ta-C Films Research Articles

Chemically Stable Group IV-V Transition Metal Carbide Thin Films in Hydrogen Radical Environments.

Hydrogen is a crucial element in the green energy transition. However, its tendency to react with and diffuse into surrounding materials poses a significant challenge. Therefore, developing coatings to protect system components in hydrogen environments (molecular, radicals (H*), and plasma) is essential. In this work, we report group IV-V transition metal carbide (TMC) thin films as potential candidates for protective coatings in H* environments at elevated temperatures. We expose TiC, ZrC, HfC, VC, NbC, TaC, and Co2C thin films, with native surface oxycarbides/oxides (TMO x C y /TMO x ), to H* at elevated temperatures. Based on X-ray photoelectron spectroscopy performed on the samples before and after H*-exposure, we identify three classes of TMCs. HfC, ZrC, TiC, TaC, NbC, and VC (class A) are found to have a stable carbidic-C (TM-C) content, with a further subdivision into partial (class A1: HfC, ZrC, and TiC) and strong (class A2: TaC, NbC, and VC) surface deoxidation. In contrast to class A, a strong carbide reduction is observed in Co2C (class B), along with a strong surface deoxidation. The H* interaction with TMC/TMO x C y /TMO x is hypothesized to entail three processes: (i) hydrogenation of surface C/O atoms, (ii) formation of CH x /OH x species, and (iii) subsurface C/O atom diffusion to the surface vacancies. The number of adsorbed H atoms required to form CH x /OH x species (i) and the corresponding thermodynamic energy barriers (ii) are estimated based on the change in the Gibbs free energy (ΔG) for the reduction reactions of TMCs and TMO x . Hydrogenation of surface carbidic-C atoms is proposed to limit the reduction of TMCs, whereas the deoxidation of TMC surfaces is governed by the thermodynamic energy barrier for forming H2O.

Contact fatigue property of diamond-like carbon films with different structure in cyclic impact conditions

Repeated contact with high loads can cause damage to the surface of the DLC films and thus affect their fatigue strength. This study systematically investigates the contact fatigue damage of the DLC films with different carbon structures (a-C, a-C:H, and ta-C) by macro-scale cyclic impact tests with alternating loads. The results reveal that the a-C film possesses significantly superior contact fatigue property compared to the a-C:H and ta-C films under high load. The graphitization transformation of the a-C and a-C:H films lead to a decrease in hardness and elastic modulus. The work-hardening of the ta-C films results in an increase in hardness and elastic modulus. The findings indicate that under cyclic load impact conditions, films need a combination of load support and fatigue resistance to achieve optimum lifetime, and solely increasing film hardness could be accompanied by brittle fracture and higher wear.

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.

Electrochemical performance of boron- and nitrogen-doped tetrahedral amorphous carbon

Tetrahedral amorphous carbon (ta-C) is one of the most versatile carbon electrode materials available. The incorporation of various dopants in ta-C alters the structural, electrochemical, physical, and chemical properties which are tunable for various applications. Furthermore, doped ta-C (ta-C:X) can be deposited on many different substrate materials (metals, ceramics, plastics) and geometries (micro-electrodes to ultra large areas at continuous coating lines). In this work, we present a comparison in electrochemical performance of ta-C electrodes prepared by filtered laser-arc (a physical vapor deposition process), doped with nitrogen (ta-C:N) at various levels, doped by boron (ta-C:B), and doped by boron and nitrogen together (ta-C:B:N). The ta-C:N films were doped via N2 gas in the deposition chamber, while ta-C:B and ta-C:B:N were deposited via incorporation of the dopant into the graphite cathode used for film synthesis. Films coated on silicon (Si) and titanium (Ti) substrates were compared through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The Fe(CN)63-/4− and Ru(NH3)63+/2+redox systems were used to investigate the electrochemical characteristics of each ta-C:X film type. Boron-doped diamond (BDD), glassy carbon (GC), and undoped ta-C were used as control electrodes. It was found that all ta-C:X films exhibited appropriate rates of electron transfer, wide working potential window (ca. 3.7 V in KCl), and low capacitance. Specifically, the boronated films produced high heterogenous rate constant (k), the lowest double layer capacitance (Cdl), and a consequentially smaller RC time constant than BDD and GC. Incorporating boron into undoped and nitrogenated ta-C films stabilizes the formation of sp3 bonded content during deposition leading to films with a smoother surface and increased hardness. It is shown that the B-incorporated films exhibit no compromise in their electrochemical performance despite the increase in sp3 content. Due to its versatile nature, ta-C stands out as a material with a wide range of tunable properties for various applications.

