TiC Surface Research Articles

Geometries and Electronic Structures for the Adsorption of CO Molecule on TiC(001) Surface

Using the first-principles method, the configurations and electronic structures for the adsorption of CO molecule on TiC(001) surface were investigated in details with a slab model. Our results showed that the CO molecule tended to occupy the atop site of the surface Ti atom through its C ending, and the predicted adsorption energy, the positions of CO valence levels and the red shift of C-O stretching frequency were in agreement with the experimental observations. By analyzing the band structures and Mulliken population, when CO molecule was adsorbed by C ending, the compositions and energy levels of the corresponding 5μ and 2* states changed obviously with respect to CO in gas-phase , especially for the case that CO sited at the bridge site between two neighbor Ti atoms. Furthermore, the role of the surface state during the CO adsorption was discussed.

Adsorption of gold on TiC(001): Au–C interactions and charge polarization

High-resolution photoemission and first-principles density-functional slab calculations were used to study the adsorption of gold on a TiC(001) surface. A positive shift in the binding energy of the C 1s core level is observed after the deposition of Au on the metal carbide surface. The results of the density-functional calculations corroborate the formation of Au-C bonds. In general, the bond between Au and the TiC(001) surface exhibits very little ionic character, but there is a substantial polarization of electrons around Au that affects its chemical properties.

Processing of carbide-derived carbon (CDC) using biomorphic porous titanium carbide ceramics

Biomorphic porous TiC and TiC/TiO 2 ceramics were covered with highly porous carbon, so-called carbide-derived carbon (CDC), by selective etching of Ti from TiC with chlorine containing gas in a temperature range 400–1000 °C. The etching rate of TiC is strongly affected by the chlorine concentration, but only slightly by the reaction temperature. No correlation was found between etching rate of TiC and specific surface area (SSA) of the resulting CDC with respect to temperature. Kinetic investigations show that for up to 80 min the etching process is controlled by the chemical reaction. At longer times a diffusion limitation has to be taken into account. Addition of hydrogen to the etching gas enhances the diffusion of the chlorine molecules, so that no diffusion limitation was observed. Microporous carbon with narrow pore size distribution was obtained at temperatures ranging from 400 to 800 °C. Treatment at higher temperatures leads to formation of mesopores. The CDC produced by chlorination of TiC at 400 °C is amorphous. Increased reaction temperature and addition of hydrogen to the chlorine gas lead to formation of regions with higher order like onion carbon and graphitic ribbons. The CDC coated TiC/TiO 2 ceramics with predominantly anatase phase show enhanced photo catalytic activity.

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Titanium doped hydrogenated amorphous carbon (Ti-C:H) was synthesized on silicon, YG8 cemented carbide by middle frequency magnetron sputtering technology. Doped Ti concentration was adjusted via allotting Ar/C2H2 flow rate to be 0–1.21 at.%. To investigate the influence of Ti concentration on mechanical properties and wear resistance to Ti6Al4V, stylus profiler, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), nanoindentation, energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and ball-on-disk friction test were applied. The results indicated that the incorporated Ti element existed as amorphous-like structure within the carbon network resulting in the decrease of sp3 fraction and increase of graphitization. The compressive stress of the film declined gradually along with the sacrifice of hardness and elastic modulus as the increase of Ti concentration. Compared with the none-doped and 1.21 at.% Ti-C:H, the friction coefficient of 0.32 at.% and 0.74 at.% Ti-C:H remained lower and more stable value of 0.07, which led to a lower wear rate of about 3.1 E-8 mm3/Nm as well. The worn surface of various Ti-C:H films after rubbing against Ti6Al4V exhibited three kinds of morphologies as adhesive wear, smooth and abrasive wear. The different morphologies and wear mechanism were discussed.

