Corrosion protection of ultra-thin ta-C films for recording slider applications at varied substrate bias

Ultra-thin tetrahedral amorphous carbon (ta-C) films have been prepared by filtered catholic vacuum arc system for recording slider applications. In order to study the corrosion behavior of the slider coatings deposited at different substrate bias, the micro-structure and surface properties of the ta-C films were respectively investigated in terms of Raman spectroscopy, atomic force microscopy, water contact angle and corrosion measurements. Results showed that a very smooth ta-C film with lowest surface energy and highest sp³ bonding content was obtained at the medium bias voltage of 100 V. However, as to the protection against corrosion, the optimum bias was found to be greater than that giving the maximum diamond-like properties.     

Plasma biasing to control the growth conditions of diamond-like carbon

It is well known that the structure and properties of diamond-like carbon, and in particular the sp³/sp² ratio, can be controlled by the energy of the condensing carbon ions or atoms. In many practical cases, the energy of ions arriving at the surface of the growing film is determined by the bias applied to the substrate. The bias causes a sheath to form between substrate and plasma in which the potential difference between plasma potential and surface potential drops. In this contribution, we demonstrate that the same results can be obtained with grounded substrates by shifting the plasma potential. This “plasma biasing” (as opposed to “substrate biasing”) is shown to work well with pulsed cathodic carbon arcs, resulting in tetrahedral amorphous carbon (ta-C) films that are comparable to the films obtained with the conventional substrate bias. To verify the plasma bias approach, ta-C films were deposited by both conventional and plasma bias and characterized by transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS). Detailed data for comparison of these films are provided.     

Effect of high substrate bias and hydrogen and nitrogen incorporation on spectroscopic ellipsometric and atomic force microscopic studies of tetrahedral amorphous carbon films

The influence of substrate bias and hydrogen/nitrogen incorporation on the optical properties [studied by spectroscopic ellipsometry (SE)] and surface morphology [studied by atomic force microscopy] of tetrahedral amorphous carbon (ta-C) films, deposited by S bend filtered cathodic vacuum arc (FCVA) process, is reported. SE spectra of the imaginary part of the dielectric constant are used to estimate carbon bonding ratios. In ta-C films, optical constants increase with substrate bias and hydrogen/nitrogen incorporation. The optical band gap (E_g) and sp³ content increase up to −200 V substrate bias and then decrease. E_g increases with hydrogen incorporation but is unchanged by nitrogen incorporation.     

Effect of the carbon ion energy on the microstructure of ta-C/Cr multilayers

The effect of the carbon ion energy applied in a pulsed arc deposition process on the morphology of the interface between Cr layers and subsequently deposited tetrahedral bonded carbon (ta-C) coatings was investigated. The carbon ion energy was varied through the bias voltage between ~ 25 and 500 eV. Small-angle and wide-angle X-ray scattering (SAXS and WAXS), high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) were used as the central experimental methods for microstructure analysis. In order to improve the scattering power of the ta-C/Cr interfaces for SAXS, the samples were prepared in the form of ta-C/Cr multilayers. SAXS revealed density and thickness of individual layers in the multilayer stack, and the roughness and morphology of the ta-C/Cr interfaces. The density of ta-C layers ranged between 2.72 and 3.15 g/cm³, which was related to the amount of sp³ bonds between 37% and 71%. The amount of the sp³ bonds calculated from the density of the ta-C layers agreed well with the amount of the sp³ bonds determined using EELS. HRTEM confirmed both the increase of the interface roughness with increasing carbon ion energy and the changes in the correlation of the interface corrugations observed by SAXS. Furthermore, HRTEM approved the crystallinity of Cr layers revealed by WAXS.     

