Corrosion and tribological properties of ultra-thin DLC films with different nitrogen contents in magnetic recording media

The corrosion spot density and contact–start–stop tribological properties that correlate to mechanical properties, electrical resistivity and lubricant bonded ratio of DLC overcoats on different disks of various surface roughness were investigated. DLC overcoats of hydrogenated carbon (CH) and nitrogenated carbon (CN) films were deposited by ion beam deposition (IBD) and sputter, respectively. Results show that the intensity ratio I(D)/I(G) increases with decreasing IBD-CH film thickness and increasing N₂ concentration of sputtered-CN layer, which implies that the films prepared at higher N₂ concentration contain a relatively lower sp³ bonded carbon. The composite hardness and Young's modulus of DLC films decrease with decreasing IBD-CH thickness and increasing N₂ concentration of sputtered-CN layers. Compared to disk overcoats deposited with only IBD-CH of comparable thickness, the lubricant bonded ratio is dramatically increased from 12 to 30% when the 0.5 nm CN is deposited on IBD-CH film. By increasing the N₂ concentration in the CN layer from 10 to 50 at.%, the electrical resistivity decreased from 3.6 to 0.8 kΩ and the lubricant bonded ratio increased from 30 to 46%. The corrosion spots density of sputtered-CN film surface decreases with increasing N₂ concentration. It is concluded that the dual layer of 1.5 nm IBD-CH/0.5 nm sputtered-CN with 30% N₂ deposition has the best integrated performance of corrosion resistance and CSS tribological properties.     

Nanoindentation and nanoscratching studies of amorphous carbon films

Hydrogen-free amorphous carbon (a-C) films prepared by RF magnetron sputtering were deposited on Si substrates in thin films, at various negative bias voltages V_b (i.e. Ar-ion energies), and in thick layered-structure films with alternative values of V_b. The main purposes of this work are to present preliminary results concerning the effect of Ar-ion bombardment during deposition on the elastic properties of thin a-C films with Ar⁺ energies in the range 30–200 eV, and the adhesion failure which limits their thickness and usefulness for practical applications, and the enhancement of hardness and scratch resistance of sputtered a-C films developed in a layered structure. The results show a significant improvement in the elastic properties of layered structure films and their stability. The combination of high hardness and relative low elastic modulus which the layered films exhibit make them more resistant to plastic deformation during contact, as confirmed by scratch testing.     

Corrosion behavior of amorphous carbon deposit in 0.89% NaCl by electrochemical impedance spectroscopy

Amorphous carbon films were deposited on SS316L substrates using a DC magnetron sputtering system, aiming at the application of the coated SS316L for biomedical implants. The biocompatibility and chemical stability of the carbon layers have been previously demonstrated. The films were deposited on top of sputtered titanium coatings introduced as a buffer layer to enhance film-substrate adhesion. The corrosion resistance of the a-C/Ti/SS316L systems was investigated by electrochemical techniques. The electrolyte used in this work was 0.89 wt.% NaCl at pH 7.4, which simulates body fluid ionic concentrations. The coated samples displayed corrosion resistance values in the saline solution much higher than the stainless steel substrates and the role of the Ti coating thickness was analysed in order to determine the optimal system for biological applications.     

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.     

Corrosion performance of thin hydrogenated amorphous carbon films prepared by magnetron sputtering

Hydrogenated amorphous carbon (a-CH_x) films were prepared by magnetron sputtering at different H₂/Ar gas flow ratios. Several sets of perpendicular magnetic recording disks with a-CH_x overcoats of 5 nm were prepared for accelerated environmental corrosion test. The corrosion spots on the disks and the corrosion products were analyzed using optical surface analyzer and time-of-flight secondary ion mass spectroscopy, respectively. Other techniques, such as Raman spectroscopy, atomic force microscopy nano-scratch and contact angle measurement, were used to characterize the properties of a-CH_x films. Compared to pure carbon overcoat, all the a-CHx overcoats have better corrosion resistance and higher polar component of surface free energy. The a-CH_x film with appropriate H content shows the highest scratching resistance, and the corresponding disk has the lowest corrosion area on the disk surface after the accelerated corrosion.     

