Hydrogen-free Diamond-like Carbon Films Research Articles

Extremely enhanced friction and wear performance of hydrogen-free DLC film at elevated temperatures via Si doping

Hydrogen-free diamond-like carbon (DLC) films have garnered significant attention as a highly efficient solid lubrication material, but poor high-temperature lubrication and wear resistance limit their further application. Here, silicon-doped DLC (Si-DLC) films with different Si contents were deposited using a superimposed HiPIMS-DCMS deposition system to improve their high-temperature lubrication and wear resistance performance. The influence of Si content on the microstructure, mechanical and high-temperature friction and wear behavior of Si-DLC films was investigated. The results showed that Si-DLC films had a dense structure and nanoscale smooth surface. The Si doping enhanced the adhesion strength and relieved the residual stress of Si-DLC films, but decreased the hardness and Young's modulus. Meanwhile, Raman and XPS spectroscopy revealed significant local structural changes in Si-doped DLC films. Furthermore, the results showed that DLC films doped with low Si content displayed excellent tribological performance at elevated temperatures due to the graphite lubricating layer on the surface of wear marks. Specifically, Si-DLC film with 3.31 at% Si had an ultra-low average friction coefficient of 0.04 at 450 °C and still provided effective lubrication at temperatures up to 500 °C. In contrast, pure DLC film failed to lubricate at 450 °C due to the severe graphitization transition and Si-DLC film with 17.47 at% Si quickly failed to lubricate at 200 °C due to the SiC and SiO2. This work sheds light for the high-temperature application of DLC films doped with low Si content by superimposed HiPIMS-DCMS deposition system.

Highly radiation-stable DLC coatings for a new class of detectors: Structural and morphological features

Within the framework of the FTM-NEXT INFN (Fast Time Micropattern gaseous detectors - next of Nuclear Physics National Institute) experiment, we produced hydrogen-free diamond-like carbon films through pulsed-laser deposition to serve as resistive layers in modern resistive micro-pattern gaseous detectors that must work in extreme radiation environments at future colliders. To obtain homogeneous diamond-like carbon coatings, over medium-to-large size (3 cm × 3 cm), with excellent adhesion to the substrate and with typical surface resistivity values in the range of 1–100 MOhm/sq, growth conditions had to be optimized. In this paper we report on the stability of resistive diamond-like carbon layers subjected to increasing doses of irradiation with proton beams accelerated to an energy of 2 MeV. The morphological, structural, and electrical properties, also at the nanoscale level, of diamond-like carbon coatings following ion irradiation were studied by electron microscopy, electron diffraction, electrical transport characterization and scanning tunneling spectroscopy.

Friction and wear behavior of molybdenum-disulfide doped hydrogen-free diamond-like carbon films sliding against Al2O3 balls at elevated temperature

Diamond-like carbon (DLC) films quickly experience lubrication failure due to oxidation and mechanical degradation when used above 300 °C, which limits their application. Molybdenum disulfide (MoS2) doping is considered as a feasible method to improve high-temperature properties of DLC films. However, the effect of MoS2 doping on the high-temperature friction behavior of MoS2-DLC composite films and its mechanism remain unclear. Here, MoS2-DLC composite films with different contents of MoS2 were synthesized by a superimposed deposition system consisting of high-power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). The MoS2 content in the films was controlled by varying the number of MoS2 pellets in the graphite-MoS2 (C–MoS2) mosaic target. The effect of MoS2 doping content on the structure and mechanical properties of MoS2-DLC films was analyzed. In addition, the friction and wear behavior and mechanism of MoS2-DLC composite films sliding against Al2O3 balls at 25–450 °C were studied. The results indicated that MoS2 was successfully incorporated into DLC matrix, and the content of Mo in the films ranged from 0 to 6.15 at. %. The as-deposited MoS2-DLC composite films exhibited a dense and featureless structure. The doping of MoS2 improved the adhesion strength and released the internal stress of MoS2-DLC composite films, but reduced the hardness and elastic modulus of the films. In particular, MoS2-DLC composite films with 3.78–6.15 at. % Mo maintained excellent anti-friction and anti-wear performances in the temperature range of 25–450 °C. Abrasive wear and fatigue wear occurred in the friction test of the films at different temperatures. However, the friction mechanism of the films varied with the temperature of friction test. At 25 °C, MoS2-DLC composite films showed excellent lubrication and anti-wear abilities due to the carbon suspension bond was passivated by H2O molecules. At 250 °C, MoS2 phase precipitated on the surface of the films, resulting in low friction. At 350 °C, MoS2-xOx solid solution was generated on the surface of the wear track to maintain lower friction and wear. Interestingly, MoS2-DLC composite films with high Mo content still achieved relatively low coefficient of friction and wear rate up to 450 °C due to the continuous availability of MoS2-xOx generated on the surface of wear track.

