Amorphous Carbon Coatings Research Articles

Influence of Carbyne Content on the Mechanical Performance of Nanothick Amorphous Carbon Coatings

This study concerns the evaluation of the coefficient of friction, at different temperatures, of amorphous carbon thin films, deposited onto nanocrystalline sputtered copper coatings by clean-technology rf magnetron sputtering. The aim is to access the capacity of carbon thin films, with different contents of sp2 and sp1 bonds, to act as a solid lubricant for copper surfaces. Raman spectroscopy revealed that all the as-deposited coatings consist of amorphous carbon with low defect content and decreasing carbyne concentration with increasing thickness. The tribological tests at 25 °C and 200 °C revealed that, for the higher temperature, the 15 nm carbon coating present 0.001 friction coefficients at 2 N load. Overall, the study presents a one-step technology for the greener production of solid lubrication systems for micro- and nano-components, avoiding the environmental impact of lubricants.

Structure and mechanical properties of Ni and Cr binary doped amorphous carbon coatings deposited by magnetron sputtering and pulse cathodic arc

This paper studies the structure and mechanical properties of amorphous carbon coatings (a-C) modified by Ni and Cr binary doping. The a-C:(Nix+Cry) coatings were deposited using pulsed cathodic carbon arc technique with direct current magnetron sputtering.Microstructure, surface morphology and chemical composition of the a-C:(Nix+Cry) coatings were analyzed via Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy. Friction parameters of the coating were studied under the condition of dry sliding using a "sphere-plane" method. It has been established that the structure of binary doped coatings mainly consists of an amorphous carbon matrix, primarily with sp2 bonds, chromium carbide and nickel metal clusters. The dependence of the Сsp3/Сsp2 ratio and microhardness on the composition in the coating has been specified. The Ni/Cr ratio in the coating with the optimal content of Csp2 and C-Cr carbide phases resulting in a decrease in the wear rate of the counterbody has been determined. At the same time, binary doping of carbon coatings substantially changes in both the phase compositions and surface morphology, which leads to the stabilization of the friction coefficient and reduces the counterbody wear rate.

New Droplet Removal Polishing Method for Diamond-Like Carbon with Carbon Fiber Brush

In this paper, we propose a new polishing method for diamond-like carbon (DLC) coatings using a carbon fiber brush (CFB). Surface finishing is an important process for DLC coating applications. A lapping process is widely used for attaining tetrahedral amorphous carbon (ta-C) coatings, which are a type of DLC coating containing many droplets, to obtain fine flat surfaces. The lapping process removes protuberant parts of droplets rather than the entire droplet. In this paper, we propose a new polish brush material made of carbon fiber, called CFB. Carbon fiber has both mechanical strength due to its hard carbonaceous material and flexibility due to its fiber structure. In polishing tests, CFB removed droplets from ta-C coatings and the removal effect increased with the shortening of the brush length. The surface profiles of the polished surfaces indicated that a shorter brush length yielded deep scratch marks on ta-C surfaces. Consequently, the arithmetic average surface roughness of the polished ta-C surfaces, Sa, had almost the same value as that of a non-polished surface. Here, we show the ability of CFB to remove the droplets without an increase in the surface roughness. The CFB with the longest brush length in the present study (12 mm) showed a ten-point average roughness SZJIS= 75 nm and Sa= 4.7 nm, which were 59% and 22% lower than those of the non-polished surface, respectively. Furthermore, the longest CFB removed the entire droplets whereas a shorter CFB merely removed the protuberant part of the droplets. The result indicates that CFB polishing can remove entire droplets, which result in abrasive wear or deterioration. From other polishing tests, the optimum polishing distance was determined. Shorter polishing distances could not remove droplets sufficiently whereas longer polishing distances caused deep scratches on ta-C surfaces due to the material transferred to the CFB. Accordingly, the polishing distance of 600 m showed the best surface finishing with SZJIN= 25 nm and Ra= 0.43 nm, which were 86% lower than and similar to those of the non-polished ta-C surface, respectively.