Chemistry of Methyl Formate with TiC(100): Comparison of Experiment with Density Functional Calculations

Titanium carbide is an important tribological material used as an antiwear coating on mechanical components. The TiC(100) surface has demonstrated interesting low-temperature chemistry with a variety of adsorbates, including methyl formate, the simplest ester and a model for this chemical functionality found in many aerospace lubricants. In an effort to identify and characterize surface adsorption and reaction products, we have undertaken a comparison of theoretical results employing density functional methods with experimental results using high-resolution electron energy loss spectroscopy (HREELS) obtained on clean, single-crystal TiC surfaces. The DFT results presented in this work have enabled the identification of an elusive surface reaction product with a characteristic vibration of 1120 cm-1 as a formyl (CHO) species bridging between a surface carbon and a titanium site. This result and others have demonstrated the importance of the lattice carbidic carbon atom in the adsorption of the breakdown products of methyl formate. Specifically, any atomic adsorbate and every reaction product with an unsatisfied valence on a carbon atom showed a theoretical preference for bonding with a lattice carbon rather than the titanium. Alternatively, reaction fragments containing oxygen preferred bonding to surface titanium sites through the oxygen atom, with the exception of carbon monoxide. The calculations show that the breakdown of methyl formate to methoxy and formyl groups, or to surface formate and methyl groups, is exothermic, while further decomposition reactions are endothermic. These results provide a rationale for the experimental observation of these surface reaction products at temperatures ranging from 150 to 300 K.

Study on hemocompatibility and corrosion behavior of ion implanted TiNi shape memory alloy and Co‐based alloys

Biomedical TiNi shape memory alloy and Co-based alloys were ion implanted, and corrosion resistance and hemocompatibility of these had been investigated with electrochemical method, dynamic clotting time, and hemolysis rate tests. The results indicated that the electrochemical stability and anodic polarization behavior of the materials were improved significantly after ion implantation. When TiNi, Co-based alloys were implanted Mo + C and Ti + C, respectively, the corrosion potentials were enhanced more than 200 mV, passive current densities decreased, and passive ranges were broadened. Dynamic clotting time of the ion implanted substances was prolonged and hemolysis rate decreased. All the results pointed out that corrosion resistance and hemocompatibility of the alloys were improved by ion implantation, and effects of dual implantation was better than that of C single implantation. X-ray diffraction analysis of the alloys after dual implantation revealed that TiC, Mo(2)C, Mo(9)Ti(4), and Mo appeared on the surface of TiNi alloy, and CoC(x), Co(3)Ti, TiC, and TiO on the surface of Co-based alloys. These phases dispersing on the alloy surface formed amorphous film, prevented dissolving of alloy elements and improved the corrosion resistance and hemocompatibility of the alloys.

TEM and DFT investigation of CVD TiN/κ–Al 2O 3 multilayer coatings

This paper investigates the interfacial structure in hot-wall CVD TiN/κ–Al 2O 3 multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the κ–Al 2O 3 layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/κ–Al 2O 3 coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/κ–Al 2O 3 coatings, are not present. However, the orientation relationships at the TiN/κ–Al 2O 3 interfaces are the same in both multilayers. The HREM investigations show that κ–Al 2O 3 (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, γ–Al 2O 3 can be grown, at least partly stabilized by the cube-on-cube orientation relationship between γ–Al 2O 3 and the underlying TiN. The DFT calculations show very similar adsorption strengths for an O monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fcc site. O adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for κ–Al 2O 3 on TiN(111) than on TiC(111) and a higher stability for κ–Al 2O 3 than for α–Al 2O 3 on both TiC and TiN. This indicates that the observed higher stability of κ–Al 2O 3 on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of κ–Al 2O 3 over α–Al 2O 3 can be partly attributed to the mismatch.

Nature of adsorption on TiC(111) investigated with density-functional calculations

