Characterization of nano-structured W-, Ti-, V-, and Zr-doped carbon films

Bonding structure of carbon and metal as well as nanostructural changes of metal-doped amorphous carbon films (a-C:Me) were investigated depending on metal type (W, Ti, V, and Zr), concentration (<25 at.%) and annealing temperature (< 1300 K, except W: < 2800 K). Pure C films exhibit ~ 2 nm distorted aromatic and graphene-like regions. Both increase in size with annealing. After deposition the metals have carbide-like bonding and are mainly distributed atomically disperse in an amorphous environment. Annealing leads to the formation of carbide crystallites (TiC, VC, ZrC, WC, W₂C, and WC_1 − x) of several nanometers. The VC particles reach the largest size up to 1300 K. All metal dopings reduce the erosion rate against oxidation (expect V) and hydrogen impact.     

Kinetics of tungsten carbidization under non-isothermal conditions

Kinetic laws of high-temperature interaction between tungsten and methane are studied under non-isothermal conditions in the temperature interval 1273-2873 K. Computer Assisted Electrothermography was applied during which thin metallic wires 100 μm in diameter were directly heated up by electric current in the carbon-containing atmosphere. Experimental data on weight gain, carbide layer growth, and microstructure of phases are obtained in the linear heating regime at heating rates from 10 to 1500 K/s. We established that the interaction between tungsten and methane in a wide interval of pressure and heating rates may proceed with simultaneous or consecutive formation of W₂C and WC carbide phases. Experimental data on weight gain and carbide layer growth were processed using calculation schemes deduced within the framework of a reaction diffusion model with the first type boundary conditions under non-isothermal conditions. Kinetic and diffusion constants are determined for tungsten carbidization with formation of the W₂C phase.     

Influence of the annealing temperature on a crystal phase of W/WC bilayers grown by pulsed arc discharge

The X-ray diffraction technique was used to study the influence of the temperature on a crystal phase of W/WC bilayer produced by the plasma-assisted pulsed arc discharge. In order to grow the films, a target of W with 99.9999% purity and stainless-steel 304 substrate were used. For the production of W layer, the reaction chamber was filled up with argon gas until reaching a 300 Pa and the discharge was performed at 270 V with 3 pulses. The WC layer was grown in a methane atmosphere at 300 Pa and 275 V discharge voltage with 4 pulses. The active and passive times of the pulsed discharge were 1 and 0.5 s, respectively. The influence of post-annealing temperature of their crystal phases was studied at the post-annealing temperatures up to 600 °C. As-grown layer comprised of mixed phases WC, W₂C and W. The post-annealed layer also comprised of the mixed phases of WC, W₂C and W at annealing temperatures below 600 °C. At the annealing temperature above 600 °C, XRD diffractograms showed only substrate and W peaks, and tungsten carbide peaks were not observed, but the presence of WO phases were detected for an annealing temperature of 600 °C. XPS analyses showed the presence of WC before the annealing process and the existence of C-C bond that is considered responsible for the high polycrystallinity of the material was also detected. The XPS showed the formation of WO₂ and WO₃ without the presence of WC for post-annealing at 600 °C.     

Influence of carbon chemical bonding on the tribological behavior of sputtered nanocomposite TiBC/a-C coatings

The tribological performance of nanocomposite coatings containing Ti-B-C phases and amorphous carbon (a-C) are studied. The coatings are deposited by a sputtering process from a sintered TiB₂:TiC target and graphite, using pulsed direct current and radio frequency sources. By varying the sputtering power ratio, the amorphous carbon content of the coatings can be tuned, as observed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The crystalline component consists of very disordered crystals with a mixture of TiB₂/TiC or TiB_xC_y phases. A slight increase in crystalline order is detected with the incorporation of carbon in the coatings that is attributed to the formation of a ternary TiB_xC_y phase. An estimation of the carbon present in the form of carbide (TiB_xC_y or TiC) and amorphous (a-C) is performed using fitting analysis of the C 1s XPS peak. The film hardness (22 to 31 GPa) correlates with the fraction of the TiB_xC_y phase that exists in the coatings. The tribological properties were measured by a pin-on-disk tribometer in ambient conditions, using 6 mm tungsten carbide balls at 1 N. The friction coefficients and the wear rates show similar behavior, exhibiting an optimum when the fraction of C atoms in the amorphous phase is near 50%. This composition enables significant improvement of the friction coefficients and wear rates (μ ∼ 0.1; k < 1 × 10⁻⁶ mm³/Nm), while maintaining a good value of hardness (24.6 GPa). Establishing the correlation between the lubricant properties and the fraction of a-C is very useful for purposes of tailoring the protective character of these nanocomposite coatings to engineering applications.     

