Amorphous Diamond-like Carbon Research Articles

Nanocomposite materials based on amorphous SiC-containing DLC structures modified with rhodium nanoparticles

From the 1980s to the present, interest in carbon has remained very high. The variety of polytypic forms of carbon is expected to have a large number of possible applications. The creation of electronic and optical devices based on them is a promising direction. One of the most interesting carbon materials could be amorphous diamond-like carbon (DLC) films. This work considers nanocomposite materials based on DLC with nanostructures of SiC components. Thin composite DLC:Si films modified with Rhodium nanoparticles (Rh NPs) were synthesized by magnetron sputtering. The structure of the films has been studied by transmission electron microscopy, Raman and X-ray photoelectron spectroscopy (XPS). It is shown that a diamond-like carbon matrix with inclusions of SiC nanostructures and Rh nanocrystals is formed. Significant influence of Rh NPs on the structures of the carbon matrix and SiC during the formation of the DLC: Si matrix is revealed. In addition, X-ray photoelectron spectroscopy shows a considerable effect of Rh NPs on the edge of the valence zone and the density of localized states. The significant effect of rhodium nanoparticles on the electronic component of the valence band of DLC:Si films can be used in the development of new optoelectronic devices.

Amorphous diamond embedded in dense boron nitride with excellent mechanical properties

Diamond and cubic boron nitride (BN) are important materials with a variety of technological and industrial applications; however, overcoming the intrinsic brittleness of these materials is still a challenge. Here, we synthesize a compound of crystalline BN and amorphous diamond-like carbon through BN nanotubes and fullerene under high pressure and high temperature conditions. The obtained composite exhibits excellent combination of a measured Vickers’ hardness of 86.2 GPa and fracture toughness of 10.2 MPa m1/2. Morphological and structural characterizations reveal that the amorphous diamond-like carbon is homogeneously embedded in a matrix of dense BN. The formation of the amorphous diamond-like carbon particles within the polycrystalline BN can effectively impede the migration of crack tips when the compound is subjected to the plastic deformation, in which most of crack tips are forced to deflect or confined near the boundaries of dense BN and amorphous diamond particles. The crystalline-amorphous composite strengthening presented here may provide a promising strategy for the further improvement of mechanical properties of hard or superhard materials.

Enhancing corrosion and tribology performance of stainless steel with DLC coatings: Effects of doping and multilayered structures

The increasing demand for energy production has expanded oil and gas extraction, particularly in harsh exploration sites with highly corrosive media. In the Brazil's pre-salt layer, rich in oil reserves, high NaCl content and CO2 presence may induce sweet corrosion. Corrosion resistant alloys are often used for better performance, and among them, austenitic stainless steels (SS) offer good balance of cost and properties. However, pre-salt acidic and salty environment may induce localized corrosion, making protective coatings a viable option to prolong the lifespan of components exposed to these environments. Diamond-like carbon (DLC) coatings are a great alternative, as besides being chemically stable, they improve mechanical properties of the coated material, such as higher hardness and wear resistance and lower friction coefficient. Therefore, the performance of different types of DLC on the anti-corrosion protection of SS was evaluated. Four DLC types were tested: a hydrogenated amorphous DLC; a multilayered film (DLC-M); and films with fluorine (DLC-F) and silicon (DLC-S). The films showed chemical and structural differences, evidenced by Raman, SEM/EDS and XPS analysis. Their tribological performances were evaluated through scuffing resistance tests; the base DLC showed the highest resistance, followed by DLC-F and DLC-M. DLC-S films didn't exhibit satisfactory tribological performance. For corrosion characterization, potentiodynamic polarization tests were conducted in NaCl and CO2-saturated NaCl solutions. In the NaCl tests, DLC films exhibited impressive anti-corrosion properties, with DLC-F delivering the highest performance, followed by DLC-M, DLC-S, and base DLC. 316 L SS showed high corrosion susceptibility in the CO2-saturated solution, but the coatings also improved corrosion resistance in this setting. The DLC, DLC-S and DLC-M showed similar Ecorr and jcorr values, while DLC-F demonstrated superior corrosion resistance with higher Ecorr and lower jcorr values.

