Hydrogenated Diamond-like Carbon Research Articles

Why is Superlubricity of Diamond-Like Carbon Rare at Nanoscale?

Hydrogenated diamond-like carbon (HDLC) is a promising solid lubricant for its superlubricity which can benefit various industrial applications. While HDLC exhibits notable friction reduction in macroscale tests in inert or reducing environmental conditions, ultralow friction is rarely observed at the nanoscale. This study investigates this rather peculiar dependence of HDLC superlubricity on the contact scale. To attain superlubricity, HDLC requires i) removal of ≈2nm-thick air-oxidized surface layer and ii) shear-induced transformation of amorphous carbon to highly graphitic and hydrogenated structure. The nanoscale wear depth exceeds the typical thickness of the air-oxidized layer, ruling out the possibility of incomplete removal of the air-oxidized layer. Raman analysis of transfer films indicates that shear-induced graphitization readily occurs at shear stresses lower than or comparable to those in the nanoscale test. Thus, the same is expected to occur at the nanoscale test. However, the graphitic transfer films are not detected in ex-situ analyses after nanoscale friction tests, indicating that the graphitic transfer films are pushed out of the nanoscale contact area due to the instability of transfer films within a small contact area. Combining all these observations, this study concludes the retention of highly graphitic transfer films is crucial to achieving HDLC superlubricity.

Temperature dependence of atomic‑oxygen irradiation mechanism of hydrogenated diamond-like carbon film

Hydrogenated diamond-like carbon (H-DLC) films are effective lubricants for aerospace moving parts, but the service of H-DLC is severely threatened by atomic oxygen (AO) in low Earth orbit (LEO). The interaction between AO and H-DLC strongly depends on ambient temperature; however, the interactive mechanism has not been fully understood. In this study, we report different temperature dependences of AO irradiation behavior on H-DLC film. Under the AO irradiation with low energy, the increase in ambient temperature can only increase the surfacial oxidation; while, the effect of temperature under high AO irradiation energy is more significant, resulting in the transition from the surficial oxidation to continuous carbon removal. Subsequent analysis of the AO trajectories shows that the different repulsion mechanisms of the H-DLC surface against high and low energy AO result in the different temperature dependence. For AO incident with low energy, the incident AO cannot touch the H-DLC surface anymore due to the repulsion of the surfacial oxygen in the later irradiation stage. Nevertheless, high energy AO could overcome the repulsion of oxidized surface to contact the H-DLC surface, the increase in temperature can continuously promote the AO irradiation on the H-DLC surface.

Atomic-scale evolution of hydrogenated fullerene-like carbon in the presence of black phosphorus

Hydrogenated diamond-like carbon (H-DLC) is widely used by the industry, but over the past few decades, its mechanical properties have been hindered by oxidation stemming from frictional contact and heat generation. Herein, we propose a solution by establishing a tribosystem that incorporates two-dimensional black phosphorus (BP) nanosheets and H-DLC. Experiments and atomistic simulations reveal that integration of BP nanosheets to the sliding interface not only shields H-DLC from oxygen erosion, but also creates reactive sites for the formation of hydrogenated fullerene-like carbon (FLC:H) via covalent and hydrogen bonding interactions, resulting in significantly reduced friction. These findings provide a novel insight into the atomic-scale evolution of FLC:H and reveal promising prospects for expanding the industrial application of hydrogenated carbon films.

The occurrence of ultralow friction even superlubricity for graphene oxide/hydrogenated diamond-like carbon composites in multi-environments

The establishment of ultralow friction and superlubricity can significantly decrease the dissipation of energy and failure of parts in moving mechanical systems. Hydrogenated diamond-like carbon (H-DLC) film can realize the frictionless state at engineering scale by the stress-induced graphitization that leads to the formation of the vital superlubricious structures: graphene-like carbon and onion-like carbon during the sliding process. Herein, we designed some composites by combination H-DLC film and graphene oxide (GO) nanosheets to provide the “pre-graphitized” sliding interface for decreasing the friction. Frictional behaviors of these composites in different atmospheres and the as-formed structures of sliding interfaces were investigated comprehensively in this work. It was demonstrated that the existence of GO on H-DLC film could significantly decrease the friction and even establish the near friction-vanishing state. The structural investigations showed that the existence of GO on H-DLC film could work as a “pre-graphitization” process, hinder the structural evolution of H-DLC film from amorphous to sp2-rich ones and endow the composite with ultralow friction even superlubricious performance. The investigation would provide some helpful advices for H-DLC film to establish ultralow even superlow friction from inert atmospheres to humid air at engineering scale by depositing GO flakes on the surface of H-DLC film.

