RF deposition of soft hydrogenated amorphous carbon coatings for adhesive interfaces on highly elastic polymer materials

Developing soft and elastic coating materials is one of the future challenges in research on new coating materials for vacuum deposition processes. This development direction is in its infancy and fully contrary to the thin film research mainstream. Such coatings - tailor-made with gradients in hardness and elastic modulus - could work as adhesive and load-supporting layers on polymers bridging the properties from soft substrate to the stiff, wear-resistant hard top coating.In this work we used the approach of a chemical vapour deposition process with plasma assistance from an unbalanced RF (13.56 MHz) powered magnetron sputtering cathode in planar parallel plate arrangement. The characterization of the amorphous hydrogenated and polymer-like carbon films showed a high influence of the used precursor gas (acetylene, butane) on hardness and elasticity. The elastic moduli were found to be between 2 and 35 GPa for the fully amorphous films with a-C:H structure. Specific growth structures were found in HR-TEM imaging of the amorphous coatings. All coatings adhere strongly on a rubber-like polymer (thermoplastic polyurethane).     

Microscopic study on the interfacial strength of hydrogenated amorphous carbon coating systems

The microstructure and the adhesion strength of two hydrogenated amorphous carbon (a-C:H) coatings on steel substrates with a Cr adhesion layer were investigated by Rockwell C testing, Focused Ion Beam and High Resolution Transmission Electron Microscopy (HRTEM).Slight variation of the coatings in the ramp layer between Cr and a-C:H resulted in a good or poor adhesion behaviour. Both coatings exhibited in the ramp layer a quasi amorphous matrix structure with short-range ordered substructures. No crystalline chromium carbides could be detected in these regions. Focused investigations of the chemical gradients with Energy Filtered TEM (EFTEM) and Energy Dispersive X-Ray (EDX) spectrometry revealed a clear difference in the chemical composition of the ramp layers. Smooth gradients together with a nanocomposite structure seem to provide a high stability of the ramp layer against delamination.     

Superlow friction behavior of Si-doped hydrogenated amorphous carbon film in water environment

Si-doped hydrogenated amorphous carbon (a-C:H:Si) film was prepared using hybrid radio frequency plasma-enhanced chemical vapor deposition (R.F. PECVD) and non-balanced magnetron sputtering deposition technique. The microstructure of the film was characterized by means of Raman spectrometry, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), while its friction behavior in water environment was investigated using a ball-on-disc tribometer. Results show that the a-C:H:Si film has typical diamond-like characteristics, and Si doped with a relative atomic concentration of about 3.9% in the film mainly exists in the form of Si, SiC, and SiO₂, while it shows a superlow friction coefficient of about 0.005 in water environment when sliding against Si₃N₄ ball. The superlow friction behavior of the hybrid diamond-like-carbon (DLC) film could be attributed to the formation of a tribochemical reaction film and boundary lubrication layer mainly consisting of colloidal silica generated from tribochemical reactions between silicides and water molecules.     

Properties of hydrogenated amorphous carbon films deposited by PECVD and modified by SF6 plasma

Hydrogenated amorphous carbon (a-C:H) films were grown at room temperature on glass and polished silicon substrates using RF-PECVD (Radio-Frequency Plasma Enhanced Chemical Vapor Deposition). Plasmas composed by 30% of acetylene and 70% of argon were excited by the application of RF signal to the sample holder with power ranging from 5 to 125 W. After deposition, the films were submitted to SF₆-plasma treatment for 5 minutes. SF₆ plasmas were generated at a pressure of 13.3 Pa by a RF power supply operating at 13.56 MHz with the output fixed at 70 W. The resulting films were characterized in terms of their molecular structure, chemical composition, surface morphology, thickness, contact angle, and surface free energy. During the SF₆ plasma treatment, fluorine species were incorporated in the film structure causing chemical alterations. The interaction of chemical species generated in the SF₆ plasmas with surface species was responsible for the decrease of the film thickness and surface energy, and for the increase of the film roughness and hydrophobicity.     

