Tungsten Carbide Films Research Articles

Effect of substrate temperature on the structural properties of Tungsten Carbide and Tungsten-rich Tungsten Carbide films

ABSTRACT The unique mechanical properties of Tungsten Carbide (WC), such as fracture toughness, exceptional hardness, high melting temperature, outstanding thermal stability, anti-oxidation qualities fatigue and creep resistance, have made it a suitable industrial material for different applications. It is also considered to be a promising material for coating on plasma-facing walls in a Tokamak reactor. The aim of the present work is to study the effect of varying substrate temperature, concentration of tungsten (W) and annealing in the structural properties of WC films. The WC films are deposited on a silicon substrate at different substrate temperatures viz. RT (300 K), 400, 500, 600 and 700 K using DC magnetron sputtering. W-rich WC films were deposited keeping the power of WC constant (100 Watt) and varying W powers (10 Watt, 20 Watt, 30 Watt, 40 Watt) at room temperature. Later, these W-rich WC films were annealed at 800°C in a reducing atmosphere. Glancing incidence X-ray diffraction (GIXRD) and Raman spectroscopy of the films are performed for structural analysis. GIXRD of the WC films reveals that as-deposited RT films are amorphous. However, films which are either deposited at elevated substrate temperature or are annealed are crystalline. This is confirmed by Raman’s measurements. It also indicates the presence of disorder and nanocrystalline phases due to temperature variations.

Deposition of tungsten in methane atmosphere using a low energy plasma focus device

Deposition of tungsten in methane atmosphere was performed using a low energy plasma focus device. The resulting compounds were synthesized and investigated during the deposition process. Carbon-rich contamination was identified as an organic viscous liquid crystal defect. Tungsten carbide (W2C) and dark-black oxygen-deficient tungsten trioxide (WO3-x) thin films were synthesized by plasma focus device using different number of focused shots. The synthesis of each thin film, which was dependent upon operation of plasma focus device was verified and discussed. Tungsten carbide W2C was synthesized during the deposition process while the dark-black WO3-x was created after the deposition process in atmosphere. All compounds were investigated by spectroscopy, crystalline, and microscopy methods. Nano indentation test disclosed higher value of hardness for the film containing tungsten carbide compound (10.01 GPa) compared to both pure tungsten (5.72 GPa) and the dark-black WO3-x thin film (6.83 GPa). The verified band gap energy of dark-black WO3-x was around 1.475 eV.

Wear properties of carbon-rich tungsten carbide films

Thin films of wear-resistant tungsten carbide were deposited on steel substrates by sputtering of WC with magnetron sputtering in a mixed argon/acetylene atmosphere. Variation of the acetylene supply during the deposition led to the growth of the tungsten carbide films with different excesses of carbon with respect to the stoichiometric WC. Variation of bias voltage influences mainly the [C]/([C]+[W]) ratio and secondary the mechanical properties hardness and Young's modulus. The films are supposed to combine good wear-resistant properties with a low friction under dry conditions. A tribometric analysis of friction and wear shows that an excess of carbon in the hard material layer can reduce the coefficient of friction by up to 40%. However, the volume of wear increased with increasing amount of carbon, while the nanoindentation measurements revealed higher hardness and lower elastic modulus.

Mesoporous WCx Films with NiO-Protected Surface: Highly Active Electrocatalysts for the Alkaline Oxygen Evolution Reaction.

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template‐controlled porosity that are surface‐modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft‐templated mesoporous tungsten oxide (mp. WO x ) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WC x ), and a subsequent shape‐conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+, leading to remarkably improved utilization of OER‐active sites in alkaline medium. The obtained Ni mass‐specific activity is about 280 times that of mesoporous NiO x (mp. NiO x ) films. The NiO‐coated WC x catalyst achieves an outstanding mass‐specific activity of 1989 A gNi −1 in a rotating‐disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte.

Phase formations in tungsten carbide films deposited by reactive magnetron sputtering

Phase formations in tungsten carbide films deposited by reactive magnetron sputtering

Epitaxial growth of cubic WCy(001) on MgO(001)

Tungsten carbide films are sputter-deposited onto MgO(001) substrates at 400 °C in 5 mTorr Ar - CH4 gas mixtures with CH4 fractions fCH4 = 0.4–6%, yielding total C-to-W ratios x = 0.57–1.25 as determined by ion beam analyses and energy-dispersive X-ray spectroscopy. The C-to-W ratio y in the cubic WCy phase is smaller than x, ranging from y = 0.47–0.68, as determined from lattice constant ao measurements in combination with first-principle calculations that predict an increasing ao = (0.4053 + 0.0295 y) nm for y = 0.3–1.0. This suggests that the cubic phase is stabilized by carbon vacancies and that the layers contain amorphous C with a volume fraction increasing from 4% to 26% for fCH4 = 0.4–6%. X-ray diffraction (XRD) θ-2θ scans, ω-rocking curves, and reciprocal space maps in combination with high-resolution transmission electron microscopy (TEM) indicate the growth of epitaxial rock-salt structure WCy(001) layers with a cube-on-cube epitaxial relationship with the substrate: (001)WC || (001)MgO and [100]WC || [100]MgO. The measured XRD out-of-plane coherence length of 8 – 14 nm is nearly independent of the film thickness d = 10 or 600 nm, suggesting that growth beyond d = 10 nm leads to an epitaxial breakdown and the nucleation of misoriented hexagonal or orthorhombic W2C grains for fCH4 ≤ 1% and cubic nanocrystalline WCy grains for fCH4> 1%.