A facile strategy for recovering spent LiFePO4 and LiMn2O4 cathode materials to produce high performance LiMnxFe1-xPO4/C cathode materials

To successfully recycle spent LiFePO4 and LiMn2O4 batteries and simultaneously produce high performances nano-LiMnxFe1-xPO4/C powders, a mechanical activation-assisted method was effectively applied in the recycling process. The technique consists of the separation and purification of spent cathode materials, high-energy mechanical mixing of raw materials and a commonly used heat treatment processes. The structural and morphological characterization results indicate that nano-sized LiMnxFe1-xPO4/C composites with uniform amorphous carbon coatings were successfully synthesized from spent LiFePO4 and LiMn2O4 batteries. The electrochemical performance testing results show that the recovered nano-LiMnxFe1-xPO4/C cathodes possess promising Li-ion storage properties. LiMn0.5Fe0.5PO4/C displays high capacities of 143.2, 138.1, and 127.6 mAh g−1 at 0.1, 1, and 2C rates, respectively. The obtained cathodes also show outstanding cycling stabilities of 98.47% and 97.58% for LiMn0.5Fe0.5PO4/C and LiMn0.8Fe0.2PO4/C after 100 cycles, respectively. This work indicates that recycling spent LiFePO4 and LiMn2O4 to produce high-performance LiMnxFe1-xPO4/C is a promising strategy for recycling spent LiFePO4 and LiMn2O4 based lithium ion batteries (LIBs) in an efficient and environmentally friendly manner.

Effect of Pillar and Mesh Structure of Tetrahedral Amorphous Carbon (ta-C) Coatings on the Wear Properties and Fracture Toughness of the Coating

Effect of Pillar and Mesh Structure of Tetrahedral Amorphous Carbon (ta-C) Coatings on the Wear Properties and Fracture Toughness of the Coating

On Friction Reduction by Surface Modifications in the TEHL Cam/Tappet-Contact-Experimental and Numerical Studies

The overall energy efficiency of machine elements and engine components could be improved by using new technologies such as surface modifications. In the literature, surface engineering approaches like micro-texturing and the application of diamond-like carbon (DLC) coatings were frequently studied separately, with focus on a specific model contact and lubrication conditions. The contribution of the current study is to elucidate and compare the underlying friction reduction mechanisms of the aforementioned surface modifications in an application-orientated manner. The study applied the operating conditions of the thermo-elastohydrodynamically lubricated (TEHL) cam/tappet-contact of the valve train. Therefore, tribological cam/bucket tappet component Stribeck tests were used to determine the friction behavior of ultrashort pulse laser fabricated microtextures and PVD/PECVD deposited silicon-doped amorphous carbon coatings. Moreover, advanced surface characterization methods, as well as numerical TEHL tribo-simulations, were utilized to explore the mechanisms responsible for the observed tribological effects. The results showed that the DLC-coating could reduce the solid and fluid friction force in a wide range of lubrication regimes. Conversely, micro-texturing may reduce solid friction while increasing the fraction of fluid friction.

Tribological studies on multi-coated forming tools

Processes of Sheet-Bulk Metal Forming combine operations of both sheet and bulk metal forming, enabling the production of closely-toleranced functional components. Locally differing friction conditions on a single tool surface are crucial to control the material flow and to reduce process forces and tool wear. In order to understand how different friction conditions on a multi-coated tool surface affect the material flow during forming operations, variants of amorphous carbon coatings (a-C:H:W) and Cr-based hard coatings (CrAlN, CrAlCN) were deposited in combinations side by side on punches of ring compression tests. Evaluating the rings’ shapes revealed the coatings’ effects on material flow in terms of ring inner contours with sections of varying curvatures. Characteristic friction mechanisms were observed, which allowed a local delimitation of friction conditions by selectively applied coatings.

Electrical contact resistance and tribological behaviors of self-lubricated dielectric coating under different conditions

Abstract In this study, the electro-tribological performance of hydrogenated amorphous carbon coatings against bearing steel in different experimental conditions was continuously monitored by means of coefficient of friction (CoF) and electrical contact resistance (ECR). The influence of tribosystem variables such as humidity, velocity and lifetime on frictional and electrical response were also compared with specific wear energy calculation method. The main findings of both tribological and electrical behaviors include the dissipated energy and the micro-slip dielectric failures that can be linked to the properties of self-lubricating layers, which depend also on the working media. In this framework, the electro-tribological evaluation provides an electrical monitoring tool for coating premature wear detection in real time.