Characterization of silicon-stabilized amorphous hydrogenated carbon

The characterization of a modified form of diamondlike carbon, referred to as silicon-stabilized amorphous hydrogenated carbon (Si-AHC), is described. Auger electron spectroscopy, electron probe microanalysis, solid-state nuclear magnetic resonance, Raman scattering, nuclear reaction analysis, and optical absorption spectroscopy were used to determine chemical composition and to monitor structural changes due to the addition of silicon. Hardness, density, residual stress, and tribological properties of AHC and Si-AHC are compared. The Si-AHC coatings show much lower stress, a friction coefficient that is insensitive to moisture, an increase in optical gap compared to AHC, improved thermal stability, and equivalent hardness and wear. Some automotive applications are discussed.     

Structure and blood compatibility of tetrahedral amorphous hydrogenated carbon formed by a magnetic-field-filter plasma stream

Tetrahedral amorphous hydrogenated carbon (ta-C:H) films with various substrate bias voltages were prepared using a magnetic-field-filter plasma stream deposition system. The microstructural and optical properties were studied using ellipsometric spectra. The refractive index n of each sample was obtained by simulating their ellipsometric spectral using Tauc-Lorentz oscillator model, and then the relative sp³ C ratio of each sample was calculated using Bruggeman effective medium approximation. The sp³ C fraction of each sample was quantified by using electron energy-loss spectroscopy (EELS). The blood compatibility of the samples was evaluated by tests of platelet adhesion, kinetic clotting time and thrombin time. The quantity and morphology of the adherent platelets on the surface of these samples were investigated using scanning electron microscopy. Results show that the spectroscopic ellipsometry is a helpful method to evaluate the sp³ carbon fraction of the carbon films. The substrate bias voltage has an obvious effect on sp³ content and blood compatibility of ta-C:H films. The sample prepared with substrate bias voltage of − 20 V showed the best blood compatibility. A simple bio-physical hypothetical model was proposed to explain the experiment results.     

Si-W-Mo coating for SiC coated carbon/carbon composites against oxidation

In order to prevent carbon/carbon (C/C) composites from oxidation at 1773 K, a Si-W-Mo coating was prepared on the surface of SiC coated C/C composites by a simple pack cementation technique. The microstructures and phase composition of the as-received multi-coating were examined by SEM, XRD and EDS. It was seen that the compact multi-coating was composed of α-SiC, Si and (W_xMo_1 − x)Si_2. Oxidation behaviour of the SiC/Si-W-Mo coated C/C composites was also studied. After 315 h oxidation in air at 1773 K and thermal cycling between 1773 K and room temperature for 17times, no weight loss of the as-coated C/C composites was measured. The excellent anti-oxidation ability of the multi-coating is attributed to its dense structure and the formation of the stable glassy SiO₂ film on the coating surface during oxidation.     

A MoSi2-SiC-Si oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a dense coating has been produced by a two-step pack cementation technique. XRD and SEM analysis shows that the as-obtained coating was composed of MoSi₂, SiC and Si with a thickness of 80-100 μm. The MoSi₂-SiC-Si coating has excellent anti-oxidation property, which can protect C/C composites from oxidation at 1773 K in air for 200 h and the corresponding weight loss is only 1.04%. The weight loss of the coated C/C composites is primarily due to the reaction of C/C substrate and oxygen diffusing through the penetration cracks in the coating.     

Mechanical properties and antibacterial activity of copper doped diamond-like carbon films

A hydrogenated diamond-like carbon (a-C:H) with a copper dopant (Cu/a-C:H) was deposited on glass substrates using a combined radio-frequency plasma and magnetron sputtering deposition process under various Ar/CH₄ gas mixtures. The effects of the Cu content on the structure and properties of the a-C:H matrix were investigated using X-ray diffraction (XRD), Raman transmission electron microscopy, high-resolution transmission electron microscopy (TEM), and nano-indentation. The bacterial activity of a Cu/a-C:H film was evaluated with Escherichia coli (E. coli). TEM images and XRD spectra demonstrated that composite films containing copper nanoparticles embedded in the a-C:H were deposited on the glass substrates. The Raman spectra showed the structure of a-C:H film was substantially changed by the incorporation of Cu. The Cu/a-C:H films offered superior antibacterial activity against E. coli indicating that they could be suitable for surface coatings in cardiovascular applications.     