Ab initio structural dynamics of pure and nitrogen-containing amorphous carbon

Amorphous carbon (a-C) has attracted considerable interest due to its desirable properties, which are strongly dependent on its structure, density and impurities. Using ab initio molecular dynamics simulations we show that the sp2/sp3 content and underlying structural order of a-C produced via liquid quenching evolve at high temperatures and pressures on sub-nanosecond timescales. Graphite-like densities (lesssim 2.7 g/cc) favor the formation of layered arrangements characterized by sp2 disordered bonding resembling recently synthesized monolayer amorphous carbon (MAC), while at diamond-like densities (gtrsim 3.3 g/cc) the resulting structures are dominated by disordered tetrahedral sp3 hybridization typical of diamond-like amorphous carbon (DLC). At intermediate densities the system is a highly compressible mixture of coexisting sp2 and sp3 regions that continue to segregate over 10’s of picoseconds. The addition of nitrogen (20.3%) (a-CN) generates major system features similar with those of a-C, but has the unexpected effect of reinforcing the thermodynamically disfavored carbon structural motifs at low and high densities, while inhibiting phase separation in the intermediate region. At the same time, no nitrogen elimination from the carbon framework is observed above simeq 2.8 g/cc, suggesting that nitrogen impurities are likely to remain embedded in the carbon structures during fast temperature quenches at high pressures.

Nanoscale friction of tetrahedral amorphous diamond-like carbon film after thermal annealing

Diamond-like carbon (DLC) coatings have demonstrated significant potential as solid lubricants for a wide range of applications. However, thermal-induced structural changes lead to alterations in its mechanical and frictional properties. In this study, we investigate the friction properties of DLC films after high-temperature annealing, by conducting reciprocating single asperity scanning experiments of self-mated hydrogen-free DLC tribopair under ultra-high vacuum (UHV) conditions. As the annealing temperature increases, the coefficients of friction (COFs) first increase owing to the desorption of water molecules; while, with the further increase of temperature, friction shows a decreasing trend because of the high-temperature-induced sp3-to-sp2 rehybridization which lowers the interfacial shear strength. The insights gained from this study provide a better understanding of the frictional properties of DLC films.

Combining multi-scale surface texturing and DLC coatings for improved tribological performance of 3D printed polymers

Polymer components fabricated by additive manufacturing typically show only moderate strength and low temperature stability, possibly leading to severe wear and short lifetimes especially under dry tribological sliding. To tackle these shortcomings, we investigated the combination of single- and multi-scale textures directly fabricated by digital light processing with amorphous diamond-like carbon (DLC) coatings. The topography of the samples and conformity of the coatings on the textures are assessed and their tribological behaviour under dry conditions is studied. We demonstrate that the surface textures have a detrimental tribological effect on the uncoated samples. This changes with the application of DLC coatings since friction substantially reduces and wear of the textures is not observed anymore. These trends are attributed to the protection of the underlying polymer substrate by the coatings and a reduced contact area. The best tribological performance is found for a coating with highest hardness and hardness-to-elasticity ratios. Moreover, multi-scale textures perform slightly better than single-scale textures due to a smaller real contact area. Summarizing, we verified that the high flexibility and low production costs of 3D printing combined with the excellent mechanical and tribological properties of DLC results in synergistic effects with an excellent performance under dry sliding conditions.

Nickel concentration dependent mechanical and tribological properties of subsurface layer of electrodeposited Ni-C nanocomposite thin films

