Surface and Corrosion Characteristics of a-C:H/Fluorocarbon Films

Fluorocarbon films were deposited on type 301 stainless steel substrates from mixtures of hexafluoroethane (HFE) or hexafluoroacetone (HFA) and acetylene and argon in a radio-frequency (13.56 MHz) plasma discharge. A 10 nm thick polysilicon interlayer was deposited prior to fluorocarbon film deposition to obtain good adhesion. To prevent film failure. a-C:H layer was deposited on the polysilicon layer prior to fluorocarbon film deposition, resulting in a-C:H/fluorocarbon composite film structures. The influence of the feed gas composition on the properties of the layered structure was investigated. Surface energies of the films were calculated from the film contact angle values obtained with water and diiodomethane. The composition of the surface layer of these films was characterized using X-ray photoelectron spectroscopy (XPS). The resistance offered by these a-C:H/fluorocarbon film structures to anodic breakdown in an electrolyte containing 0.1 M NaCl and 0.1 M Na₂SO₄ was studied using a potentiostatic technique. The anodic current density for the coated type 301 stainless steel samples was at least 3 orders of magnitude smaller than that for the bare sample and more than an order of magnitude smaller than that observed with samples coated with only the (equally thick) a-C:H layer. The resistance offered by the layered coatings to solution penetration increased with increasing fluorine content in the films.     

Structural and chemical characterization of fluorinated amorphous carbon films (a-C:F) as a liquid crystal alignment layer

a-C:F thin films with varying fluorine content were prepared by plasma CVD and the sputtering method as inorganic alignment layers for overcoming the disadvantages of conventional liquid crystal (LC) alignment layers. The material and structural properties were investigated by X-ray photoelectron spectroscopy, Fourier transform infrared absorption, and contact angle measurement. For elucidation of the liquid crystal alignment layers, LC cells with a-C:F films were fabricated, followed by examination of the textures of the LC and electro-optical characteristics. The fluorine concentrations of a-C:F films were controlled by changing the mixture gas ratio (R_G) in CVD and applied power ratio (R_P) in the sputter system. An increase in R_G and R_P led to increase fluorine incorporation, and the film microstructure changed from a diamond-like to a polymer-like structure. In addition, the sputtered a-C:F films showed a higher fluorination than the CVD sample since the PTFE target was only composed of CF₂ functional groups. Surface composition influenced the surface energy of thin films and an extremely hydrophobic property was obtained in the case of fluorine-rich a-C:F films. LC orientations were observed in various compositions of a-C:F films, and the vertically self-aligned LC textures confirmed that a sputtered a-C:F film is a good candidate for an alignment layer without any post-treatment.     

Mechanical properties of a-C:H multilayer films

Multilayered amorphous hydrogenated carbon (a-C:H) films consisting of alternating sublayers with different mechanical properties have been deposited by an electron cyclotron resonance microwave-plasma chemical vapor deposition (ECR MP-CVD) system and modulating substrate bias voltage. The mechanical properties of the multilayer films were determined using nanoindentation and nanoscratch experiments with reference to single a-C:H layers of which the multilayer structure were composed. In nanoindentation tests, the relationship between the film hardness and indentation depth has been obtained over an indentation depth range of 20–500 nm. Since the films tend to fracture under high load in nanoindentation tests, their critical fracture loads were determined. The critical loads for fracturing the multilayered a-C:H films were higher than those of single a-C:H layers. The nanoscratch tests also showed that the multilayered a-C:H films required a higher critical load for scratching fracture. This study implies that the mechanical properties of a-C:H film can be improved by engineering suitable multilayer structures.     

Improved biocompatibility of parylene‐C films prepared by chemical vapor deposition and the subsequent plasma treatment

The purpose of this study is to prepare the thin film of C‐type parylene (C‐type polyxylylene, parylene‐C) with improved biocompatibility for the biomedical applications, since in spite of the popularity, the parylene‐C has been known to have the less biocompatibility than the N‐type or D‐type parylene. To prepare the well‐designed parylene films through the chemical vapor deposition (CVD) process and the subsequent plasma surface treatment, the parameters of deposition and surface modification were controlled to obtain optimized physical and surface properties. Using CVD, the thin films of parylene‐C as thick as 5 μm were prepared under different deposition pressures. When increasing the deposition rate of parylene film or the deposition pressure, the tensile strength of film increased, whereas the properties such as the surface contact angle and permeability, and the elongation decreased. The deposition rate could be controlled to optimize the physical and physiochemical properties of films. The hydrophilicity of the parylene‐C film increased after plasma surface treatment by showing the larger water contact angle than untreated one. When the radio frequency power was above 100 W in the plasma process, the thin film obtained reveals an excellent cytotropism. It shows the improved biocompatibility with living cells. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Polymeric amorphous carbon films with an extended range of optical gaps