Synthesis of hydrogenated amorphous carbon films with a line type atmospheric-pressure plasma CVD apparatus

In this study, hydrogenated amorphous carbon (a-C:H) films were synthesized on polyethylene terephthalate (PET) substrates with a line type atmospheric-pressure plasma chemical vapor deposition (CVD) apparatus. Acetylene and nitrogen mixture gas was used for process gas and a-C:H films having two different thickness were synthesized with varying the C₂H₂ mixing rates. We investigated the effect of chemical bonding structure on the ultraviolet ray shielding property of the films. The deposition rate increased as a function of the C₂H₂ mixing rates. Increasing the C₂H₂ mixing rate from 2.5 to 10% caused an increase in the deposition rates from 13 to 22 nm/s. The deposition rate under atmospheric pressure was faster than that of low-pressure plasma CVD (~ 5–16 nm/s). Ultraviolet transmittance of 2 μm thick a-C:H film synthesized at the C₂H₂ mixing rate of 10% on 100 μm thick PET substrates ranged from 0 to 3% as the UV wavelength ranged from 310 to 400 nm, while that of uncoated PET substrates ranged from 0 to 80%. From the result of X-ray photoelectron spectroscopy (XPS) analysis, the component of sp²-hybridized C, such as CC and CO bonds increased as the C₂H₂ mixing rate and thickness of a-C:H films increased. A decrease of sp³-hybridized C, such as CC and CO bonds and an increase of sp²-hybridized C bonds lead to an improvement of ultraviolet ray shielding property.     

Corrosion and tribo-corrosion enhancement of SS301 and Ti–6Al–4V substrates by amorphous hydrogenated SiN/SiC/a-C multilayer coating architecture

Amorphous hydrogenated silicon-based multilayer coatings were deposited on 301 stainless steel (SS301) and Ti–6Al–4V alloy substrates using plasma enhanced chemical vapor deposition (PECVD), in order to integrate the advantages of the respective layers. Corrosion and tribo-corrosion behaviors of the complete coating/substrate system on different substrates were investigated. The SiN/SiC double-layer coating substantially improved the corrosion resistance of the metals: For SS301, the corrosion current, i_corr, was reduced by more than three orders of magnitude, and the breakdown voltage was increased from 0.34 to 1.37 V. For Ti–6Al–4V, the i_corr was decreased by a factor of ~ 50. Particularly, the Ti–6Al–4V/SiN/SiC multilayer system exhibited excellent anti-corrosion properties according to potentiodynamic polarization measurements, due to the superior corrosion resistance of both the Ti–6Al–4V substrate and the silicon-based coatings. Further enhancement of the tribo-corrosion resistance has been achieved by applying an amorphous hydrogenated carbon (a-C) coating as a top layer in the three-layer system. In the tribo-corrosion test in 1 wt.% NaCl solution, the SiN/SiC/a-C coating reduced the wear rate and the friction coefficient by a factor of ~ 175 and ~ 4, respectively, compared with the bare Ti–6Al–4V. The Ti–6Al–4V/SiN/SiC/a-C multilayer system integrates in synergy the advantages of the respective layers, and its versatility makes it a particularly attractive candidate for applications in different harsh working environments.     

