ta-C Surface Research Articles

Preferential orientation of diamond formation on TaC: Diamond(111)//TaC(111)

Experimental results showed that for the diamond film prepared by hot filament chemical vapor deposition (HFCVD) using Ta filament, TaC existed between diamond and the silicon substrate, and diamond grew directly on TaC, while the inherent mechanism was not clear. Here, a special coherent interface Diamond(111)//TaC(111) is observed using high resolution transmission electron microscopy, and then we explore the effects of the TaC with different lattice planes on the diamond formation by first-principle calculations. The results show that C tends to adsorb on the TaC(111) C-terminated surface. The strong covalent bond between C from diamond and Ta from TaC is formed in the Diamond(111)//TaC(111) interface, while only C–C covalent bonds are formed at the Graphite(002)/TaC(111). This makes diamond thermodynamically more stable than graphite on the TaC surfaces. Our investigations provide critical information to understand the complex diamond formation mechanism, especially with the presence of TaC.

Tailoring intrinsic properties for tetrahedral carbon/copper composites construction via electronic interface engineering

The electronic structure induced interface properties are important for active control of the advanced equipment performance. Nevertheless, it is difficult to reveal the intrinsic mechanism by experiments. In this paper, the interfacial properties and electronic characteristics of eight different stacking ta-C/Cu interfaces were studied via first principles method. The work of adhesion of ta-C/Cu(110)-2nd interface the largest (6.19 J/m2) and its interfacial energy is the smallest (3.74 J/m2), which indicates that it is the most stable structure with the smallest interface formation resistance. In addition, the polar covalent bonds dominated by electronic structures are major contributor of the ta-C/Cu interface properties. During tensile process, the critical strain of ta-C/Cu(110)-2nd is the largest (17.0 %), and the tensile strength of ta-C/Cu(100)-2nd is the largest (61.59 GPa). When all interfaces are fractured under tensile force, the atoms on Cu surfaces are transferred to ta-C surfaces and separated into two free parts. When the energy of one of the wave valleys first reaches the threshold of the lowest plane-averaged potential, the interfacial structure breaks at the interfacial gap corresponding to the wave valley. Changes in interfacial bonding properties can therefore be judged by changes in the magnitude of the electrostatic potential.

A density functional theory study of CO2 hydrogenation on carbon-terminated TaC (111) surface

A density functional theory study of CO2 hydrogenation on carbon-terminated TaC (111) surface

Thermodynamic and Kinetic Study of Carbon Dioxide Hydrogenation on the Metal-Terminated Tantalum-Carbide (111) Surface: A DFT Calculation

The need to reduce our reliance on fossil fuels and lessen the environmentally harmful effects of CO2 have encouraged investigations into CO2 hydrogenation to produce useful products. Transition metal carbides exhibit a high propensity towards CO2 activation, which makes them promising candidates as suitable catalysts for CO2 hydrogenation. Here, we have employed calculations based on the density-functional theory to investigate the reaction network for CO2 hydrogenation to product molecules on the tantalum-terminated TaC (111) surface, including two routes from either HCOOH* or HOCOH* intermediates. Detailed calculations of the reaction energies and energy barriers along multiple potential catalytic pathways, along with the exploration of all intermediates, have shown that CH4 is the predominant product yielded through a mechanism involving HCOOH, with a total exothermic reaction energy of −4.24 eV, and energy barriers between intermediates ranging from 0.126 eV to 2.224 eV. Other favorable products are CO and CH3OH, which are also produced via the HCOOH pathway, with total overall reaction energies of −2.55 and −2.10 eV, respectively. Our calculated thermodynamic and kinetic mechanisms that have identified these three predominant products of the CO2 hydrogenation catalyzed by the TaC (111) surface explain our experimental findings, in which methane, carbon monoxide, and methanol have been observed as the major reaction products.

