Hydrogen-free Amorphous Carbon Films Research Articles

Tribological performances of a-C films tested on dry nitrogen environment with various Hertz pressures: Failure mechanisms and in-situ formation graphene

The tribological properties of hydrogen-free amorphous carbon (a-C) films are affected by the test environment and load condition. The wear mechanisms of a-C films sliding against Si3N4 ball under inert atmosphere exhibits different mode during different sliding cycles. The results show that the tribological behaviors for a-C films exhibit adhesive wear and it did not wear out even after sliding test 40,000 cycles when the normal load is 1 N. As the normal load increases, the wear mechanisms mainly show fatigue wear and three-body wear induced by fatigue wears due to the increasing of alternating stress. The emergence of three-body wear will drastically change the local stress distribution of a-C film. The position obtaining maximum Von-Mises stress shifts from the subsurface of substrate to subsurface of a-C film, and the maximum Von-Mises stress and Hertzian contact pressure increases sharply. The structures of in-situ formation graphene originated from the transformation of amorphous carbonaceous structure are detected on the wear scars. The formation of carbonaceous transfer film is beneficial to facilitate the graphitization transition with increasing Hertz pressure from 0.65 GPa to 1.35 GPa. On the other hand, the transition temperature of graphitization is sharply reduced to the flash temperature of contact zone due to the emergence of three-body wear obtaining the instantaneous Hertz pressure reached to 29.5 GPa. These conclusions possibly provide valuable references for designing film thicknesses, reveal tribological mechanism for a-C films under dry inertness atmosphere and broaden industrial application.

Laser-induced graphitized periodic surface structure formed on tetrahedral amorphous carbon films

Femtosecond laser-induced periodic surface structure (LIPSS), graphitization and swelling observed on ultra-hard, hydrogen-free tetrahedral amorphous carbon (ta-C) films are examined and compared with those on hydrogenated amorphous carbon (a-C:H) films, nitride films, and glassy carbon plates. The threshold fluence for LIPSS formation on ta-C is approximately twice as high as that for other specimens, and the LIPSS period Λ near the threshold is very fine at ca. 80 nm. Λ gradually increases with increasing fluence, and rapidly increases to ca. 600 nm at a high fluence. The ablation rate also increases rapidly at this fluence. In addition, ta-C and a-C:H are graphitized by irradiation and expand in volume. The surface layer of ta-C film changes to nanocrystalline graphite as the fluence increases and the crystallinity is improved; however, at higher fluence, the crystallinity deteriorates suddenly similar to that at low fluence. At high fluence, the rapid increase in Λ and the ablation rate, and the sudden deterioration in crystallinity are determined as common phenomena for these disordered carbons. LIPSS formation and swelling over a large area by scanned spot irradiation produces submicron height flat hills with conductivity and surface functionality on the insulating surface.

Depth-resolved study of hydrogen-free amorphous carbon films on stainless steel

A method to obtain depth-resolved structural information from hydrogen-free amorphous carbon films of high roughness, high thickness and grown on unpolished substrates was developed. The characterization was based on combining secondary ion mass spectrometry (SIMS) sputtering for craters at defined depth with X-ray photoelectron spectroscopy (XPS) and visible Raman spectroscopy analysis. After determination of the etching rate for the carbon film, areas with a defined end position corresponding to different known depths of the films were obtained. The SIMS etching process used did not significantly affect the structure of the amorphous carbon layer. XPS measurements of the crater bottoms provided information about the composition of the films. Visible Raman spectroscopy measurements inside of the craters were correlated with the XPS results taking into account the penetration depths of both techniques, and models aimed at predicting the sp2/sp3 ratio from Raman measurements were evaluated.