Electrodeposited nickel- carbon (Ni-C) nanocomposite thin films are found to have varying mechanical and tribological properties. Ni-C nanocomposites thin films of varying nickel content were synthesized by a simple electrodeposition method. Annealed thin films possess a relatively hard and adherent subsurface layer. In order to have an understanding of the mechanical and tribological characteristics of the subsurface layer, the relatively soft and fragile outer layer of the thin films was removed by scraping. The sp2 and sp3 bondings of carbon atoms in the thin films were explored by Raman spectroscopy. The Fourier- transform infrared spectra of the diamond-like carbon (DLC) and Ni-C thin films also consist of sp2 and sp3 hybridized carbon bonds. Nanocrystalline nickel nanoparticles distributed in carbon matrix were observed in Transmission electron microscopy study of the films. Decrease in coefficient of friction of the films is observed with increase in nickel content in thin films having DLC structure. The highest nickel content Ni-C composite thin film shows the maximum hardness of 7.6 GPa while the amorphous DLC thin film possesses the least hardness of 2.19 GPa. The subsurface layers demonstrate a clear demarcation of mechanical properties with unmodified overlayer pointing towards an extra care must be exercised before applying these electrodeposited coating on technological appliances.

Highly Stable Electrochemical Supercapacitor Performance of Self-Assembled Ferromagnetic Q-Carbon.

Novel phase Q-carbon thin films exhibit some intriguing features and have been explored for various potential applications. Herein, we report the growth of different Q-carbon structures (i.e., filaments, clusters, and microdots) by varying the laser energy density from 0.5 to 1.0 J/cm2 during pulsed laser annealing of amorphous diamond-like carbon films with different sp3-sp2 carbon compositions. These unique nano- and microstructures of Q-carbon demonstrate exceptionally stable electrochemical performance by cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy for energy applications. The temperature-dependent magnetic studies (magnetization vs magnetic field and temperature) reveal the ferromagnetic nature of the Q-carbon microdots. The saturation magnetization and coercive field values decrease from 132 to 14 emu/cc and 155 to 92 Oe by increasing the temperature from 2 to 300 K, respectively. The electrochemical performances of Q-carbon filament, cluster, and microdot thin-film supercapacitors were investigated by two-electrode configurations, and the highest areal specific capacitance of ∼156 mF/cm2 was observed at a current density of 0.15 mA/cm2 in the Q-carbon microdot thin film. The Q-carbon microdot electrodes demonstrate an exceptional capacitance retention performance of ∼97.2% and Coulombic efficiency of ∼96.5% after 3000 cycles due to their expectational reversibility in the charging-discharging process. The kinetic feature of the ion diffusion associated with the charge storage property is also investigated, and small changes in equivalent series resistance of ∼9.5% and contact resistance of ∼9.1% confirm outstanding stability with active charge kinetics during the stability test. A high areal power density of ∼5.84 W/cm2 was obtained at an areal energy density of ∼0.058 W h/cm2 for the Q-carbon microdot structure. The theoretical quantum capacitance was obtained at ∼400 mF/cm2 by density functional theory calculation, which gives an idea about the overall capacitance value. The obtained areal specific capacitance, power density, and impressive long-term cyclic stability of Q-carbon thin-film microdot electrodes endorse substantial promise in high-performance supercapacitor applications.

Influence of Si atoms on the structure and electronic properties of amorphous DLC films

The paper considers the effect of silicon on the formation of the structure and properties of amorphous diamond-like carbon (DLC) films. Thin films of silicon-modified amorphous diamond-like carbon (DLC:Si) were synthesized by magnetron ion-plasma co-sputtering of a combined target. Studies of the structure and properties of amorphous DLC:Si films were carried out in the range of silicon concentrations up to 6.75 at.%. A change in the formation of the film structure is shown using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS) revealed a significant change in the local structure of amorphous DLC:Si films. Which, in turn, leads to a change in the density of states of π-electrons in the band and, as a rule, to a change in the electronic properties.

Influence of nuclear quantum effects on the electronic properties of amorphous carbon

We carry out quantum simulations to study the physical properties of diamond-like amorphous carbon by coupling first-principles molecular dynamics with a quantum thermostat, and we analyze multiple samples representative of different defective sites present in the disordered network. We show that quantum vibronic coupling is critical in determining the electronic properties of the system, in particular its electronic and mobility gaps, while it has a moderate influence on the structural properties. We find that despite localized electronic states near the Fermi level, the quantum nature of the nuclear motion leads to a renormalization of the electronic gap surprisingly similar to that found in crystalline diamond. We also discuss the notable influence of nuclear quantum effects on band-like and variable-hopping mechanisms contributing to electrical conduction. Our calculations indicate that methods often used to evaluate electron-phonon coupling in ordered solids are inaccurate to study the electronic and transport properties of amorphous semiconductors composed of light atoms.