First principle study of adsorption and desorption behaviors of NH<sub>3</sub> molecule on the TaC (0001) surface

The adsorption and desorption behaviors of ammonia on TaC(0001) surface are studied by employing spin-polarized density function theory calculations. The surface energy calculation results show that the TaC (0001) terminating with Ta is the most stable surface. According to the optimized structural and energetic properties, it is found that NH<sub>3</sub> prefers to adsorb on the top site, whereas NH<sub>2</sub>, H prefer to adsorb on the triple hcp site and NH, N prefer to stay on the triple fcc site. In addition, three transition states are found for analyzing the mechanism of dehydrogenation of NH<sub>3</sub>, and the N recombination reaction is also considered. The results show that the desorption of nitrogen atoms is the rate-determining step in the overall reaction. Finally, in order to further elucidate the mechanism of NH<sub>3</sub> adsorption and dissociation on the surface of Ta-TaC, the electronic structure of the most stable adsorption position is analyzed from the perspective of charge density distribution and electron density of states. The results of electronic structure calculation show that NH<sub>3</sub> molecule is adsorbed on the surface through the mixture of 2p<i><sub>z</sub></i> orbital of N atom and <inline-formula><tex-math id="Z-20211229103116-1">\begin{document}$ 5{\rm d}_{z^2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20210400_Z-20211229103116-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20210400_Z-20211229103116-1.png"/></alternatives></inline-formula> orbital of substrate Ta. With the progress of dehydrogenation, the charge transfer phenomenon becomes more and more serious. The charge transfer between adsorbate and substrate plays an important role in accelerating NH<sub>3</sub> dehydrogenation catalytic process.

Ta2C precipitation after low pressure carburizing of tantalum

Low pressure carburizing cycles were carried out on tantalum samples. A Ta2C precipitates layer is present under the TaC and Ta2C surface layers. As the TaC and Ta2C layers, the Ta2C precipitates layer (TPL) grows by carbon diffusion in tantalum matrix. Specific zones with low precipitates content were observed at the bulk surface and in the tantalum grain boundaries. Other zones with lower precipitates content are also present in carburized and annealed samples. All the zones with low precipitates content are induced by the Ta2C precipitation mechanism described by Dahmen et al. [1,2]. Indeed, Ta2C needs vacancies to precipitate. If vacancy sinks are present at the bulk surface, at a grain boundary or in a dislocation, then Ta2C precipitation is highly reduced. Another effect of the annealing treatment is an increase of the Ta2C precipitates layers growth rate.

XPS studies of surface oxidation of metal carbides

Metal carbides MC (HfC, NbC, ZrC, TaC and TiC) that were exposed to air under the ambient temperature condition for a period of time were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and Raman. Fresh MC samples exposed to the air for different time (fresh, one day, one week and one quarter) illustrated unexpected oxidation degree of the surfaces. XRD tests provided very limited information, since the oxidation process occurred from the surface at the nanometer scale. However, the XPS tests provided more comprehensive information for the MOx (HfO2, ZrO2, Nb2O5, Ta2O5 and TiO2) film and MC (HfC, NbC, ZrC, TaC and TiC) interlayer. Meanwhile, the HRTEM and Raman tests could give some additional or supporting information for XPS tests, when the samples were exposed to the air for longer time (one week or one quarter). Meanwhile, this research is the first to show that the oxidation progress of the metal surfaces of HfC, NbC, ZrC, TaC and TiC can take place in air under the ambient temperature condition and the oxidation film is about 2–5 nm. Meanwhile, a new oxidation mechanism of metal carbides MC (HfC, NbC, ZrC, TaC and TiC) exposed to air under the ambient temperature condition is also proposed, considering the obtained Raman, TEM and XPS results that could clearly confirm the formation of HfO2 and free carbon.

Simultaneously improved bio-tribological and biological corrosion properties of taC film coated on Ti6Al4V by sealing with ALD

The failure of the implant biomedical materials in the physiological environment mainly depends on the synergistic effect of the corrode and wear. The implant-bone interface is the critical position where the corrosion and wear are often occurred. Most of the work have focused on developing films with anti-corrosion and anti-wear while ignoring the inherent shortcomings in deposition methods. The present paper is therefore focused on investigating the anti-corrosion and bio-tribological performance of the taC composite, in which taC film was firstly fabricated on the Ti6Al4V substrate via hybrid arc ion plating (AIP) system, then a TiO2 layer was deposited on taC surface through atomic layer deposition (ALD). Structural, biological corrosion resistance and bio-tribological properties of the composites were systematically investigated. Results show that the wear resistance of Ti6Al4V is enhanced by the designing taC composite: the friction coefficients of Cr/taC-TiO2 and Ti/taC-TiO2 are decreasing by 6.5 times in relation to the Ti6Al4V substrate; The wear rates of Cr/taC-TiO2 and Ti/taC-TiO2 are 1.7 × 10−6 mm3/Nm and 6.5 × 10−7 mm3/Nm respectively, which are lower than that of Ti6Al4V. Excellent anti-corrosion property was achieved by TiO2 sealed specimen, as evidenced by an order of magnitude higher corrosion potential and notable reduced corrosion current density compared to bare substrate. This is due to the high sealing efficiency of TiO2 layer to reduce the porosity of the taC film when compared with the Cr/taC and Ti/taC film. These composite films on Ti6Al4V substrates, which simultaneously prove an excellent wear resistance and good corrosion resistance, thus could have prospective application in biomedical implants.