Thermal stability of ultrathin amorphous carbon films synthesized by plasma-enhanced chemical vapor deposition and filtered cathodic vacuum arc

Ultrathin hydrogenated amorphous carbon (a-C:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and hydrogen-free amorphous carbon (a-C) films of similar thickness deposited by filtered cathodic vacuum arc (FCVA) were subjected to rapid thermal annealing (RTA). Cross-sectional transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) were used to study the structural stability of the films. While RTA increased the thickness of the intermixing layer and decreased the sp3 content of the a-C:H films, it did not affect the thickness or the sp3 content of the a-C films. The superior structural stability of the FCVA a-C films compared with PECVD a-C:H films, demonstrated by the TEM and EELS results of this study, illustrates the high potential of these films as protective overcoats in applications where rapid heating is critical to the device functionality and performance, such as heat-assisted magnetic recording.

Characterization and properties of duplex a-C:H/MAO coatings on magnesium alloy using combined microarc oxidation and hybrid magnetron sputtering

The combined microarc oxidation (MAO) and magnetron sputtering deposition process was used to deposit duplex a-C:H/MAO and Ti-a-C:H/MAO coatings on AM80 magnesium alloy. The microstructure, mechanical properties and tribological behavior of the two duplex coatings were investigated. The experimental results showed that the a-C:H and Ti-a-C:H top films on Si substrates were dense and had a low G peak position and ID/IG ratio, compared with the hydrogen-free amorphous carbon films. Numerous micropores were found on the duplex a-C:H/MAO and Ti-a-C:H/MAO coatings together with low values of hardness (H) and elastic modulus (E), which also showed good binding strength with the Mg alloy substrates. Compared to MAO treated substrate used for the protection of the Mg alloy, the duplex a-C:H/MAO and Ti-a-C:H/MAO coatings still had stable and low value of friction coefficient, even though the surface of the duplex coatings was rough and porous. Furthermore, the mechanism of friction reduction of the two duplex coatings on the Mg alloy substrates was discussed.

The Optical Properties of DLC Films Processed by Laser Radiation

Diamond-like carbon (DLC) films have being widely applied in various domains owing to their excellent optical and mechanical properties, chemical inertness and bio-compatibility[1,2]. But it generally exhibits low a laser induced damage threshold (LIDT).As to infrared protective films, DLC films were asked to be a high laser damage threshold. So far, the real reason of laser damage is unclear. In order to understand the real reason of the laser damage, it is necessary to deeply research the relationship between optical properties and laser radiation. In this paper, optical properties post irraditing by various laser energy were inverstigated. Hydrogen-free amorphous carbon films were deposited by unbalanced magnetron sputtering with a graphite target on Si substrate in this paper. The DLC films were radiated with raster scans methods by a 1064nm and 8–10 ns laser in this work. The transmittance, refractive index, extinction coefficient and surface morphology of DLC before and after radiation were examined by fourier transform infrared spectrometer, ellipsometer and talysurf respectively. The influences were considered as function of laser energy density.The optical properties before and after laser radiation were compared. It is found that as laser fluences rise from 0 to 0.5 J/cm2, the transmittance peak reduce from 67.06% to 58.27%, the refractive index decrease from 1.88 to 1.67(λ=1200), the extinction coefficient increase from 0.07 to 0.32(λ=1200), and the surface roughness become 149.28nm in this procedλure. Especially, the reasons of change were expounded in this paper.

Nanosecond UV laser graphitization and delamination of thin tetrahedral amorphous carbon films with different sp3/sp2 content

We have produced hydrogen-free tetrahedral amorphous carbon films with different densities and Young's modulus by coating silicon with a filtered vacuum arc under different angles. The films were modified with a pulsed laser (wavelength 355nm) into sp2 rich amorphous carbon and nano crystalline carbon films. The graphitization threshold of the films depends on the film thickness as well as on the carbon density. Simulations of the optical absorption of the different carbon films permitted to confirm the experimental results. On the other side, the delamination threshold of carbon films increases with the film thickness and was found to be controlled by thermal properties of the film. The thin film graphitization and delamination is investigated by optical microscopy, atomic force microscopy, scanning electron microscopy and Raman spectroscopy.

Acetylene–argon plasmas measured at an rf-biased substrate electrode for diamond-like carbon deposition: II. Ion energy distributions**This paper was presented as an invited talk at the 19th European Sectional Conference on Atomic and Molecular Physics of Ionized Gases, Granada, Spain, 15–19 July 2008. See stacks.iop.org/PSST/18/3.