Effect of composition on mechanical behaviour of diamond-like carbon coatings modified with titanium

In this study, diamond-like carbon (DLC) films modified with titanium were deposited by plasma decomposition of metallorganic precursor, titanium isopropoxide in CH₄/H₂/Ar gas atmosphere. The obtained films were composed of amorphous titanium oxide and nanocrystalline titanium carbide, embedded in an amorphous hydrogenated (a-C:H) matrix. The TiC/TiO₂ ratio in the DLC matrix was found to be dependent on the deposition parameters. The dependence of the films chemical composition on gas mixture and substrate temperature was investigated by X-ray photoelectron spectroscopy, whereas the crystallinity of TiC nanoparticles and their dimension were evaluated by X-ray diffraction. The size of TiC crystallites varied from 10 to 35 nm, depending on the process parameters. The intrinsic hardness of 10-13 GPa, elastic modulus of 170-200 GPa and hardness-to-modulus ratio of obtained coatings were measured by the nanoindentation technique. Obtained results demonstrated a correlation of mechanical properties with the chemical composition and the ratio of amorphous/crystalline phases in the films. In particular, the formation of nanocrystalline TiC with atomic concentration not exceeding 10% and with grain size between 10 nm and 15 nm resulted in significantly enhanced mechanical properties of composite material in comparison with ordinary DLC films.     

Preparation and electrocatalytic activity of tungsten carbide and titania nanocomposite

Tungsten carbide and titania nanocomposite was prepared by combining a reduced-carbonized approach with a mechanochemical approach. The samples were characterized by X-ray diffraction, transmission electron microscope under scanning mode and X-ray energy dispersion spectrum. The results show that the crystal phases of the samples are composed of anatase, rutile, nonstoichiometry titanium oxide, monotungsten carbide, bitungsten carbide and nonstoichiometry tungsten carbide, and they can be controlled by adjusting the parameters of the reduced-carbonized approach; tungsten carbide particles decorate on the surface of titania support, the diameter of tungsten carbide particle is smaller than 20 nm and that of titania is around 100 nm; the chemical components of the samples are Ti, O, W and C. The electrocatalytic activity of the samples was measured by a cyclic voltammetry with three electrodes. The results indicate that the electrocatalytic activities of the samples are related to their crystal phases and the property of electrolyte in aqueous solution. A synergistic effect between titania and tungsten carbide is reported for the first time.     

Electrochemical deposition of carbon films on titanium in molten LiCl-KCl-K2CO3

Electrodeposition of carbon films on the oxide-scale-coated titanium has been performed in a LiCl-KCl-K₂CO₃ melt, which are characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction analysis. The electrochemical process of carbon deposition is investigated by cyclic voltammetry on the graphite, titanium and oxide-scale-coated titanium electrodes. The particle-size-gradient carbon films over the oxide-scale-coated titanium can be achieved by electrodeposition under the controlled potentials for avoiding codeposition of lithium carbide. The deposited carbon films are comprised of micron-sized ‘quasi-spherical’ carbon particles with graphitized and amorphous phases. The cyclic voltammetry behavior on the graphite, titanium and oxide-scale-coated titanium electrodes shows that CO₃^2 − ions are reduced most favorably on the graphite for the three electrodes. Lithium ions can discharge under the less negative potential on the electrode containing carbon compared with titanium electrode because of the formation of lithium carbide from the reaction between lithium and carbon.     

Mechanical alloying and sintering of nanostructured tungsten carbide-reinforced copper composite and its characterization

Elemental powders of copper (Cu), tungsten (W) and graphite (C) were mechanically alloyed in a planetary ball mill with different milling durations (0–60 h), compacted and sintered in order to precipitate hard tungsten carbide particles into a copper matrix. Both powder and sintered composite were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and assessed for hardness and electrical conductivity to investigate the effects of milling time on formation of nanostructured Cu–WC composite and its properties. No carbide peak was detected in the powder mixtures after milling. Carbide WC and W₂C phases were precipitated only in the sintered composite. The formation of WC began with longer milling times, after W₂C formation. Prolonged milling time decreased the crystallite size as well as the internal strain of Cu. Hardness of the composite was enhanced but electrical conductivity reduced with increasing milling time.     