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.

Effect of defects on wear behavior in ta-C coating prepared by filtered cathodic vacuum arc deposition

This study examined the tribological behavior of a ta-C coating fabricated at a substrate bias of −100 V via filtered cathodic vacuum arc (FCVA) deposition. Defects in the ta-C coating were categorized as droplets, pores, and spikes, considering the morphological, structural, and mechanical properties of the ta-C surface. A friction test was conducted at 170 °C in ambient air using a ball-on-disk tribometer. The friction coefficient was 0.08 during the steady state and the wear rate was 4.3 × 10−6 mm3/Nm. To explain the friction and wear behavior, the morphological and structural properties of the defects in the designated area were compared among different sliding cycles.

The AJ519 antibody labels the human TAC/IL2RA protein by immunofluorescence

The recombinant antibody AJ519 detects by immunofluorescence the human TAC surface protein in paraformaldehyde-fixed cells.

Understanding X-ray Spectroscopy of Carbonaceous Materials by Combining Experiments, Density Functional Theory, and Machine Learning. Part I: Fingerprint Spectra

Carbonaceous materials, especially tetrahedral amorphous carbon (ta-C), can form complex functionalized surface structures and are thus promising candidates for applications in biomedical devices and electrochemistry. Functional groups at ta-C surfaces have been widely studied by spectroscopic techniques; however, interpretation of the experimental data is extremely difficult, especially in the case of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The assignments of experimental XPS and XAS signals are normally based on references obtained from molecular or crystalline samples, which are simplified approximations for the far more complex amorphous structures. Here, we use extensive density functional theory (DFT) simulations to predict XAS and XPS signatures for carbon-based materials in more realistic environments, building on large data sets of structural models generated by a machine-learning (ML) interatomic potential. The results indicate clear signatures: individual...

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.

ОПТИМИЗАЦИЯ ПАРАМЕТРОВ БАЗИСНОГО НАБОРА ДЛЯ DFT МОДЕЛИРОВАНИЯ НАНОМАТЕРИАЛОВ

Atomic-like orbital basis sets allow efficient calculation of nanomaterial’s surface properties within the density functional theory. However, unlike plane wave basis sets, they require thorough optimization on a reference system before modelling systems of interest. We considered the basis set optimization procedure for several structures: bulk tantalum carbide, oxygen molecule, bulk lithium, and α -carbyne. We showed that during the optimization procedure not only the total energy of a reference system should be monitored but other physical characteristics (bond length and atomic charges) too. Moreover, optimal basis parameters could not correspond to the minimum total energy of a reference system to get the correct physical properties. We obtained optimal orbital parameters, which can be used for modelling of the following systems: oxygen adsorption on tantalum carbide surface, and Li-functionalized carbyne. Considering oxygen adsorption on TaC surface and Li-functionalization of carbyne, we also demonstrated that the basis set optimization influences binding energies and atomic charges of an adsorbent and a surface.

Computational Surface Chemistry of Tetrahedral Amorphous Carbon by Combining Machine Learning and Density Functional Theory

Tetrahedral amorphous carbon (ta-C) is widely used for coatings because of its superior mechanical properties and has been suggested as an electrode material for detecting biomolecules. Despite extensive research, however, the complex atomic-scale structures and chemical reactivity of ta-C surfaces are incompletely understood. Here, we combine machine learning, density functional tight binding, and density functional theory simulations to shed new light on this long-standing problem. We make atomistic models of ta-C surfaces, characterize them by local structural fingerprints, and provide a library of structures at different system sizes. We then move beyond the pure element and exemplify how chemical reactivity (hydrogenation and oxidation) can be modeled at the surfaces. Our work opens up new perspectives for modeling the surfaces and interfaces of amorphous solids, which will advance studies of ta-C and other functional materials.

