Tetrahedral Amorphous Carbon Films Research Articles

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.

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.

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.

Improved electrochemical detection of copper ions in nitrogen doped ta-C films for marine corrosive monitoring

The corrosion of copper-based alloy in the harsh marine environment threatens the reliable operation and service life of marine engineering equipment. In this work, the nitrogen modified tetrahedral amorphous carbon (ta-C:N) film electrodes were prepared for marine-oriented copper ions detection. The relationship between structural evolution and electrochemical performance of ta-C:N film electrode tailored by N2 flux was evaluated. The sp2 cluster and the content of sp2-bonded carbon in ta-C:N film were promoted with the increase of N2 flux. XPS results indicated that the variation of N2 flux modified the nitrogen-containing groups of the ta-C:N film electrodes. The optimized ta-C:N film electrode deposited at N2 flux of 3 sccm showed a quasi-reversible electrode behaviors. Moreover, the ta-C:N film electrode exhibited a linear range from 8 × 10−3 mM to 5 mΜ, a detection limit of 8 × 10−3 mM, and good repeatability and reproducibility for Cu2+ detection in 3.5 wt% NaCl solution. The improved Cu2+ sensing properties were ascribed to the enhanced sp2 cluster and the introduction of multiple nitrogen-containing functional groups, especially the pyrrolic nitrogen and graphitic nitrogen. DFT computation and wavefunction analysis revealed that the pyrrolic nitrogen possessed the strongest interaction for copper ions among different nitrogen configurations.

Atomic insights on nanoindentation-induced plastic-deformation behavior of tetrahedral amorphous carbon film

The nanoindentation-induced deformation behavior of the Cr-doped diamond-like carbon (DLC) films was clarified with Cr concentration varying from 0 to 16.5 at.% by experiment and molecular dynamics simulation. The Cr-DLC films were fabricated by a direct cathode vacuum arc technique and Cr dopants existed in the form of stripes owing to the deposition features. Results showed the addition of Cr facilitated the transformation of sp3-C bonds to sp2-C bonds, resulting in a decrease of the hardness and elastic modulus. Especially, the sp2 nano-crystallites with the lamellar structure were found in the Cr (16.5 at.%)-DLC film. During nanoindentation, the ring-like cracking on the surface and lateral crack at the interface are exhibited in the Cr-free DLC film, while the introduction of Cr makes the DLC film alleviate the cracking trend. According to the atomic-scale analysis, Cr can significantly improve the plasticity of the DLC film because the Cr-rich cluster with a high free volume can facilitate the shear transformation zones percolation and the shear bands (SBs) nucleation by collective coordinated vortex-like movements of atoms. The Cr-free DLC film can hardly accommodate high plastic strain with very few fine SBs and blockage of the SBs.

Microstructure evolution and mechanical properties of Al doped ta-C films prepared by filtered cathodic vacuum arc

Al-doped tetrahedral amorphous carbon (ta-C) films were prepared by filtered cathodic vacuum arc technique. Effect of Al content on the structure and properties of the ta-C film were systematically investigated. Results showed that Al content in films increased from 0 to 39.5 at. % as the Al target current altered from 0 to 2 A. Without Al doping, ta-C film mainly presented amorphous characteristic with disordered arrangement and compact structure. With a small amount of Al doping, the film maintained the amorphous phase structure, and Al atoms mainly existed in the carbon tetrahedral interstitial space in the form of solid solution. At high Al doping content, the formed Al nanoparticles embedded in the carbon matrix network. Meanwhile, the addition of Al could promote the graphitization of films. The residual stress of ta-C films decreased gradually with increase of Al content. Furthermore, the hardness and elastic modulus of films also decreased with increasing Al content. That's ascribed to the reduction of residual stress and graphitization of the carbon structure. In addition, the ta-C film with Al addition could effectively improve the friction and wear properties of films at both room and high temperature. The friction coefficient decreased with the increase of Al content at room temperature (RT). At 400 °C, the film containing 7.6 at. % Al obtained the lowest friction coefficient and wear rate, corresponding to the values of 0.29 and 12.3 × 10−8 mm3/N⋅m, respectively.