Study on particle size and field effect with sp2/sp3 ratio of hydrogenated diamond-like carbon

Hydrogenated diamond-like carbon (HDLC) films were synthesized through a reactive gas-plasma process employing methane (CH4) and hydrogen (H2) as precursor gases on a silicon (100) wafer substrate, conducted at room temperature. The deposition process utilized a biased enhanced nucleation technique, varying the flow rate ratio of hydrogen and methane. The investigations revealed that increasing the methane flow rate led to a reduction in grain size and an augmented nucleation density of HDLC, as evidenced by contact-mode atomic force microscopy (AFM) images. This study demonstrated the effective control of diamond grain growth by introducing high-methane-concentration pulses during deposition. The field emission characteristics of HDLC samples were analyzed, revealing threshold fields of 12.2 V/μm for nanocrystalline films, 8.5 V/μm for subcrystalline films and 4.1 V/μm for microcrystalline films, corroborated by Raman spectra. Surface energy measurements indicated hydrophobic behavior in the samples. Notably, a decrease in the hydrogen/methane ratio was found to increase the sp2 character, which correlated with the emission field. AFM analysis of HDLC samples yielded surface roughness values ranging from 0.2 nm to approximately 0.01 nm, confirming the continuous, non-porous and smooth nature of the surfaces.

Tribochemistry of Diamond-like Carbon: Interplay between Hydrogen Content in the Film and Oxidative Gas in the Environment.

The lubricity of hydrogenated diamond-like carbon (HDLC) films is highly sensitive to the hydrogen (H) content in the film and the oxidizing gas in the environment. The tribochemical knowledge of HDLC films with two different H-contents (mildly hydrogenated vs highly hydrogenated) was deduced from the analysis of the transfer layers formed on the counter-surface during friction tests in O2 and H2O using Raman spectroscopic imaging and X-ray photoelectron spectroscopy (XPS). The results showed that, regardless of H-content in the film, shear-induced graphitization and oxidation take place readily. By analyzing the O2 and H2O partial pressure dependence of friction of HDLC with a Langmuir-type reaction kinetics model, the oxidation probability of the HDLC surface exposed by friction as well as the removal probability of the oxidized species by friction were determined. The HDLC film with more H-content exhibited a lower oxidation probability than the film with less H-content. The atomistic origin of this H-content dependence was investigated using reactive molecular dynamics simulations, which showed that the fraction of undercoordinated carbon species decreased as the H-content in the film increased, corroborating the lower oxidation probability of the highly-hydrogenated film. The H-content in the HDLC film influenced the probabilities of oxidation and material removal, both of which vary with the environmental condition.

Tribological behavior of H-DLC and H-free DLC coatings on bearing materials under the influence of DC electric current discharges

In automotive industry, most of the moving components and lubricants are currently being reengineered in order to meet the much harsher operating conditions of electric vehicles (EVs). Diamond-like carbon (DLC) coatings have been widely used in automotive field mainly because of their lower friction and wear coefficients, high chemical inertness, corrosion resistance, and hardness. It is also possible that DLC coatings can afford very desirable tribological properties to the bearings and gears of electric vehicles. In addition, their superior dielectric properties could also be very ideal for such applications. In EVs, current leakages in the powertrain are very common and these can adversely impact the tribological behavior of bearings and/or other moving mechanical elements. Although DLC coatings provide superior tribological performance in IC engine applications, they have not yet been evaluated under electrified sliding conditions of EVs. In this work, two different types of DLC coatings (Hydrogenated DLC (H-DLC) and hydrogen-free DLC (H-free DLC) were produced on AISI 52100 steel substrates and tested using a ball-on-disc tribometer under non-electrified and electrified (DC, 3A) both dry and lubricated sliding conditions. Coefficient of friction (CoF), wear volumes, wear modes and mechanisms have been assessed and reported. The results showed that electrification can cause a decrease in CoF for baseline steel-on-steel contacts, but an increase for H-free DLC in lubricated condition. The change in CoF of H-DLC interface was negligible. However, electrification caused a significant increase in the wear volume of steel-on-steel and H-free DLC-coated surfaces, but little or no wear was observed for the H-DLC-coated cases in both dry and lubricated test conditions. Overall, the tribological behavior of H-DLC was almost unaffected by electrification, suggesting that one has to select the right kind of DLC for applications involving electrified contact conditions.