A new type of hydrogenated amorphous carbon (a-C:H) film is prepared under low bias voltage and an extended range of plasma density in a radio-frequency plasma enhanced chemical vapor deposition system (RF-PECVD). The obtained a-C:H samples are grown on electrically floating substrates instead of substrates mounted on the powered or the grounded electrode of RF-PECVD, and have structure and properties that are significantly different from regular a-C:H films. The samples have an optical gap ranging from 1 to 4 eV, while maintaining low intrinsic stress between 0.1 and 0.2 GPa. Fractions of all types of CH_x carbon–hydrogen groups of the obtained samples are measured, analyzed, and used to locate these samples in the carbon-hydrogen ternary phase diagram. The obtained samples are located in the same general area as the regular a-C:H films, indicating configurational rather than compositional structural differences. The hydrogenation ratio of sp³ carbons in the obtained samples is found to remain at a very high level, and is used to explain their unique properties.     

Nanotribological study of PECVD DLC and reactively sputtered Ti containing carbon films

Amorphous carbon film, also known as DLC film, is a promising material for tribological application. It is noted that properties relevant to tribological application change significantly depending on the method of preparation of these films. These properties are also altered by the compositions of these films. DLC films are well known for their self-lubricating properties, as well. In view of this, the objective of the present work is to compare the tribological properties of diamond like carbon (DLC) film obtained by plasma enhanced chemical vapour deposition (PECVD) with the Ti containing nanocrystalline carbon (Ti/a-C:H) film obtained by unbalanced magnetron sputter deposition (UMSD) in nN load range. Towards that purpose, DLC and Ti/a-C:H films are deposited on silicon substrate by PECVD and UMSD processes respectively. The microstructural features and the mechanical properties of these films are determined by scanning electron microscope (SEM), transmission electron microscope (TEM) and nano indenter. The surface topographies and the friction force surfaces of these films are evaluated by means of an atomic force microscope (AFM). The results show that although PECVD DLC film has higher elastic modulus and higher hardness than UMSD Ti/a-C:H film, the surface roughness and the friction coefficient of PECVD film is significantly higher than that of UMSD Ti/a-C:H film.     

Ellipsometric study of a-C: H films

The optical constants, porosity and thickness of amorphous hydrogenated carbon (a-C: H) films were determined by multiple-angle ellipsometry at an optical wavelength of 632.8 nm. The films were produced by a plasma-activated CVD process in a dc glow discharge of acetylene, toluene and octane. The results indicate that the refractive index of the a-C: H films can be changed over the interval 2.35–1.55 by increasing the deposition rate and an appropriate choice of hydrocarbon precursor. A reduction of the refractive index correlates with a decrease in the extinction index in the range 0.3–0.01. The influence of the chemical nature of the hydrocarbon precursor on optical constants in the visible region is found from the results of the ellipsometric measurements. The variations of the experimental data are reviewed in view of the a-C: H structure model proposed previously. The porosity of the a-C: H films has been determined by a new technique based on the ellipsometric measurements.     

Water-repellency and surface free energy of a-C:H films prepared by heat-treatment of polymer precursor

Amorphous hydrogenated carbon (a-C:H) films with high water-repellency were prepared on Si substrates by a simple heat-treatment of the poly(phenylcarbyne) polymer at various temperature in Ar atmosphere. The contact-angle (CA) was measured by the sessile-drop technique. The use of CA for different liquid (water, ethylene glycol, and formamide) analysis for the evaluation of surface energy including the dispersion and polar components was described. The influences of surface roughness, chemical composition, and microstructure of carbon films on water CA and surface energy were investigated by atomic force microscopy (AFM), Fourier transforms infrared spectrometry (FTIR), and Raman spectroscopy, respectively. As the results, the a-C:H films exhibit good hydrophobicity and low surface free energy. The CA of a-C:H films slight increases and the surface free energy of a-C:H films reduces with the increase of the heat-treatment temperature. The a-C:H films are hydrophobic for not only pure water but also corrosive liquids, such as acidic and alkali solutions. The roughness of the films has no obvious effect on the CA and the reduction of surface energy with the increase of heat-treatment temperature is related to increase of sp² content in the film.     