Hydrogen-free DLC films fabricated using superimposed HiPIMS-DCMS deposition system: Bias voltage effects

Diamond-like carbon (DLC) films were grown using a superimposed high-power pulse magnetron sputtering and direct current magnetron sputtering (HiPIMS-DCMS) deposition system at various bias voltages from 0 to −350 V. The microstructure, mechanical and tribological properties of the films were studied in relation to bias voltage. The results suggested that the deposition rate of DLC films decreased gradually with the enhancement of bias voltages. The surface roughness of DLC films decreased from 2.81 nm to 0.54 nm. The values of ID/IG of DLC films decreased first and increased later with the enhancement of bias voltages, and the value of ID/IG was the minimum at −150 V. Therefore, the highest sp3 CC fraction of DLC film was obtained when deposited at −150 V. Moreover, the hardness and internal stress of DLC films also show the law of increasing first and decreasing later. DLC film obtained the maximum hardness and highest internal stress of 32.0 GPa and − 2.08 GPa at −150 V. Finally, the friction test suggested that the films had excellent tribological performance. In particular, the film deposited at −150 V had excellent tribological properties, including low friction coefficient (0.095) and low wear rate (1.36 × 10−7 mm3/N·m).

Probing Friction Properties of Hydrogen-Free DLC Films in a Nitrogen Environment Based on ReaxFF Molecular Dynamics.

In this paper, ReaxFF molecular dynamics simulations were used to look at how load and the number of nitrogen molecules affect how friction behavior in hydrogen-free diamond-like carbon (DLC). The presence of nitrogen molecules will inhibit the formation of C-C covalent bonds between the contact surfaces of the upper and lower DLC, thereby effectively suppressing the increase in friction during the initial friction phase. After the initial friction stage, the mechanical mixing of the contact surfaces brought on by the diffusion of nitrogen molecules results in considerable shear stress, which has significant impacts on the friction force. In addition, due to the existence of nitrogen molecules, the effect of graphitization of hydrogen-free DLC on friction is almost negligible.

Tailoring sheet resistance through laser fluence and study of the critical impact of a V-shaped plasma plume on the properties of PLD-deposited DLC films for micro-pattern gaseous detector applications

Diamond-like carbon (DLC) is an attractive inexpensive alternative to diamond. The control of the C (sp3)/C(sp2) ratio, influencing the material properties, is fundamental for its use in dedicated applications like, for example, the fabrication of a particular class of gas detectors for ionizing particles termed Micro-Pattern Gaseous Detectors (MPGDs).A deposition technique enabling control on the amount of C(sp3) hybridized bonds, hydrogen content and disorder is pulsed lased deposition (PLD).In this paper, we report on the preparation of hydrogen-free DLC films by nanosecond excimer laser-based ablation of a graphite target correlating the laser fluence with the electrical properties of the DLC films to be used as protective resistive layers in MPGD detectors.Moreover, unprecedented physical insight and discussion are provided about the critical aspects of both occurrence of a V-branched plume and its composition on the spatial distribution of C(sp3) bonds and film uniformity. Improvement strategies are demonstrated by comparing and discussing two classes of PLD experiments: ON-axis (coaxial plasma expansion axis and substrate symmetry axis) and OFF-axis (substrate-axis shifted with respect to the plume-axis and rotating substrate). Optimally working OFF-axis configuration conditions are shown to lead to fine tuning of resistivity and uniformity of composition of C(sp3) bonding over a few cm2 extended areas. Sheet resistance (3.1 ± 0.3) × 109 Ω/sq., corresponding to C(sp3) bonding concentration of ~86%, is reported in OFF-axis experiments at a fluence of 5.5 J/cm2 onto kapton substrates which are used for the realization of MPGDs.