CORROSION PROTECTION OF AMORPHOUS CARBON COATING FOR THE BIPOLAR PLATES OF PEMFCs

A carbon precursor film was formed on a titanium plate by a hydrothermal method using glucose, and an amorphous film was obtained by carbonization at 400∘C under an Ar atmosphere. The morphology and composition of the surface was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), and the interface contact resistance (ICR) under different pressures by simulating the working mode of the fuel cell. The corrosion resistance of amorphous carbon coatings was tested by simulating the proton exchange membrane fuel cells (PEMC). The amorphous coating showed excellent interfacial conductivity and great corrosion resistance, with high potential application in bipolar plates of PEMFCs

Carbon dots as an additive for improving performance in water-based lubricants for amorphous carbon (a-C) coatings

As a new type of carbon material, carbon dots (CDs) with exceptional properties are regarded as a type of potential water-based lubricant additive. In this study, it is first demonstrated that CDs perform excellently when used as lubricant additives for water-lubricated amorphous carbon (a-C) contacts. The results show that the introduction of CDs into water at a concentration of 0.1 wt% could reduce friction and wear in a-C contacts by 33% and 80%, respectively. Additionally, the lubricating performance of CDs is highly sensitive to its adding concentration. The excellent tribological performance of CDs results from the minimized CDs sediment particles (deposited in situ on worn a-C surfaces) and the reduced friction and wear via nano-bearing and nano-filling mechanisms. With high CDs concentrations, however, micron and submicron-sized sediment particles form when friction occurs and cause undesirable abrasive wear. For water-lubricated a-C contacts, therefore, the CDs concentration is the crucial determinant of lubrication performance that has to be appropriately controlled.

Silicon-Core-Carbon-Shell Nanoparticles for Lithium-Ion Batteries: Rational Comparison between Amorphous and Graphitic Carbon Coatings.

Silicon-core-carbon-shell nanoparticles have been widely studied as promising candidates for the replacement of graphite in commercial lithium-ion batteries. Over more than 10 years of R&D, the many groups actively working in this field have proposed a profusion of distinctive nanomaterial designs. This broad variety makes it extremely challenging to establish mechanistic insight into how fundamental material structure and properties affect battery performance. In particular, the interplay between the character of the carbon encapsulation layer and the electrochemical performance of the composite is still poorly understood. In this work, we aim to address this lack of knowledge through the development of a modified chemical vapor deposition approach that enables precise control of the degree of graphitization of the carbon coating. We provide a comparison between core-shell structures maintaining identical silicon cores with different types of carbon shells, that is, graphitic carbon and amorphous carbon. A highly graphitic carbon layer is not only characterized by higher electrical conductivity but markedly favors the transport of lithium ions into the silicon core with respect to an amorphous one. This advantageous property confers better cycling stability to the composite material. We also demonstrate that the graphitic-carbon-coated particles display excellent electrochemical performance even when used as a simple "drop-in" additive in graphite-dominant anodes for current generation Li-ion batteries. Replacement of 10% by weight of graphite in the electrode composition results in an increase of 60% in the storage capacity with a first cycle Coulombic efficiency of 91% and capacity retention over 100 cycles of 86%.

Hydrogen existence state of a hydrogenated amorphous carbon coating and its thermal stability

Hydrogenated amorphous carbon (a-C:H) coatings were deposited by plasma-enhanced chemical vapor deposition at a bias voltage of 740 V. The coated 316 L sample was annealed in 5 Pa argon media. The hydrogen content, chemical bond structure, and morphology of the coating and their evolution during annealing were characterised by elastic recoil detection analysis, mass spectrometry, Fourier transformation infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. It was found that the hydrogenated a-C:H coating had ~50 at. % hydrogen. However, a large part of the hydrogen probably exists in the form of hydrogen molecules, some of which was encapsulated in some “closed” voids. Obviously, this special structure is an effective way to improve the sp3 CC fraction in the a-C:H with high H content. The molecular hydrogen could stay in the “closed” voids stably during annealing below 490 °C. For these a-C:H coatings with specific structure, when annealed below 330 °C, the transformation of C sp3 to sp2 was controlled by the so-called active CH bond reaction in the a-C:H coating. Annealed above 330 °C, these active CH bonds were depleted out quickly, and carbon clusters started to grow. At annealing temperatures >490 °C, the rapid transformation of C sp3 to C sp2 was attributed to C sp3 bond breakage and atomic rearrangement induced by overcoming energy barrier.

