Hydrogen-free Tetrahedral Amorphous Carbon Research Articles

The structure and electronic properties of tetrahedrally bonded hydrogenated amorphous carbon

We have synthesized hydrogenated and deuterated amorphous carbon materials that have a density, 2.7 ± 0.1 g/cm3, consistent with almost entirely tetrahedral bonding. In hydrogen-free tetrahedral amorphous carbon, the presence of a minority of sp2 bonded atoms leads to localized states that could be passivated with hydrogen by analogy with hydrogenated amorphous silicon. Neutron diffraction analysis demonstrated that the local bonding environment is consistent with ab initio models of high density hydrogenated tetrahedral amorphous carbon and with the related tetrahedral molecular structure neopentane. The optical bandgap of our material, 4.5 eV, is close to the bandgap in the density of states determined by scanning tunneling spectroscopy (4.3 eV). This bandgap is considerably larger than that of hydrogen-free tetrahedral amorphous carbon, confirming that passivation of sp2 associated tail-states has occurred. Both the structural and electronic measurements are consistent with a model in which the tetrahedrally bonded carbon regions are terminated by hydrogen, causing hopping conductivity to dominate.

Effect of doping elements to hydrogen-free amorphous carbon coatings on structure and mechanical properties with special focus on crack resistance

The doping of amorphous carbon is one way of specifically modifying its properties. The present work is a contribution to understand the effect of dopant selection on structure, mechanical properties and crack resistance of hydrogen-free tetrahedral amorphous carbon (ta-C) coatings. For this purpose, nonmetallic (B, Si) and metallic (Mo, Fe, Cu) dopants were selected. The coatings were produced by the Laser-Arc technique and investigated by Raman spectroscopy, transmission electron microscopy, nanoindentation, bending, and scratch tests. It was found that doping generally affected the sp2 content and cluster size of the sp2 carbon phase, and thus hardness, Young's modulus, and intrinsic compressive stresses, but also the defect content and the crack resistance. The addition of metallic dopants led to an increase and clustering of the sp2-bonded carbon and, consequently, to a significant reduction in hardness and Young's modulus. This effect was much less pronounced when doping with non-metallic elements. The failure behavior changed toward higher ductility and crack resistance when metals were added. The crack resistance was found to be directly dependent on the H3/E2 value, with H3/E2 > 0.2 leading to a more brittle behavior with unstable crack propagation, while at H3/E2 < 0.2 the material was more ductile and exhibited stable crack propagation and thus higher crack resistance.

Low-Friction of ta-C Coatings Paired with Brass and Other Materials under Vacuum and Atmospheric Conditions.

Vacuum environments provide challenging conditions for tribological systems. MoS2 is one of the materials commonly known to provide low friction for both ambient and vacuum conditions. However, it also exhibits poor wear resistance and low ability to withstand higher contact pressures. In search of wear-resistant alternatives, superhard hydrogen-free tetrahedral amorphous carbon coatings (ta-C) are explored in this study. Although known to have excellent friction and wear properties in ambient atmospheres, their vacuum performance is limited when self-paired and with steel. In this study, the influence of the paired material on the friction behavior of ta-C is studied using counterbodies made from brass, bronze, copper, silicon carbide, and aluminum oxide, as well as from steel and ta-C coatings as reference materials. Brass was found to be the most promising counterbody material and was further tested in direct comparison to steel, as well as in long-term performance experiments. It was shown that the brass/ta-C friction pair exhibits low friction (µ < 0.1) and high wear in the short term, irrespective of ambient pressure, whereas in the long term, the friction coefficient increases due to a change in the wear mechanism. Al2O3 was identified as another promising sliding partner against ta-C, with a higher friction coefficient than that of brass (µ = 0.3), but considerably lower wear. All other pairings exhibited high friction, high wear, or both.

Interplay of mechanics and chemistry governs wear of diamond-like carbon coatings interacting with ZDDP-additivated lubricants

Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear.

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.