Microstructure and tribological behavior in ta-C films prepared by laser-induced filtered cathodic vacuum arc technique: Effect of charge voltage

As an important power supply parameter, charge voltage has a significant influence on the properties of deposited films. In this work, the ta-C films were fabricated by the laser-induced filtered cathodic vacuum arc technique (LI-FCVA). The effect of charge voltage on microstructure, mechanical and tribological properties of ta-C films was investigated. Although macro-particles with various sizes were clearly visible on surfaces of films, the surface roughness was declined with increase of charge voltage. The sp3-C bond ratio in films was closely dependent on the charge voltage. At 160 V charge voltage, the sp3-C bond ratio was 54.1 %, then reaching to 61.2 % at the charge voltage of 190 V. With further increasing to 250 V, the sp3-C ratio slowly down to 57.1 %. This is ascribed to the change in energy for incident carbon ions which governed by the charge voltage. The highest hardness value, 54.5 ± 2.1 GPa, was obtained at the 190 V charge voltage, which was due to the high sp3-C content and relatively high residual stress. Similarly, the film deposited at 190 V possessed both low coefficient of friction (COF) and wear rate under various load, indicating that the excellent tribological performance of film could be achieved at 190 V. In addition, the COFs for all films decreased with an increase of load. The present work revealed that the moderate charge voltage value is beneficial for the enhanced wear property of film.

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.

Probing the role of sp2-C in high-temperature tribology of a-C films by a comparative study of ta-C film and a-C film

The high-temperature tribological properties of ta-C and a-C films are comparatively studied. The ta-C film shows friction coefficients of around 0.10 at 450 °C and 500 °C, but the a-C film shows high friction coefficients above 0.5 at 450 °C. Surprisingly, the lubrication failure of the a-C film at 450 °C is not due to the failure (wear-out) of the film. Furthermore, the graphitization degree of the wear track of a-C film at 450 °C is higher than those of ta-C films at 450 °C and 500 °C. The underlying frictional mechanism is clarified, and the role of sp2-C atoms at the rubbing surfaces is highlighted. The higher sp2-C content is, the more water needs to achieve sufficient passivation.

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.

Microstructure, mechanical and tribological properties of Ti doped ta-C films deposited by a hybrid coating system

Ti doped ta-C films were deposited by a hybrid coating system with filtered cathodic vacuum arc and pulsed DC magnetron sputtering. The microstructure, mechanical and tribological properties of ta-C: Ti films were investigated. Results showed that the Ti content in the films linearly increased from 0 to 19.9 at. % by increasing the Ti target current from 0 to 2 A. Without and with small amount of Ti doing, ta-C films presented typical amorphous characteristic. TiC phase was formed in the film at Ti content of 5.5 at. %, and as the Ti content further increased, the microstructure of the films gradually transformed to nanocomposite structure, where TiC nanocrystalls were embedded in amorphous carbon matrix. The sp3 bonding fraction was decreased with increasing Ti content, accordingly, the hardness and residual stress of films also decreased with diminishing sp3 content. In addition, the Ti doping had a positive impact on the adhesion force between films and substrates. ta-C film with Ti doping content of 5.5 at. % had the lowest friction coefficient and the lowest wear rate under different friction temperatures, corresponding to 0.09 and 3.6 × 10−8 mm3/N·m at room temperature, and 0.25 and 22.5 × 10−8 mm3/N·m at 400 °C, respectively, which was mainly attributed to the relative high hardness, lower residual stress, good adhesion of the 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.