Understanding the DLC film – Polyamide 12 substrate interrelation during pulsed laser deposition

High-quality hydrogen-free diamond-like carbon (DLC) films that are free of any detectable signs of cracks and delamination are produced on polyamide 12 (PA) substrates using the Pulsed Laser Deposition (PLD) technique, resulting in subplantation of the polymeric substrate by carbon species and the formation of an interphase between the DLC film and the substrate. The interrelation between the DLC films and PA substrates of different crystallinity degree is investigated using spectroscopic (NMR, IR, XPS, and Raman) techniques and the morphology and topography of the films are evaluated using SEM and AFM. It is demonstrated that the laser fluence (16 and 20 J cm−2) directly affects the subplantation intensity and the DLC film structure. Additionally, it is shown that by changing the physical characteristics of the polymer substrate, such as the degree of crystallinity, the characteristics of the DLC layer and the response of the substrate to the energetic carbon species can be influenced. The studies open up new avenues for research aimed at understanding the interaction of energetic species with polymer substrates and enable the identification of parameters that are important in obtaining hard thin films of high quality on such substrates.

Microstructure evolution and wettability regulation of air-exposed hydrogen-free diamond-like carbon films

Regulation of surface wettability is of vital significance to diamond-like carbon (DLC) serving as multi-functional coatings in many important fields. In this study, hydrogen-free DLC films with highly adjustable surface wettability induced by surface local hybridized structures were deposited on silicon substrates by RF magnetron sputtering under different substrate temperatures and RF powers. A non-monotonic dependency of surface wettability on the hybridization states, which is just inverse for polar and non-polar liquids, was found on the homogeneous DLC films with nanoscale surface roughness. In particular, the wetting transition from hydrophilicity to hydrophobicity occurred on the DLC films in contact with water. A clear picture about the essential correlation of surface wettability to microstructures of DLC films was drawn in terms of detailed analysis of surface energy components associated with the evolution of sp2-C bonds or clusters in content, type and distortion. Further, a possible mechanism accounting for the wetting transition of the DLC films was also proposed. This work aims to reveal the wetting nature of hydrogen-free DLC films versus graphite and diamond, and provide an available strategy to fabricate high-performance DLC films with special surface wettability used in aqueous environment.

Microstructure, erosion and tribological behaviour of thick DLC coatings by the filtered cathodic vacuum arc combined with a high-voltage pulse power

Hydrogen-free diamond-like carbon (DLC) films with thickness of 36 μm and 50 μm have been prepared on Si and AISI 304L stainless steel substrates, respectively, by filtered cathodic vacuum arc (FCVA) together with a high-voltage pulse power. The structure, chemical bonding state, mechanical properties and wear, corrosion resistance and anti-erosion performance were evaluated. The thick DLC films with compressive stress of 0.71 GPa and hardness of 3160 HV were obtained. The preparation of the low stress but high hardness coating mainly due to the appropriate high negative bias and short pulse duration, which could result in an alternate ion bombardment and deposition. The 50 μm-thick coating showed corrosion resistance with high polarization resistance of 4.69 × 108 Ω·cm2 and low corrosion current density of 5.47 × 10−8 A·cm−2. Thick coating exhibited outstanding sand particle erosion resistance with low average mass loss rate of 0.035 mg g−1. Under high-speed sand erosion conditions, the coating remains unbroken, without peeling or cracking were observed on the erosion center. In addition, DLC coatings showed good tribological performance under dry, water- and oil-lubricated condition.

On the Origin of Reduced Cytotoxicity of Germanium-Doped Diamond-Like Carbon: Role of Top Surface Composition and Bonding.

