Hydrogenated Diamond-like Carbon Films Research Articles

Novel method for quantifying the ratios of sp3/sp2 hybridized carbon in diamond-like carbon films using soft X-ray emission spectroscopy

This study presents a pioneering method for quantifying the ratios of sp3/sp2 hybridized carbon in diamond-like carbon (DLC) films. The novelty lies in using soft X-ray emission spectroscopy (SXES), a technique not widely employed in this context. The DLC films, obtained through various deposition methods, were characterized for hydrogen content, film density, and sp3/sp2 ratio using Rutherford backscattering spectroscopy/Elastic recoil detection analysis (RBS/ERDA), X-ray reflectivity (XRR), and SXES, respectively. To ensure the reliability of the SXES method, it was validated through comparative analysis with near-edge X-ray absorption fine Structure (NEXAFS) spectroscopy, providing confidence in its accuracy. While NEXAFS provided consistent sp3/sp2 ratio measurements in hydrogen-free DLC films, its accuracy in hydrogenated DLC films was impacted due to poor accuracy when applied to this film group. In contrast, SXES demonstrated greater precision in hydrogenated and hydrogen-free DLC films, revealing sp3/sp2 ratios ranging from 36 % to 62 %, corresponding variations in hydrogen contents and film densities. These findings highlight the superior utilization of SXES in accurately determining the sp3- and sp2-hybridized carbon ratios, particularly in hydrogenated DLC films, offering a robust alternative to NEXAFS and paving the way for enhanced characterization and application in various industrial fields.

The establishment of superlubricity in engineering field for H-DLC composite in multi-environments

The application of hydrogenated diamond-like carbon (H-DLC) with superlubricity can significantly decrease friction and largely save energy dissipation in mechanical systems. Thus, the realization of the near-frictionless state at the engineering scale in both inert and active atmospheres for H-DLC film is of interest. However, the film can achieve the state only under inert and vacuum conditions, while it fails in active atmospheres, such as oxygen and moist air, which seriously hinders wide application of the superlow friction. In previous studies, we have successfully solved this challenge by combining H-DLC film with MoS2 flakes. Nevertheless, the life of the superlubricity achieved by the MoS2/H-DLC composite in an inert gas was extremely short, which was attributed to the failure of MoS2-rich transfer films. To establish the near-frictionless state in various atmospheres, graphene oxide with numerous oxygen-containing groups was used to prepare a GO/MoS2/H-DLC triple composite in this study. The tribological performances were investigated systematacially. The results demonstrated that the triple composite could establish superlubricity in inert gases and ultralow friction in active atmosphere by forming robust transfer films at sliding interfaces. The structures at frictional interfaces including wear tracks and scars were thoroughly investigated. This work provides a valuable approach for the H-DLC composite to achieve an extremely low friction in various atmospheres and can guide the application of superlow friction in engineering fields.

Comparative study of dry high-temperature tribological performance of hydrogen-free and hydrogenated DLC films deposited by HiPIMS in DOMS mode

Solid lubricants are crucial for industries operating at temperatures beyond 300 ºC, where liquid lubricants encounter limitations. Diamond-like carbon (DLC) films, known for exceptional solid lubrication and mechanical properties, need higher thermal stability for effective use in high-temperature applications. This study focuses on developing DLC films with the required thermal stability and solid lubricating properties. Hydrogen-free and hydrogenated DLC films were deposited utilizing deep oscillation magnetron sputtering (DOMS). Thermal characterizations revealed both films surpassed 500 ºC in thermal stability, rendering them suitable for high-temperature tribological applications. However, the hydrogenated DLC film exhibited superior solid lubricating properties, achieving an ultra-low friction coefficient below 0.05 at elevated temperatures, along with enhanced wear resistance, while effectively protecting its counterpart up to 500 ºC.

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.

Tribological behavior of DLC film models in base oils: Analysis of influence of sp3/sp2 ratio and hydrogen content

DLC coatings have been proven in formulated oils used in the automotive industry. The development of electric cars and its consequences on the future of the internal combustion engine makes it increasingly necessary that these coatings take their place in other applications where lubrication is generally of lower quality such as for example strips cold rolling where DLC shows significant potential both for production and finishing cold rolling. As a result, the interest of the behavior of these coatings in base oils takes on a new dimension. In this study, we explore the influence of the composition and the sp3/sp2 ratio on tribological behavior of DLC coatings in mineral and synthetic base oils. Hydrogenated DLC films deposited by conventional PECVD with various levels of hydrogen measured by Elastic Recoil Detection Analysis (ERDA) and non-hydrogenated DLC films deposited by PLD in the nanosecond mode (ns-PLD) and by cathodic arc evaporation, thus allowing to have different sp3/sp2 ratios confirmed by Multiwavelength Raman spectrometry, are tested and compared. The tests are performed on a ball-on-flat device with coated balls to better evaluate wear level. The behavior of highly hydrogenated DLC is of particular interest; with a very high level of durability in both base oils and friction level which comparatively varies between low and very low depending on the base oil.

Plasma Chemical Deposition of Hydrogenated DLC Films with Different Hydrogen and sp3-Hybrid Carbon Content

Plasma Chemical Deposition of Hydrogenated DLC Films with Different Hydrogen and sp3-Hybrid Carbon Content

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.