SiC nanowire-toughened SiC-MoSi2-CrSi2 oxidation protective coating for carbon/carbon composites

To prevent carbon/carbon (C/C) composites from oxidation, a dense SiC nanowire-toughened SiC-MoSi₂-CrSi₂ multiphase coating was prepared by the two-step technique composed of chemical vapor deposition (CVD) and pack cementation. The coatings were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). SiC nanowires could decrease the dimension of cracks and improve the oxidation and thermal shock resistance of SiC-MoSi₂-CrSi₂ multiphase coating. Oxidation test shows that, after introducing SiC nanowires, the weight loss of the coated sample can be reduced from 1.06% to 0.64% after oxidation at 1773 K for 155 h and decreased from 6.92% to 3.42% after thermal cycling between 1773 K and room temperature for 30 times.     

Effects of plastic strain of diamond-like carbon coated stainless steel on the corrosion behavior in simulated body fluid environment

We have investigated the effect of plastic deformation of diamond-like carbon (DLC) coated and uncoated stainless steel on the corrosion resistance in a simulated body fluid environment to measure its protective efficiency as a biomedical coating material. We deposited the DLC film on 304 stainless steel specimens by radio frequency plasma assisted chemical vapor deposition(R.F.-PACVD) method, followed by a tensile test to apply plastic strain on the coated specimen. Corrosion behavior in the simulated body fluid environment was studied by a potentiodynamic polarization test. As the tensile deformation progressed, the cracks of the film were observed to be perpendicular to the tensile axis. Further deformation increased both cracking and the spallation. Estimated porosity and corrosion current density increased, and thus the protective efficiency decreased at a strain of 2%. In spite of the degradation, the anti-corrosion properties were significantly improved relative to the uncoated stainless steel. However, a significant increase in porosity and corrosion current density was observed at a strain of 4%. This study showed that increasing the thickness of the Si interlayer of film improved the corrosion resistance with reduction of spallations and cracks.     

Effects of nickel coating thickness on electric properties of nickel/carbon hybrid fibers

In this work, nickel/carbon hybrid fibers were prepared by the electrolytic plating on carbon fibers in order to improve electric conductivity of the carbon fibers; the effects of nickel content and coating thickness on the electric conductivity of the fibers were studied. The structural properties and surface morphologies of the hybrid fibers were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The electric conductivity of the fibers was measured using a 4-point probe method. As for experimental results, it was observed that the electric conductivity of the nickel/carbon hybrid fibers was dramatically increased in the presence of metallic nickel particles, with the best result in the condition of over 0.75 μm of the thickness due to the minimization of the inner pores.     

Photosensitivity and optical performance of hydrogenated amorphous carbon films processed by picosecond laser beams

In this work we investigate the photosensitivity of hydrogenated amorphous Carbon films (a-C:H) and the changes in their structural features and optical properties, when they are exposed to picosecond laser beams of various wavelengths (Nd:YAG, 1st harmonic, λ = 1064 nm and 4th harmonic, λ = 266 nm). The different light-matter interactions, which take place for the various laser wavelengths, are considered and discussed. The main findings include the formation of SiC at the a-C:H film/Si interface and the film graphitization, when the 1064 nm and 266 nm beams, respectively, are used. Finally we managed to vary locally the refractive index in the range of 1.60-1.95 (20% variations) by laser processing, a fact that is very important for various applications in photonics.     

Friction and wear behavior of plasma assisted chemical vapor deposited nanocomposites made of metal nanoparticles embedded in a hydrogenated amorphous carbon matrix

Nanocomposite coatings consisting of preformed silver or chromium nanoparticles embedded into a hydrogenated amorphous carbon matrix (a-C:H) were synthesized by Electron Cyclotron Resonance plasma assisted Chemical Vapor Deposition (ECR-CVD). In a first step, the nanoparticles were distributed on silicon substrates by dipping in an ethanol suspension. In a second step, the ECR-CVD deposition of the a-C:H layer was done. The effect of the incorporation and the concentration on the friction and wear behavior was derived from unlubricated reciprocating sliding tests performed in ambient air. A decrease in the coefficient of friction, more intense with Cr incorporation, is induced by the preferential metal interaction with environment. In addition, for both metals, the coefficient of friction becomes lower as the metal concentration increases. A gradual increase in the coefficient of friction is detected for increasing the number of sliding cycles, which is attributed to the combined effect of surface smoothing and oxidation in the sliding contact. In conclusion, the valuable protective properties of the fullerene-like a-C:H coatings are enhanced by metal addition. As a consequence, a considerable reduction of the surface roughness and the volume loss in the wear tracks is especially noticeable for 10,000 cycles tests.     