Thickness dependence of the structure of diamond-like carbon films by Raman spectroscopy

The thickness dependence on structure of Diamond-like carbon films of a-C:H deposited by ECR-CVD and ta-C by FCVA has been studied by visible and UV Raman spectroscopy. The results show that the evolution of structure as a function of the thickness for a-C:H films contains two stages: when thickness is less than 50 Å, the film contains less sp³ sites and not continuous; and when thickness is up to 50 Å, the film contains more sp³ sites and become continuous. However, for ta-C films, it includes three stages. In the first stage of thickness lower than 20 Å, the film is not continuous, and also contains less sp³. In the second stage of thickness between 20 Å and 50 Å, the sp³ site abruptly shifts a higher value in 20 Å and then keeps stable. In the third stage of thickness over 50 Å, the sp³ site has a little increase and then almost not changed. Thus, the fundamental limitation thickness in using DLC as an ultrathin overcoat for ta-C films is 20 Å (> 10 Å), and for a-C:H films is 50 Å. The implications of result on the mechanisms proposed for the film growth mode were also discussed.     

Nanomechanical and nanotribological properties of carbon-based thin films: A review

Carbon-based thin films possess unique and adjustable combination of properties such as high hardness and wear resistance, chemical resistance and good tribological performances. Among critical variables to tailor a-C film’s properties for specific application is the distribution of the carbon hybridization states (sp¹, sp² and sp³ bonds), the atomic H content, the content in dopants such as Si, F, N, B and O. Here we focus on: (i) a-C and hydrogenated amorphous carbon (a-C:H) films with a mixture of sp² and sp³ bonding, highly sp³-boned material (ta-C) and sp²-bonded carbon, (ii) carbon nitride (CN_x) coatings and (iii) metal/amorphous carbon (a-C:M) composite films.The study is focused on the review of the nanomechanical properties and analysis of the nanoscratching processes at low loads to obtain quantitative analysis, the comparison of their elastic/plastic deformation response, and nanotribological behavior of the a-C, ta-C, a-C:H, CN_x, and a-C:M films. For ta-C and a-C:M films new data are presented and discussed.     

Effects of tetrahedral amorphous carbon film deposited on dental cobalt–chromium alloys on bacterial adhesion

The dental cobalt–chromium alloys are an important biomaterial used in making artificial dentures. Bacterial adhesion to cobalt–chromium alloys usually results in severe complications such as periodontal infection, secondary caries, and denture stomatitis, which have severe adverse impacts on human health. Therefore, an effective method is needed to reduce the bacterial adhesion to dental cobalt–chromium alloys. The aim of this study was to investigate the effects of ta-C films deposited on a dental cobalt–chromium alloy on the adhesion of Streptococcus mutans (ATCC175), Actinomyces viscosus (ATCC19246) and Candida albicans (ATCC76615). A filtered cathodic vacuum arc (FCVA) technique was used to coat the cobalt–chromium alloy with a ta-C film. Atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to analyze the surface characteristics of the coating. Surface roughness was detected. Surface free energy and its components were calculated by measuring the contact angle. The results showed that the maximum sp³ fraction was achieved at 200 V substrate bias voltage. Compared with uncoated specimens, the ta-C film coated specimens had a lower surface roughness, a higher surface energy and a higher hydrophilicity. Most importantly, the adhesion of the three tested bacterial strains to the ta-C film coated cobalt–chromium alloy was significantly decreased. These results showed that ta-C film surface treatment could significantly reduce the bacterial adhesion to dental cobalt–chromium alloys, suggesting the potential of ta-C film surface treatment in artificial denture applications.     

Corrosion behavior of nitrogen doped diamond-like carbon thin films in NaCl solutions

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were grown on p-Si(1 1 1) substrates using filtered cathodic vacuum arc (FCVA) deposition by varying nitrogen flow rate. The effect of nitrogen flow rate on the corrosion performance of the films was investigated through potentiodynamic polarization and immersion tests in 0.6 M NaCl solutions. The polarization results showed that the corrosion resistance of the films dropped with increased nitrogen flow rate due to formation of more sp² bonds. The immersion tests revealed that the pH value of the solutions had a significant effect on the corrosion behavior of the ta-C:N films.     