Enhancement of discharge and mechanical properties of ta-C films by pulse-enhanced cathodic arc deposition

A home-made straight-duct filter cathodic vacuum arc system was used to synthesize tetrahedral amorphous carbon (ta-C) films. The ta-C films were fabricated using both direct-current cathodic arc evaporation (DC-CAE) and pulse-enhanced cathodic arc evaporation. A series of films were prepared as a function of pulse current ranging from 500 to 1000 A compared with 80 A direct current. The introduction of pulse current could markedly increase instantaneous input power (Wiip). The pulse-enhanced Wiip led to the increase in average substrate current from 0.46 to 1.07 A. Moreover, compared with DC-CAE, the pulse current of 1000 A significantly increased the intensity ratios of C II/(C I + C II) and C II/Ar II by about 65% and 130%, respectively. All the results showed that the pulse-enhanced Wiip substantially improved the evaporation and ionization rate of graphite. However, due to thermal-induced graphitization conversion, the threshold pulse current was 800 A. Under this pulse current, the film exhibited the smallest ID/IG (0.26) as well as the highest sp3 fraction (64.3%). Correspondingly, the highest hardness (≈40 GPa) and the preferable toughness (H/E* of 0.12 and H3/E*2 of 0.52) were simultaneously achieved.

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.

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.

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.

Interfacial Layer Effect on the Adhesion of the Ultra-Hard Thick TAC Film Deposition

Carbon-based thin film tool coatings, such as diamond-like carbon (DLC), have excellent lowfriction and anti-sticking properties. These thin films are widely used for the cutting and machining of increasingly widely-used lightweight non-metallic and non-ferrous metal materials, as a part of countermeasures against global warming. However, non-metallic and non-ferrous metal materials are significantly inferior in strength and heat resistance compared to iron-based metals. Therefore, they are primarily employed in high-content fiber reinforced composite materials, which significantly improves their mechanical and thermal properties. Tetrahedral amorphous carbon (TAC) coating has a hardness level similar to diamond coating. However, when TAC is deposited as a thick film, delamination of the coating layer may occur because of the high internal compressive stress between the carbide-based substrate and coating layer, thereby restricting its scalability to other applications. Other factors to be controlled for thick film TAC deposition include minimizing droplets generated during the coating process, and improving interfacial properties like hardness and fatigue resistance. Here, C in the form of CH4, which has high solubility over Cr and forms various compounds, was added during the interfacial deposition process, between the carbide and TAC, to improve interfacial strength and adhesion by precipitation of carbide at the interface. This eventually led to thick TAC film with the thickness and adhesion of commercially viable thick film.

Effects of interlayer bias voltage on the mechanical properties of tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) films with bias-regulated (namely from −50 V to −200 V) Ti interlayers were prepared on AISI 440C stainless steel substrates, followed by systematic investigation on the microstructure, mechanical, and adhesion properties of such films. In addition, molecular dynamics simulation has been employed to study the formation of Ti films deposited with different incident energy, which was found to be influenced by the bias voltage significantly. The experimental results demonstrated that varying bias voltage on the Ti interlayer exerted minor influence on the sp3C fraction and mechanical properties of ta-C films. Molecular dynamics simulations showed further support for the experimental results. The Ti interlayer exhibited the amorphous-like microstructure, and increasing Ti-atom incident energy resulted in increasing Ti/Fe interlayer thickness and a firstly rising and then descending Fe/Ti interfacial bonding strength. The adhesive failure mode can be divided into buckling crack and interfacial spallation, which was primarily attributed to the tensile-to-compressive transformation of the Ti-interlayer internal stress. Optimised adhesion property of ta-C films can be achieved with a Ti-interlayer bias voltage of ∼ -100 V.

