of DLC films remains difficult using conventional methods. Particularly, uniform deposition technology on 3D components has not been developed for tetrahedral amorphous carbon (ta-C) films, which possess high CC sp3 bonds with high hardness. In this study, the filtered cathodic vacuum arc (FCVA) method was used to synthesize ta-C films on Si substrates placed on the sidewalls of trench-shaped and one-side tilted samples at various tilt angles and investigated their microstructure and hardness by Raman spectroscopy and nanoindentation. The results showed that the G peak position, full width at half maximum of G peak (FWHM (G)), and hardness of the ta-C film on the trench sidewall decreased as the depth from the top surface increased, similar to previous research. In contrast, ta-C films deposited on one-side tilted sample showed little depth dependence, and the film properties were determined by the tilt angle. We revealed that the film property dependence on the trench depth direction is due to the sputtered particles from the opposing trench sidewalls, resulting in graphitization of the film.