Structural analysis of a-C:H and a-C:H:Si films under high-pressure and high-temperature by synchrotron X-ray diffraction

Amorphous carbon films have several outstanding tribology characteristics, including high hardness, surface smoothness, and low friction. Under tribological conditions, their surface is generally exposed to high-temperature and pressure. Although the structure of amorphous carbon films is likely changed by high temperature and pressure, there have been no reports on such structural changes of the films. To obtain information about their structural changes, synchrotron X-ray diffraction was used to analyze two kinds of amorphous carbon films, a-C:H and a-C:H:Si, under high-temperature and high-hydrostatic pressure conditions. Synchrotron X-ray diffraction was applied to films pressurized by a multi-anvil press installed in the PF-AR NE5C beamline at KEK at room temperature and at a high-temperature around 200 °C. The pair distribution functions derived by Fourier transformation of the obtained scattering intensity profiles showed that the sp²/sp³ ratios for both films decreased as the pressure increased and that the sp²/sp³ ratio for the a-C:H film increased as the temperature increased. This indicates that high-pressure creates sp³ stabilization in a-C:H and a-C:H:Si films while high-temperature creates sp² transition in a-C:H film. The sp²/sp³ ratio for the a-C:H:Si film did not change much even at high-temperature due to the high thermal-oxidative stability of a-C:H:Si.     

Amorphous hydrogenated carbon films as barrier for gas permeation through polymer films

The influence of amorphous hydrogenated carbon (a-C:H) coatings on the gas permeation through polymer films was investigated. a-C:H films were deposited from a 13.56-MHz RF glow discharge in methane or acetylene atmosphere. Thin poly(ethylene terephthalate) and polyimide foils were used as substrates. The permeation of the gases H₂, N₂, O₂ and CO₂ was measured and the reduction of the permeability coefficient was correlated to composition and density of the a-C:H films. The stoichiometry of the layers was analyzed using ion-beam techniques on films deposited onto silicon samples. The a-C:H/PET surfaces were analyzed using optical microscopy and atomic force microscopy (AFM). Multilayer structures comprising different types of a-C:H films were also investigated. A reduction of the permeability coefficient by 80% for hard, dense and 94% for soft, polymer-like layers was found. Surprisingly, the barrier efficacy of the coating decreases with increasing a-C:H film density. This unexpected result is attributed to the appearance of a network of deep cracks spread out over the whole coating.     

Hydrogenated amorphous carbon films deposited on 316L stainless steel

Research on hydrogen amorphous carbon films (a-C:H), which possess the diamond-like characteristic, has been stimulated for many years by need to simultaneously optimizing the mechanical, optical and biological properties, and by challenges related to the deposition of a-C:H films on medical implants. In the present work, we investigate the structure, optical and mechanical properties (hardness, elastic modulus and stress) of a-C:H films deposited on 316L stainless steel substrate by the radio frequency plasma enhanced chemical vapor deposition (RF PECVD). The negative self-bias voltages significantly influence on temperature of steel substrates during the deposition process and films properties. Specifically, the high energetic deposition leads also to stabilization of the sp² content and thermally-activated relaxation in the stress of a-C:H films. Presented correlation between the obtained results and literature analysis let deem the Raman spectra as a good tool to control the properties of implants made of 316L stainless steel with a-C:H film for general use.     

Surface and interface morphology and structure of amorphous carbon thin and multilayer films

We review the implementation of X-ray reflection (reflectivity and scattering) techniques for the study of amorphous Carbon (a-C, a-C:H, ta-C) thin and multilayer films and in particular in the determination of the film density and surface and interface morphology, which are intrinsically significant for ultra-thin films. We present studies of various a-C and a-C:H films, which include in particular: i) the morphology of a-C/Si interface, ii) the surface morphology and density evolution during sputter growth of a-C, iii) the morphology of the sp²-rich a-C/sp³-rich a-C interfaces in multilayer a-C films, iv) the universal correlation between the film density and the refractive index of a-C and a-C:H films. We also compare and validate the experimental results with relative results from Monte-Carlo simulations within an empirical potential scheme. The computational results shed light on the atomistic mechanisms determining the structure and morphology of the a-C interfaces between individual sp²- and sp³-rich a-C layers and between a-C and Si substrates.     