Influence of deposition parameters on the internal stress in a-C:H films

Amorphous hydrogenated carbon (a-C:H) films were deposited from a mixture of C₂H₂ and Ar by the r.f. plasma-enhanced chemical vapor deposition technique. The internal stress was measured by the substrate bending method. The films were characterized by IR and positron annihilation spectroscopies. The influence of substrate bias and C₂H₂ content in the feedgas on the internal stress and structure were studied. It was found that the hydrogen content and sp³/sp² ratio decrease with an increase in substrate bias and a decrease in C₂H₂ content in the feedgas. However, the variations in H content and sp³/sp² ratio with C₂H₂ content are much less than that with substrate bias. The void density, which increases monotonically with the increase of C₂H₂ content, initially decreases and then increases with the increase of substrate bias. The internal stress in a-C:H films decreases with the increase of substrate bias and C₂H₂ content in the feedgas. The decreases of the H content and sp³/sp² ratio in the films are the main factors that cause the reduction in internal stress. The void density has a minor effect on the internal stress.     

Cross-linked graphene layer embedded carbon film prepared using electron irradiation in ECR plasma sputtering

A new path to prepare cross-linked graphene layers embedded carbon (GLEC) films has been reported by introducing electron irradiation during the mirror-confinement electron cyclotron resonance (ECR) plasma sputtering process. The electron irradiation in the ECR plasma was identified by using Langmuir single probe equipped with a designed simulated substrate and the irradiation mode was found to be controlled directly by altering the substrate bias voltage. Cross-linked GLEC film was prepared using the electron irradiation in the pressure of 0.04 Pa and positive bias voltage of 50 V, and the nanostructure and binding configuration of the film were analyzed by high resolution transmission electron microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS). The results showed that GLEC film contains cross-linked graphene layers grown normally to the substrate surface when the content of sp² hybridized carbon atoms in the film is more than 70%. The tribological behaviors of both cross-linked GLEC films and amorphous carbon films were compared using a Pin-on-Disk tribometer, and the mechanism for low friction coefficient was discussed by using HRTEM observation on wear track. The HRTEM results indicated that the cross-linked GLEC film has the potential to achieve low friction at the beginning of the friction.     

Exploring the potential of high power impulse magnetron sputtering for growth of diamond-like carbon films

Amorphous carbon films are deposited employing high power impulse magnetron sputtering (HiPIMS) at pulsing frequencies of 250 Hz and 1 kHz. Films are also deposited by direct current magnetron sputtering (dcMS), for reference. In both HiPIMS and dcMS cases, unipolar pulsed negative bias voltages up to 150 V are applied to the substrate to tune the energy of the positively charged ions that bombard the growing film. Plasma analysis reveals that HiPIMS leads to generation of a larger number of ions with larger average energies, as compared to dcMS. At the same time, the plasma composition is not affected, with Ar⁺ ions being the dominant ionized species at all deposition conditions. Analysis of the film properties shows that HiPIMS allows for growth of amorphous carbon films with sp³ bond fraction up to 45% and density up to 2.2 g cm^− 3. The corresponding values achieved by dcMS are 30% and 2.05 g cm^− 3, respectively. The larger fraction of sp³ bonds and mass density found in films grown by HiPIMS are explained in light of the more intense ion irradiation provided by the HiPIMS discharge as compared to the dcMS one.     

Density changes with substrate negative bias for ta-C films deposited by filter cathode vacuum arc

Specular X-ray reflectivity (XRR) measurements were used to study the density and cross-section information of tetrahedral amorphous carbon (ta-C) films deposited by filter cathode vacuum arc(FCVA) system at different substrate bias. According to the correlation between density and substrate negative bias, it is found that the value of density reaches a maximum at -80 V bias. As the substrate bias increases or decreases, the density tends to lower gradually. Based on the density of diamond and graphite, sp3 bonding ratio of ta-C films was obtained from their corresponding density according to a simple equation between the two. And a similar parabolic variation was observed for ta-C films with the sp3 content changes with substrate negative bias. The mechanical properties such as hardness and elastic modulus were also measured and compared with the corresponding density for ta-C films. From the distribution of data points, a linear proportional correlation between them was found, which shows that the density is a critical parameter to characterize the structure variation for ta-C films.     