Influence of mechanical properties of coating and substrate on wear performance of HDLC and TiN-coated AISI 5140 alloy steel

This paper investigates the mechanical and wears resistance properties of AISI 5140 alloy steel coated with thin films of hydrogenated diamond-like carbon (HDLC) and titanium nitride (TiN). HDLC and TiN films were sputtered onto AISI 5140 alloy steel samples using the magnetron sputtering technique. In addition to coatings, the metallic substrates were also evaluated for hardness and wear resistance. The findings on wear losses, patterns, and wear trace widths were determined after the wear tests. In tribological tests, a variety of wear patterns were observed on both thin film coatings. The primary wear mechanisms of TiN coatings were found to be tribochemical wear, substrate material loss, and wear debris. However, the HDLC coating experienced wear due to ploughing action, with minor patches of the thin film being removed from the underlying material, but scanning electron microscopy observations showed no detectable wear. Using Raman spectroscopy, the structural properties of HDLC were analyzed, and the findings were compared to those of wear patterns. The results showed that the coating significantly improved the wear resistance of the AISI 5140 alloy steel and that the TiN-coated AISI 5140 alloy steel sample was more comparable to wear than the HDLC-coated AISI 5140 alloy steel sample. It is also shown that, as compared to an uncoated metallic substrate, the resilience of thin film coated components is a key factor impacting component life.

Distinct effects of endogenous hydrogen content and exogenous hydrogen supply on superlubricity of diamond-like carbon

Hydrogenated diamond-like carbon (HDLC) has drawn significant interest as a solid lubricant coating due to its superlubricity. However, HDLC exhibits drastically different frictional behavior depending on hydrogen (H) content or sp2/sp3 carbon ratio. Various structural aspects of HDLC can be analyzed with Raman spectroscopy; but analyzing the wear track on the HDLC surface could not provide insightful information about the shear plane structure because the probe depth of Raman is not shallow enough to discriminate the contribution from the bulk film. When a dissimilar material is rubbed on HDLC, the transfer film is always formed on the counter-surface. Since the transfer film is the direct outcome of frictional shear, one can assume that analyzing the transfer film can depict characteristic features of the shear plane during friction without convolution from the bulk contribution. This study employed microscopic Raman analysis to capture hyperspectral images of the entire transfer films on a stainless-steel counter surface formed from HDLCs with different endogenous H-contents (30 and 40 at.% in the film) in gas environments of N2 and H2 (exogenous supply). This analysis provided statistically meaningful data showing how the degrees of graphitization and hydrogenation of the shear plane during the run-in and steady-state friction periods vary with the HDLC structure as well as the environment condition, which is critically needed information to understand how the superlubricity is induced and altered when HDLC is used as a solid lubricant film.

Novel Tribometer for Coated Self-Lubricating Spherical Plain Bearings in a Vacuum

Coated self-lubricating spherical plain bearings (SSPBs) are a fairly key component of the space-swing mechanism. To examine the operation status and tribological properties of coated SSPBs, a tribometer with a temperature control module in a vacuum condition was developed. The tribometer was mainly composed of a fixture system, reciprocating rotational motion system, environment control system, etc. First, the tribometer was verified with the self-made hydrogenated diamond-like carbon (H-DLC) SSPBs. The sensor signals indicated that the tribometer conformed to the design specifications. Then, the influence of friction heat on the tribological properties of H-DLC SSPBs was analyzed. The results showed that friction torque and temperature increased with the overall test time. Although the temperature had reached 48 °C, the frictional heat had little effect on the H-DLC SSPBs’ lifespans. The damage mechanism of H-DLC SSPBs was dominated by abrasive wear and fatigue wear in vacuum conditions.