Carbide and nanocomposite thin films in the Ti-Pt-C system

Thin films in the Ti-Pt-C system were deposited by non-reactive, DC-magnetron sputtering. Samples were characterised using X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy. A previously not reported metastable solid solution carbide, (Ti_1 − xPt_x)C_y with a Pt/Ti ratio of up to 0.43 was observed. This solid solution phase was present both as single phase in polycrystalline samples, and together with amorphous carbon (a-C) in nanocomposite samples. Annealing of nanocomposite samples leads to the decomposition of the solid solution phase and the formation of a nc-TiC_x/a-C/nc-Pt nanocomposite. Test sensors for automotive gas exhausts manufactured from such a three-phase material suffer from complete oxidation of the coating at 400 °C with no observed sensor activity.     

An easy soft-template route to synthesis of wormhole-like mesoporous tungsten carbide/carbon composites

Wormhole-like mesoporous tungsten carbide/carbon (WC/C) composites can be prepared by an easy method that combines emulsion processing with triblock copolymer self-assembly strategy, followed by a high-temperature carbothermal reduction. X-ray diffraction, transmission electron microscopy, X-ray spectroscopy, thermogravimetric analysis and N₂ sorption techniques were employed to characterize the mesoporous WC/C composites. The results show that the resultant materials have a wormhole-like mesostructure containing nanoscale (∼40 nm) tungsten carbide particles, and high surface areas (up to 314.9 cm²/g). It is proposed that a general assemble procedures are responsible for the wormhole-like mesoporous WC/C composites.     

Structure and mechanical properties of W-Se-C/diamond-like carbon and W-Se/diamond-like carbon bi-layer coatings prepared by pulsed laser deposition

Bi-layer W-Se-C/diamond-like carbon (DLC) and WSe_x/DLC coatings were obtained by standard and shadow-masked pulsed laser co-deposition from WSe₂ and graphite targets. W-Se-C coatings appeared as nanocomposites containing quasi-amorphous WSe₂, WC, spherical β-W nanocrystalline particles encapsulated in WSe₂ amorphous shell, and amorphous carbon phases. In WSe_x/DLC coatings, the formation of chemical bonds between W and C atoms was noticed at the interface. An increase of the C concentration over 40 at.% increases hardness and elasticity (up to 2 times at ~ 60 at.%C), and the Se/W ratio was always close to 1.4. The use of shadow-masked configuration avoids the deposition of micro- and nanoparticles. However, this method leads to a substantial increase of the Se content (Se/W ≥ 4), and the coatings became softer.     

Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition

Nanostructured tungsten (W) and tungsten trioxide (WO₃) films were prepared by glancing angle deposition using pulsed direct current magnetron sputtering at room temperature with continuous substrate rotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectron spectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and high resolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibit nanostructured morphologies with an extremely high surface area, which may potentially increase the sensitivity of chemiresistive WO₃ gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasma at room temperature crystallized into a predominantly simple cubic β-phase with <100> texture although evidence was found for other random grain orientations near the film/substrate interface. Subsequent annealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO₃ structure and the nanorod morphology was retained. Substoichiometric WO₃ films grown in an Ar/O₂ plasma at room temperature had an amorphous structure and also exhibited nanorod morphology. Post-deposition annealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO₃ phase and also caused a morphological change from nanorods into a nanoporous network.     

Characterization of magnetron co-sputtered W-doped C-based films

In this paper, W-doped C-based coatings were deposited on steel and silicon substrates by RF magnetron sputtering, using W and C targets, varying the cathode power applied to the W target and the substrate bias. The chemical composition was varied by placing the substrates in a row facing the C and W targets. W content in the films increased from 1 to 2 at.% over the C target to ∼ 73 at.% over the W target. The coatings with W content lower than ∼ 12 at.% and ∼ 23 at.%, for biased and unbiased conditions, respectively, showed X-ray amorphous structures, although carbide nanocrystals must exist as shown by the detection of the WC_1−x phase in films with higher W content. C-rich films were very dense and developed a columnar morphology with increasing W content. An improvement in the hardness (from 10 GPa, up to 25 GPa) of the films was achieved either when negative substrate bias was used in the deposition, or when the WC_1−x phase was detected by X-ray diffraction. The adhesion of the coatings is very low with spontaneous spallation of those deposited with negative substrate bias higher than 45 V. Varieties in cathode power (90 W or 120 W) applied to the W target showed no observable influence on the characteristics of the films.     