Extensive density-functional calculations are performed for chemisorption of atoms in the three first periods (H, B, C, N, O, F, Al, Si, P, S, and Cl) on the polar TiC(111) surface. Calculations are also performed for O on TiC(001), for full O(1x1) monolayer on TiC(111), as well as for bulk TiC and for the clean TiC(111) and (001) surfaces. Detailed results concerning atomic structures, energetics, and electronic structures are presented. For the bulk and the clean surfaces, previous results are confirmed. In addition, detailed results are given on the presence of C-C bonds in the bulk and at the surface, as well as on the presence of a Ti-based surface resonance (TiSR) at the Fermi level and of C-based surface resonances (CSR's) in the lower part of the surface upper valence band. For the adsorption, adsorption energies E-ads and relaxed geometries are presented, showing great variations characterized by pyramid-shaped E-ads trends within each period. An extraordinarily strong chemisorption is found for the O atom, 8.8 eV/adatom. On the basis of the calculated electronic structures, a concerted-coupling model for the chemisorption is proposed, in which two different types of adatom-substrate interactions work together to provide the obtained strong chemisorption: (i) adatom-TiSR and (ii) adatom-CSR's. This model is used to successfully describe the essential features of the calculated E-ads trends. The fundamental nature of this model, based on the Newns-Anderson model, should make it apt for general application to transition-metal carbides and nitrides and for predictive purposes in technological applications, such as cutting-tool multilayer coatings and MAX phases.

Study on Grain Refining and Metamorphism of ZL111 Aluminium Alloys

Abstract Vacuum sintering and ball milling methods were employed in the preparation process of Ti-C grain refinement and the ability of refiners with varying ratios of Ti and C to refine ZL111 crystal grains was tested. The refinement effect of the Ti-C ratios on tensile strength, elongation percentage, Brinell hardness, pro-eutectoid αAl and the size of the Si phase of ZL111, after modification by rare-earth and strontium nitrate, were studied by means of metallographic examination, SEM and mechanical property tests. The results show that there is an obvious increase in the tensile strength and elongation percentage of refined ZL111 with these new Ti and C refiner compounding powders, while Brinell hardness remained more or less constant. The pro-eutectoid αAl is considerably reduced in size and the Si phase shows a finer and rounder structure. The refiner exhibits a good grain refining performance when the Ti-C ratio is 25:1, for Al crystals can favorably easily form nuclei and grow up along the TiC surface thanks to the TiAl 3 generated by surplus Ti and Al. The mechanical properties have clearly been improved by the addition of strontium nitrate to ZL111. The effective factors in the modification of mechanical properties of ZL111 are in order of importance: strontium nitrate, Ti-C ratio and rare earth.

Microstructure of Fe–TiC surface composite produced by cast-sintering

Fe–TiC surface composite was prepared in situ on a surface of cast steel by means of cast-sintering technique. The microstructure of this material was investigated by means of SEM, electron probe and XRD. Results show that the TiC and (Fe,Cr) 7C 3 carbides in an iron matrix were achieved on the surface of cast steel during cast-sintering. From the top surface of sample to that of the master-steel, the concentration of Ti, Cr and Ni as well as the quantity of both TiC and (Fe,Cr) 7C 3 carbides decreased gradually, and the morphology of (Fe,Cr) 7C 3 transforms from strip-chunky into make-and-break reticulation. There was an excellent metallurgy-bond between the surface composite layer and the master-steel.

Role of Kinetics in the Selective Surface Oxidations of Transition Metal Carbides

The different oxidation behavior of TiC and VC(100) surfaces by molecular oxygen has been investigated by density functional theory with a slab model. From the thermodynamic stability of the final states that involve dissociated O(2), one cannot well explain the experimental observations. Two different oxidation pathways of TiC and VC(100) surfaces have been explored in this work, and the results indicate that two channels share the same precursor state. However, from the precursor, only the pathway leading to the formation of a C-O bond is energetically feasible for the TiC(100) surface, while on VC(100) the O atoms tend to occupy the metal surface sites due to a smaller energy barrier for this channel. Further band structure calculations reveal that the additional d electron of V atom favors the stability of the molecularly adsorbed species. The oxidation mechanism unveiled from the present calculations clearly evidences that the kinetic effects introduced by one additional d electron of the V atom play a crucial role in explaining the different surface chemistry between TiC and VC (100) surfaces.