Effect of deposition temperature and hydrogen as a process gas on mechanical properties and specific electrical resistivity of thick a-C:H obtained by means of PACVD

DLC coatings are widely used for their protective properties such as high wear resistance, low friction coefficient as well as chemical inertness. However, their electrical resistance is usually very high, which limits their utilization in electrotechnical applications. To improve electrical conductivity, DLC films are typically doped with nitrogen or metals. This study, however, investigates the mechanical and electrical properties of un-doped, hydrogenated DLC films deposited using temperatures above 450 °C. To further enhance the coating's properties, hydrogen gas was added during deposition.The DLC coatings were produced by means of PA-CVD using a pulsed DC discharge. Temperatures of 450 °C, 500 °C and 550 °C were used to deposit a-C:H films on steel substrate. The process gas consisted of a mixture of argon and acetylene. Additionally, coatings were deposited with hydrogen added to the gas mixture. A silicon-based interlayer served as an electrical insulator between substrate and coating and was deposited with HMDSO as a precursor. To measure the specific electrical resistivity of the films, the van der Pauw method was performed. The mechanical properties of the coatings were determined through nanoindentation. Raman spectroscopy was performed to analyze the structure of the DLC coatings.The films showed a significant decrease in specific electrical resistivity with increasing deposition temperature. Values dropped to <104 μΩ cm at 550 °C, attaining levels close to graphite. Hardness and Young's modulus increased up to 147 % with rising deposition temperature. The addition of 18 % hydrogen gas during deposition resulted in at least 60 % further reduction in specific electrical resistivity, while also slightly raising coating hardness for deposition temperatures above 450 °C. With this new distinct deposition method, electrically conductive a-C:H coatings with improved mechanical properties can be produced only by increasing the deposition temperature and the utilization of hydrogen as process gas.

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.

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.

Evaluation of DLC, MoS2, and Ti3C2Tx thin films for triboelectric nanogenerators

Due to their cost-effective fabrication, easy integration, and low frequency working range, triboelectric nanogenerators (TENGs) demonstrate tremendous potential in green energy harvesting to power smart devices and the internet of things (IoT). However, there is an urgent need to synergistically maximize their output and improve their durability to ensure a long-lasting high performance. This study aims at elucidating the performance of protective thin films deposited on the wear-prone PTFE surface of TENGs including doped and undoped, single- and multi-layer hydrogenated DLC films, MoS2 coatings fabricated by physical vapor deposition and multi-layer Ti3C2Tx (MXene) films. The deposited coatings are characterized by electron microscopy, and Raman spectroscopy. Their triboelectric performance is analyzed for TENGs operating in contact separation and freestanding sliding modes. We verified that MXenes outperformed the other films in contact separation mode due to the good electron gain ability of functional oxygen and fluorine groups. In sliding mode, the undoped a-C:H coating performed on a comparable level to the uncoated reference and superior to the tungsten-doped DLC and MoS2 films. The film withstood long-term tests without notable signs of wear; merely the output slowly decreased with time due to graphitization and thus potential material transfer to the mating body.

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.

In-situ thermal stability analysis of amorphous Si-doped carbon films

Doping enhances diamond like carbon (DLC) coatings for extreme high-temperature applications. However, understanding the enhancement mechanism is elusive. This study employs a novel system integrating Raman spectroscopy and depth-sensing indentation with a heating chamber to monitor chemical structural, and mechanical properties of doped and un-doped hydrogenated DLC films, with temperature. This in-situ investigation represents extreme working conditions, revealing how doping enhances DLC films thermal stability. It is shown that the thermal stability of the a-C:H:Si films could be maximized by increasing the Si content. The film with the highest Si content was stable until 650 ̊C while the films with lower Si or No–Si content graphitized at lower temperatures. This study presents hardness measurement under high-temperature conditions that were not available before. In-situ observations reveal how Si doping correlates with stability of mechanical properties at elevated temperatures. The correlation of the mechanical properties with WG reveals that the mechanism of thermal stability is that Si doping makes the film resistant to graphitization by promoting sp3 hybridization. Furthermore, our in-situ approach makes it possible to conduct characterization that emulates real service conditions, and therefore, our results show great potential of the industrial application of DLC coatings in extreme thermal conditions.

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.

Effect of Soft X-ray Irradiation on Film Properties of a Hydrogenated Si-Containing DLC Film.

The effect of soft X-ray irradiation on hydrogenated silicon-containing diamond-like carbon (Si-DLC) films intended for outer space applications was investigated by using synchrotron radiation (SR). We found that the reduction in film thickness was about 60 nm after 1600 mA·h SR exposure, whereas there was little change in their elemental composition. The reduction in volume was attributable to photoetching caused by SR, unlike the desorption of hydrogen in the case of exposure of hydrogenated DLC (H-DLC) film to soft X-rays. The ratio of the sp2 hybridization carbon and sp3 hybridization carbon in the hydrogenated Si-DLC films, sp2/(sp2 + sp3) ratio, increased rapidly from ~0.2 to ~0.5 for SR doses of less than 20 mA·h. SR exposure significantly changed the local structure of carbon atoms near the surface of the hydrogenated Si-DLC film. The rate of volume reduction in the irradiated hydrogenated Si-DLC film was 80 times less than that of the H-DLC film. Doping DLC film with Si thus suppresses the volume reduction caused by exposure to soft X-rays.

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.

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.