Carbon-rich Surface Layer Research Articles

Effect of Europium and Gadolinium Alloying Elements on the Tribological Response of Low Hydrogen Content Amorphous Carbon.

Dopants and alloying elements are commonly introduced in amorphous carbon (a-C) materials to tailor their mechanical and tribological properties. While most published studies have focused on doping and alloying a-C coatings with metals or metalloids, doping a-C films with rare-earth elements has only recently been explored. Notably, our understanding of the shear-induced structural changes occurring in rare-earth-element-containing a-C films is still elusive, even in the absence of any liquid lubricants. Here, the friction response of Eu- and Gd-containing a-C films with low hydrogen content deposited by HiPIMS on silicon was evaluated in open air and at room temperature. The load-dependent friction measurements indicated that the introduction of Gd ((2.3 ± 0.1) at.%) and Eu ((2.4 ± 0.1) at.%) into the a-C matrix results in a significant reduction of the shear strength of the sliding interfaces ((41 ± 2) MPa for a-C, (16 ± 1) MPa for a-C:Gd2.3at.%, and (11 ± 2) MPa for a-C:Eu2.4at.%). NEXAFS spectromicroscopy experiments provided evidence that no stress-assisted sp3-to-sp2 rehybridization of carbon atoms was induced by the sliding process in the near-surface region of undoped a-C, while the amount of sp2-bonded carbon progressively increased in a-C:Gd2.3at.% and a-C:Eu2.4at.% upon increasing the applied normal load in tribological tests. The formation of an sp2-bonded carbon-rich surface layer in a-C:Gd2.3at.% and a-C:Eu2.4at.% films was not only proposed to be the origin for the reduced duration of the running-in period in tribological test, but was also postulated to induce shear localization within the sp2-carbon-rich layer and transfer film formation on the countersurface, thus decreasing the interfacial shear strength. These findings open the path for the use of Gd- and Eu-containing a-C even under critical conditions for nearly hydrogen-free a-C films (i.e., humid air).

Formation of carbon and oxygen rich surface layer on high purity magnesium by atmospheric carbon dioxide plasma

Formation of carbon and oxygen rich surface layer on high purity magnesium by atmospheric carbon dioxide plasma

Two-step synthesis of few layer graphene using plasma etching and atmospheric pressure rapid thermal annealing

A two-step process using halogen based plasma etching combined with atmospheric pressure rapid thermal annealing has been used to synthesize few layer graphene films on 6H-SiC (0001) surfaces. In this process, the 6H-SiC substrates were etched under different plasma conditions to produce a carbon rich surface layer. This was followed by rapid thermal annealing in a flow of atmospheric pressure Ar to produce the graphene films. The effects of different etching conditions, heating rates, and average and maximum annealing temperatures were investigated. Changes in surface and near-surface composition after each step were characterized by x-ray photoelectron spectroscopy and overall film quality was assessed using Raman spectroscopy. Film defects associated with this synthesis method included a buffer layer between the SiC substrate and graphene as well as oxygen-based defects on the graphene surface. Electrical characterization of these films was performed using both two and four point methods. In the two point measurements, these films exhibited back-to-back Schottky behavior from which the Schottky barrier height, carrier density and mobility were determined. The four point measurements were used to determine the contact and film resistivity as well as the transfer length.

Design and Synthesis of Hierarchical SeS2/C Nanocomposite Cathodes for High Performance Lithium Batteries with Ether-Based Electrolytes

