Nanoindentation-induced deformation behaviour of tetrahedral amorphous carbon coating deposited by filtered cathodic vacuum arc

The nanoindentation-induced deformation behaviour of a ta-C (tetrahedral amorphous carbon) coating deposited on to a silicon substrate by a filtered vacuum cathodic vapour arc technique was investigated. The 0.17-μm-thick ta-C coating was subjected to nanoindentation with a spherical indenter and the residual indents were examined by cross-sectional transmission electron microscopy. The hard (~ 30 GPa) ta-C coatings exhibited very little localized plastic compression, unlike the softer amorphous carbon coatings deposited by plasma-assisted chemical vapour deposition. However, neither through-thickness cracks nor delamination was observed in the coating for the loads studied. Rather, the silicon substrate exhibited plastic deformation for indentation loads as low as 10 mN and at higher loads it showed evidence of both phase transformation and cracking. These microstructural features were correlated to the observed discontinuities in the load-displacement curves. Further, it was observed that even a very thin coating can modify the primary deformation mechanism from phase transformation in uncoated Si to predominantly plastic deformation in the underlying substrate.     

Deposition of iron containing amorphous carbon films by filtered cathodic vacuum arc technique

Iron containing amorphous carbon (a-C:Fe) films have been deposited with an Fe/graphite composite target with different Fe content by filtered cathodic vacuum arc (FCVA) technique. X-Ray induced photoelectron spectroscopy (XPS) was used to analyze the Fe content in the films. Micro-Raman spectroscopy was employed to characterize the structural changes of a-C:Fe films. The properties of the a-C:Fe films such as the intrinsic stress, morphology and roughness investigated by the profiler, atomic force microscope (AFM). The XPS results show that there exists small amount of oxygen in the form of FeO in the films and the Fe fraction in the films is always larger than that in the target. Compared with pure amorphous carbon films the intrinsic stress was effectively reduced by incorporating Fe into the films, and decreases with increasing Fe content. As increasing the Fe content, the clusters in the films become finer and the roughness increases The studies of Raman spectra show that the positions of G peak and D peak shift to low and high wavenumbers, respectively, and the ratio of the intensity of D and G peaks increases with an increase in Fe content, that suggests that the sp²-bonded carbon and the size of the sp²-bonded cluster increases with an increase in the Fe content.     

Techniques for filtering graphite macroparticles in the cathodic vacuum arc deposition of tetrahedral amorphous carbon films

Filtered cathodic vacuum arc (FCVA) deposition has been found to be a reliable technique for the production of high quality tetrahedral amorphous carbon films (ta-C). These coatings can be used as protective coatings for different applications ranging from cutting tools to human hip joint prosthesis. The FCVA technique is widely used in different laboratories around the world with somewhat different technical implementations. A serious disadvantage in the FCVA technique is the graphite particles that are emitted from the solid graphite cathode during the arc-discharge. A variety of different techniques exist to diminish their production and transport. However, some of the magnetic filtering designs that are used to reduce the macroparticle transport into the substrate do not work well with high melting point cathode materials such as graphite. Although the influence of graphite particles for the ta-C coating performance in some applications is controversial, many applications demand that the produced ta-C film is practically particle-free. This is especially important in corrosion resistance, electrical and optical applications. In this paper an introduction to different FCVA devices is presented. Different magnetic filtering designs together with control techniques for macroparticle generation have been reviewed and their advantages and disadvantages in the plasma transport and particle filtering efficiency have been discussed.     

Surface energy of metal containing amorphous carbon films deposited by filtered cathodic vacuum arc