Improving the corrosion resistance of a-C:H film by a top a-C:H:Si:O layer

During the deposition of a-C:H film, defects (pinholes or discontinuities) caused by excessive stress will inevitably appear, which will reduce the corrosion resistance of the a-C:H film. In this study, top a-C:H:Si:O layers (thickness of approximately 0.3 μm) on the surface of a-C:H films were deposited on a large scale by PACVD technology using acetylene (C₂H₂) and/or hexamethyldisiloxane (HMDSO) as reactants, to improve the corrosion resistance of a-C:H films while ensuring the appropriate overall hardness of the films. The corrosion behaviors of the films were studied by electrochemical impedance spectroscopy (EIS) and Tafel polarization. We found that the a-C:H/a-C:H:Si:O films possess a lower electrolyte penetration rate due to their stronger capacitance characteristics. In addition, the corrosion current density of the a-C:H/a-C:H:Si:O films (10^?10 A cm^?2) were reduced by 2 orders of magnitude compared to the a-C:H film (10^?8 A cm^?2), and by 3 orders of magnitude compared to 316 stainless steel (10^?7 A cm^?2). The impedance results obtained by EIS were simulated using appropriate equivalent circuits, and the corresponding electrical parameters were used to further verify the electrochemical protection behavior of the top a-C:H:Si:O layer.     

Etching process of hydrogenated amorphous carbon (a-C:H) thin films in a dual ECR–r.f. nitrogen plasma

Hydrogenated amorphous carbon (a-C:H) films deposited from CH₄ in a dual electron cyclotron resonance (ECR)–r.f. plasma were treated in N₂ plasma at different r.f. substrate bias voltages after deposition. The etching process of a-C:H films in N₂ plasma was observed by in situ kinetic ellipsometry, mass spectroscopy (MS), and optical emission spectroscopy (OES). Ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the etched film surface. XPS analysis proves that the nitrogen treatment on the a-C:H film, induced by r.f. substrate bias, causes a direct nitrogen incorporation in the film surface up to 15–17 at.% to a depth of about 20–40 Å depending on the r.f. bias. Various bonding states between carbon and nitrogen, such as tetrahedral sp³ C–N, and trigonal sp² C–N were confirmed by the deconvolution analysis of C 1s and N 1s core level spectra. The evolution of etching rate and the surface roughness in the film measured by AFM exhibit a clear dependence on the applied r.f. bias. MS and OES show the various neutral species in the N₂ plasma such as HCN, CN, and C₂N₂, which may be considered as the chemical etching products during the N₂ plasma treatment of a-C:H film.     

Structure,gas-barrier properties and overall migration of poly(lactic acid) films coated with hydrogenated amorphous carbon layers

Hydrogenated amorphous carbon (a-C:H) films were grown on a poly(lactic acid) (PLA) substrate by means of a radiofrequency plasma-enhanced chemical vapour deposition (rf-PECVD) technique with different deposition times (5, 20 and 40 min). The main goal of this treatment was to increase the barrier properties of PLA, maintaining its original transparency and colour as well as controlling interactions with food simulants for packaging applications. Morphological, chemical, and mechanical properties of PLA/a-C:H systems were evaluated while permeability and overall migration tests were performed in order to determine the effect of the plasma treatment on the gas-barrier properties of PLA films and their application in food packaging. Morphological results suggested a good adhesion of the deposited layers onto the polymer surface and the samples treated for 5 and 20 min only slightly darkened the PLA film. X-ray photoelectron spectroscopy revealed that the structural properties of the carbon layer deposited onto the PLA film depend on the exposure time. PLA/a-C:H system treated for 5 min showed the highest barrier properties, while none of the studied samples exceeded the migration limit established by the current legislation, suggesting the suitability of these materials in packaging applications.     