The hydrogen reduction of cobalt-tungsten mixed oxides

The hydrogen reduction of two non-stoichiometric samples of cobalt tungstate (one cobalt-rich, the other heavily tungsten-rich) has been studied over the temperature range 600 to 1100° C using thermogravimetry, X-ray diffraction analysis and scanning electron microscopy. The products are shown to be non-equilibrium at most reduction temperatures. In order to explain the experimentally observed X-ray diffraction data it is postulated that the reduction process occurs via the formation of an amorphous phase which contains cobalt, tungsten and oxygen. The amorphous phase becomes unstable at low oxygen potentials and precipitates either, or both, Co₃W and Co₇W₆ depending upon the degree of cobalt enrichment of the amorphous phase. These are the only two cobalt-containing crystalline phases in the products of reduction and are not detected before at least 53% reduction has occurred. During the early stages of reduction either tungsten (for near stoichiometric, cobalt-rich oxide) or WO₂ (for tungsten-rich oxide-CoWO₄ plus WO₃) are the only crystalline products of reduction.     

In-situ energy-dispersive X-ray diffraction of metal sulfide assisted crystallization of strongly (001) textured photoactive tungsten disulfide thin films

Highly (001) textured tungsten disulphide (WS₂) thin films were grown by rapid metal (Ni, Pd) sulfide assisted crystallization of amorphous reactively sputtered sulfur-rich tungsten sulfide (WS_3 + x) and by metal sulfide assisted sulfurization of tungsten metal films. The rapid crystallization was monitored by real-time in-situ energy dispersive X-ray diffraction (EDXRD). Provided that a thin nickel or palladium film was deposited prior to the deposition of WS_3 + x or W, the films crystallized very fast (about 20 nm/s) at temperatures above the metal sulfide eutectic temperature. After crystallization, isolated MeS_x crystallites are located on the surface of the WS₂ layer, which was proved by scanning electron microscopy. The metal sulfide assisted crystallized WS₂ layers exhibit a pronounced (001) orientation with large crystallites up to 2 µm. The in-situ EDXRD analysis revealed distinct differences of the two crystallization routes from tungsten and from amorphous, sulfur-rich WS_3 + x precursors, respectively. The crystallized WS₂ films showed photoactivity. Combined with the high absorption coefficient of 10⁵ cm^− 1 and a indirect band gap of 1.8 eV these properties make such films suitable for absorber layers in thin film solar cells.     

Effect of oxygen partial pressure on the microstructural and physical properties on nanocrystalline tin oxide films grown by plasma oxidation after thermal deposition from pure Sn targets

In this work, microstructural and physical properties were studied in the tin oxide films deposited by thermal evaporation of Sn films on stainless steel substrates followed by in situ D.C. plasma oxidation at 200 °C substrate temperature. The surface properties were studied by scanning electron microscopy, X-ray diffraction, atomic force microscopy and four-point probe electrical resistivity. The typical calculated grain size of the films deposited by thermal evaporation was between 28 nm and 66 nm and the texture structure was found to be dependent on the thermal deposition pressure. A cassiterite structure of SnO₂ was produced by D.C. plasma oxidation with the main diffraction peaks of the (101), (200), (211), (310) and (221) planes at the 25% and 50% O₂ partial pressure conditions. However, at 12.5% O₂ partial pressure oxidation conditions, amorphous tin oxide structure and crystalline SnO phases were detected. Increasing thermal deposition pressure resulted in preferential texture formation at (211) and (310) planes. The surface structure investigation of the produced films by SEM and AFM studies showed large SnO₂ islands with approximately 1.0 μm and 1.5 μm sized nodules, and they are called as grape-like structures. The grape-like grains possess nano grains, which are between 20 nm and 30 nm in diameter calculated by Scherer's formula. The grape-like grains were seen to be separated by large cavities and the size of these cavities and nano grains was seen to be larger when the O₂ partial pressure is increased. The four-point probe resistivity of the films, grown at different oxidation temperatures, decreased with the increase in oxygen partial pressure. The values of resistivity for SnO₂ phase were measured as low as 10⁻⁵ Ω-cm and observed to decrease with increasing thermal deposition pressure and oxygen partial pressure.     