Tribological performance of hydrogenated diamond-like carbon coating deposited on superelastic 60NiTi alloy for aviation self-lubricating spherical plain bearings

It is imperative to develop a novel matching of metallic substrate and self-lubricating coating for aircraft spherical plain bearing in a wide range of service conditions. As a new type of superelastic material, 60NiTi alloy meets the performance requirements of aerospace bearing materials, but exhibits poor tribological performance, especially under the conditions of dry sliding friction. A Hydrogenated Diamond-Like Carbon (H-DLC) coating was deposited on the 60NiTi alloy to improve its tribological performance. The microstructure and mechanical behavior of the 60NiTi alloy and its H-DLC coating were explored. Results show that improvement of friction and wear performance of the H-DLC coating deposited on the 60NiTi substrate is mainly achieved by graphitization at the friction interface and the transfer film produced on the counterpart ball. The increased friction load leads to intensification of graphitization at the friction interface and formation of continuous and compact transfer film on the surface of the counterpart ball.

The effects of Cr and B doping on the mechanical properties and tribological behavior of multi-layered hydrogenated diamond-like carbon films

To enhance the adhesion strength and tribological properties of hydrogenated diamond-like carbon (H-DLC) films, the multi-layered H-DLC films consisting of a function layer with chromium and boron doping, Cr-C and Cr-H-DLC transition layer and a Cr bonding layer were prepared on 52100 steel through the unbalanced magnetron sputtering system. The influence of Cr and B doping on the mechanical properties and tribological performance were investigated. The results indicated that the structure of H-DLC, Cr-H-DLC and B-H-DLC films was compact. There were no peelingflake, delamination, and crack between the films and substrate. Raman spectra showed that the value of ID/IG increased with the Cr and B doping, which indicated that the sp3 content inside the H-DLC film decreased. Compared with the atmospheric condition, the coefficient of friction and wear rate of those films were higher than that in vacuum. According to the Raman spectra of the wear track, it was found that the graphitization of sliding interface theory had its limitations. The mechanical wear of those films was dominated by the abrasive and oxidative wear in the atmospheric environment. Owing to the passivation of the hydrogen atoms, the abrasive wear and adhesive wear was the main failure mechanism in vacuum.

Optimized study of the annealing effect on the electrical and structural properties of HDLC thin-films

The plasma-enhanced chemical vapor deposition (PECVD) technique has been utilized for the facile surface deposition of hydrogenated diamond-like carbon (HDLC) thin-films onto Si(100) substrates. The as-deposited film surface is homogenous, free of pinholes, and adheres to the substrate. Annealing of the synthesized HDLC surface in a vacuum was performed in the temperature range of 200 to 1000 °C. A host of instrumental techniques, viz. FTIR spectroscopy, AFM, STM, and EC-AFM, were successfully employed to detect the morphological transformation in the HDLC films upon annealing. EC-AFM studies show irreversible biased behavior after undergoing a surface redox couple reaction and morphological change. Raman spectroscopy was carried out along with STM and EC-AFM to determine the functional nature and conductivity of the annealed surface.

Environmental effects on superlubricity of hydrogenated diamond-like carbon: Understanding tribochemical kinetics in O2 and H2O environments