Microstructure and tribological properties of the a-C:H films deposited by magnetron sputtering with CH4/Ar mixture

Hydrogenated amorphous carbon (a-C:H) films were deposited on steel and silicon wafers by unbalanced magnetron sputtering under different CH₄/Ar ratios. Microstructure and properties of the a-C:H films were investigated via Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Atomic force microscopy (AFM) and substrate curvature method. The results revealed that CH₄/Ar ratio played an important role in the H content but acted a little function on the sp³/sp² ratio of the films. Also, the internal stress of those films was relatively low (< 1 GPa), and the deposition rate decreased firstly and then increased with the decrease of the CH₄ fraction. The film deposited under CH₄/Ar = 1/1 (55 sccm/55 sccm) with moderate sp³ C-H / sp³ C-C had the best tribological properties. The composition, microstructure and properties of the a-C:H films were strongly dependent on the deposition process and composition of reactant gases.     

Diamond-like carbon thin films produced by femtosecond pulsed laser deposition of fullerite

Diamond-like carbon films have been deposited from a fullerite target by ultra-short pulsed laser deposition technique. The results indicate that the films morphology and structure, determined by scanning electron microscopy, atomic force microscopy and energy dispersive X-ray diffraction, depend strongly on the substrate temperature. X-ray photoelectron, X-ray Auger electron, Raman and surface enhanced Raman scattering spectra indicate that the fs-DLC films composition involves a mixed sp, sp² and sp³ carbon network consisting of aromatic rings and sp³ diamond-like structures linked by chains of different lengths and composition. The films deposited at room temperature, presenting the higher content of sp³ carbon (48%), also contain C₆₀ crystalline phase and show a very high hardness of 49 GPa.     

Deposition of diamond-like films of hydrogenized carbon

Diamond-like films of hydrogenized carbon on metal substrates were produced by deposition from methane activated by capacitive electric discharge (AD) within the frequency range of 5–250 Hz and with a relative pulse duration of the charge of 1.5–17. Variations in the charge parameters (frequency, relative pulse duration, current density, methane pressure, the value of gas flow), substrate temperature, and geometry of the vacuum chamber affect the deposition rate, properties, and structures of films within a wide range. Diamondlike films of hydrogenized carbon represented nanocomposite material. The size of sp ² clusters was 5–6 nm, whereas the sp ³/sp ² ratio of carbon and the content of bound hydrogen decreased with increasing substrate temperature and current density and decreasing methane pressure.     

Effect of bias voltage on microstructure and properties of Ti-doped graphite-like carbon films synthesized by magnetron sputtering

Ti-doped graphite-like carbon (GLC) films with different microstructures and compositions were fabricated using magnetron sputtering technique. The influence of bias voltages on microstructure, hardness, internal stress, adhesion strength and tribological properties of the as-deposited GLC films were systemically investigated. The results showed that with increasing bias voltage, the graphite-like structure component (sp² bond) in the GLC films increased, and the films gradually became much smoother and denser. The nanohardness and compressive internal stress increased significantly with the increase of bias voltage up to −300 V and were constant after −400 V. GLC films deposited with bias voltages in the range of -300--400 V exhibited optimum adhesion strength with the substrates. Both the friction coefficients and the wear rates of GLC films in ambient air and water decreased with increasing voltages in the lower bias range (0--300 V), however, they were constant for higher bias values (beyond −300 V) . In addition, the wear rate of GLC films under water-lubricated condition was significantly higher for voltages below −300 V but lower at high voltage than that under dry friction condition. The excellent tribological performance of Ti-doped GLC films prepared at higher bias voltages of −300--400 V are attributed to their high hardness, tribo-induced lubricating top-layers and planar (2D) graphite-like structure.     