Enhanced cycling stability of Ni-MH battery by depositing amorphous carbon film on the Ti1.4V0.6Ni negative electrode

Amorphous carbon (a-C) films with various thicknesses depending on the reaction time are deposited on the surface of Ti_1.4V_0.6Ni alloy electrodes for Ni-MH (nickel-metal hydride) battery by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). With the increasing deposition time, the Raman spectra show a gradually disordered sp²-bonding change of the films and the changing trend of sp²/sp³ is obtained by X-ray photoelectron spectroscopy. The a-C film of depositing for 30 min with the thickness of 400 nm shows a favorable stability in alkaline electrolyte, the capacity is enhanced by 36.2% after 50 cycles than the bare electrode, and the charge voltage is 80 mV lower than the bare one. The a-C film with high sp²-bonded carbon content effectively reduces the charge transfer resistance, and as a coating layer, the dissolution of V of the alloy is also inhibited. In particular, to get a proper discharge voltage and a stable capacity simultaneously, covering completely and an appropriate thickness of the a-C film are crucial for an expected performance.     

Tribological study of hydrogenated amorphous carbon films with tailored microstructure and composition produced by bias-enhanced plasma chemical vapour deposition

We investigated the mechanical and tribological properties of hydrogenated amorphous carbon (a-C:H) films on silicon substrates by nanoindentation, ball-on-disc tribotesting and scratch testing. The a-C:H films were deposited from an argon/methane gas mixture by bias-enhanced electron cyclotron resonance chemical vapour deposition (ECR-CVD). We found that substrate biasing directly influences the hardness, friction and wear resistance of the a-C:H films. An abrupt change in these properties is observed at a substrate bias of about ?100 V, which is attributed to the bias-controlled transition from polymer- to fullerenelike carbon coatings. Friction coefficients in the range of 0.28–0.39 and wear rates of about 7 × 10^?5 mm³/Nm are derived for the polymeric films when tested against WC–Co balls at atmospheric test conditions. On the other hand, the fullerenelike hydrogenated carbon films produced at ion energies > 100 eV display a nanohardness of about 17 GPa, a strong reduction in the friction coefficient (～ 0.10) and a severe increase in the wear resistance (～ 1 × 10^?7 mm³/Nm). For these films, relative humidity has a detrimental effect on friction but no correlation with the wear rate was found.     

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.     

Studies of physical and chemical properties of styrene-based plasma polymer films deposited by radiofrequency Ar/styrene glow discharge

Styrene-based plasma polymer (SPP) films having thickness in the range of 900–1800 nm are deposited from radiofrequency (RF) Ar/styrene glow discharge. Depositions of the SPP films are carried out at working pressure of 1.2 × 10^?1 mbar and in the RF power range of 40–130 W. The physical and chemical properties of the SPP films are investigated as a function of RF power. Optical emission spectroscopy (OES) studies on Ar/styrene glow discharge reveal that the relative concentrations of active plasma species are strongly dependent on the variation of RF power. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) are used to analyze the internal chemical structures of the films. It is revealed that the SPP film with highest carbon content exhibits enhanced scratch and corrosion resistance behavior along with stable thermal properties. The thermogravimetric (TGA) and gel permeation chromatography (GPC) results suggest the presence of both aliphatic and aromatic units in the SPP films. Attempts are made to correlate the results obtained from OES, FT-IR and XPS analyses with the deposited films properties. The possibility of using SPP films as protective coatings is also explored.     

Temperature effect on bonding structures of amorphous carbon containing more than 30at.% silicon

Bonding evolution of amorphous carbon incorporated with Si or a-C(Si) in a thermal process has not been studied. Unhydrogenated a-C(Si) films were deposited by magnetron sputtering to undergo two different thermal processes: i) sputter deposition at substrate temperatures from 100 to 500 °C; ii) room temperature deposition followed by annealing at 200 to 1000 °C. The hardness of the films deposited at high temperature exhibits a monotonic decrease whereas the films deposited at room temperature maintained their hardness until 600 °C. X-ray photoelectron spectroscopy and Raman spectroscopy were used to analyze the composition and bonding structures. It was established that the change in the mechanical property is closely related to the atomic bonding structures, their relative fractions and the evolution (conversion from C–C sp³ → CC sp² or CC sp² → C–Si sp³) as well as clustering of sp² structures.     