Investigation of Vacuum Arc-Deposited ta-C and ta-C:N Thin Films on Silicon and Stainless-Steel Foil Substrates Using Raman Spectroscopy

In this paper, we investigated the vacuum arc-deposited tetrahedral amorphous carbon (ta-C) and nitrogen-doped ta-C (ta-C:N) thin films grown on silicon and stainless-steel foil substrates by the visible Raman Spectroscopy. We found that carbon films grown on silicon surfaces prefer more sp3 hybridized carbon compared to the stainless-steel foil, which prefer more sp2 hybridized carbon even though the films are grown concurrently under the same conditions. The impact energy of plasma ions modifies the interfacial layer formation, giving a strong dependence of the sp2/sp3 ratio with the substrate bias, behaving differently for different substrates. However, nitrogen doping in amorphous carbon thin films helps to increase sp2 contents uniformly on both the surfaces for a fixed substrate bias. Understanding such interfaces are of interest for many electronic, optoelectronic, and energy storage devices as these interfaces get buried and can influence the properties significantly.

Insights on high temperature friction mechanism of multilayer ta-C films

In this work, the high temperature friction mechanism of the tetrahedral amorphous carbon (ta-C) film was elucidated. The multilayer ta-C film with alternating hard and soft sub-layers exhibited a low friction coefficient of 0.14 at 400 °C before a sudden failure occurred at 4600 cycles. The wear failure was attributed to the gradual consumption of the ta-C film at the contact region. The design of a hard or soft top layer effectively regulated the high temperature friction properties of the multilayer ta-C. The addition of a hard top layer contributed to a low friction coefficient (0.11) and a minor wear rate (4.0 × 10−7 mm3/(N m)), while a soft top layer deteriorated the lubrication effect. It was proposed that the passivation of dangling bonds at the sliding interface dominated the low-friction mechanism of the ta-C film at high temperature, while the friction induced graphitization and the formation of sp2-rich carbonaceous transfer layer triggered CC inter-film bonding, resulting in serious adhesion force and lubrication failure. Moreover, the multilayer ta-C film with hard top layer obtained excellent friction performance within 500 °C, while the high temperature induced oxidation and volatilization of carbon atoms led to the wear failure at 600 °C.

Nanoindentation-Induced Plastic-Deformation Behavior of Tetrahedral Amorphous Carbon Film with Different Cr Contents

Nanoindentation-Induced Plastic-Deformation Behavior of Tetrahedral Amorphous Carbon Film with Different Cr Contents

Laterally controlled ultra-low energy ion implantation using electrostatic masking

We describe the ongoing development of an ultra-low energy (ULE) ion implantation system for the implementation of a laterally controlled ion incorporation into 2D materials like graphene or transition metal dichalcogenides (TMDs). For this purpose, we use ion optics simulations to demonstrate how the generation of an electrostatic potential gradient laterally along the sample surface can be used to control the size of the implanted sample area. The simulations are complemented by experiments on the incorporation of 25eV Mn ions into thin tetrahedral amorphous carbon (ta-C) films on Si substrates. With the help of RBS measurements, it is shown how potential gradients of different strengths influence the amount of introduced Mn.

Nanoindentation-induced deformation behaviors of tetrahedral amorphous carbon film deposited by cathodic vacuum arc with different substrate bias voltages

The diamond-like carbon (DLC) thin films were deposited using a direct cathode vacuum arc (DCVA) technology at a variety of substrate bias voltages from 0 to −50 V. The cross-sectional morphologies, compositions, chemical state, and surface roughness of the prepared DLC films were investigated and highly controlled by the bias voltage. The detailed microstructure of the DLC films and interfaces were observed by transmission electron microscope (TEM). The mechanical properties of the DLC films were obtained by nano-indentation test and significantly depended on the applied bias voltages. The deformation mechanisms and crack initiation/propagation of the DLC films at different substrate bias voltage (SBV) in the nano-indentation process were analyzed by TEM and clarified by evolution models. There were three forms to release the energy induced by indenter for the DLC films with an amorphous structure, as follows: (i) the plastic deformation stemmed from the flow of a large number of C atoms; (ii) the formation and slip of the shear bands; (iii) the cracking at the position of stress concentration.

Evaluation of Anti-Adhesion Characteristics of Diamond-Like Carbon Film by Combining Friction and Wear Test with Step Loading and Weibull Analysis.