Tribological behaviour of titanium carbide/amorphous carbon nanocomposite coatings: From macro to the micro-scale

The tribological behaviour of nanocomposite coatings made of nanocrystalline metal carbides and amorphous carbon (a-C) prepared by PVD/CVD techniques is found to be very dependant on the film deposition technique, synthesis conditions and testing parameters. Focusing in the TiC/amorphous carbon-based nanostructured system, this paper is devoted to an assessment of the factors governing the tribological performance of this family of nanocomposites using a series of TiC/a-C films prepared by magnetron sputtering technique varying the power applied to each target (titanium or graphite) as model system to establish correlations between film microstructure and chemical compositions and tribological properties measured by a pin-on-disk tribometer. The film microstructure goes from a quasi-polycrystalline TiC to a nanocomposite formed by nanocrystals of TiC embedded in an amorphous carbon matrix as observed by transmission electron microscopy (TEM). The nanocrystalline/amorphous ratio appears to be the key-parameter to control the tribological properties and its quantification has been done by electron energy-loss spectroscopy (EELS). A significant change in the tribological performance is observed for nanocomposites with amorphous carbon phase contents above 60–65%. The friction coefficient decreases from 0.3 to 0.1 and the film wear rates by a factor of 10. Examination of the wear scars on ball and film surfaces by laser micro-Raman spectroscopy has allowed to determine the presence of metallic oxides and carbonaceous compounds responsible of the observed friction behaviour. The revision of the literature results in view of the conclusions obtained enabled to explain their apparent dispersion in the tribological performance.     

Corrosion protection ability of plasma-deposited amorphous hydrogenated carbon and fluorocarbon films

Hydrogenated diamond-like carbon and fluorocarbon films, deposited in a radio-frequency (rf) plasma reactor, have high chemical inertness and high electrical resistivity. These films, deposited on aluminum and type 301 stainless steel substrates at several rf power and feed gas flow rates using different gas phase precursors, were characterized for their pinhole density and stability with exposure to 0.6 M NaCl and 0.1 M NaCl and 0.1 M Na₂SO₄ solutions using electrochemical impedance spectroscopy and potentiostatic techniques, respectively. The results from electrochemical characterizations with salt water exposure indicated that films with high effective pore resistances (>10⁸ Ω · cm²)* and high stability with exposure (<10% changes in capacitance values) can be obtained over a narrow range of process conditions and gas phase compositions.     

Improvement of a-C:H film adhesion by intermediate layers and sputter cleaning procedures on stainless steel, alumina and cemented carbide

The adhesion of amorphous hydrogenated carbon (a-C:H) films deposited in a radio frequency (r.f.) plasma discharge on stainless steel, alumina and cemented carbide with different intermediate layers (Ni, Ti and TiC) and sputter cleaning procedures was studied. The composition of the carbon films and the intermediate layers as well as the interface between the coating and the substrate was determined by secondary ion mass spectroscopy (SIMS). The adhesion experiments were carried out using a scratch tester. Tested specimens were also studied by scanning electron microscopy (SEM) to reveal the morphology of the coatings and the scratches.

Without any intermediate layer, the a-C:H coatings generally had insufficient adhesion to the substrate materials studied. For stainless steel and cemented carbide substrates, the TiC intermediate layer and, for alumina substrates, the titanium intermediate layer gave the best adhesion values evaluated by the scratch test. Also, the sputter cleaning of the substrates prior to deposition was necessary for sufficient adhesion of the coating. The intermediate layers also change the failure mode of the coating in the scratch test in some cases.