Metal containing amorphous carbon (a-C:Me) films including a-C:Al, a-C:Ti, a-C:Ni, a-C:Si were prepared by the filtered cathodic vacuum arc (FCVA) technique with metal-carbon (5 at.% metal) composite targets. The substrate bias ranging from floating to 1000 V was applied. The wettability of the films was examined using the VCA Optima system from AST Products, Inc. Three types of liquid with different polarities were used to study the surface energy changes of the films. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition and chemical state of the films. Atomic force microscopy (AFM) was employed to characterize the morphology and roughness of the films. The contact angle of the a-C:Me films remains relatively constant with different substrate bias. The Al containing films show the highest contact angle with water, which reaches as high as 101°. The Si containing films show the lowest contact angle approximately 64°. The contact angles of Ni and Ti containing films are approximately 80°, 97°, respectively. The harmonic-mean method was used to calculate the polar and depressive component of the surface energy. The absorption of oxygen on the surface plays an important role on the polar component of the a-C:Me films. The formation of AlO and TiO bonds is responsible for their lower polar component. The metal state Ni results in higher polar component. However, the SiO bond is contributed to the high polar component of a-C:Si films. As all films are atomic scale smooth, the RMS roughness is below 0.5 nm, the roughness does not have obvious effect on the surface energy.     

Properties of tetrahedral amorphous carbon prepared by vacuum arc deposition

The structural, optical, electrical and physical properties of amorphous carbon deposited from the filtered plasma stream of a vacuum arc were investigated. The structure was determined by electron diffraction, neutron diffraction and energy loss spectroscopy and the tetrahedral coordination of the material was confirmed. The measurements gave a nearest neighbour distance of 1.53 Å, a bond angle of 110 and a coordination number of four. A model is proposed in which the compressive stress generated in the film by energetic ion impact produces pressure and temperature conditions lying well inside the region of the carbon phase diagram within which diamond is stable. The model is confirmed by measurements of stress and plasmon energy as a function of ion energy. The model also predicts the formation of sp²-rich materials on the surface owing to stress relaxation and this is confirmed by a study of the surface plasmon energy. Some nuclear magnetic resonance, infrared and optical properties are reported and the behaviour of diodes using tetrahedral amorphous carbon is discussed.     

Heat treatment of tetrahedral amorphous carbon films grown by filtered cathodic vacuum-arc technique

Tetrahedral amorphous carbon (ta-C) is a potential low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. The thermal stability of tetrahedral amorphous carbon (ta-C) films has been investigated by heat treatment of the films at temperatures from room temperature to 450°C in high vacuum, low vacuum and oxygen ambient. It was found that heat treatment in oxygen ambient leads to a much more prominent variation in film thickness, stress and hardness than in both low and high vacuum. Raman studies also show an increase of the G-band frequency to higher values, an increase of the integrated intensity ratio and a narrowing of the G bands for films annealed in oxygen ambient with increasing temperature. By contrast, ta-C films exhibit a high resistance to degradation during treatment in low and high vacuum. They sustain their structure, thickness, stress and hardness for temperatures up to 400°C.     

Nitrogenated carbon films deposited using filtered cathodic arc

Nitrogenated carbon films were deposited on various substrates using filtered cathodic arc. Non-uniformity of the film thickness was less than 5% over a 15 cm diameter area. Mechanical, optical (refraction index, extinction coefficient versus wavelength) and electrical properties were investigated as a function of nitrogen flow rate. Deposited coatings demonstrated high hardness of 40–65 GPa, Young's modulus 200–285 GPa, excellent elastic recovery, high critical pressure for scratch formation, and surface smoothness. While the hardness showed a relatively small decrease with nitrogen flow increase, the stress decrease was more significant (8–3.8 GPa). Extremely low wear rates were observed, even at high contact pressures, and no substantial debris was detected indicating that carbon is oxidized during wear. Clear correlation was found between transparency, electrical resistivity and stress of the films. Transparency and resistivity showed a significant rise with an increase of stress. An explanation of the film properties is based on the assumption that the basic characteristics of the deposited films were determined by the relative proportion of two three dimensional complementary type of bonds; the tetrahedral sp³ bonds leading to stiff networks, and the trigonal sp² arrangments close to fullerene-like, or nanotube-like, structures.     