Gas-phase Interaction of Thiophene with the Ti8C12+ and Ti8C12 Met-Car Clusters

The reactivity of the Ti(8)C(12)(+) met-car cation toward thiophene was investigated using density functional theory (DFT) and mass selective ion chemistry. It is shown that the experimentally observed mass spectrum can be well described by the DFT calculations. In contrast to the weak bonding interactions seen for thiophene on a TiC(001) surface, the Ti(8)C(12)(+) met-car cation is able to interact strongly with up to four thiophene molecules with the cluster staying intact. In the most stable conformation, the thiophene molecules bond to the four low-coordinated Ti(0) sites of Ti(8)C(12)(+) via a eta(5)-C,S coordination. The stability and the activity of the Ti(8)C(12)(+) met-car is observed to increase with an increasing number of attached thiophene molecules at the Ti(0) sites, which is associated with a significant transfer of electron density from thiophene to the cluster. The additional electron density on the Ti(8)C(12)(+) cation cluster, however, is not sufficient to cleave the C-S bonds of thiophene and the dissociation reaction of thiophene is predicted to be a highly activated process. By contrast, DFT calculations for the neutral Ti(8)C(12) met-car predict that the dissociation reaction leading to adsorbed S and C(4)H(4) fragments is energetically favorable for the first thiophene molecule. The binding behavior for subsequent addition of thiophene molecules to the neutral met-car is also presented and compared to that of the cation.

Nature of chemisorption on titanium carbide and nitride

Extensive density-functional calculations are performed to understand atomic chemisorption on the TiC(1 1 1) and TiN(1 1 1) surfaces, in particular the calculated pyramid-shaped trends in the adsorption energies for second- and third-period adatoms. Our previously proposed concerted-coupling model for chemisorption on TiC(1 1 1) is tested against new results for adsorption on TiN(1 1 1) and found to apply on this surface as well, thus reflecting both similarities and differences in electronic structure between the two compounds.

Flux-assisted wetting and spreading of Al on TiC

The effect of a K–Al–F-based flux on the spreading of Al on TiC, at temperatures up to 900 °C, in Ar and in air has been studied. Whilst obtuse contact angles were observed without flux, the flux facilitated rapid spreading to a perfect wetting condition, in both Ar and in air. The atmosphere was found to have a weak effect on the spreading kinetics as the liquid flux provides a locally protective atmosphere by spreading over the TiC surface and also on the solid surface of Al. The flux dissolves the aluminium oxide, covering Al, so that when Al melts, and the oxide layer has been removed or weakened, intimate contact occurs between liquid Al and the TiC substrate facilitating spontaneous spreading and instantaneous wetting of liquid Al on TiC. Since flux-assisted spreading is very rapid and occurs without the formation of a reaction layer at the Al/TiC interface, this process is very different to the reactive wetting behaviour previously reported in the Al–TiC system.

In situ production of Fe–TiC surface composite coatings by tungsten-inert gas heat source

In the present study, AISI 1045 steel surfaces were alloyed with pre-placed graphite, ferrotitanium and Fe–Cr–B–Si powders by using a tungsten-inert gas (TIG) heat source. The effects of welding parameters and thickness of the pre-placed powder layers on the microstructure and properties of the coatings were also investigated. The results indicated that TiC particles can be obtained by direct metallurgical reaction between ferrotitanium and graphite during the TIG welding process. Most of TiC particles were uniformly distributed in the surface coating. The microhardness showed a gradient variation from the molten boundary to the top surface of the coatings, and it was influenced by the thickness of the pre-placed powder layer and the welding parameters. The surface composite coating exhibited a higher hardness and lower wear rate than that of the substrate due to the formation of TiC carbides.

Comparative studies of methanol decomposition on carbide-modified V(1 1 0) and Ti(0 0 0 1)

The reaction pathways of methanol on V(1 1 0), carbide-modified V(1 1 0), Ti(0 0 0 1), and carbide-modified Ti(0 0 0 1) have been studied using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and temperature-programmed desorption (TPD). On V(1 1 0) and C/V(1 1 0), methanol undergoes complete dissociative adsorption producing a methoxy intermediate for exposures less than 2 L at 100 K. On Ti(0 0 0 1) and C/Ti(0 0 0 1), methanol dissociates to produce methoxy at exposures of 3 L at 100 K. The combination of TPD and AES reveals that the number of methoxy per surface metal atom is 0.13 on C/V(1 1 0) and 0.41 on C/Ti(0 0 0 1) at 100 K. All methoxy species undergo further decomposition on C/V(1 1 0) at higher temperatures. However, a significant fraction (∼39%) of methoxy species undergo recombinatory reaction to produce gas-phase methanol on C/Ti(0 0 0 1). The reaction pathways on the C/V(1 1 0) and C/Ti(0 0 0 1) surfaces are compared with previous studies of methanol on other carbide-modified surfaces, as well as on single crystal VC and TiC surfaces.