Hydrogenated diamond-like carbon (H-DLC) exhibits superlubricity in inert conditions, making it ideal for protective coating in machines, but its superlubricity is often lost in humid air. Knowing reaction mechanisms of such oxidation processes would be a prerequisite to incorporate environment tolerance into the H-DLC chemistry for superlubricious performance in ambient air. But mechanistic understanding has been hampered by air-oxidation of the tribo-tested H-DLC surface during the sample transfer for ex-situ analyses. As an alternative approach, this study derived a Langmuir-type kinetics model to analyze tribochemical processes occurring at the H-DLC surface in oxidative environments. The model unveiled very distinct environmental effects of O2 and H2O on tribochemistry of H-DLC. The O2-oxidized H-DLC surface retains low friction after the run-in period (μ≈0.057) but is susceptible to frictional wear (wear rate ≈ 32 μm3/(N·mm)). In contrast, the H2O-oxidized surface is susceptible to the adsorption of water which acts like boundary lubricant layers protecting the surface from wear (∼3.4 μm3/(N·mm)) but with relatively high friction after the run-in period (μ≈0.22). The oxidation probability of H-DLC by O2 or H2O (∼10−4 Torr−1 s−1) is found to be comparable to the reaction probability of hydrocarbons on catalytically-active metals (∼10−3 Torr−1 s−1).

Durable superlubricity of hydrogenated diamond-like carbon film against different friction pairs depending on their interfacial interaction

Superlubricity of hydrogenated diamond-like carbon (H-DLC) film in vacuum was achieved against four different friction pairs (ZrO2, Al2O3, Si3N4 and SiC) due to the formation of graphene and nanoscrolls in transfer layer as well as the graphitization of contacted substrate; however, the durability of superlubricity was significantly diverse. The Al2O3/H-DLC pair presents much longer superlubricity lifetime than other three tribology systems. Combining with the experimental characterizations and the first principles calculation results, the synergistic effects from mechanical action and interfacial adhesion interaction between two sliding surfaces were found to mainly respond to the superlubricity failure of H-DLC films. High mechanical action or/and strong interfacial adhesion interaction facilitated the wear of counterface or H-DLC substrate, either of which would largely weaken the durability of superlubricity. Through with the similar mechanical action compared to the Si3N4/H-DLC interface, the Al2O3/H-DLC interface was capable of durable superlubricity since the much weaker interfacial adhesion interaction helps maintain the stable transfer layer on the ball surface and form smoothly graphitized substrate surface in the contacted region.

Effects of water molecules on the formation of transfer films and the occurrence of superlow friction

The establishment of superlow friction in moist air is very important for the engineering application of hydrogenated diamond-like carbon (H-DLC) films. Nevertheless, water molecules in the surrounding atmosphere always result in the failure of the near-frictionless state. This work aims to explore the effects of water molecules in the environment and the material of the counterparts on the tribological performance of a composite structure prepared by depositing MoS2 on a H-DLC film. The results indicated that the existence of water molecules in the atmosphere is beneficial for achieving stable superlubricity for the material system because it helps retain the in-situ formed MoS2 transfer film on the counterpart. In the presence of water molecules, the wear interface was replaced by a robust and incommensurate MoS2 tribolayer/H-DLC sliding interface, which was responsible for the superlow friction achieved in this work. The results also revealed that the ZrO2 counterpart was capable of retaining the as-formed MoS2 transfer film and establishing long-lasting superlow friction even in dry air. The mechanisms behind this phenomenon are also discussed in this paper.

The pivotal role of oxygen in establishing superlow friction by inducing the in situ formation of a robust MoS2 transfer film

The achievement of superlow friction is vital for the engineering application of hydrogenated diamond-like carbon (H-DLC), but it always fails in an oxygen atmosphere. In this paper, robust superlow friction was achieved by MoS2 flakes and H-DLC composite films in a large range of atmospheres, especially in oxygen. The results showed that the composite structure could only retain the superlow friction for an short time in pure argon, nitrogen and carbon dioxide; surprisingly, oxygen was capable of remaining in the near frictionless state with a friction coefficient as low as 0.002, and the duration was prolonged significantly by the introduction of oxygen in those other gases. The stability of the transfer film that induced the near frictionless state was also studied comprehensively. The experimental results and first-principle calculations demonstrated that oxygen could bond with the molybdenum, sulfur and aluminum atoms to form bridge bonds that fixed the MoS2 transfer film on the counterface; this led to the formation of incommensurate contact between the MoS2 tribo-layer and H-DLC film, which enabled robust superlow friction. This finding supports a simple strategy to resolve the challenge of superlubric failure and opens a path for the actual application of H-DLC in oxygen-rich environments.