Deposition of carbon nitride films by filtered cathodic vacuum arc combined with radio frequency ion beam source

Atomically smooth carbon nitride films were deposited by an off-plane double bend filtered cathodic vacuum arc (FCVA) technique. A radio frequency nitrogen ion source was used to supply active nitrogen species during the deposition of carbon nitride films. The films were characterized by atomic force microscopy (AFM), XPS and Raman spectroscopy. The internal stress was measured by the substrate bending method. The influence of nitrogen ion energy (0–1000 eV) on the composition, structure and properties of the carbon nitride films was studied. The nitrogen ion source greatly improves the incorporation of nitrogen in the films. The ratio of N/C atoms in the films increases to 0.40 with an increase in the ion beam energy to 100 eV. Further increase in the ion beam energy leads to a slight decrease in the N/C ratio. XPS results show that nitrogen atoms in the films are chemically bonded to carbon atoms as C---N, C=N, and CN bonds, but most of nitrogen atoms are bonded to sp² carbon. The increase in nitrogen ion energy leads to a decrease in the content of nitrogen atoms bonded to sp² carbon, and an increase in the content of nitrogen atoms bonded to sp³ and sp¹ carbon. Raman spectra indicate an increase in the sp² carbon phase in carbon nitride films with an increase in nitrogen ion energy. The increase in sp² carbon fraction is attributed to the decrease in internal stress with increasing nitrogen ion energy.     

Preparation and microstructure properties of tetrahedral amorphous carbon films by pulsed laser deposition using camphoric carbon target

The properties of tetrahedral amorphous carbon (ta-C) films grown by pulsed laser deposition (PLD) using camphoric carbon (CC) target and their respective effects of diamond percentages by weight in the target (Dwt.%) are discussed. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Visible-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the Dwt.% noticeably modified the sp³ bonds content and the morphology of the ta-C films. The optical gap (E_g) and electrical resistivity (ρ) increase with Dwt.% up to 1.6 eV and 5.63×10⁷ (Ω cm), respectively, for the ta-C films deposited using target with higher of 50 Dwt.%. We found that the Dwt.% has modified the surface morphological, structural, bonding and physical properties of the camphoric carbon films.     

Cathodoluminescence of fluorine doped amorphous carbon nanoparticles deposited by a filtered cathodic arc plasma system

The fluorine doped amorphous carbon nanoparticles (a-C:F NPs) films with sizes 50-100 nm were deposited on polyethylene terephthalate in an atmosphere of CF₄ by a 90°-bend magnetic filtered cathodic arc plasma system. The surface morphology of a-C:F NPs films was observed by field emission scanning electron microscope and atomic force microscope. The microstructure and chemical bonding nature of the a-C:F NPs films were investigated by Raman, X-ray diffraction and X-ray photoelectron spectroscopy. This work presents cathodoluminescence (CL) spectra of a-C:F NPs films obtained at 1.9-2.4 eV and verifies luminescence from a-C:F NPs films in the visible region. The incorporation of fluorine into the carbon network results in orange emission (∼2.03 eV) due to the transitions between fluorine-related electron levels and σ* states, and the red emission (∼1.97 eV) results from the recombination of carriers in the valence π and conduction π* states. The peak at ∼2.10 eV may result from the defects of the structures in a-C:F NPs films.     

Electrical properties of a-C: Mo films produced by dual-cathode filtered cathodic arc plasma deposition

Molybdenum-containing amorphous carbon (a-C:Mo) thin films were prepared using a dual-cathode filtered cathodic arc plasma source with a molybdenum and a carbon (graphite) cathode. The Mo content in the films was controlled by varying the deposition pulse ratio of Mo and C. Film sheet resistance was measured in situ at process temperature, which was close to room temperature, as well as ex situ as a function of temperature (300–515 K) in ambient air. Film resistivity and electrical activation energy were derived for different Mo and C ratios and substrate bias. Film thickness was in the range 8–28 nm. Film resistivity varied from 3.55 × 10^− 4 Ω m to 2.27 × 10^− 6 Ω m when the Mo/C pulse ratio was increased from 0.05 to 0.4, with no substrate bias applied. With carbon-selective bias, the film resistivity was in the range of 4.59 × 10^− 2 and 4.05 Ω m at a Mo/C pulse ratio of 0.05. The electrical activation energy decreased from 3.80 × 10^− 2 to 3.36 × 10^− 4 eV when the Mo/C pulse ratio was increased in the absence of bias, and from 0.19 to 0.14 eV for carbon-selective bias conditions. The resistivity of the film shifts systematically with the amounts of Mo and upon application of substrate bias voltage. The intensity ratio of the Raman D-peak and G-peak (I_D/I_G) correlated with the pre-exponential factor (σ₀) which included charge carrier density and density of states.     