Lithium/sulfur (Li/S) batteries have attracted much attention as one of the most promising next-generation energy storage systems due to the high theoretical specific capacity (1,675 mAh g−1), low cost, natural abundance and environmentally benign nature of sulfur.1 However, the poor electrical conductivity of elemental S (and its reaction product Li2S) often results in low utilization and capacity of S, thus insufficient specific energy of Li/S batteries.2 Selenium (Se), as a congener of S, has been studied as an alternative to S, and exhibited much improved electrode kinetics due in part to higher conductivity of Se than S.3 , 4 However, the high cost, toxic nature and lower gravimetric capacity of Se hinder the adoption of Li/Se batteries. Thus, it is crucial to rationally combine the complementary properties of Se and S for practically viable energy storage systems. Selenium disulfide (SeS2), with a theoretical specific capacity of 1342 mAh g−1, is a promising novel cathode material because of its attractive merits beyond individual S and Se. In this report, we first designed and investigated hierarchical network architectures of carbon as effective hosts for SeS2. By a facile two-step heat treatment method, we synthesized SeS2@KetjenBlack 600 (KB600) nanocomposites in which amorphous SeS2 were uniformly infused/distributed inside porous carbon while creating the carbon-rich surface layer. This unique hierarchical structure with well-interconnected carbon network can effectively confine dissolved SeS2 as well as provide excellent transportation pathways of both Li-ions and electrons. To further enhance the rate capability as well as protect lithium metal anode in Li/SeS2 batteries, we employed LiNO3 and ionic liquids (N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, Py14TFSI) as additives in ether-based electrolytes. It was observed that LiNO3 contributed to higher discharge capacity at a given discharge/charge rate while Py14TFSI was beneficial in suppressing the dissolution of polysulfides/polyselenides of Li/SeS2 batteries. Under optimized composition of electrolytes, hierarchical SeS2@KB600 electrodes showed good initial discharge capacity of 873 mAh g−1 at 250 mA g−1 and high initial Coulombic Efficiency (CE) of 98.3 %. The effects of LiNO3 and Py14TFSI on the electrochemical performance of Li/SeS2 batteries were further correlated with the changes in chemical composition of Li metal surface after cycling by combined electrochemical and microstructural analysis. Symmetrical Li/Li cells were also assembled in order to investigate the chemical/morphological stability of Li metal with various electrolytes in this work. Reference: Song, M. K.; Zhang, Y.; Cairns, E. J. Nano Lett 2013, 13, (12), 5891-9.Fang, R.; Zhao, S.; Sun, Z.; Wang, D. W.; Cheng, H. M.; Li, F. Adv Mater 2017.Eftekhari, A. Sustainable Energy Fuels 2017, 1, (1), 14-29.Cui, Y.; Abouimrane, A.; Lu, J.; Bolin, T.; Ren, Y.; Weng, W.; Sun, C.; Maroni, V. A.; Heald, S. M.; Amine, K. J Am Chem Soc 2013, 135, (21), 8047-56.

Understanding the hydrogen and oxygen gas pressure dependence of the tribological properties of silicon oxide–doped hydrogenated amorphous carbon coatings

Silicon oxide-doped hydrogenated amorphous carbons (a–C:H:Si:O) are amorphous thin films used as solid lubricants in a range of commercial applications, thanks to its increased stability in extreme environments, relative to amorphous hydrogenated carbons (a–C:H). This work aims to develop a fundamental understanding of the environmental impact on the tribology of a–C:H:Si:O. Upon sliding an a–C:H:Si:O film against a steel counterbody, two friction regimes develop: high friction in high vacuum and low gas pressure (oxygen pressure<10mbar; hydrogen pressure<50mbar), and a low friction regime at higher gas pressures (10mbar<oxygen pressure<500mbar; 50mbar<hydrogen pressure<1000mbar). Scanning electron microscopy (SEM) revealed that the tribological behavior of a–C:H:Si:O is governed by adhesive junctions at the sliding interface. At low gas pressures, material transfer from the steel pin to the a–C:H:Si:O flat occurs. At higher gas pressures, a tribofilm forms on the steel countersurface. Raman and near edge X-ray absorption spectroscopy (NEXAFS) spectroscopies demonstrate that upon sliding under the higher gas pressure, low friction regime, a surface layer with an elevated fraction of sp2-bonded carbon atoms forms. These changes indicate that these gases favor the release of the adhesive junctions by dissociatively reacting with the mechanically-stressed sp2 carbon-rich surface layer.

Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide

After the pioneering investigations into graphene-based electronics at Georgia Tech, great strides have been made developing epitaxial graphene on silicon carbide (EG) as a new electronic material. EG has not only demonstrated its potential for large scale applications, it also has become an important material for fundamental two-dimensional electron gas physics. It was long known that graphene mono and multilayers grow on SiC crystals at high temperatures in ultrahigh vacuum. At these temperatures, silicon sublimes from the surface and the carbon rich surface layer transforms to graphene. However the quality of the graphene produced in ultrahigh vacuum is poor due to the high sublimation rates at relatively low temperatures. The Georgia Tech team developed growth methods involving encapsulating the SiC crystals in graphite enclosures, thereby sequestering the evaporated silicon and bringing growth process closer to equilibrium. In this confinement controlled sublimation (CCS) process, very high-quality graphene is grown on both polar faces of the SiC crystals. Since 2003, over 50 publications used CCS grown graphene, where it is known as the "furnace grown" graphene. Graphene multilayers grown on the carbon-terminated face of SiC, using the CCS method, were shown to consist of decoupled high mobility graphene layers. The CCS method is now applied on structured silicon carbide surfaces to produce high mobility nano-patterned graphene structures thereby demonstrating that EG is a viable contender for next-generation electronics. Here we present for the first time the CCS method that outperforms other epitaxial graphene production methods.