This work attempts to understand the behaviour of Ge-induced cytotoxicity of germanium-doped hydrogen-free diamond-like carbon (DLC) films recently thoroughly studied and published by Jelinek et al. At a low doping level, the films showed no cytotoxicity, while at a higher doping level, the films were found to exhibit medium to high cytotoxicity. We demonstrate, using surface-sensitive methods—two-angle X-ray-induced core-level photoelectron spectroscopy (ARXPS) and Low Energy Ion Scattering (LEIS) spectroscopy, that at a low doping level, the layers are capped by a carbon film which impedes the contact of Ge species with tissue. For higher Ge content in the DLC films, oxidized Ge species are located at the top surface of the layers, provoking cytotoxicity. The present results indicate no threshold for Ge concentration in cell culture substrate to avoid a severe toxic reaction.

Dramatically enhanced mechanical properties of diamond-like carbon films on polymer substrate for flexible display devices via argon plasma pretreatment

Hydrogen-free diamond-like carbon (DLC) films have been deposited on polyimide (PI) substrate at room temperature using the sputtering with a novel nanodiamond target. Also, argon plasma pretreatment procedure for PI substrate was optimized to improve the adhesion and characteristics of the as-prepared DLC film. The deposited DLC film on the PI substrate pretreated with −400 V of bias voltage was found to possess the highest sp3 bonding fraction of 53%, the pencil hardness of 3H, and the optical transmittance of 82%, respectively. The research work demonstrates prosperous applications of DLC films in the protective coating for commercialized optoelectronic devices.

Gas phase lubrication on diamond-like carbon film: Tribochemical reactions under isobutane condition

Abstract Diamond-like carbon (DLC) film, with a host of intriguing properties, has emerged as a unique tribological material to be used in many fields. However, incomplete knowledge about the crucial parts, which control the frictional behavior under different gases, limits the further use of them. Here, we show that the tribological performance of hydrogen-free DLC film changed over a wide range by introducing hydrocarbon molecules into the frictional system. And the friction coefficient increased while the wear decreased with the increasing of isobutane amount. It was that the adsorbates which dissociate from isobutane molecules during tribochemical process on sliding interfaces caused the increasing trend of friction coefficient and provided a layer to prevent the sliding surface from wear.

Surface and in-depth distribution of sp2 and sp3 coordinated carbon atoms in diamond-like carbon films modified by argon ion beam bombardment during growth

Carbon atom coordination at diamond-like carbon (DLC) film surfaces and in sub-surface regions has been determined nondestructively from high-energy resolved C 1s photoelectron spectra, X-ray induced C KVV Auger electron spectra, and angular-resolved C 1s spectra (ARXPS) aided by maximum entropy method (MEM). The spectra were recorded from hydrogen-free DLC films prepared by a pulsed laser deposition under medium energy Ar ion beam assisted growth. The sp3 and sp2 fractions determined from C 1s and C KVV spectra recorded at the normal emission angle differ substantially. This indicates an inhomogeneous depth-resolved distribution of the fractions. The result is validated by the analysis of angular-resolved C 1s spectra using the MEM approach. In-depth reconstructions of the carbon bonding states show that sp2 coordination is dominant at the surfaces. We found that Ar ion beam assisted growth induces a C sp2 peak beneath the surface. The peak shifts towards the surface and is growing with Ar ion energy. C sp3 hybridization is dominant in deeper layers. The in-depth reconstruction is further supported by the depth-dependent mass density determined from the low-loss electron spectra excited at various primary electron energy. The results are discussed within the subplantation model.

Quantifying Macroscopic Friction of Diamond-like Carbon Films by Microscopic Adsorption and Removal of Water Molecules.