Anti-adhesion characteristics are important requirements for diamond-like carbon (DLC) films. The failure load corresponding to the anti-adhesion capacity varies greatly on three types of DLC film (hydrogen-free amorphous carbon film (a-C), hydrogenated amorphous carbon film (a-C:H), and tetrahedral hydrogen-free amorphous carbon film (ta-C)) in the friction and wear test with step loading using a high-frequency, linear-oscillation tribometer. Therefore, a new method that estimates a representative value of the failure load was developed in this study by performing a statistical analysis based on the Weibull distribution based on the assumption that the mechanism of delamination of a DLC film obeys the weakest link model. The failure load at the cumulative failure probabilities of 10% and 50% increased in the order ta-C < a-C:H < a-C and ta-C < a-C < a-C:H, respectively. The variation of the failure load, represented by the Weibull slope, was minimum on ta-C and maximum on a-C:H. The rank of the anti-adhesion capacity of each DLC film with respect to the load obtained by a constant load test agreed with the rank of the failure load on each DLC film at the cumulative failure probability of 10% obtained by Weibull analysis. It was found to be possible to evaluate the anti-adhesion capacity of a DLC film under more practical conditions by combining the step loading test and Weibull analysis.

Modification of graphene-like, hydrogenated amorphous, hydrogenated tetrahedral amorphous carbon and amorphous carbon thin films by UV-C light

The study aims to explore the effect of low fluence UV-C radiation on the structural quality of thin carbon films. We have modified single to few-layered nano-sized graphene-like films deposited by pulsed laser deposition (PLD) on ~300 nm SiO2/Si substrates and additionally different hydrogenated amorphous carbon a-C:H, tetrahedral amorphous carbon (ta-C:H) and amorphous carbon (a-C) thin carbon films. The modification was carried out by irradiation of the samples with UV-C lamps (Hg lamps λ= 254 nm wavelength and fluence of about 2 × 10−3 W/cm−2) for 5- 30 min in air-atmosphere for graphene-like and up to 60 min for thin carbon films. The irradiated graphene-like films were oriented either at about 2 arcdeg to the impinging light, i.e. the light was almost parallel to the honey-comb plane of graphene film or perpendicular to the UV-C light. The thin carbon films were treated only by a light directed perpendicularly to the films' surface for 60 min. Films were studied before and right after UV-C modification by ellipsometry or profilometry, X-ray photoelectron and Raman spectroscopies to clarify the influence of the UV-C treatment. The most pronounced influence of the UV-C irradiation on the structural quality of the films was established on a-C:H and ta-C:H films where the sp3 and the oxygen-containing radicals content decrease moderately and drops significantly, respectively. The influence of the UV-C irradiation directed almost parallel to the films’ surface on the structural quality of the graphene-like films was slightly higher than that directed perpendicularly. No significant influence on the quality of a-C films synthesized or a-C:H films annealed at high temperatures (1020–1050) °C was observed.

Improving Aluminum Ultraviolet Plasmonic Activity through a 1 nm ta-C Film.

Aluminum (Al) can actively support plasmonic response in the ultraviolet (UV) range compared to noble metals (e.g., Au, Ag) and thus has broad applications including UV sensing, displays, and photovoltaics. High-quality Al films with no oxidation are essential and critical in these applications. However, Al is very prone to fast oxidation in air, which critically depends on the fabrication process. Here, we report that by leveraging the in situ sputter etching and sputter deposition of a 1 nm tetrahedral amorphous carbon (ta-C) film on the Al nanostructures, Al plasmonic activity can be improved. The prior sputter etching process greatly reduces the oxidized layer of the Al films, and the subsequent sputter deposition of ta-C keeps Al oxidation-free. The ta-C film outperforms the naturally passivated Al2O3 layer on the Al film because the ta-C film has a denser structure, higher permittivity, and better biocompatibility. Therefore, it can effectively improve the plasmonic response of Al and be beneficial to molecule sensing, which is proved in our experiments and is also verified in simulations. Our results can enable the various applications based on plasmon resonance in the UV range.