The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis

Thin films of iridium oxide deposited by reactive magnetron sputtering have been investigated as catalysts for electrochemical water splitting in a polymer electrolyte membrane (PEM) cell. The sputtered films possess excellent mechanical stability and corrosion resistance at the high anodic potentials where oxygen evolution takes place. Their catalytic activity has been assessed using the conventional electrochemical methods of cyclovoltammetry and steady state polarisation techniques. A morphology factor assessing the catalyst active surface for a series of sputtered samples with varying thickness/loading has been determined and correlated to the catalytic efficiency. It has been proven that iridium oxide is a very efficient catalyst for oxygen evolution reaction (OER). The best performance with anodic current density of 0.3 A cm⁻² at potential of 1.55 V (versus RHE) has shown the 500 nm thick film containing 0.2 mg cm⁻² catalyst. The results obtained have also demonstrated the advantages of the reactive magnetron sputtering as simple and reliable method for deposition of efficient and cost effective catalysts for PEM electrolysis application.     

Mechanical and tribological properties of TiC/amorphous hydrogenated carbon composite coatings fabricated by DC magnetron sputtering with and without sample bias

TiC/amorphous hydrogenated carbon (a-C:H) composite films were deposited by Ti DC magnetron sputtering using argon and acetylene as the carrier gas and precursor, respectively. The working pressure was maintained at 4 × 10^− 1 Pa and the composition of the films was modulated by controlling the partial pressure of acetylene. The composition and structure of the films were evaluated by X-ray photoelectron spectroscopy and glancing angle X-rays diffraction, whereas the hardness and elastic modulus values of the films fabricated using different sample biases were measured by nano-indentation. Ball-on-disk tribometry was used to measure the tribological properties, and secondary electron microscopy was used to analyze the wear tracks. The results show that the friction coefficients and wear rates do not vary significantly with the Ti concentrations when the Ti concentration is above 39.7 at.% or below 20 at.% but increase with increasing titanium concentrations between 20 at.% and 39.7 at.%. The wear mechanism depends on the relative amounts of TiC and a-C:H. At high Ti concentrations, the mechanism resembles that of TiC due to the thin a-C:H matrix surrounding the TiC grains. At low Ti concentrations, the mechanism is similar to that of DLC as the effects of the a-C:H matrix dominates over those of the TiC grains.     

Nucleation and growth of amorphous carbon film on tungsten-implanted stainless steel substrates

Amorphous carbon (a-C) films were deposited on W-implanted (20 kV, 3 × 10¹⁷ ions cm^− 2) and un-implanted steel substrates by plasma immersion ion implantation and deposition (PIII&D). The W implantation pretreatment changes the surface structure and impacts film nucleation. Consequently, the growth mechanism of the a-C film is altered resulting in different surface morphologies and roughnesses even though the films deposited on the un-implanted steel substrates possess similar a-C structures as revealed by Raman spectroscopy. The structural differences are probed by X-ray photoelectron spectroscopy and X-ray diffraction. Moreover, microstructural observations were carried out by transmission electron microscopy. A model based on the statistical formation theory is proposed to explain the growth of the a-C films on the implanted and un-implanted substrates.     

Photovoltaic properties of an amorphous carbon/fullerene junction

This paper demonstrates all-carbon photovoltaic devices made of amorphous carbon (a-C) and C₆₀ thin films. C₆₀ film is deposited by the sublimation in vacuum and a-C film is synthesized by exposing N₂ radicals to C₆₀ during the deposition. C₆₀ is converted into a-C when the rf power is larger than 150 W and the optical band gap decreases with increasing the power. Photovoltaic properties of device with the structure of Al/C₆₀/a-C/indium tin oxide/glass are presented. It is shown that the present cell has a strong spectral response in the wavelength range shorter than 550 nm and a small response at around 620 nm.     