The Effects of Accelerated Aging and Contact with Food Simulants on the Adhesion of Amorphous Hydrogenated Carbon Films Deposited on Clarified Polypropylene

This work focuses mainly on the influence of time, temperature, and contact with food simulants on the adhesion of amorphous hydrogenated carbon (a-C:H) films obtained by plasma-enhanced chemical vapor deposition (PECVD) in clarified polypropylene (cPP). Two types of films prepared by PECVD were studied: diamond-like carbon (DLC) and polymer-like carbon (PLC) films that can both act as a functional barrier. The adhesion between the film and polymer substrates is critical in relation to the barrier effectiveness during the packaging shelf life. Therefore, the adhesion was analyzed by a tape test and scanning electron microscopy (SEM). The films were exposed to Food and Drug Administration (FDA) listed food simulants and were submitted to an accelerated aging test to evaluate the long-term adhesion performance of a-C:H films. The chemical alterations on the surface related to the accelerated aging test and the liquid simulants were analyzed by a contact angle test. It showed that the polarity of a-C:H films increased after immersion in liquid simulants, indicating a change on the surface. Before the accelerated aging test, the SEM micrographs and the tape test indicated that the PLC film has a structure with lower surface tension and, therefore, regions with fewer detachment points in relation to the DLC film. The results obtained in this study showed that the adhesion behavior and preservation of the a-C:H structure (DLC or PLC) are related to intrinsic factors such as the type of film structure (flexible or rigid) and the polymeric substrate.     

Effects of acetylene addition on mechanical and dielectric properties of amorphous carbon films

The para-xylene added with acetylene from 15% to 50% was plasma polymerized at 50 to 150 W to deposit the a-C:H films. After the films were annealed from 200 to 400 °C, the network structure, hardness and dielectric constant of films were analyzed by FT-IR, Raman, nanoindentor and capacitance–voltage plot, respectively. Those measured results suggest that hydrocarbon bonds and oxygen related bonds of the a-C:H film effectively reduce and the number of ordered aromatic rings increases with decreasing the deposition power after annealing at 400 °C. In addition, both the dielectric constant and the hardness, respectively, increase up to 2.82 and 2.37 GPa, but the adhesion strength decreases with increasing the C₂H₂ concentration and deposition power. Therefore, the a-C:H films not only have a lower dielectric constant, but also have enough mechanical strength for the IC processing.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Enhancement of fluorine doped amorphous carbon thin films from microwave surface wave plasma activated above room temperature

Fluorinated amorphous carbon (a-C:F) thin films were synthesized above room temperature by microwave surface wave plasma chemical vapour deposition (MW SWP CVD). The effect of deposition temperature on optical, electrical, chemical and bonding properties of the a-C:F films were studied by ultraviolet–visible spectroscopy (UV–VIS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometry (XPS), Raman spectrometry and TEM measurements. The film exhibits high transparency and decrease in optical band gap with increasing deposition temperature. FTIR study shows the increase in CC and decrease in C–Fx bonds of the films with increasing deposition temperature. Raman study shows some important structural changes in the films due to fluorine incorporation. XPS result shows the shift of carbon peak to higher binding energy due to carbon fluorine link to the films. TEM shows the increasing graphitic layer in the films with increasing deposition temperature.     

Electrochemical behavior of the Ti–6Al–4V alloy coated with a-C:H films

In this work diamond-like carbon films were deposited on the Ti–6Al–4V alloy, which has been used in aeronautics and biomedical fields, by electrical discharges using a magnetron cathode and a 99.999% graphite target in two different atmospheres, the first one constituted by argon and hydrogen and the second one by argon and methane. Films deposited using the argon/hydrogen mixture were called a-C:H, while films deposited using the argon/methane mixture were called DLC. Raman spectroscopy was used to study the structure of the films. The Raman spectra profile of the a-C:H films is quite different from that of the DLC films. The disorder degree of the graphite crystalline phase in a-C:H films is higher than in DLC films (a-C:H films present small values for the the I_D/I_G ratio). Potentiodynamic corrosion tests in 0.5 mol l⁻¹ NaCl aqueous solution, pH 5.8, at room temperature (≈25 °C) were carried out as for the a-C:H as for the DLC coated surfaces. Comparison between the corrosion parameters of a-C:H and DLC coated surfaces under similar deposition time, showed that DLC coated surfaces present bigger corrosion potential (E_corr) and polarization resistance than those coated with a-C:H films. Electrochemical impedance spectroscopy (EIS) was also used to study the electrochemical behavior of a-C:H and DLC coated surfaces exposed to 0.5 mol l⁻¹ aqueous solution. The EIS results were simulated with equivalent electrical circuit models for porous films. The results of these simulations showed similar tendency to the one observed in the potentiodynamic corrosion tests. The DLC film resistance and the charge transfer resistance (R_ct) for the DLC coated surface/electrolyte interface were bigger than the ones determined for the a-C:H coated surfaces.