SiC-SiO2 nanocomposite films prepared by laser CVD using tetraethyl orthosilicate and acetylene as precursors

SiC-SiO₂ nanocomposite films were prepared by laser chemical vapor deposition (LCVD) using a CO₂ laser with tetraethyl orthosilicate (TEOS) and acetylene (C₂H₂) as precursors. The effects of laser power on the crystal phase and microstructure of the SiC-SiO₂ nanocomposite films were investigated. Films produced with laser power below 150 W (below 1523 K) had an amorphous structure, while those produced above 200 W (above 1673 K) were a mixture of crystalline SiC and amorphous phase. At 245 W (1774 K) the film contained 3C-SiC nanocrystals 100 to 200 nm in diameter dispersed in an amorphous matrix having high-density stacking faults formed on the (1?1?1?) and (111?) planes.     

Highly (100)-oriented Ce1 − xFexO2 − δ solid solution films prepared by laser chemical vapor deposition

Ce_1 − xFe_xO_2 − δ solid solution films were prepared on amorphous silica substrates by laser chemical vapor deposition using metal dipivaloylmethanate precursors and a semiconductor InGaAlAs (808 nm in wavelength) laser. X-ray diffraction revealed the formation of single Ce_1 − xFe_xO_2 − δ phase at x ≤ 0.15, while CeO₂ and Fe₂O₃ phases were found for higher Fe content. Highly (100)-oriented Ce_1 − xFe_xO_2 − δ (x = 0.02) films were obtained at laser power, P_L = 50-200 W and deposition temperature, T_dep = 800-1063 K. Lotgering factor (200) was calculated to be above 0.8 for films prepared at P_L = 50-150 W. X-ray photoelectron spectroscopy revealed the presence of Fe³⁺, Ce⁴⁺ and Ce³⁺ on solid solution films. Cross-sectional transmission electron microscope images disclosed a film columnar feather-like structure with a large number of nano-scale interspaces. Deposition rates were 2 or 3 orders of magnitude higher than those reported for conventional metal organic chemical vapor deposition of CeO₂.     

Deposition and characterization of magnetron sputtered amorphous Cr-C films

Thin films in the Cr-C system with carbon content of 25-85 at.% have been deposited using non-reactive DC magnetron sputtering from elemental targets. Analyses with X-ray diffraction and transmission electron microscopy confirm that the films are completely amorphous. Also, annealing experiment show that the films had not crystallized at 500 °C. Furthermore, X-ray spectroscopy and Raman spectroscopy show that the films consist of two phases, an amorphous CrC_x phase and an amorphous carbon (a-C) phase. The presence of two amorphous phases is also supported by the electrochemical analysis, which shows that oxidation of both chromium and carbon contributes to the total current in the passive region. The relative amounts of these amorphous phases influence the film properties. Typically, lower carbon content with less a-C phase leads to harder films with higher Young’s modulus and lower resistivity. The results also show that both films have lower currents in the passive region compared to the uncoated 316L steel substrate. Finally, our results were compared with literature data from both reactively and non-reactively sputtered chromium carbide films. The comparison reveals that non-reactive sputtering tend to favour the formation of amorphous films and also influence e.g. the sp²/sp³ ratio of the a-C phase.     

Microstructural characterisation of nanocomposite nc-MeC/a-C coatings on oxygen hardened Ti-6Al-4V alloy

Nanocomposite coatings are novel, important systems composed of two or more nanocrystalline, or nanocrystalline and amorphous, phases. Such coatings offer a possibility of tailoring the coating microstructure and achieving new improved properties of coated materials. In this work a duplex surface treatment, consisting of an oxygen diffusion treatment and deposition of low friction nanocomposite nc-MeC/a-C (Me = transition metal, Ti, W or Cr) coatings, was applied for improvement of the Ti-6Al-4V alloy properties. The coatings composed of nanocrystallites of transition metal carbides (TiC or Cr_xC_y or WC) embedded in hydrogen-free amorphous carbon (a-C) matrix were deposited onto the surface of an oxygen hardened Ti-6Al-4 V alloy substrate by means of a simple DC magnetron sputtering. A nano/microstructure of the substrate material and coatings has been examined by scanning- and transmission electron microscopy complemented with the results of X-ray diffraction analyses.It was found that the nanocomposite coatings are composed of different carbide nanocrystals (with sizes of a few nanometres) embedded in an amorphous carbon matrix. The results of qualitative and quantitative analyses of the nanocrystalline phase in the coatings with use of high-resolution transmission electron microscopy combined with image analysis are given in the paper.An effect of the nano/microstructure parameters of the coated alloy onto its micro-mechanical (nanohardness and Young's modulus) and tribological properties (wear resistance and friction coefficient) is discussed in the paper.