MoDTC Tribochemistry in Steel/Steel and Steel/Diamond-Like-Carbon Systems Lubricated With Model Lubricants and Fully Formulated Engine Oils

This work focuses on the tribochemistry of molybdenum dithiocarbamate (MoDTC) oil additive to improve friction behavior of diamond-like-carbon (DLC) coated systems lubricated in boundary regime. Raman microscopy has been used to investigate surface tribolayers formed on coated (hydrogenated a-C:H and non-hydrogenated ta-C) and steel surfaces when lubricated with model lubricants and commercial engine oils. The effect of the additive package and the type of DLC played a crucial role in the development and composition of the tribolayer and the friction performance. The additive package contained in the fully formulated (FF) oils limited the friction reduction capabilities of MoDTC additive for every material pair. Accelerated a-C:H coating wear related to MoDTC tribochemistry was found. For the first time, it has been shown that a distinctive MoS2-containing tribolayer can be formed on the ta-C surface, leading to a coefficient of friction lower than 0.04. The underlying mechanisms of MoDTC/surface interactions and their effect on friction and wear are discussed.

The adsorption and dissociation of O2 on Pd and Pt modified TaC (1 0 0) surface: A first principles study

The adsorption and dissociation of O2 on the palladium and platinum modified TaC (1 0 0) surfaces were investigated based on the density functional theory calculations. It is found that the adsorption sites of O2 are the Ta-Ta bridge sites on both the partially covered TaC (1 0 0) surfaces by Pd and Pt, M4/TaC (1 0 0) (M = Pd and Pt), while the 4-fold metal hollow sites and the metal-metal bridge sites are preferred on the fully covered TaC (1 0 0) surfaces by Pd and Pt monolayer, MML/TaC (1 0 0), respectively. The deposition of Pd or Pt can enhance the oxidation resistance of TaC (1 0 0). Meanwhile, the TaC (1 0 0) decorated by monolayer Pd still exhibited outstanding catalytic activity for O2 dissociation. Our study might be useful to designing efficient catalysts for the oxygen reduction reaction.

Effect of test atmosphere on the tribological behaviour of the tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C-F) coatings against steel

Tetrahedral amorphous carbon based coatings generally exhibit a low coefficient of friction (COF) while sliding against steel and aluminum, but their tribological properties are sensitive to the test environment. In this study, tribological behaviour of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C-F) coatings were investigated under different atmospheres including ambient air (53% relative humidity, RH) and dry N2 (3% RH) against a steel counterface. The ta-C showed a period of low COF of 0.27 in ambient air and a C transfer layer was formed on the counterface, but the COF increased with sliding duration to ~0.4 due to transfer of Fe2O3 debris to ta-C surface. A high COF of ~0.60 with high fluctuations occurred in dry N2 as no carbon transfer layers were formed on the counterface. The ta-C-F coating exhibited a steady-state behaviour with low COF (μs) values under both ambient (μs=0.25) and dry N2 (μs=0.23) atmospheres. Formation of an F containing carbonaceous transfer layer on the counterface provided low COF for ta-C-F.

Tribological Performance of Halogen-Free Ionic Liquids in Steel–Steel and DLC–DLC Contacts

ABSTRACTThe tribological performance of halogen-free ionic liquids at steel–steel and diamond-like carbon (DLC)–DLC contacts was investigated. Hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (ta-C) were used as test specimens. Friction tests were carried out on steel–steel, a-C:H–a-C:H, and ta-C–ta-C contacts by using a reciprocating cylinder-on-disk tribotester lubricated with two different types of halogen-free ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([BMIM][DCN]) and 1-butyl-3-methylimidazolium tricyanomethanide ([BMIM][TCC]). From the results of friction tests, the ta-C–ta-C tribopair lubricated with [BMIM][DCN] or [BMIM][TCC] exhibited an ultralow friction coefficient of 0.018–0.03. On the other hand, ultralow friction was not observed at the steel–steel and a-C:H–a-C:H contacts. Measurements obtained with a laser scanning microscope and an atomic force microscope (AFM) showed that a chemical reaction film, derived from the ionic liquid lubricant used, was formed on the steel surfaces. However, this chemical reaction film was not observed on either of the DLC surfaces. The AFM results showed that there were high-viscosity products on the ta-C surfaces, that the wear tracks on the ta-C surfaces exhibited low frictional properties, and that the ta-C surfaces were extremely smooth after the friction tests. Based on these results, it was concluded that an ionic liquid–derived adsorbed film formed on the ta-C surface and resulted in the ultralow friction when lubricated with a halogen-free ionic liquid.