Mechanical characterization of ultra-thin fluorocarbon films deposited by R.F. magnetron sputtering

Fluorocarbon films were deposited on silicon substrate by R.F. magnetron sputtering using a polytetrafluoroethylene (PTFE) target. Structure of the deposited films was studied by X-ray photoelectron spectroscopy (XPS). Hardness, elastic modulus and scratch resistance were measured using a nanoindenter with scratch capability. -CF_x (x = 1, 2, 3) and C-C units were found in the deposited fluorocarbon films. The hardness and elastic modulus of the films are strongly dependent on the R.F. power and deposition pressure. The film hardness is in the range from 0.8 GPa to 1.3 GPa while the film elastic modulus is in the range from 8 GPa to 18 GPa. Harder films exhibit higher scratch resistance. Differences in nanoindentation behavior between the deposited fluorocarbon films, diamond-like carbon (DLC) films and PTFE were discussed. The fluorocarbon films should find more applications in the magnetic storage and micro/nanoelectromechanical systems.     

Semiconducting amorphous carbon thin films for transparent conducting electrodes

Semiconducting amorphous carbon thin films were directly grown on SiO₂ substrate by using chemical vapor deposition. Raman spectra and transmission electron microscopy image showed that the a-C films have a short-range ordered amorphous structure. The electrical and optical properties of the a-C thin films were investigated. The films have sheet resistance of 3.7 kΩ/□ and high transmittance of 82%. They exhibit metal-oxide-semiconductor field effect transistor mobility of 10–12 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to previous reported mobility of amorphous carbon. The optical band gap was calculated by Tauc’s relationship and photoluminescence spectra showed that the films are semiconductor with an optical band gap of 1.8 eV. These good physical properties make the a-C films a candidate for the application of transparent conducting electrodes.     

Advanced TiC/a-C:H nanocomposite coatings deposited by magnetron sputtering

TiC/a-C:H nanocomposite coatings have been deposited by magnetron sputtering. They consist of 2–5 nm TiC nanocrystallites embedded in the amorphous hydrocarbon (a-C:H) matrix. A transition from a columnar to a glassy microstructure has been observed in the nanocomposite coatings with increasing substrate bias or carbon content. Micro-cracks induced by nanoindentation or wear tests readily propagate through the column boundaries whereas the coatings without a columnar microstructure exhibit substantial toughness. The nanocomposite coatings exhibit hardness of 5–20 GPa, superior wear resistance and strong self-lubrication effects with a friction coefficient of 0.05 in air and 0.01 in nitrogen, under dry sliding against uncoated bearing steel balls. Especially, reversible transitions from low to ultra-low friction are observed if the atmosphere is cycled between ambient air and nitrogen. The lowest wear rate is obtained at high humidity.     

Microstructure,mechanical and tribological properties of Ti–B–C–N films prepared by reactive magnetron sputtering

Quaternary Ti–B–C–N thin films are deposited on high-speed steel substrates by the reactive magnetron sputtering (RMS) technique. The microstructure, mechanical and tribological properties of Ti–B–C–N films with different carbon contents (from 28.9 at.% to 54.2 at.%) are explored systematically. The microstructure of Ti–B–C–N films deposited by RMS is consisted mainly of Ti(C, N) nano-crystals embedded into an amorphous matrix of a-C/a-CN/a-BN/a-BC. As the carbon content increases, the crystalline size of the films diminishes, but the hardness linearly increases from 14 GPa to 26 GPa. The friction coefficient of the films sliding against steel GCr15 balls in air decreases with the increase of carbon content, which shows that Ti–B–C–N films with both higher hardness and lower friction coefficient can be obtained by means of increasing the carbon concentration in the films.     

Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at various bias voltages V_b applied on Si substrate. We studied the optical properties of the films using in situ spectroscopic ellipsometry (SE) measurements in the energy region 1.5–5.5 eV. From the SE data analysis the dielectric function ε(ω) of the a-C films was obtained, providing information about the electronic structure and the bonding configuration of a-C films. Based on the SE data the films are classified in three categories. In Category I and II belong the films developed with V_b≥0 V (rich in sp² bonds) and −100≤V_b<0 V (rich in sp³ bonds), respectively. The dielectric function of the films belonging in these two categories can be described with two Lorentz oscillators located in the energy range 2.5–5 eV (π–π^*) and 9–12 eV (σ–σ^*). A correlation was found between the oscillator strength and the sp² and sp³ contents. The latter were calculated by analyzing the ε(ω) with the Bruggeman effective medium theory. In films deposited with V_b<−100 V (Category III), the formation of a new and dense carbon phase was detected which exhibits a semi-metallic optical behavior and the ε(ω) can be described with two oscillators located at ∼1.2 and ∼5.5 eV.