Hydrogen-free Amorphous Carbon Research Articles

Macroscale, humidity-insensitive, and stable structural superlubricity achieved with hydrogen-free graphene nanoflakes

Achieving solid superlubricity in high-humidity environments is of great practical importance yet remains challenging nowadays, due to the complex physicochemical roles of water and concomitant oxidation on solid surfaces. Here we report a facile way to access humidity-insensitive solid superlubricity (coefficient of friction 0.0035) without detectable wear and running-in at a humidity range of 2–80%. Inspired by the concept of structural superlubricity, this is achieved between Au-capped microscale graphite flake and graphene nanoflake-covered hydrogen-free amorphous carbon (GNC a-C). Such GNC a-C exhibits reduced pinning effects of water molecules and weak oxidation, which demonstrates stable structural superlubricity even after air exposure of the surfaces for 365 days. The manufacturability of such design enables the macroscopic scale-up of structural superlubricity, achieving the leap from 4 μm × 4 μm contact to 3 mm ball-supported contact with a wide range of materials. Our results suggest a strategy for the macroscale application of structural superlubricity under ambient condition.

Structural changes of doped ta-C coatings at elevated temperature

Doped hydrogen-free amorphous carbon coatings, ta-C, ta-C:B, ta-C:Si, a-C:Fe and a-C:Mo, deposited from laser-assisted vacuum arc evaporation, were annealed in air up to 600 °C and in vacuum up to 800 °C. Subsequently they were studied regarding mechanical properties and structure, using nanoindentation, Raman spectroscopy, light microscopy, EDS and wafer curvature method. It was found that the coatings with high ratio of sp3 bonded carbon, high hardness and amorphous structure were stable up to 800 °C in vacuum, with a small increase in hardness (+5%–15%) above 400 °C, while compressive stresses were relieved significantly. In air, undoped ta-C was fully oxidized first, followed by ta-C:B and ta-C:Si, which improved temperature stability best. Metal dopands led to a decrease of sp3 in the as deposited coating, with structural change and hardness loss above 400 °C in vacuum. While a-C:Fe formed clear graphite peaks in the Raman spectrum, a-C:Mo showed only small changes in the amorphous structure. In air, a-C:Fe fully decomposed with remaining iron oxides at 500 °C whereas a-C:Mo formed a MoO3 overlayer at 600 °C. Failure mechanism at 500 °C in air was studied in detail for ta-C:Si. It was found that failure occurred from both degradation of the chromium interlayer as well as from oxidation of the topmost coating, both initiated at growth defects found in the respective layer.

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.

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.

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.

Mechanism of superlubricity of a DLC/Si3N4 contact in the presence of castor oil and other green lubricants

To meet the surging needs in energy efficiency and eco-friendly lubricants, a novel superlubricious technology using a vegetable oil and ceramic materials is proposed. By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp2 and sp3 hybridized carbon in presence of a commercially available silicon nitride bulk ceramic, castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most. Besides a partial liquid film possibly separating surfaces in contact, local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds. High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities, thus helping both the chemical polishing of surface and its chemical passivation by H and OH species. Particularly, the formation of the formation of −(CH2−CH2)n− noligomers have been evidenced to have a major role in the friction reduction. Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp2-hybridized carbon atoms in the amorphous carbon structure. Hence, tuning sp2-carbon content in hydrogen-free amorphous carbon, in particular, on the top layers of the coating, provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.

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.

Comparison of fracture properties of different amorphous carbon coatings using the scratch test and indentation failure method

In this work, fracture properties of different types of hydrogen-free amorphous carbon coatings were studied in detail, using ta-C, ta-C:B and a-C:Mo coatings deposited at nominal coating thickness of 1 μm and 6 μm. Two common test methods with additional advanced evaluation techniques were used. Scratch testing was conducted with load and indenter size scaled to the coating thickness, evaluating critical loads and relative area of delamination. Instrumented indentation failure testing at different final loads was carried out, analyzing the load-depth curve for pop-ins, and acoustic emission for fracture events. Afterwards, indentation sites were studied using SEM and FIB cross sections, following a new approach where crack pattern imaging was directly correlated with fracture events obtained from indentation. Observed crack modes were assigned to radial, circumferential, parallel and lateral cracks. The crack types were classified with respect to location, timing, causative stress situation, amount of plastic substrate deformation, and conclusion about the coating behavior. Furthermore, the causes for the different crack types were discussed in detail.In the case of the studied coatings, a specific fracture behavior was found for each type of carbon coating, which cannot be simply attributed to the coating hardness. Instead, it is shown that during indentation not only the first fracture events but also the entire behavior up to the maximum indentation load should be considered, allowing a more distinct differentiation of fracture behavior of different types carbon coatings.