Hybrid HiPIMS + controlled pulsed arc for deposition of hard coatings

A new hybrid PVD deposition system was developed based on the combination of HiPIMS pulsed glow discharge and pulsed cathodic arc discharge (HiPIMS+ARC). The presence of cathodic arc discharge in the HiPIMS is a typical situation when the glow discharge randomly transits to an arc discharge during the active part of the pulse cycle. In these typical cases, the transition appears randomly and cannot be controlled. The new hybrid HiPIMS+ARC discharge works with modified pulsed high power supplies which allow the initiation of pulsed arc discharge during HiPIMS pulse at the defined time. The initiated arc during the active part of the HiPIMS pulse is quenched at the end of this active part of the pulse when the magnetron cathode is disconnected from the negative voltage of the power supply. Several modifications with HiPIMS sequence of active pulses with arc were tested as well. The hybrid HiPIMS+ARC was applied for the deposition of ta-C thin films by use of a graphite magnetron target. Carbon ta-C films were deposited on silicon substrates at different pulse configurations. Deposited films were analyzed by DUV Raman scattering and the ratio of sp3/sp2 bonds was qualitatively determined. Microhardness and modified Young's modulus was analyzed by microindenter. The surface morphology was observed by optical microscopy and the presence of microdroplets was analyzed for particular conditions. Achieved ta-C film parameters for this deposition source were compared with other deposition methods.

Growth of self-aligned nonlayered TaC nanosheets on liquid copper by a solid phase diffusion strategy

Compared with layered 2D materials, nonlayered 2D materials combine atomic thickness and high-activity surface, leading to many unique properties. Herein, a solid phase diffusion strategy is raised to obtain self-aligned nonlayered TaC single crystals on liquid copper. Because of the immiscibility of Cu and Ta, a β-Ta nanolayer with a thickness of ∼7 nm would form on the surface of liquid copper by a long-time solid phase diffusion process. After introducing CH4, the β-Ta nanolayer could be converted into 10 μm TaC single crystals with a thickness of 10 nm. With the extension of the reaction time, TaC single crystals would form an ordered self-aligned state spontaneously under the action of lateral capillary force. Finally, the self-aligned TaC nanosheets are assembled into large-size TaC films with a thickness of 10 nm. This strategy paves a new way to grow large-size nonlayered 2D transition metal carbides.

Adhesion and corrosion of Al–N doped diamond-like carbon films synthesized by filtered cathodic vacuum arc deposition

Diamond-like carbon (DLC) films (thickness: ∼118 nm) doped with Al and N on AISI 4140 steel were synthesized using a filtered cathodic vacuum arc technique. The adhesion and corrosion of the ta-C, ta-C:N, ta-C:Al, and ta-C:Al:N films were examined in terms of structure and bonding. X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy revealed that the Al-doped and Al and N co-doped films had lower densities and internal stresses than the other films, as indicated by their higher ID/IG ratios and graphite cluster size (La) and lower sp3 relative fractions. All DLC films exhibited improved corrosion resistance as compared to AISI 4140, as indicated by their decreased corrosion rate and increased corrosion potential and polarization resistance. The corrosion resistances of ta-C:Al:N and ta-C:Al films were high owing to the combined effect of sp3 C–N and Al2O3, as confirmed by near-edge X-ray absorption fine structure spectroscopy and XPS. All doped DLC films exhibited improved adhesion performance compared to the ta-C film owing to their improved lubrication performance (increased sp3-hybridized C–N bonds) and improved toughness (formation of Al2O3).

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.

Structure and properties of ta-C films prepared by vacuum cathodic arc with an unbalanced external electromagnetic field

ta-C thin films were deposited by vacuum cathodic arc technology with an unbalanced external electromagnetic field. The intensity of the magnetic field was adjusted by changing coil current and its effect on structure and properties of ta-C films was studied. FEM simulation results showed that as coil currents increased, the transverse, normal magnetic field and the magnetic intensity from arc cathode to substrates holder gradually increased. The UBM system increases the magnetic field intensity of the entire vacuum chamber by mainly improving the normal magnetic flux density near the arc cathode. The surface morphology, microstructure, tribo-mechanical properties of ta-C films were analyzed by SEM, Raman spectrometer, XPS, 3D surface profiler, Nano-indentation and ball-on-disk tribometer. Results showed that as the coil current increased, the average target voltage increased from 21.14 V to 25.97 V and target power rose from 845 W to 1043 W. Changes of magnetic field affected the plasma in vacuum chamber and the number of ions reaching the substrates, which had significant influences on film structure and properties. With a proper magnetic field (at coil current of 4A), ta-C films perform the highest hardness of 58.8 GPa, and the highest sp 3 content of about 71% with the lowest ratio of I D /I G (0.38). During the wear process, the transfer layer was formed by oxidation and graphitization of contact surfaces. High hardness provided good support for transfer film, which leads to a low wear rate.

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.