PET surface modification by 0.2 keV and 2.5 keV argon ions

PET foils have a high potential as a material for biomedical and electrical industries. PET foils were irradiated by ions for variable irradiation time. The effects of low (2.5, 0.2 keV) energy argon ion flux irradiation on the surfaces of polyethylene terephthalate thin foils (PET) were studied. The source of ions was an ECR Ion Gun with settable acceleration voltages. The modified foils were investigated by in-situ X-ray Photoelectron Spectroscopy (XPS) and ex-situ Fourier transform infrared spectroscopy (FTIR). The significant changes in the chemical composition of the surface layer were quantitatively studied by XPS. The scission of the chains in the surface layer of PET foil was induced by ion flux interaction with PET surface. The strong selective sputtering of oxygen atoms in PET film was observed. The atomic ratio O/C was decreased by 0.2keV and 2.5keV argon ion flux from 0.40 to 0.25 and 0.04 respectively. The oxygen atoms in ester bonds are detached first. This phenomenon is responsible for the creation of carbon-rich surface layer. The FTIR analyses identified changes in chemical composition but with no obvious correlation to surface changes. PET volume changes in the spectra were probably results of photons from the ion source influence on PET foils.

XPS and AFM surface study of PMMA irradiated by electron beam

The effect of low energy electron (LEE) irradiation on polymethylmethacrylate (PMMA) film has been studied. The PMMA film of thickness 20 μm is exposed by a 10 keV electron beam with fluence 2 × 10 14 e/cm 2. The irradiated film was characterized by X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The significant changes in the chemical composition of the surface layer were confirmed by XPS quantitatively. The scission of the chain in the surface layer of PMMA film was induced by electron irradiation and the atomic ration (O 1s/C 1s) was decreased. This phenomenon is responsible for the creation of carbon-rich surface layer. TM-AFM showed hills of nano size surrounded by crater-type features on the irradiated film. The root-mean-square (rms) surface roughness of the sample increased from 3.715 nm to 5.020 nm due to the irradiation, which shows that the surface became rougher after irradiation.

Interaction of O and H Atoms with low-k SiOCH films pretreated in He plasma

AbstractThe effect of He plasma pretreatment on interaction of O and H atoms with SiCOH low-k materials is studied using a special experimental system designed for this purpose. The experimental system allowed separate study of the effects of He plasma, VUV light and He 21S0 metastable atoms. It is shown that the carbon depletion by oxygen atoms can be significantly reduced by He plasma pretreatment. Considerable increase of CH and CH2-CH2 groups in the surface area of low-k films is observed when the films were exposed to VUV light and metastable atoms generated by He plasma. FTIR and ellipsometry showed formation of densified surface layer. This carbon rich densified surface layer decreases damage of low-k film when it is exposed in O2 plasma. The impact of H atoms on low-K surface noticeably differs from O atoms effect. The H atoms saturate all unbounded remaining carbon bonds thereby promoting improvement of SiOCH structure.

ICP-MS, SKPFM, XPS, and Microcapillary Investigation of the Local Corrosion Mechanisms of WC – Co Hardmetal

hardmetal exhibits high corrosion susceptibility in aqueous solutions, related to complex microscale reaction mechanisms. This paper presents developed methods to characterize the local distribution of surface reactions difficult to assess by conventional electrochemical methods. Laterally resolved electrochemical potential distributions measured using scanning Kelvin probe force microscopy (SKPFM) under controlled humidity identified the more noble nature of and the microscale galvanic coupling with the Co areas acting as anodes. Inductively coupled plasma mass spectrometry (ICP-MS) element analysis, carried out using an online flow cell, provided simultaneous, time-resolved detection and quantitative concentration measurement of the dissolved elements. W ions in solution at the open-circuit potential indicated chemical dissolution due to the pH increase on the cathodes in addition to electrochemical anodic Co dissolution. Various mechanisms attributed to homogeneous dissolution of microscale phases or dissolution transients related to localized corrosion attack are identified. X-ray photoelectron spectroscopy (XPS) revealed a carbon-rich surface layer on the grains supporting a mechanism of selective W dissolution. These different techniques provided information on the microscale reactions on surfaces in aqueous solution and allowed construction of a comprehensive model.