Synthesis of Graphene Oxide from Hydrogenated Diamond Like Carbon and Protein Immobilization onto It: Characterization and Study of Practical Utility

In the last few years, Graphene oxide material and biomolecules studies have increased. The various synthesis methods of graphene oxide are constantly pursued to improve and provide safer and more effective alternatives. Though the preparation of graphene oxide from Graphite powder or Graphite flake through Hummers method is one of the oldest techniques but still now it is one of the most suitable methods. Here, Graphene Oxide has been prepared from a tunable material Hydrogenated diamond like carbon (HDLC) which is an atomically smooth surface that can be deposited on high-surface area Silicon (100) wafer plate. The HDLC film was heated at a fixed temperature of 900°C for 30 min in high vacuum ~1 × 10−6 torr and oxygenated at room temperature. A synthetic sequence is described involving Oxidation of annealed HDLC (A-HDLC). Raman measurements confirm the G and D peak by Oxidation of A-HDLC and FTIR confirms functional groups. Atomic force microscopy (AFM) images describe the surface of A-HDLC, Oxidized Graphene and BSA immobilized GO. This GO onto Silicon substrate offers many technical advantages than as oxidized graphene Synthesis from other Chemical methods.

Origin of low friction in hydrogenated diamond-like carbon films due to graphene nanoscroll formation depending on sliding mode: Unidirection and reciprocation

The origins of low friction of hydrogenated diamond-like carbon (H-DLC) films depending on unidirectional and reciprocating motions were detected by the analyses of transmission electron microscopy (TEM), Raman spectroscopy and atomic force microscopy (AFM). Under the comparable experimental conditions, the reciprocating sliding of H-DLC films against a steel ball in humid air enables the stable friction coefficient to lower more than 70% but the surface wear to increase 50% compared to the unidirectional sliding. The uniformity and structure of transfer layer and worn H-DLC surface strongly depend on sliding motions. Two-way movements in the reciprocating sliding help to keep the transfer layer within contact region, enhancing its graphitization. TEM image of transfer layer shows the formation of abundant graphene nanoscrolls in reciprocating sliding but only few graphite layers in unidirectional sliding. AFM results and Raman spectra suggest that not only the transfer layer but also the graphitization of the rough worn H-DLC surface induced by high alternating stress plays a significant role in the low friction under reciprocating motion. Combining the atomic-scale, nanoscale and macroscale results, a comprehensive model is proposed for the statement of low friction sliding contact of H-DLC films depending on the sliding motions.

Tribological properties improvement of H-DLC films through reconstruction of microstructure and surface morphology by low-energy helium ion irradiation

The hydrogenated diamond-like carbon (H-DLC) films were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique and implanted with 60 keV He ion (He+) over a wide dose ranges. A two-steep model for the evolution of tribological properties of H-DLC films after irradiation has been proposed and discussed. Firstly, at the He+ doses range from 0 to 3 × 1016 cm−2, irradiation treatment induced structural transformation of graphitization in the H-DLC films modification layer due to the increase of sp2/sp3 ratio, which led to the upgradation of tribological properties owing to the lubrication effect of graphite. The decreases of the root mean square roughness (Sq) led to a decrease in the resistance to the relative movement of the friction pair. Therefore, the reduction of friction coefficient and wear rate of H-DLC was contributed to be the combined effect of graphite phase transition and low Sq value. As the dose was increased to 1 × 1017 cm−2, the degradation of tribological properties was evidenced, which could be ascribed to the formation of irradiation-induced He bubbles and holes on the surface of films. In addition, when the irradiation dose increased from 1 × 1016 cm−2 to 3 × 1016 cm−2 and then to 1 × 1017 cm−2, the variation of H/Ef value also indicated that the anti-wear properties of H-DLC film first increased and then decreased. Based on the experimental results, the H-DLC film that irradiated by the He+ at the dose of 3 × 1016 cm−2 shows the best tribological properties with low friction coefficient and high anti-wear properties while maintaining agreeable mechanical properties.