Study of the influence of palladium nanoparticles on the structure of DLC films synthesized on silicon (100) substrates

This work reports about the study of the structure of films of amorphous diamond-like carbon modified with palladium nanoparticles and synthesized on silicon (100) substrates (a-C <Pdx>). Results of Raman studies indicate the change in the ratio of sp2/sp3 sites depending on the Pd concentration in the carbon films. It was found that palladium nanoparticles affect the formation of carbon bonds and increase the number of sp2 sites, and at concentrations of more than 1 at.%, the film structure becomes graphite-like. In addition, atomic force microscopy methods revealed the effect of palladium on the surface morphology of a-C <Pdx> films and non-uniform distribution of electron density. The size of the particles which form the structure of the a-C <Pdx> films was estimated by using the “threshold method of grains analysis” program.

Structure, properties and applications of diamond-like carbon coatings prepared by reactive magnetron sputtering

AbstractMetal containing and metal free amorphous diamond-like carbon films were prepared by reactive magnetron sputter deposition using targets from transition metals or graphite and acetylene as reactive gas. Both types of coating materials are well known for their low friction coefficients. However, metal containing films exhibit clearly lower wear resistances and lower hardness values than metal free films. To study the differences between both materials, tungsten containing and metal free coatings were compared. The deposition experiments were carried out in an industrial scale d.c. magnetron sputter machine. Transmission electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy as well as hardness measurements and wear tests were performed to characterize and to compare composition, structure, mechanical, and tribological properties of both coating materials.

Effect of a-MoS2 and a-MoS3 nanophases on tribological properties of nanocomposite coatings containing diamond-like carbon

Using the modes of reactive and vacuum pulsed laser deposition, nanocomposite coatings consisting of alternating layers of amorphous molybdenum sulfide (a-MoSx) and diamond-like carbon (a-C) were created. For solid lubricating layers, the compositions a-MoS2 and a-MoS3 were chosen. The study of tribological properties was carried out according to the method of sliding a ball on a disk in very different environmental conditions: humid air, dry air at +25 °C and –100 °C. A significant effect of the composition of the a-MoSx phase on the friction and wear of the nanocomposite coatings in humid air and at low temperature has been established. The use of the a-MoS3 polymer-like phase provided better antifriction properties and increased durability of the coatings. In dry air, the coatings with the a-MoS2 nanophase exhibited a lower coefficient of friction than those with a-MoS3 with approximately the same wear resistance between them. The results of the study of tribo-induced changes in coatings by micro-Raman spectroscopy are presented.

Formation of wear-resistant graphite/diamond-like carbon nanocomposite coatings on Ti using accelerated C60-ions

The nanostructural and tribological characteristics of carbon nanocomposite coatings (CNCs), deposited by accelerated C 60 ions at high temperatures, were studied. The CNCs were successfully fabricated on Ti-alloy using a flux that contained C 60 ions with 5 and 10 keV energies. Unlike the previous studies, ion mass-separator was not implemented to the deposition process. This enabled deposition of sp 2 -rich CNCs with a lower coefficient of friction (COF) at a higher deposition rate. Laser confocal microscopy , atomic force microscopy transmission electron microscopy , Raman spectroscopy , X-ray photoelectron spectroscopy, and X-ray Auger Electron Spectroscopy were used to investigate the structure of the coatings and friction test results. A carbon nanocomposite film consisting of graphite nanocrystals enclosed in an amorphous diamond-like matrix was formed at a deposition temperature of 300–400 °C. Upon deposition of the CNC, a COF of ~0.06 was achieved, which was almost one order of magnitude lower than that of bare Ti-alloy substrate. The surface wear was reduced by 360 times. The wear mechanism was changed from abrasive wear (on bare Ti-alloy) to a burnishing smoothening of the surface roughness (on CNCs). The transfer of graphite nanocrystals from the coating to the counter surface was identified and observed to act as a solid lubricant . • Carbon nanocomposite coatings were deposited on Ti using accelerated C 60 -ions. • Graphite nanocrystals embedded in amorphous diamond-like carbon matrix were formed. • The coatings were deposited without the implementation of ion mass-separator. • Deposition temperature significantly affected the size of graphite nanocrystals. • The coatings reduced the COF and wear rate of Ti by 8 and 360 folds, respectively.