The adsorption and desorption of water molecules, which affect the physical and chemical properties of the sliding interface, are critical for the friction behaviors of two solid contacts in atmosphere environment. The amount of water adsorbed on the open surface is a function of gas pressure according to an adsorption equation. However, for a confined sliding interface, the variation of surface fraction covered by gas molecules with water vapor pressure and its induced effects on friction have not been figured out. In this work, the macroscopic friction of diamond-like carbon (DLC) films in a water vapor atmosphere is quantified on the basis of microscopic adsorption and removal of water molecules. The studies correlate the fraction of water molecules adsorbed on the interface of self-mated DLC films with water vapor pressure to illustrate the direct relationship between friction coefficient and gas pressure by first-principles calculations and model fitting. The calculated results revealed that chemisorption and physisorption of water molecules occur on the surfaces of hydrogen-free DLC films (ta-C) and hydrogenated DLC films (HCF). Then, the relation between friction and gas pressure was built by employing a fractional coverage model based on the linear adsorption equation and gas removal. The obtained model agrees well with the typical experimental results about the steady-state friction coefficient of both highly hydrogenated DLC film (HCF) and tetrahedral amorphous carbon (ta-C) film during sliding at various water vapor pressures. In addition, it gave the curves of fractional surface coverage as a function of water vapor pressure. These results show that the frictional coefficient of DLC films could be predicted on the basis of fractional surface coverage as well as the intrinsic characters on surface chemistry. We suggest that the model may be thus extended to understand and predict the friction of DLC films under a specific gas pressure at a low load and speed.

Friction and wear of hydrogenated and hydrogen-free diamond-like carbon films: Relative humidity dependent character

In this study, tribological properties of hydrogenated and hydrogen free diamond-like carbon films at various relative humidity (RH) were investigated to understand the friction mechanism in the presence of water molecules. At normal load of 2N, DLC-H film’s friction coefficient was 0.06 at RH14% while DLC film’s friction coefficient was 0.19 at RH17%. With the increase of RH, their friction coefficient converged to about 0.15. This character remained unaltered when the normal load was 5N. Results show that low friction of DLC-H film at low RH was attributed to the low shear force aroused by graphitic tribofilm at wear care center. However, the high friction of DLC film was mainly endowed by the high adhesive force aroused by σ dangling bonds. At high RH, solid-to-solid contact was isolated by water molecules confined between the counterfaces, where capillary was a dominant factor for friction. In addition to the capillary force, the absence of tribofilm was also accountable. These two factors lead to the level off of friction coefficient for DLC-H and DLC films. Moreover, for both DLC-H and DLC films, tribo-oxidization was proved to be closely related to wear rate with the assist of H2O molecules during sliding.

Effect of a hydrogenated interface on the wear behavior of a diamond-like carbon film in a water environment

Hydrogenated diamond-like carbon (DLC) films or DLC films with hydrogenated interlayers tend to undergo early stage failure in water environments for reasons that are not fully understood. In this paper, a hydrogenated interface was prepared between a hydrogen free DLC film and a CoCrMo substrate, and the effects of the hydrogenated interface on the film structure, hardness, adhesion, wear behavior and corrosion resistance were investigated. The hydrogen free DLC films on the hydrogenated and the hydrogen free substrates exhibited similar structures and mechanical properties. The DLC films on both the hydrogenated and the virgin CoCrMo substrates exhibited good adhesion. The adhesions of the DLC films on both the substrates were not deteriorated after 150days of immersion in 0.9% NaCl solution. The wear resistance of the DLC film on the hydrogenated CoCrMo substrate was worse than that on the virgin CoCrMo substrate in water. The presence of hydrogen improved the activity of the interface, restrained the formation of a passivation layer, and promoted the corrosion of the interface, which caused early stage failure of the DLC film in water. Therefore, the hydrogenated interface has a negative effect on the wear behaviors of DLC films in water environments.

Dry machining of metal using an engraving cutter coated with a droplet-free ta-C film prepared via a T-shape filtered arc deposition