Ion energy distributions (IEDs) have been determined at the radio frequency (rf)-biased electrode in an inductively coupled acetylene–argon plasma for various substrate bias voltages and frequencies under conditions suitable for diamond-like carbon (DLC) and polymer-like film deposition. These are compared with those obtained at a capacitively coupled plasma grounded wall. In the former, for pressures <25 mTorr, the IEDs exhibit bimodal structures with peak separation values that follow the expected voltage and frequency dependences. At higher pressures, 120 mTorr, the bimodal structure is replaced by a single peak. For all conditions the dominant ion is Ar+ or ArH+ despite the set flow ratio of C2H2 : Ar of 2 : 1 and this can be attributed to the high electron dissociation of the parent molecule. DLC films indicate a peak hardness at an ion energy of around 90 eV and a very sharp fall in hardness is noted beyond this value. This is similar to the observed sp3-bond formation in hydrogen-free tetrahedral amorphous carbon or bias-sputtered films. However, due to the lack of carbon-based ions, an alternative mechanism is likely based on argon knock-on implantation of surface adsorbed carbon species. The results have shown that the use of high-frequency bias or bias harmonics may lead to much narrower IEDs.

The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance

Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R a and R z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the “Skewness” ( R sk) and “Kurtosis” ( R ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R a = 10 nm. This phenomenon is explained by analyzing the R sk and R ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.

Hydrogen-free carbon thin films prepared by new type surface-wave-sustained plasma (SWP)

In this paper is described the simple structure of a new type surface-wave-sustained plasma (SWP) source without a magnetic field surrounding the chamber wall. In the source, the plasma is excited and sustained by 2.45 GHz microwaves, and the plasma density is measured by a single Langmuir probe in the target direction. The results indicate that the electron density obtained in this system is as high as 9 × 10 11 cm − 3 even at a low pressure of 2.8 Pa. A graphite target (99.998%) and argon (99.999%) are used for depositing hydrogen-free amorphous carbon films by the new SWP source. The Raman spectra of the carbon films were obtained, and the results denote that the structure of the carbon films prepared by SWP is typical of diamond-like carbon; the Raman intensity ratio I D/ I G is 2.97. The surface morphology was investigated by using an atomic force microscope (AFM). The images demonstrate that the hydrogen-free carbon films deposited by SWP have a very smooth surface, with a grain size of about 20 nm and surface roughness R a of about 0.778 nm.

Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment

Friction and wear behaviors of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) films sliding against Si 3N 4 balls were investigated in different testing environments. The result showed that two films with extreme chemical disparity (one hydrogenated, and the other hydrogen free) showed distinct different friction and wear behaviors, and the friction and wear behaviors of the both films were strongly dependent on the environment. For a-C:H films, much low friction coefficient and wear rate were obtain in dry N 2. In the water and/or oxygen containing environments, the friction coefficient and wear rate of a-C:H films were obviously increased. On the contrary, a-C films only provided low friction coefficient and wear rate in the presence of water and/or oxygen in the test chamber. In dry N 2, the highest friction coefficient and wear rate were observed for a-C films. By investigating the worn surfaces of the films using XPS, it was proposed that the environment dependence of the friction and wear behaviors of the films was closely related with the friction-induced chemical interactions between the films and water and/or oxygen molecules. The specific roles of hydrogen, water and oxygen molecules and their tribochemical effects on the friction and wear mechanism of the films are discussed.

Room temperature photoluminescence from nanostructured amorphous carbon

Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (PAr). By varying PAr from 5to340mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp2 bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp2 rich phase.