Anomalous characteristics of nanostructured hydrogenated carbon thin films

Hydrogenated diamond-like carbon (DLC) films are sought for several technological applications including electronics, optics, mechanics, and tribology. However, conventional hydrogenated DLC films, that commonly deposit at low base pressure (~10−5 to 10−8 Torr), have many intrinsic limitations in terms of moderate hardness (15–25 GPa), low electrical conductivity (in insulating regime), and they display amorphous morphology. Here we develop high-performance hydrogenated carbon-based films using a cost-effective and fast deposition approach. We report the room temperature synthesis of nitrogen incorporated nanostructured hydrogenated carbon films (n-C:N:H) at a high base pressure of ~5 × 10−3 Torr, employing a non-conventional radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) system equipped with only a primary pump. The n-C:N:H films show appealing properties such as wide band gap (2.35–2.9 eV), high optical transparency, high hardness (up to ~ 40 GPa), ultrahigh elasticity (elastic recovery ~95%), reasonably good electrical conductivity, and diode-like behavior when examined in n-C:N:H/Si heterojunction device configuration. Interestingly, some of n-C:N:H films revealed the multifunctional activities with properties surpassing to those of many low base pressure grown traditional amorphous DLC films. This discovery solves many fundamental concerns of hydrogenated DLC films and opens new paths for their enhanced commercial applications.

High Pressure Carbon Polymers from Impact Melt Rock of the Giant Popigai Astrobleme (Siberia, Russia)

The Popigai astrobleme is a unique geological object with huge storages of impact diamonds of high hardness quality being perspective as an abrasive material. In spite on very well-known after-graphitic nature of the Popigai diamonds, it was predicted that a part of them might have non-graphitic precursor as the target of the meteorite crater partly consist of sedimentary rocks with coals and bitumens. In this study by use of a complex of modern methods we have analyzed unusual carbon particles from impact melt rocks of the Popigai astrobleme. The studied carbons according to the provided Raman studies belong to high pressure polymers with hydrogenated carbons - hydrogenated amorphous tetrahedral diamond-like carbon (ta-C:H) and rare polymeric a-C:H. A list of typomorphic features of the high pressure non-crystalline carbons are described in the paper on the basis of UV Raman spectroscopy, scanning electron microscopy, microprobe and 813C isotopic composition analysis.

Machine learning driven simulated deposition of carbon films: From low-density to diamondlike amorphous carbon

Amorphous carbon (a-C) materials have diverse interesting and useful properties, but the understanding of their atomic-scale structures is still incomplete. Here, we report on extensive atomistic simulations of the deposition and growth of a-C films, describing interatomic interactions using a machine learning (ML) based Gaussian approximation potential model. We expand widely on our initial work [M. A. Caro et al., Phys. Rev. Lett. 120, 166101 (2018)] by now considering a broad range of incident ion energies, thus modeling samples that span the entire range from low-density ($s{p}^{2}$-rich) to high-density ($s{p}^{3}$-rich, ``diamondlike'') amorphous forms of carbon. Two different mechanisms are observed in these simulations, depending on the impact energy: low-energy impacts induce $sp$- and $s{p}^{2}$-dominated growth directly around the impact site, whereas high-energy impacts induce peening. Furthermore, we propose and apply a scheme for computing the anisotropic elastic properties of the a-C films. Our work provides fundamental insight into this intriguing class of disordered solids, as well as a conceptual and methodological blueprint for simulating the atomic-scale deposition of other materials with ML driven molecular dynamics.