Heat treatment of binder jet printed 17–4 PH stainless steel for subsequent deposition of tribo-functional diamond-like carbon coatings

Diamond-like carbon (DLC) coatings deposited on additively manufactured steel greatly improve the tribological properties. However, a high substrate hardness is crucial to sustaining high mechanical loads in the tribological contact. Herein, the heat treatment of binder jet printed 17–4PH enhances the hardness from 24 to 39 HRC. Binder jet printed 17–4 PH substrates are coated by DLC of the types hydrogen-free amorphous carbon (a-C) of ∼23 GPa and hydrogenated amorphous carbon (a-C:H) of ∼20 GPa. The influence of the heat treatment on the tribo-mechanical properties of the DLC coatings is investigated. 17–4 PH demonstrates high friction and wear against steel counterparts, but the wear rate is reduced from 693 ± 43 × 10–6 mm3/Nm to 492 ± 41 × 10-6 mm3/Nm by heat treating the steel. Both a–C and a–C:H are effective in reducing the friction and wear with wear rates below 0.3 × 10–6mm3/Nm. The a–C and a–C:H coatings demonstrate lower plastic wear on heat treated 17–4 PH due to the higher substrate hardness. Consequently, the heat treatment is an essential process step to ensure maximum tribological functionality of the DLC coating on additively manufactured 17–4PH steel.

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.

Evaluation of Anti-Adhesion Characteristics of Diamond-Like Carbon Film by Combining Friction and Wear Test with Step Loading and Weibull Analysis.

Anti-adhesion characteristics are important requirements for diamond-like carbon (DLC) films. The failure load corresponding to the anti-adhesion capacity varies greatly on three types of DLC film (hydrogen-free amorphous carbon film (a-C), hydrogenated amorphous carbon film (a-C:H), and tetrahedral hydrogen-free amorphous carbon film (ta-C)) in the friction and wear test with step loading using a high-frequency, linear-oscillation tribometer. Therefore, a new method that estimates a representative value of the failure load was developed in this study by performing a statistical analysis based on the Weibull distribution based on the assumption that the mechanism of delamination of a DLC film obeys the weakest link model. The failure load at the cumulative failure probabilities of 10% and 50% increased in the order ta-C < a-C:H < a-C and ta-C < a-C < a-C:H, respectively. The variation of the failure load, represented by the Weibull slope, was minimum on ta-C and maximum on a-C:H. The rank of the anti-adhesion capacity of each DLC film with respect to the load obtained by a constant load test agreed with the rank of the failure load on each DLC film at the cumulative failure probability of 10% obtained by Weibull analysis. It was found to be possible to evaluate the anti-adhesion capacity of a DLC film under more practical conditions by combining the step loading test and Weibull analysis.

Effect of long carbon bombardment step on the adhesion of thick amorphous carbon coating deposited by cathodic arc evaporation

The main restriction in the application of thick hydrogen-free amorphous carbon (a-C) coatings is the adhesion to the substrate, since high compressive stresses tend to delaminate the coating when a certain thickness is reached. In the present work, amorphous carbon films were deposited by unfiltered cathodic arc over a gas nitrided stainless steel substrate with metallic chromium as bond layer. An additional step was included, where carbon ions were accelerated by high bias potential before the deposition of functional a-C, to produce a carbon transition layer to act as a buffer layer. Different thicknesses of this transition layer were produced, including a sample without this step, for reference purposes. The coating adhesion was evaluated by scratch test. In addition, the microstructure of the interface was analyzed by scanning transmission electron microscopy (STEM) and the sp-type bonds were quantified by electron energy loss spectroscopy (EELS). The analysis of the microstructure close to the interface with the chromium bond layer revealed a mixing layer of Cr and C. Furthermore, it was observed the formation of the transition layer characterized by a homogeneous carbon layer with high sp2 content, when compared to the functional amorphous carbon layer. Results indicate a significant enhancement of adhesion for the samples prepared with the carbon bombardment step, which can be correlated to the presence of the mixing layer and the carbon transition layer. Moreover, a decrease in adhesion was observed for an increase in the thickness of the transition layer, which can be attributed to the lower shear strength of this sp2 rich layer.