Trends in the chemical properties of early transition metal carbide surfaces: A density functional study

In this paper we present density functional theory (DFT) investigations of the physical, chemical and electronic structure properties of several close-packed surfaces of early transition metal carbides, including β-Mo2C(0001), and the (111) surfaces of TiC, VC, NbC, and TaC. The results are in excellent agreement with experimental values of lattice constants and bulk moduli. The adsorption of atomic hydrogen is used as a probe to compare the chemical properties of various carbide surfaces. Hydrogen adsorbs more strongly to the metal-terminated carbide surfaces than to the corresponding closest-packed pure metal surfaces, due to the tensile strain induced in the carbide surfaces upon incorporation of carbon into the lattice. Hydrogen atoms were found to adsorb more weakly on carbide surfaces than on the corresponding closest-packed pure metal surfaces only when there were surface carbon atoms present, and in some cases very stable C–H species were formed. The DFT results indicated that the hydrogen adsorption energy could be correlated to the d-band center of the carbide surfaces, although the correlation was not as good as our previous studies on bimetallic surfaces.

Surface segregation of transition metal impurities on the TiC(1 0 0) surface

The segregation energies of 3d (Sc–Cu), 4d (Y–Ag) and 5d (La–Au) transition metal impurities on the (1 0 0) surface of TiC have been obtained using first-principles electronic structure calculations. The results are in agreement with available experimental data and show that the difference in atomic size between the impurity and host species, as well as the difference in surface energies determines if the impurity will segregate towards the surface or not. The results indicate that the difference in size is the dominant factor for the trends in segregation of transition metal impurities towards the (1 0 0) surface of TiC.

Electronic structure of the Ti suboxide layer formed on a TiC(1 0 0) surface: Angle-resolved photoemission study

Angle-resolved photoemission spectroscopy (ARPES) utilizing synchrotron radiation has been used to study the electronic structure of a TiO-like layer formed on a TiC(100) surface. When the TiC(100) surface exposed to 500L of O2 is heated at 1000°C, a TiO-like layer with the thickness of 1.3–2.0Å is formed on the surface. ARPES measurements show that the Ti 3d band and O 2p band are formed in the TiO-like layer at around the Fermi level and at 6–7eV, respectively. Both bands show clear dispersions along both 〈100〉 (Γ¯M¯) and 〈011〉 (Γ¯X¯) symmetry directions of TiC(100), and the dispersive features are compatible with a (1×1) periodicity of TiC(100). These results are compatible with the model that the oxide layer is a well ordered TiO(100) film which is epitaxially grown on the TiC(100) surface.

Reactions of water and ethanol with polycrystalline TiC surfaces

The adsorption and reaction of water and ethanol with polycrystalline TiC coatings have been investigated and compared with those of the nonpolar (100) face of single crystal TiC. This work is pursued to develop a fundamental understanding of the surface bonding and reaction properties, thus enabling the use of TiC as a tribological coating material. Temperature-programmed desorption has been used to characterize the desorption behavior of these model adsorbates as well as that of products resulting from their reaction with the TiC surface. Following adsorption at 100K, molecular desorption as well as desorption of reaction products is evident for both water and ethanol. Approximately 70% of the water in the monolayer is judged to irreversibly react with the TiC surface, producing a surface oxide and gaseous hydrogen and carbon monoxide. Approximately 92% of the ethanol in an adsorbed monolayer reacts, predominantly producing gaseous ethene. These results are similar to those on TiC(100) surfaces, although a greater extent of surface reactivity is observed on the polycrystalline surface.