Molecular dynamics simulations of CF 3 etching of SiC

Molecular dynamics simulations were performed to investigate CF 3 continuously bombarding SiC surfaces with energies of 100, 150 and 200 eV at normal incidence and room temperature. The simulated results show that the etching rates of Si and C atoms increase linearly with the incident energy. The etch rate of Si atoms is much more than that of C atoms. A carbon-rich surface layer is observed which is in good agreement with experiments. Under bombarding by CF 3, an F-containing reaction layer is formed through which Si and C atoms are removed. In reaction layer, SiF and CF species are dominant. The formation mechanisms of ejected products are discussed. In etching products, SiF 3 is dominant. It is found that etching of C atoms in SiC is controlled by physical sputtering, while etching of Si atoms in SiC is controlled by chemical sputtering.

Photoemission study of ternary to penternary Fe-based metallic glasses: Chemical analysis of surface and bulk

Bulk metallic glasses consisting of Fe, Mo, Cr, C, B, and Er have been investigated by x-ray photoelectron spectroscopy, aimed to elucidate the local atomic structure of the amorphous phase. In order to examine the electronic properties of this class of material, photon energy dependent measurements in combination with argon-ion irradiation were employed to identify and separate surface and bulk contributions to the spectra. The core levels suggest the presence of a carbon-rich surface layer with oxidized boron and metals, and metal carbides and borides in the bulk. Exposure to molecular oxygen and annealing experiments probe the chemical reactivity of the material. Formation of boron oxides at comparably low temperatures (300°C) might have consequences for the stability of the amorphous phase. We observe variations in binding energy of the Fe 3p core level with respect to the alloy composition, which indicate changes in the chemical state of iron.

Etching of SiC by energetic F 2: Molecular dynamics simulation

Molecular dynamics (MD) simulations were performed to investigate F 2 continuously bombarding silicon carbide (SiC) surfaces with energies in the range of 50–200 eV at normal incidence and room temperature. The Tersoff–Brenner form potential was used. The simulation results show that the uptake of F atoms, the etch yields of C and Si from the initial substrate, and the surface structure profile are sensitive to the incident energy. Like occurrence in Si etching, steady-state etching is observed and an F-containing reaction layer is formed through which Si and C atoms are removed. A carbon-rich surface layer after bombarding by F 2 is observed which is in good agreement with experiments. In the reaction layer, SiF in SiF 2 species are dominant; with increasing incident energy, the total fraction of SiF and SiF 2 increases, while the amount of SiF 3 and SiF 4 decreases. Finally, etching mechanisms are discussed.

Reactive etching of SiC by energetic CF3: molecular dynamics simulation

Molecular dynamics simulations were performed to investigate CF 3 continuously bombarding SiC surfaces with energies of 10, 50 and 100 eV at normal incidence and room temperature. In the simulation, the deposition yield of C and F atoms from CF 3 , the etching yield and rate of C and Si from the initial substrate, and the surface structure profile are discussed. Accompanying the saturation of F atoms from the incident CF 3 molecules deposited on the surface, a balance between C removal from the substrate and C deposition from CF 3 is established, while Si and C etching from the initial substrate reaches a steady state. The simulated results show that the etch rate of Si is more than that of C. A carbon-rich surface layer is observed which is in good agreement with experiments. In the F-containing reaction layer, SiF in SiF x species is dominant.

Influence of surface roughness of PVD coatings on tribological performance in sliding contacts

The influence of surface roughness on the tribological performance, i.e. friction, wear and material pick-up tendency, of two different commercial PVD coatings, TiN and WC/C, in sliding contact with ball bearing steel has been evaluated using two different types of sliding wear laboratory tests. Post-test characterisation using SEM/EDS, AES, ToF-SIMS and XPS was used to evaluate the prevailing friction and wear. The results show that the surface roughness of the coating is of importance in order to control the initial material pick-up tendency and thus the friction characteristics in a sliding contact. Once initiated, the material pick-up tendency will increase, generating a tribofilm at the sliding interface. For steel–TiN sliding couples a FeO-based tribofilm is generated on the two surfaces and FeO/FeO becomes the sliding interface (interfilm sliding) resulting in a high friction coefficient. For steel–WC/C sliding couples the WC/C displays a pronounced running-in behaviour which generates a WO 3-based tribofilm on the steel surface while a carbon rich surface layer is formed on the WC/C surface, i.e. WO 3/C becomes the sliding interface (interface sliding) resulting in a low friction coefficient.