Crystalline transformation from ta-C to graphene induced by a catalytic Ni layer during annealing

As an alternative strategy to fabricate high-performance graphene over a large area, metal catalysis has been attempted at elevated temperatures with various solid carbon sources such as polymers and amorphous carbon. Due to the diversity of amorphous carbon materials and the variety of metal catalysts used in the processes, the crystalline transformation from the amorphous carbon state to the graphene phase is not yet clearly understood. Here, we fabricated hydrogen free tetrahedral amorphous carbon (ta-C) as a solid carbon source, and nickel was chosen for the catalytic metal layer, which thereafter were integrated into a designed Si/SiO2/ta-C/Ni sandwich structure. The effect of thermal annealing in the range of 400 °C to 900 °C on the transformation of atomic carbon bonds was investigated. The results showed that the formation of the graphene phase emerged at an annealing temperature of 600 °C. Further increasing the temperature to 900 °C stimulated enhanced quality of graphene, but the quality was poor due to the oversaturated carbon content and the agglomeration of the Ni layer. Decreasing the thickness of ta-C to 2 nm led to graphene with 3–4 layers at a temperature of 800 °C, where metal-induced layer exchange was proposed as the key factor for the crystalline transformation from ta-C to graphene during thermal annealing. The results introduce a new strategy for the fabrication of high-quality graphene from tetrahedral amorphous carbon.

Comparative study on effects of load and sliding distance on amorphous hydrogenated carbon (a-C:H) coating and tetrahedral amorphous carbon (ta-C) coating under base-oil lubrication condition

In this study, the lubricated friction and wear associated with diamond-like coatings (DLCs) of amorphous hydrogenated carbon (a-C:H) and hydrogen-free tetrahedral amorphous carbon (ta-C) were comparatively analyzed. The effects of load and sliding distance on the wear and friction behaviors of the coatings were studied under a boundary-lubrication condition using a poly-alpha-olefin base oil. The DLC-coated cylindrical specimens were slid with their central axes perpendicular to the sliding direction against a rotating steel counterface. The results show that the friction coefficient of the ta-C coating was lower than that of the a-C:H coating, though a graphitized layer was detected both inside and outside the wear scar of the a-C:H coating. Under a relatively low applied load (5N), the specific wear rate of the ta-C coating was considerably lower than that of the a-C:H coating. When the load was increased to a critical value of 10 N, the specific wear rate of the ta-C coating increased rapidly to 3–5 times that of the a-C:H coating. However, the specific wear rate of the a-C:H coating decreased significantly when the load exceeded 10N, which was consistently maintained thereafter. With the increase in the sliding distance, the specific wear rate of the ta-C coating increased rapidly. In comparison to the ta-C coating, the specific wear rate of the a-C:H coating decreased only slightly with the increase in the sliding distance; moreover, when the sliding distance exceeded 125m, the specific wear rate of the a-C:H coating became much lower than that of the ta-C coating. The results of Raman spectroscopy, atomic force microscopy, and scanning electron microscopy show that the coatings were influenced by different types of wear mechanisms when slid under pure base-oil boundary-lubrication conditions.

Electrical conduction of ion tracks in tetrahedral amorphous carbon: temperature, field and doping dependence and comparison with matrix data

This paper gives an extended overview of the electrical properties of ion tracks in hydrogen-free tetrahedral amorphous carbon (ta-C) with a sp3 bond fraction of about 80%. The films were grown by mass selected ion beam deposition of 100 eV 12C+ ions. The ion tracks are generated by irradiation of ta-C films with uranium ions of 1 GeV kinetic energy. Along the ion path a conversion from diamond-like (sp3) carbon to graphite-like (sp2) carbon takes place. Topography and current measurements of individual ion tracks were performed by atomic force microscopy at ambient temperature. The temperature dependence of the electric conductivity was studied between 15 and 390 K by means of 0.28 mm2 large contact pads averaging over about 107 tracks. For each sample and at each temperature the conductivity as a function of the applied electrical field (non-ohmic behaviour) was measured separately and the data were extrapolated to field zero. In this way, the zero-field conductivity was determined independent from the field dependence. In spite of large differences in the absolute values, the temperature dependence of the zero-field conductivities is found to be very similar in shape for all samples. The conductivities follow a law up to temperatures slightly below room temperature. At higher temperatures a transport mechanism based on over-barrier hopping dominates with an activation energy of about 220 meV for tracks and 260 meV for the ta-C matrix. The field dependence measurements show that the deviation of the I–V characteristics from ohmic behaviour decreases with increasing zero-field conductivity. We also tested Cu-doped ta-C samples and found that they conduct significantly better than pure ta-C. However, the doping also increases the zero-field conductivity resulting in a weaker contrast between the track and matrix. The data are interpreted within the so-called ‘barrier model’ where the electrons are assumed to move fairly freely in well-conducting sp2 regions but encounter barriers in track sections consisting of more sp3-like bonds.