Optical measure of disorder: Why Urbach analysis works for amorphous silicon but fails for amorphous carbon

We investigate the basis for the Paradox where the Urbach-slope disorder parameter disagrees with the Raman width in amorphous diamond-like carbon (DLC) materials (lower Urbach-Eo yet greater Raman width). We examined the bandgap and Urbach-slope measurement issues. While significant errors are identified, ultimately these cannot resolve the Paradox. This resolution involves large changes in shape of DLC's absorption band resulting from large energy shifts. To solidify the understanding of band energy shifts, we examined their properties using a-Si:H, well known for its agreement between Raman and Urbach-slope disorder parameters. Using Tauc-Lorentz (T-L) dispersion functions, the Urbach method works for a-Si:H because small changes in shape occur, as measured by (i) peak position, (ii) peak width, (iii) peak amplitude, and (iv) and gap energies. Moreover, for a-Si:H, a 5th condition (v) there is no other feature below the principal band to interfere with its fall. All 5 of these conditions fail for DLCs. Because T-L dispersions accommodate asymmetry, we traced the Paradox to a steepening of the band shape as it pushes toward lower energy. This steepening is unrelated to disorder, but to a boundary value problem where energy cannot be negative. We conclude that Urbach analysis is invalid for DLC materials, but the T-L width is a good measure instead. We also examined another disorder measure, where π bonds should be considered as disorder relative to the σ bonds in an idealized “amorphous diamond” sp3 network: superconvergence can be used to quantify these π transitions. Finally, further molecular orbital calculations are needed to properly interpret the changes in T-L widths.

Hollow-cathode chemical vapor deposition of thick, low-stress diamond-like carbon films

A radio-frequency (RF), hollow-cathode plasma source with confining magnetic field is described for the chemical vapor deposition of thick ( > 10 µm), amorphous diamond-like carbon ablator films for inertial confinement fusion applications. Plasma is characterized by optical emission spectroscopy, while properties of the resultant films are measured by a combination of profilometry, Rutherford backscattering spectrometry, elastic recoil detection analysis, X-ray diffraction, Raman spectroscopy, and atomic force microscopy. The dependence of the deposition rate, film density, elemental composition, self-bias and residual stress is reported as a function of RF power. Higher density films were found when using Ar plasma, than N2 or H2 plasma. The coatings produced are x-ray amorphous, exhibit low compressive stress ( ~ 100 MPa), high density ( < 1.7 g/cm3), hydrogen content of ~ 30 at.%, and a low average roughness of 0.75 nm. Applications of these films as tunable-density ablators for inertial confinement fusion experiments are discussed.

Superlow friction of amorphous diamond-like carbon films in humid ambient enabled by hexagonal boron nitride nanosheet wrapped carbon nanoparticles

The friction and wear of amorphous diamond-like carbon (DLC) films will dramatically increase with the increment of ambient humidity. This drawback can lead to lubrication failure, which greatly impedes the practical applications of DLC films in humid environment. Herein, a simple method is proposed to improve the tribological performance of DLC films in humid environment by adding hexagonal boron nitride nanosheet wrapped carbon nanoparticles (CNP@h-BNNSs) as lubricants. The reciprocating friction tests demonstrate that DLC films with CNP@h-BNNSs as lubricants possess excellent antifriction and antiwear performance at the relative humidity (RH) of 7.5–55%. Especially, it can achieve a superlow friction and wear at the RH of 45–55% which decrease by respectively 13 and 25 times as compared to those obtained from DLC films without lubricants. The investigation on the steel ball surface after friction test shows that a protective tribo-layer containing carbon, boron and nitrogen elements is formed. The existence of this tribo-layer can effectively reduce the friction and protect the steel ball surface from excessive wear. Noteworthy, the h-BNNSs are exposed at the friction interface acting as lubricating layer while the carbon nanoparticles are adsorbed on steel ball surface playing as binding binder. Furthermore, the ball movement mode of CNP@h-BNNSs and the interaction between water molecules and CNP@h-BNNSs are also considered as key factors to reduce friction. All these results indicate that CNP@h-BNNSs which can be massively produced have the potential to reduce the friction and wear of DLC films in humid environment.