Ultraviolet and visible Raman analysis of thin a-C films grown by filtered cathodic arc deposition

Amorphous carbon thin films with a wide range of sp² fraction from 20 to 90% grown by filtered cathodic arc deposition have been examined by ultraviolet (UV) at 325 nm and visible Raman spectroscopy at 457 nm excitation wavelength. The comprehensive study of behaviour of G, D and T band with sp²/sp³ content has been carried out. The upwards shift of the G peak with sp³ content was observed for both excitation wavelengths. It was also found that the I(D)/I(G) ratio decreases with sp³ content for UV and visible excitations, and for high sp³ content I(D)/I(G) tends to zero. The dispersion of the G peak is also investigated in this work as a function of sp² content.     

Temperature sensibility of amorphous diamond films prepared by filtered arc

The low temperature sensibility and the thermal stability in air of amorphous diamond (a-D) films deposited by filtered arc have been investigated with visible Raman spectroscopy. The films cooled in a range of room temperature to − 190 °C by a liquid nitrogen pump were in-situ measured on a Linkam stage. The films for thermal stability were heated in a furnace. It has been found that the films are thermally very stable in air. They can hold their hardness up to 400 °C and their structure up to 500 °C. However, complete material loss occurs if the temperature is continuously raised. Raman measurements show a shift of the G-peak frequency to higher values with increasing temperature. The films are not sensitive to low temperature and remain stable as temperature is decreased down to − 190 °C.     

The carbon nitride films prepared at various substrate temperatures by vacuum cathodic arc method

Carbon nitride films have been grown by vacuum cathodic arc method in the substrate temperature range of 100–500 °C. The bonding structure of the films was investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and infrared (IR) spectroscopy. With increasing substrate temperature, the films indicate various characteristics. At 100 °C, it can be described as a network similar to DLC in which aromatic sp²C phase is cross-linked by sp³C phase. Between 200 and 400 °C, with increasing substrate temperature the films become graphitized and the sp²CN phase increases, meanwhile the non-aromatic sp²CN phase appears at the edges of aromatic clusters in planar position as well as in out-of-planar regions. While at 500 °C the non-aromatic sp²CN phase almost comes to the same level as the aromatic sp²CN phase. So in the network of the film the aromatic sp²C phase is cross-linked by the non-aromatic sp²C phase. Based on the variation of the microstructure of the films, a comprehensive assignment pattern for the XPS C1s and N1s at different substrate temperature is proposed. In addition, the interpretation of p electron band in valence band spectra at various substrate temperatures is also discussed.     

Properties and tool performance of ta-C films deposited by double-bend filtered cathodic vacuum arc for micro drilling applications

This paper describes the results of the application of ta-C films to micro drilling operation for deep and small machining boreholes. Tetrahedral amorphous carbon (ta-C) films were successfully deposited on WC-Co substrates by a double-bend filtered cathodic vacuum arc (FCVA) system. The structure, mechanical and tribological properties of both pure ta-C and ta-C incorporated argon gas (ta-C:Ar-flow) films were systematically investigated. And then, high-speed through-hole drilling tests were performed on the PCB (printed circuit board) workpiece to investigate the machining performance of ta-C coated micro drills. The experimental results show that the ta-C:Ar-flow (2 sccm) coated micro drill has excellent microstructure, microhardness, and friction coefficient properties and represents the optimal coatings for micro drilling applications.     