Generalized approach of scratch adhesion testing and failure classification for hard coatings using the concept of relative area of delamination and properly scaled indenters

Failure characteristics and adhesion evaluation of hydrogen-free amorphous carbon coatings (a-C and ta-C) were examined by scratch testing using different indenter tip radii. Based on previous work of some of the authors, this article provides an extended description of main failure modes occurring for brittle coatings, with respect to the ratio of indenter radius R to coating thickness h. For best control over expected coating failure, it is shown that the failure mode of the coating can be reliably predicted and adjusted by the choice of the indenter radius. Failure modes of interest are predominant spallations (failure mode II) and predominant bending cracks (failure mode II). Same failure modes were typically found within certain ranges of R/h: Failure mode II occurred for 20 < R/h < 40 and failure mode III for 100 < R/h < 200.For an enhanced adhesion evaluation, the concept of adhesion assessment by comparing the relative size of delaminated area was expanded and generalized for variable indenter sizes and failure mechanisms. Thus, this approach enables direct comparison of scratch results in case of similar R/h, especially allowing adhesion comparison for a wide range of coating thicknesses.

Mechanical properties and adhesion behavior of amorphous carbon films with bias voltage controlled TixCy interlayers on Ti6Al4V

Amorphous carbon is a promising functional film material to enhance the surface properties of Ti-based alloys for orthopaedic applications. However, high adhesion of the amorphous carbon film on the orthopaedic implants is essential to fully exploit its potential under high load bearings. Interlayer films are generally employed to improve the adhesion. The applied bias voltage is a decisive deposition parameter and significantly influences the structure and mechanical properties during the interlayer growth, which in turn affect the properties of the amorphous carbon film. Therefore, chemically graded titanium carbide (TixCy) interlayers were deposited using bias voltages of −50, −100, and −150 V with a subsequent hydrogen-free amorphous carbon (a-C) top layer on Ti6Al4V by magnetron sputtering. The mechanical properties and adhesion behavior of single TixCy interlayers were evaluated to analyze the interaction effect of TixCy on bilayered TixCy/a-C structures.A high bias voltage generates dense TixCy of a more disordered and defected structure with high stresses, high hardness of ~16 GPa, and high elastic modulus of ~170 GPa. However, high compressive stresses provoke a low adhesion strength, while low compressive stresses ensure a good adhesion behavior of TixCy. Highly stressed TixCy interlayers lead to overall higher stresses for the entire TixCy/a-C film. Independently of TixCy, the a-C top layer exhibits hardness and elastic modulus values of ~16 and ~160 GPa, respectively. The TixCy/a-C films with TixCy interlayers deposited at high bias voltages possess a low adhesion strength, while a lower bias voltage favors a good adhesion of TixCy/a-C on Ti6Al4V. Therefore, a moderate bias voltage is crucial to deposit lowly stressed TixCy interlayers, which ensure a high adhesion of TixCy/a-C on Ti6Al4V. Consequently, the bias voltage allows controlling the mechanical properties and adhesion behavior of the interlayer and, hence, the adhesion strength of the entire amorphous carbon film structure on Ti-based alloys for orthopaedic applications.

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.

Correlation study of nanocrystalline carbon doped thin films prepared by a thermionic vacuum arc deposition technique

The synthesis of Ag, Mg and Si nanocrystalline, embedded in a hydrogen-free amorphous carbon (a-C) matrix, deposited by a high vacuum and free buffer gas technique, were investigated. The films with compact structures and extremely smooth surfaces were prepared using the thermionic vacuum arc method in one electron gun configuration, on glass and silicon substrates. The surface morphology and wettability of the obtained multifunctional thin films were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and free surface energy (FSE) by See System. The results from the TEM measurements show how the Ag, Mg and Si interacted with carbon and the influence these materials have on the thin film structure formation and the grain size distribution. SEM correlated with EDX results reveal a very precise comparative study, regarding the quantity of the elements that morphed into carbides nanostructures. Also, the FSE results prove how different materials in combination with carbon can make changes to the surface properties.

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.

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.

Green Superlubrication by Hydrogen-Free Amorphous Carbon with Human Friendly Lubricants

Green Superlubrication by Hydrogen-Free Amorphous Carbon with Human Friendly Lubricants