Enhanced Diamond-Like Coatings Boost Fuel Efficiency

Abstract A new laser arc method is being used to apply a hard carbon coating to engine components on the production line. This article reports on work to produce hydrogen-free tetrahedral amorphous carbon coatings on an industrial scale with consistent quality.

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.

Surface modification of thin tetrahedral amorphous carbon films by means of UV direct laser interference patterning

In this work, direct laser interference patterning of hydrogen-free tetrahedral amorphous carbon (ta-C) thin films using an ultraviolet (UV) nanosecond pulsed laser is investigated. Using this method, line-like arrays with submicrometer resolution were fabricated. In particular, a 180 nm grating period was successfully produced. In addition, it was found that depending on the laser energy density, the tetrahedral carbon film either graphitizes or crystallizes locally at the interference maxima positions. Furthermore, the crystallization is accompanied with a delamination of the film and even the formation of carbon fiber-like structures. The electrical properties of graphitized and delaminated ta-C were measured.

Session 01: DLC coatings OA 01 P2009-245 evaluation of adhesion strength and wear resistance of DLC films

Diamond-like carbon (DLC) coatings have many outstanding properties such as low friction, high wear resistance and corrosion resistance. Therefore, DLC coatings have been applied in various fields in recent years, such as cutting tools, dies, sliding parts, automotive engine parts and biomedical parts. Although the scratch test and the Rockwell test are commonly used today to evaluate the adhesion strength of DLC coatings, these techniques are insufficient under actual sliding conditions. A few papers have reported methods for evaluating the adhesion strength and wear resistance of DLC coatings under repeated sliding conditions. In this study, a sliding test in which the load was continuously increased was conducted to evaluate the adhesion strength and wear resistance of DLC coatings under practical sliding conditions. The substrate disk was made of steel (AISI 52100) with a hardness of 60 HRC and polished to a surface roughness of Ra 0.01 μm. An alumina (Al 2O 3) ball held in a jig to prevent it from rotating was pressed against the toric sliding surface of the rotating disk coated with the DLC coating. Three types of DLC coatings formed by different processes were evaluated: a hydrogen-free amorphous carbon (a-C, formed by unbalanced magnetron sputtering), a hydrogen-containing amorphous carbon (a-C:H, formed by RF plasma-enhanced CVD), and a hydrogen-free tetrahedral amorphous carbon (ta-C, formed by a filtered vacuum arc). The friction coefficient and the acoustic emission signal of the DLC coatings were simultaneously measured in the sliding test under a continuously increasing load and the condition of the sliding track was observed. It was found that the a-C:H coating showed damage with clear, large fragmentation, the a-C coating showed damage due to wear, and the ta-C coating showed damage with fine fragmentation. Different damage modes were clearly seen according to the different characteristics of the DLC coatings. The test results and observations are described in detail.

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.

Development of X-Shaped Filtered-Arc-Deposition (X-FAD) Apparatus and DLC/Cr Film Preparation

An X-shaped filtered-arc-deposition (X-FAD) apparatus was developed in order to prepare hydrogen-free tetrahedral amorphous carbon (ta-C), which is a kind of diamond-like carbon (DLC), and metal film as a binding interlayer on the superhard alloy using a plasma beam irradiated from the same direction. First of all, the droplet-reduction performance was verified, and then, the appropriate duct-bias voltages and deposition rate were measured. Optima of duct biases for chromium (Cr) and DLC were found to be 25 and 15 V, respectively. From the result of X-ray diffraction analysis, it was found that Cr film that is prepared at a higher substrate-bias voltage was well crystallized and has less internal stress. The appropriate substrate bias for preparing ta-C was -100 to -200 V. DLC film was also prepared with substrate heating. It was found that ta-C could be prepared at a substrate temperature of less than 200degC, and the film was graphitized at higher temperature. By following these results, 2.5-mum-thick ta-C film was prepared on a superhard alloy with an interlayer by X-FAD.