Effect of hydrogen flow on the properties of hydrogenated amorphous carbon films fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition

The hydrogenated amorphous carbon films (a-C:H, so-called diamond-like carbon, DLC) have exceptional physical and mechanical properties and have wide applications. In the present study, amorphous hydrogenated carbon films (a-C:H) have been deposited on a Si (100) substrate at different hydrogen flow using electron cyclotron resonance chemical vapor deposition (ECR-CVD). The flow of hydrogen changed from 10 sccm to 40 sccm and the flow of acetylene was fixed at 10 sccm. The microstructure and properties of the a-C:H were measured using visible Raman spectra, Fourier transform infrared (FTIR) spectroscopy, UV-VIS spectrometer,surface profilometer and nano-indentation. The results showed that the sp³ content and sp³-CH₂ structure in the amorphous hydrogenated carbon films increased with the hydrogen flow. The deposition rate decreased with the hydrogen flow. The residual stress and the nano-hardness of the amorphous hydrogenated carbon films increased with the hydrogen flow. Consequently, the a-C:H film become more diamond-like with the increase of hydrogen flow.     

Properties of Zr-ZrC-ZrC/DLC gradient films on TiNi alloy by the PIIID technique combined with PECVD

The Zr-ZrC-ZrC/DLC gradient composite films were prepared on TiNi alloy by the techniques combined plasma immersion ion implantation and deposition (PIIID) and plasma enhanced chemical vapor deposition (PECVD). With this method, the Zr-ZrC intermixed layers can be obtained by the ion implantation and deposition before the deposition of the ZrC/DLC composite film. In our study, an optimal gradient composite film has been deposited on the NiTi alloys by optimizing the process parameters for implantation and deposition. The surface topography was observed through AFM and the influence of the deposition voltage on the surface topography of the film was investigated. XPS results indicate that on the outmost layer, the Zr ions are mixed with the DLC film and form ZrC phase, the binding energy of C 1s and the composition concentration of ZrC depend heavily on the bias voltage. With the increase of bias voltage, the content of ZrC and the ratio of sp³/sp² firstly increases, reaching a maximum value at 200 V, and then decreases. The nano-indentation and friction experiments indicate that the gradient composite film at 200 V has a higher hardness and lower friction coefficient compared with that of the bare NiTi alloy. The microscratch curve tests indicate that gradient composite films have an excellent bonding property comparing to undoped DLC film. Based on the electrochemical measurement and ion releasing tests, we have found that the gradient composite films exhibit better corrosion resistance property and higher depression ability for the Ni ion releasing from the NiTi substrate in the Hank's solution at 37°C.     

Effect of metal ion implantation on thermal instability of diamond-like carbon films

In this work, molybdenum and tungsten ions were implanted onto the DLC films deposited by filtered cathodic vacuum arc. We investigated the effects of ion species and doses on carbon related bonding property such as the ratio of sp³ carbon to sp² phase, the chemical composition and tribological properties of the DLC films in the range of 200 to 600 °C. The oxidation starting temperature decreased with an increasing ion dose and ion mass owing to higher sp² carbon fraction. Oxidation of the implanted-metal element, however, keeps the DLC film from carbon sublimation by oxidation, offering stable tribological characteristics by covering it with a metal oxide layer at the high temperature.     

Argon Implantation in Tetrahedral Amorphous Carbon Deposited by Filtered Cathodic Vacuum Arc

The implantation of argon in tetrahedral amorphous carbon (ta-C), deposited by the filtered cathodic vacuum arc technique and concurrently bombarded with argon ions (Ar⁺), is investigated in this study. The ta-C films were prepared with a 5-ms DC-pulsed arc, a current of 190 A, and a frequency of 3 Hz, and they were deposited on a ground substrate holder. The argon atoms were implanted into the film by simultaneously bombarding the films with a beam of Ar⁺ in the 0-180 eV energy range. The concentration of argon, determined by Rutherford backscattering spectroscopy, was investigated as a function of the Ar⁺ energy. Raman scattering spectroscopy was used to investigate the structure of the films. The stress of the films depends on the Ar⁺ energy and reduces significantly as a function of the annealing temperature. A study of argon effusion, ranging from room temperature up to 1000 °C, shows that the argon atoms evolve from the films at different temperatures depending on the Ar⁺ energy. Scanning electron microscopy revealed the formation of bubbles after argon effusion. It was observed that the structural transformations that promote the relaxation of the carbon matrix and the argon effusion are different from each other.