Effect of pressure on the deposition of hydrogen-free amorphous carbon and carbon nitride films by the pulsed cathodic arc discharge method

Hydrogen-free amorphous carbon (a-C) and carbon nitride (a-C:N) films were deposited using the pulsed cathodic arc discharge at different argon and nitrogen pressures. The surface and mechanical properties of these films were found to strongly depend on the gas pressure. The tetrahedral amorphous carbon and hard a-C:N films with smooth surfaces (rms roughness: 0.15nm) were prepared at lower gas pressures (&amp;lt;6×10−2Pa). Incorporation of an increasing amount of nitrogen in a-C:N films caused a decrease in film hardness. All the films were covered with the thin (0.3–2nm) graphite-like surface layers. The film hardness was correlated to the soft surface layer thickness, and the films with thinner surface layers exhibit higher hardness. The mean energies of pulsed plasma beams were measured as the functions of argon and nitrogen pressures. The mean energies of plasma beams decrease in an exponential fashion with increasing gas pressure due to the carbon ion collisions with the neutral gas species. The effects of mean energies of deposited species on the film deposition were explained in terms of the thermal spike migration of surface atoms. The formation of graphite-like surface layers is associated with the low-energy deposition process. The low-energy (&amp;lt;1–3eV) species diffusing on film surface lead to the formation of graphite-like a-C films with plenty of grains. The higher-energy (&amp;gt;10eV) species may produce the strong thermal spike at film surface, and contribute to the formation of sp3 bonded structure at a sp3 bonded matrix.

Mechanical and tribological properties of amorphous carbon films deposited on implanted steel substrates

Hydrogen-free amorphous carbon (a-C) films were deposited using unbalanced magnetron sputtering technique from graphite targets on AISI 440C steel substrates implanted with (1) carbon (C), (2) titanium (Ti), and (3) titanium followed by carbon (Ti+C), respectively. After deposition, the adhesion strength of the films was examined by scratch test and Rockwell-C indentation test. The tribological performance of the films was evaluated by a typical ball-on-disk tribometer and a reciprocating wear tester. A dynamic impact tester was also carried out to study the fatigue strength of the films. In order to study the effect of the pre-treatment of steel substrates by means of ion implantation on the actual performance of a-C films, the implanted substrates were investigated by using X-ray photoelectron spectroscopy and nano-indentation, from which the composition depth profile as well as the hardness ( H) and elastic modulus ( E) depth profiles could be accurately obtained. As a result, due to higher contents of carbide bonds appeared at the outmost surface of the C and Ti+C implanted substrates, a critical load over 65 N was obtained, indicating good scratch resistance of the films. The combination of high interfacial strength and high plastic deformation resistance ( H 3/ E 2) of the Ti+C implanted substrates led to a higher load-carrying capacity and longer duration lifetime in the sliding wear test. In the dynamic impact test, the good adhesion strength and high toughness of C and Ti+C implanted substrates improved the impact resistance of the films.

Pulsed arc deposition of super-hard amorphous carbon films

Hydrogen-free amorphous carbon films with hardness up to 75 GPa have been deposited by special pulsed arc techniques. The influence of plasma and deposition conditions on the film properties is discussed and some applications are shown.

THE DETERMINATION OF SP3 FRACTION IN TETRAHEDRAL AMORPHOUS CARBON FILMS BY RAMAN AND X-RAY PHOTOELECTRON SPECTROSCOPY

Tetrahedral amorphous carbon films have been prepared by magnetic filtered plasma deposition system. The samples were deposited with negative bias voltage V b range from +20 V to -95 V. The relative fraction of sp 3-bonded carbon in these films was qualitatively and quantitatively estimated by a fitting of the Raman and XPS spectra, respectively. Raman results show that the sp 3 fraction of the films deposited in the range from -10 V to -50 V was estimated to be more than 80%. The C 1s spectra were consistently decomposed into the two gaussian components, one is at 284.4±0.1 eV corresponding to sp 2-hybridized bonds, and another is at 285.2±0.1 eV corresponding to sp 3-hybridized bond. The sp 3 content can be quantitatively calculated from the integrated area of the respective gaussian peak divided by the total area of the spectra. The comparison of XPS with Raman analysis, the trend of qualitative variation of sp 3 content with V b is in agreement. We concluded that XPS analysis is a useful method for quantitatively calculating sp 3 content in hydrogen-free tetrahedral amorphous carbon films.