Optimization of wear–corrosion properties of a–C:N films using filtered cathodic arc deposition

Applying a 4-factor (negative substrate bias voltage, arc current, Ar flow and N₂/Ar ratio) 3-level (L9) orthogonal array design using the Taguchi method to optimize the wear–corrosion properties of a–C:N in Filtered Cathodic Vacuum Arc (FCVA) deposition was investigated. The influence of N₂/Ar ratio, over the range of 1/6 to 2/3, is shown by the increase in the following: the wear–corrosion current density changes from 36 to 78 nA/cm², and the friction coefficient changes from 0.042 to 0.086. A higher N₂/Ar ratio induces the a–C:N film's structure to reduce the sp³ content (sp² rich) which implies the formation of loose networks due to N₂ incorporation. Based on the analysis, the N₂/Ar ratio is the most significant control factor as its percentage contribution is 69% for wear–corrosion current density and 32% for frictional coefficient, respectively. There is a tendency for a higher friction coefficient by increasing the following three factors; level of negative substrate bias voltage, Ar flow and N₂/Ar ratio. We observed an increase of N content and sp² bonding which correlated to the decrease of hardness and an also rougher a–C:N surface with increasing the factors level. Over the design range of 4 factors, the optimum wear–corrosion properties and friction coefficient obtained from the Taguchi analysis were obtained using a 20 V negative substrate bias, 30 A arc current, 30 sccm Ar flow and 1/6 N₂/Ar ratio respectively. Overall, the results show an optimum design when compared with the current design as it was able to reduce 84% of the wear–corrosion current density (35.7 down to 5.6 nA/cm²) and 58% of the frictional coefficient (0.060 down to 0.025), respectively.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Diamond-like carbon overcoat for TFMH using filtered cathodic arc system with Ar-assisted arc discharge

The deposition system described for sub-30 Å and thicker carbon (ta-C) overcoat that includes two RF ion beam guns and Filtered Cathodic Arc (FCA) module mounted on a single vacuum chamber. The system is capable of flattening the Thin Film Magnetic Heads (TFMH) surface by ion beam etching; smoothing scratches, trenches, steps on boundaries of different materials, and enhancing the adhesion by ion assisted ion beam sputtering. It provides the highly controllable deposition of carbon using an FCA module with Ar-assisted arc discharge. Low-level particulates are achieved on the deposited film surface (< 5/cm² ). It was shown that crucial impact on filtering the particles with size < 1 μm has the electrostatic field distribution across the plasma guide that can be controlled by duct bias. Mechanical and electrical properties, optical and Raman spectra of ta-C films were investigated as a function of Ar flow in the arc discharge area. At Ar flow rates 0–12 sccm, stress of the films was varied in a range 2.9–7.5 GPa while hardness and Young's Modulus stayed in ranges of 45–60 GPa, and 230–300 GPa, respectively. Density of the obtained films was greater than 2.8 g/cm³. Optical absorption and electrical conductivity of ta-C films showed a significant rise while stress came down with Ar flow. Raman G-peak was higher for ta-C films with lower stress and shifted to lower energy. The low stress films versus high stress films showed a few orders reduced electrical resistance and anisotropy of specific resistance with respect to substrate plane: ρ_⊥ ≫ ρ_∥. In situ ellipsometric control of growing film thickness was implemented on the system. Run-to-run standard deviation was less than 1 Å for 20–25 Å thick films. High corrosion resistance of FCA coatings was exhibited. The impact of Ar gas–carbon plasma interaction on the deposition conditions and microstructure of ta-C films was discussed.     

Cathodoluminescence of the a-C:N films deposited by a filtered cathodic arc plasma system

Amorphous carbon nitride (a-C:N) films were successfully synthesized on silicon using a 90°-bend magnetic filtered cathodic arc plasma (FCAP) system. ESCA analysis demonstrated that the N/C ratios reached 1.08. Many investigations on photoluminescence (PL) spectra of diamondlike carbon films have been performed, but cathodoluminescence (CL) spectra have seldom been discussed. This work investigated a-C:N films using the CL spectra at 300 K. This work presents CL spectra of a-C:N films obtained at 1.5–3.5 eV and verifies luminescence from the a-C:N films in the visible region. The most prominent luminescence in the CL spectra from the a-C:N films has two peaks centered ∼2.67 eV (blue light) and ∼1.91 eV (red light), but the a-C film yielded only the band at ∼1.91 eV. These optical emissions are intrinsic and extrinsic to the a-C:N film and are sensitive to the content of nitrogen.