Elasticity, strength, and toughness of single crystal silicon carbide, ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon

In this work, the authors report the mechanical properties of three emerging materials in thin film form: single crystal silicon carbide (3C-SiC), ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon. The materials are being employed in micro- and nanoelectromechanical systems. Several reports addressed some of the mechanical properties of these materials but they are based in different experimental approaches. Here, they use a single testing method, the membrane deflection experiment, to compare these materials’ Young’s moduli, characteristic strengths, fracture toughnesses, and theoretical strengths. Furthermore, they analyze the applicability of Weibull theory [Proc. Royal Swedish Inst. Eng. Res. 153, 1 (1939); ASME J. Appl. Mech. 18, 293 (1951)] in the prediction of these materials’ failure and document the volume- or surface-initiated failure modes by fractographic analysis. The findings are of particular relevance to the selection of micro- and nanoelectromechanical systems materials for various applications of interest.

Young's modulus, Poisson's ratio and failure properties of tetrahedral amorphous diamond-like carbon for MEMS devices

The elastic and failure mechanical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) MEMS structures were investigated via in situ direct and local displacement measurements by a method that integrates atomic force microscopy (AFM) with digital image correlation (DIC). On-chip MEMS-scale specimens were tested via a custom-designed apparatus that was integrated with an AFM to conduct in situ uniaxial tension tests. Specimens 10 µm and 50 µm wide and of 1.5 µm average thickness were used to measure the elastic properties while 340 µm wide tension specimens with a central elliptical perforation resulting in a stress concentration factor of 27 were tested to investigate local effects on material strength. The Young's modulus, Poisson's ratio and tensile strength were measured as 759 ± 22 GPa, 0.17 ± 0.03 and 7.3 ± 1.2 GPa, respectively. In an effort to understand the effect of local defects and assess the true material strength, the local failure stress at sharp central elliptical notches with a stress concentration factor of 27 was measured to be 11.4 ± 0.8 GPa. The AFM/DIC method provided for the first time local displacement fields in the vicinity of microscale perforations and these displacement fields were in accordance with those predicted by linear elasticity.

An overview on the tribological behavior of diamond-like carbon in technical and medical applications

Diamond-like carbon (DLC) coatings are a class of materials and, depending on the deposition conditions and the tribological system, different outstanding tribological properties can be obtained. The most important subclasses of DLC are hydrogenated amorphous carbon (a-C:H) and hydrogen free tetrahedral amorphous carbon (ta-C). When DLC slides against a metal, the formation of the so-called transfer layer on the metallic counterpart can protect the metal from further wear. Adhesion and cohesion of this transfer layer is a critical issue and is also influenced by the environmental atmosphere including the relative humidity. When DLC is running against a hard and chemically inert counterface, such as sapphire, the formation of a transfer layer is not observed but low wear is still obtained. This is attributed to the lubricating properties of the graphitic wear residues. Due to its unique combination of properties, DLC is already used in different industrial applications such as magnetic storage media, diesel injection pumps, sliding bearings, car valve rockers, gears, tappets of racing motorcycles, laser barcode scanner windows in supermarkets, VCR head drums, textile industry parts, motor cycle forks, razor blades, etc. In medical applications, i.e. the coating of load bearing joints that slide against ultra high molecular weight polyethylene (UHMWPE), the different in vitro experiments apparently showed contradicting results, mainly due to the different experimental setups and especially the different liquids used as lubricants. However, when DLC slides against DLC in medical applications low wear rates could be demonstrated in different in vitro tests.

Surface properties of ultra-thin tetrahedral amorphous carbon films for magnetic storage technology

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Film thickness below 2 nm and roughness well below 1 nm are needed for storage density of 200 Gbit/inch 2. We use atomic force microscopy to study the roughness evolution vs. thickness of highly sp 3 hydrogen-free tetrahedral amorphous carbon (ta-C). The roughness of films r generally follows fractal scaling laws, increasing with film thickness h as r= ah β, where β is the growth exponent. For a fixed film thickness and scan length l, the roughness varies as l α, where α is the roughness exponent. We find α∼0.39 and β∼0–0.1. We performed Monte Carlo simulations, modelling the smoothing effects caused by the thermal spike. The simulation results closely match the experimental findings and define a new growth mechanism for ta-C films. The structural evolution of these ultra-thin films is monitored by Raman spectroscopy. A linear relation between G-peak dispersion and Young's modulus is found. The 2-nm-thick ta-C films are pin-hole free, corrosion resistant, have a Young's Modulus of ∼100 GPa, sp 3 content of ∼50% and roughness of ∼0.12 nm. So, data storage density of 1 Tbit/inch 2 could be achieved.

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.