Electron field emission from hydrogen-free amorphous carbon-coated ZnO tip array

Enhanced electron field emission from hydrogen-free amorphous carbon (a-C) films deposited on ZnO-tip-array-coated Si substrates was studied. Both the ZnO tip array and the a-C films were deposited by filtered arc deposition. Compared with the a-C film deposited on Si substrates, the a-C film on ZnO had a lower threshold field (4.5 V/μm), higher field emission current density (0.215 mA/cm2 at 14.3 V/μm) and higher emission site density (&amp;gt;2×103/cm2 at 20 V/μm). The introduction of the ZnO tip array provides high local geometric electric-field enhancement for the a-C film. On the other hand, the amount of excess Zn in the ZnO film provides better conductivity. The result is that the electrons can be easily penetrated into the a-C films, which enhances the field emission properties.

Differentiating the tribological performance of hydrogenated and hydrogen-free DLC coatings

Diamond-like carbon (DLC) coatings cover a wide range of different types of carbon-based coatings, which generally have properties such as low friction and high wear resistance. DLC films can be divided into two major groups based on their hydrogen content, namely hydrogenated and hydrogen-free carbon coatings. This presentation describes the research work on amorphous hydrogenated carbon films (a-C:H) and hydrogen-free tetrahedral amorphous carbon films (ta-C). These coating types offer low friction performance and good wear resistance, but they also have some dissimilarities in the friction and wear properties due to their different hydrogen content and microstructure. The a-C:H films deposited for this study were produced by the rf plasma deposition technique and the ta-C films by the pulsed vacuum arc technique. The tribological performance of the a-C:H and ta-C coatings was evaluated by pin-on-disc tests carried out in normal atmosphere. Selected coatings were also evaluated in dry atmosphere. The wear resistance of the ta-C films was higher compared to the a-C:H films, but the ta-C films caused higher wear of the counterpart. The increase in normal load and sliding velocity decreased the friction coefficient of the a-C:H coating against the steel and alumina counter-face, whereas the ta-C coating showed more stable friction performance. The micro-Raman studies showed clear graphite formation for the a-C:H film, whereas the graphite formation on the ta-C film was not so evident. In dry conditions the ta-C had a high friction coefficient, which could be reduced by doping the film with hydrogen. The results show that the hydrogen content together with graphitisation plays an important role in the friction performance of DLC films.

Effect of annealing on electron field emission properties of hydrogen-free amorphous carbon films

In this paper, a series of 200-nm-thick, hydrogen-free, amorphous carbon (a-C) films deposited on heavily doped Si (111) by filtered arc deposition (FAD) were isothermally annealed at 200, 400, 600, 800, and 1000°C for 30 min. Electron field emission from these films was studied by using a diode structure device. It was shown that field emission properties of the films were degraded with increasing annealing temperatures. But after being annealed at 800°C for 30 min, the a-C film showed enhanced field emission properties compared with the unannealed a-C film. Atomic force microscopy (AFM) showed that a-C film annealed at 800°C had dense protrusions on the surface. It was shown that annealing can have a remarkable effect on field-emission properties of the a-C films.

Fluctuation Microscopy Studies of Amorphous Diamond-Like Carbon Films

Hydrogen-free amorphous diamond-like carbon films have stimulated great interest because of their useful properties, such as high hardness, chemical inertness, thermal stability, wide optical gap, and negative electron affinity[l]. Consequently, they may have various potential applications in mechanical and optical coatings, MEMS systems, chemical sensors and electronic devices. Amorphous diamond-like carbon films often contains significant amounts of four-fold or sp3 bonded carbon, in contrast to amorphous carbon films prepared by evaporation or sputtering which consist mostly of three-fold or sp2 bonded carbon. The ratio and the structure configurations of these three-fold and four-fold carbon atoms certainly decide the properties of these amorphous diamond-carbon films. Although the ratio of three-fold and four-fold carbon has been studied with Raman spectroscopy and electron-loss-energy spectroscopy, very little has been understood regarding key questions such as how the three-fold and the four-fold carbon atoms are integrated in the film, and what structures those three-fold carbon atoms take.