Reprint of “Effect of substrate bias on biocompatibility of amorphous carbon coatings deposited on Ti6Al4V by PECVD”

Long service life of implants is of high importance to patients. This study deposited amorphous carbon (a-C) coatings on biomedical grade Ti6Al4V substrate using plasma-enhanced chemical vapor deposition (PECVD). The effect of substrate bias power on microstructure and corrosion of the coatings is investigated and compared with the bare Ti6Al4V substrate. The experimental results show that the substrate bias power of 30 W leads to amorphous carbon in microstructure. With increased bias nanocrystalline graphite appears. All coated samples have significant improvements in corrosion resistivity as compared to Ti6Al4V alone. The sample deposited at bias power of 30 W with a thickness of 150 nm demonstrated the lowest corrosion current of 6.47 nA/cm2 and the lowest corrosion rate of 8.05 nm/year, which can postpone the corrosion of the Ti6Al4V substrate for at least 18 years. From the in-vitro cell culture studies, the best sample also registered the higher cell viability and biocompatibility as compared to bare Ti6Al4V, thus could be helpful in promoting healing.

Molecular descriptions of functionalised multi layered diamond like/ amorphous carbon coatings

Diamond-like carbon (DLC) coatings are increasingly used in multiple applications due to excellent biocompatibility and outstanding mechanical, optical, and electrical properties. However, many aspects of the synthesise and functionality of these films, especially at the surface, are still uncertain. Here, we were successful in synthesising a multi functionality multi-layer DLC and amorphous carbon(a-C:H) coatings, deposited on Si wafers, to create different materials attributes. Time-of-Flight Secondary Ions Mass Spectroscopy (ToF-SIMS), combined with Principal component analysis (PCA) analysis, was used to provide molecular characterisation of the coatings that were synthesized using plasma immersion ions implantation and deposition (PIII&D) technique. We used acetylene (C2H2), methane (CH4) and oxygen (O2) and their mixtures as precursor in the plasma treatment process. The organic layer (s) deposition’s uniformity, thickness, chemical composition, molecular structure, organics and inorganics contaminants in the film were revealed and linked to the plasma deposition conditions. The thickness of the deposited films can be determined according to their Si and O2 content. Samples with higher O content also have higher SiO and NOx contents. ToF-SIMS is successful as a stand-alone technique to provide information and knowledge regarding the surface compositions and the depth interface profile of plasma deposited DLC and a-C:H coatings.

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACTTribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.

Effect of oxygen on degradation of defects on ta-C coatings deposited by filtered arc deposition

Among diamond-like carbon (DLC) coatings, tetrahedral amorphous carbon (ta-C) coatings show particularly high thermal stability and oxidation resistance due to their high sp3 content and hydrogen-free structure. On the other hand, it is well known that high-temperature annealing leads to degradation of the ta-C structure, resulting in a release of stress, decrease of hardness, increase in surface roughness, and transformation of the coating nature. Although optical observation is a powerful candidate for imaging the surface conditions of ta-C coatings, strong light emission due to high temperature prevents the acquisition of clear images. In this paper, we establish a novel in situ observation method using environment scanning electron microscopy (ESEM), which allows us to view ta-C surface defects and droplets at 650 °C. A heater embedded in the ESEM increased the temperature on ta-C coatings to 650 °C at gas pressure of 120 Pa. Using a variety of gas conditions including ambient air, dry nitrogen, or dry oxygen, we clarified the effect of the gases on the degradation of the defects. In a dry oxygen atmosphere, defects grew dramatically with annealing time, whereas nitrogen caused no significant transformation to ta-C surfaces. Raman spectroscopy analysis revealed significant structural change in one type of defect, while no noticeable Raman shift was detected in the other type. A focused ion beam created a cross-section of an annealed defect on the ta-C coating. Subsequently, energy dispersive X-ray spectroscopy (EDS) revealed the characteristics of defect growth; one type of defect was associated with a droplet on the interlayer, and the other with graphitization due to oxidization of ta-C on the droplets.

Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C

Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol’s three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule’s complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.

Amorphous carbon modification on implant surface: a general strategy to enhance osteogenic differentiation for diverse biomaterials via FAK/ERK1/2 signaling pathways.

Bone implants play a crucial role in bone repairing. Nevertheless, low capability of osteoinductivity and osteogenic differentiation for bone regeneration are disadvantages of bone implants. Therefore, it is imperative to develop a general and facile technology to promote the bioactivity of existing implants. Herein, a facile amorphous carbon-coating approach was developed to stimulate osteogenesis on diverse biomaterials, including bioceramics, biometals, and biopolymers via magnetron sputtering deposition. The results confirmed that the amorphous carbon-coating-modified surfaces could significantly enhance osteogenesis of bone marrow mesenchymal stem cells (BMSCs) on every kind of biomaterial surface. Furthermore, it was demonstrated that the FAK/ERK1/2 signaling pathways were involved in the osteogenic effects of this amorphous carbon coating. The bone regeneration ability using the calvarial bone defect model of rats confirmed that the amorphous carbon coating induced faster bone formation and mineralization, which suggested the effect of amorphous carbon coating on stimulating osteogenesis in vivo. These results suggest that the approach involving modifying a surface with amorphous carbon provides a general and simple strategy to enhance the osteogenesis for diverse biomaterials, and this has promising potential for bone repairing applications.

The clarification of low friction mechanism in base oil lubrication of ta-CNx coating by in-situ observation with reflectance spectroscopy

Among DLC (Diamond-Like Carbon), ta-C (tetrahedral amorphous Carbon) coatings have been applied to improve fuel efficiency in automobile engines. Furthermore, it has been reported that a ta-CNx (tetrahedral amorphous Carbon Nitride) coating showed more than 50% lower friction coefficient in base oil lubrication compared to ta-C coating. On the other hand, the low friction mechanism of the ta-CNx coating has not been clarified. From the previous research, low friction mechanism of the ta-CNx coating in oil lubrication is considered that the increment of oil film thickness in accordance with increasing the oiliness of the transformed layer is important. However, the effects of oil film thickness and oiliness of the transformed layer on friction coefficient have not been quantitatively clarified, because oil film thickness and oiliness of the transformed layer have not been measured during friction. Therefore, in this research, the objective was set to clarify the low friction mechanism of the ta-CNx coating in oil lubrication by in-situ observation with reflectance spectroscopy, and oil film thickness and physical properties (polarizability, chemical bonding fraction) of the transformed layer was measured by in-situ observation with reflectance spectroscopy. Using in-situ observation with reflectance spectroscopy, film thickness, refractive index n and extinction coefficient k can be measured during friction test. And polarizability and chemical bonding fraction of the transformed layer were estimated from refractive index and extinction coefficient. From in-situ observation with reflectance spectroscopy, low friction mechanism of the ta-CNx coating in oil lubrication was proposed as shown below. The transformed layer was formed during friction and the polarizability of the transformed layer increased in accordance with decreasing C-Csp2 bonding fraction of the transformed layer, and PAO4 oil molecules were physically adsorbed on the transformed layer. As a result, oil film thickness increased and low friction occurred.

Properties of ta-C coatings prepared by pulsed cathodic arc source at various distances

Tetrahedral amorphous carbon (ta-C) coatings have been prepared on silicon substrates by the pulsed cathodic arc source. For the limitation of the heat flux of the carbon plasma to the sample, the diaphragm of 27 mm diameter has been used. The deposition process has been carried out at three different distances between the arc source and the substrate: 150, 215 and 265 mm. The properties of ta-C coatings have been studied by Raman spectroscopy and dynamic nanoindentation techniques. The analysis of Raman spectra parameters has revealed that the concentration of ordered aromatic rings in Csp2 cluster decreases with distance between the arc source and the substrate while the concentration of chain groups increases. Nanoindentation has shown the decrease in nanohardness and Young’s modulus with the distance. Nanohardness value falls from 21 GPa to 16 GPa and Young’s modulus value goes down from 197 GPa to 177 GPa. So, it’s possible to control the formation of ta-C coatings structure by changing distance between the pulsed cathodic arc source and the substrate.