In situ photoelectron spectroscopy analysis of tetrahedral amorphous carbon films

In the present study, we describe an in situ investigation of the electronic structure of ta-C (tetrahedral amorphous carbon) films with photoelectron spectroscopy in the X-ray (XPS) and ultraviolet regime (UPS). The carbon films were deposited with a filtered arc source and their structure controlled through adjustment of the ion energy by the application of a dc-bias to the substrate. The different information depths of UPS and XPS are exploited to distinguish between variations in the electronic structure and sp 3/sp 2 ratio at the surface and in the bulk. It is shown that the sp 3/sp 2 carbon ratio determined by fitting the C1s peak reflects under certain conditions the true change of this ratio in the bulk, but can yield distorted values if the presence of the sp 2 carbon rich surface layer is neglected. The reliability regime of the core level analysis is established and it is demonstrated how values of sp 3 concentrations in the bulk and at the surface can be distinguished. The analysis of the position of the plasmon loss feature supports the results from the XPS analysis. The UPS-valence band spectra are dominated by the sp 2 surface layer, and reflect the density of states at the surface. The highest surface sp 3 concentration is not equivalent to the optimum ion energy required to achieve a maximum sp 3 content in the bulk, a fact which influences consideration of contact formation.

The long-term evolution of and transport processes in a self-sustained final cover on waste deposits

A new principle for confinement of waste based on a self-sustained seal is presented. The top cover is considered to consist of two main layers; an organic carbon rich surface layer that is able to support vegetation and an inorganic layer beneath it. The function of the cover is to mitigate oxidation and acidification of landfilled waste and hence the release of toxic metals. It is suggested that forest soil formation and soil development could prove to be valuable information sources for the study of the long-term behaviour of a final cover on waste deposits. Since the cover is expected to develop in northern temperate climate the focus is on Spodosol soil. A number of simulations of the long-term behaviour of the final self-sustained landfill cover are made, including the rates of influx of oxygen into the cover. A cover having a large portion of organic matter compared with a cover with no organics can considerably decrease the oxygen concentration and thus the influx of oxygen into a landfill. The calculated oxygen intrusion rate for the former case is of the order of 0.05 kg m −2 year −1. Degradation of the organics produces acids. Our simulations indicate that the pH-buffering capacity of the mineral layer, represented by calcite and primary rock minerals, will last for many thousands of years.

Changes in chemical states of PET films due to low and high energy oxygen ion beam

The effects of low (1.0, 3.0 keV) and high (1.0 MeV) energy reactive oxygen ion beam irradiation on surface nano-morphology and chemical state of the constituent elements in the surface layer of a polyethylene terephthalate thin film (thickness 25 μm) are studied in this paper. PET thin film, which has a high potential as a material for biomedical and electrical industries, was irradiated by oxygen ions of variable doses. The treated films were investigated by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). AFM observation revealed that an increase in dose increased the surface roughness. The significant changes in the chemical composition of the surface layer were confirmed by XPS quantitatively. The scission of the chains in the surface layer of PET film was induced by oxygen ion beam irradiation, and the atomic ratio (O1s/C1s) was decreased. The selective sputtering of oxygen atoms in PET film was occurred. This phenomenon is responsible for the creation of carbon-rich surface layer.

Effects of the ion–solid interaction in glow discharge vapour deposition polymerization of pyromellitic dianhydride

Low energy He ion bombardment of pyromellitic dianhydride monomer used in glow discharge vapour deposition polymerization (GDVDP) of polyimide coatings and its effects on the film deposition process have been studied. The sublimation of the monomer molecules and the simultaneous formation of a damaged, carbon-rich surface layer on the target are discussed from a theoretical point of view based on simulations of the ion–solid interaction. Optical emission and mass spectrometry have been used to analyse the species emitted from the target. In order to study the time evolution of the PMDA target damage, the deposition rate of monomer molecules has been monitored. FT-IR spectroscopy has been used to determine the molecular damaging of the target monomer and deposited films.

Titanium carbide etching in high density plasma

Titanium carbide etching in fluorine-containing (CF 4, CHF 3, and SF 6) high density plasmas was investigated. The dependence of etch rates on gas composition and bias power was studied using the response surface method and the effects of source power and pressure were also examined. Argon was found to enhance the etch rate but the effect diminished when etching was controlled by the availability of the reactive species. The increase of the etch rate with increasing pressure indicated that the reactive radicals are the main etching species. The etch rate in SF 6 plasma was found to be up to four times faster than in CF 4 plasma because of the abundance of the active fluorine species. Electron spectroscopy for chemical analysis (ESCA) detected a carbon-rich surface layer for samples etched with CF 4, CHF 3 and even a non-polymerizing SF 6 plasma.The results suggested that titanium was extracted preferentially from TiC during reactive ion etching in fluorinated plasmas.