Abstract Tetrahedral amorphous carbon (ta-C) films were prepared on cutters which are used for engraving. Engraving tests were performed on metal plates using non-coated and ta-C-coated cutters, under dry machining conditions, without oil. Generally, the metal plate is engraved using a cemented carbide (WC-Co) cutter, which uses oil for the cutting process, to fabricate a nameplate. The oil causes a high environmental load and complicates the machining process. Therefore, from an industrial viewpoint, there is a need for a dry machining process. The ta-C film is a hydrogen-free diamond-like carbon film with a high mechanical hardness. In this study, droplet-free ta-C films were prepared on WC-Co cutters using a T-shape filtered arc deposition method. The work materials used in the engraving tests were aluminum, copper, and brass. The results of the engraving tests demonstrated suppression of metal adhesion to the cutting edge, prevention of chipping of the cutting edge, and a decrease in cutting resistance during engraving when a ta-C film had been coated onto the cutting edge. In addition, burr and residue on engraved grooves were also suppressed by using a ta-C-coated cutter. The ta-C film coating offered a remarkable improvement in the engraving performance of a WC-Co cutter in the dry machining process.

Understanding the unusual friction behavior of hydrogen-free diamond-like carbon films in oxygen atmosphere by first-principles calculations

The friction coefficients of diamond and hydrogen-free diamond-like carbon films were reported to be in an intermediate range, from 0.2 to 0.4, in oxygen atmosphere, but the mechanism has not been explored. Herein, by using first-principles calculations, we attempt to fill this gap by providing atomistic insights into the bonding behaviors of oxygen atom(s) at interfaces constructed by two self-mated diamond (001) or (111) surfaces. The effect of the interfacial oxygen coverage on the optimized interfacial structures is highlighted. The results show that if the oxygen coverage is not higher than 0.5, the two surfaces tend to be connected by oxygen atoms by formation of C–O–C bonds, and thus high friction is expected. Otherwise, the two surfaces keep being separated, and thus low friction is expected. The interfacial electron distributions are manifested to underlie the structural optimizations. Insights gained here together with the findings of T. E. Derry et al. concerning oxygen coverage on diamond surfaces contribute to understanding the unusual frictional behavior of hydrogen-free diamond-like carbons in oxygen atmosphere.

金属 3D プリンティングと DLC 膜を併用した新規摺動面の創製

The objective of this study is to materialize a high-performance tribo-surface realizing excellent tribological properties such as low friction, high wear resistance and so on. In this study, We designed a high-performance tribo-surface named oozing tribo-surface (OTS): lubricant oozes out from internal structures beneath the sliding surfaces to stabilize a low friction. As a fundamental research, disk specimens with the OTS were manufactured by selective laser sintering (SLS) 3D printer. Moreover, to obtain a synergistic effect, hydrogen-free Diamond-like Carbon (H-free DLC) films were coated on the OTS (OTS-D). In this report, tribological properties of the OTS and OTS-D were compared. Friction coefficients were evaluated by using a cylinder-on-disk tribo-tester. Examinations were carried out under the lubrication with an engine oil and dry condition. The OTS-D lubricated with the oil reached stable a low friction coefficient of approximately 0.085 due to a synergistic effect of oozed out lubricant system and low friction property of DLC film. In summary, OTS-D could be one of effective surface modification to stable a low friction behavior.

Characterization of physical and biomedical properties of nitrogenated diamond-like carbon films coated on polytetrafluoroethylene substrates

A technique to coat hydrogen-free diamond-like carbon (DLC) films on polytetrafluoroethylene (PTFE) substrates has been developed by sputtering of a negatively biased graphite target in a mixture of argon and nitrogen plasma. The coated films were characterized by various methods to investigate their chemical, electronic features, and particularly their biomedical properties. DLC films produced by this method have up to 20% sp3 carbon bonds depending on the nitrogen concentration in the plasma. Raman spectroscopy revealed that, bond-disorder increases with nitrogen doping. The average grain size of DLC decreases in the nitrogen doped samples by almost 30%. The roughness of the uncoated PTFE substrate surfaces decreased dramatically from 660nm to 170nm after DLC coating. However, the nitrogen contents in the plasma have little effects on the roughness, the cluster size, and shapes. Electronic band gap of the samples decreases with adding nitrogen from ~2eV in nitrogen-free samples to ~1eV in nitrogenated samples. Lower adhesion and aggregation of platelets on PTFE surfaces coated with DLC-10% nitrogen and DLC-20% nitrogen have been observed while there is greater adhesion of platelets on DLC-30% nitrogen and DLC-40% nitrogen.