Deposition of thin tungsten carbide films by dual ion beam sputtering deposition

Hard materials coating fabricated by physical vapor deposition techniques methods are wildly used because of their excellent mechanical strength, stability and chemical resistivity. In this article, compact and high Young's modulus tungsten carbide films with ∼100 nm thickness were prepared on Si (100) substrates by dual ion beam sputtering deposition method, and elucidates the influence of deposition temperature and assistant ion source on the adhesion strength and Young's modulus of tungsten carbide thin films. Morphological analysis, composition, and structure of tungsten carbide films were investigated by Atomic force microscope, X-ray photoelectron spectrometry, Scanning electron microscope. The X-ray diffraction analysis shows that the thin films comprise of hexagonal structure W2C. The adhesion strength is improved due to ion beam source bombardment and elevated deposition temperature. The Young's modulus of the film is codetermined by the density, microstructure and crystal structure. The influence of density, microstructure and crystal structure on Young's modulus was discussed in detail. The Young's modulus is improved due to the more compact structure caused by appropriate assistant source bombardment and elevated deposition temperature, and gets degradative due to the disorganized, coarse and granular structure caused by overlarge assistant source bombardment.

(Invited) Atomic Layer Deposition of Tungsten-Rich Tungsten Carbide Films Using WCl6 and AlH2(tBuNCH2CH2NMe2) as Precursors

Atomic layer deposition (ALD) is a thin film deposition procedure where growth occurs in a layer-by-layer fashion. Growth of a binary material AB occurs first by passing a gaseous stream of a precursor containing element A over the substrate, to afford a monolayer of the precursor on the surface. Then, an inert gas purge is carried out to remove excess precursor and any reaction products. Next, a gaseous stream of a precursor containing element B is passed over the substrate. This precursor reacts with the monolayer on the surface containing element A to afford the desired material AB. Finally, a second inert gas purge is conducted to remove any excess precursor and reaction products. These four steps constitute a growth cycle. A key feature of ALD is its self-limiting nature, in which the precursor containing element A reacts with surface reactive sites until they are all consumed. At this point, growth ceases, to afford a monolayer of the precursor on the substrate surface. Self-limiting growth gives excellent conformal coverage of high aspect ratio features because the growth is controlled by surface reactive sites, and also allows sub-nanometer control of film thicknesses by adjusting the number of growth cycles. The success of ALD is directly related to the chemical precursors that are used to deliver the desired elements for a given material. ALD precursors must combine good volatility, thermal stability at the deposition temperature, as well as high reactivity toward a second precursor to afford the desired material. The reaction products must be volatile so that they are not incorporated into the growing film. Moreover, chemical precursors must have chemical reaction paths built into them so that the desired materials are obtained. These properties are difficult to incorporate into molecules, and lack of appropriate precursors represents a major bottleneck for the wider applications of ALD. This tutorial will review ALD precursors that have been reported for a variety of materials. Emphasis will be focused on obtaining volatility, high thermal stability, and high reactivity toward a second precursor. Examples will be chosen where chemical reactivity is designed into the precursors to obtain desired materials with high purities. Difficult current challenges in ALD include design of precursors for the growth of electropositive element films, integration of appropriate ALD precursor chemistry into manufacturing processes, and the possibility of precursor design to achieve inherent area-selective deposition. Additional future challenges related to ALD precursors and chemistry encompass the emerging atomic layer etching (ALE) and atomic layer clean (ALC) techniques. The importance of precursors and chemistry in future ALD, ALE, and ALC processes will be highlighted.

(Invited) Atomic Layer Deposition of Tungsten-Rich Tungsten Carbide Films Using WCl6 and AlH2(tBuNCH2CH2NMe2) as Precursors

The thermal atomic layer deposition (ALD) of tungsten-rich tungsten carbide thin films is described using WCl6 and the aluminum dihydride co-reactant AlH2(tBuNCH2CH2NMe2). Smooth and stable films were grown at temperatures >200 °C. An ALD window was observed between 275-350 °C with a growth rate of 1.6 Å/cycle, according to SEM and XRR. Self-limiting growth was demonstrated at 300 °C for both precursors. XPS analysis showed that high purity tungsten-rich tungsten carbide (WxC, x = 2.8-3.7) films were deposited at 300 °C, with low levels of nitrogen (< 4 at.%), oxygen (< 4 at.%), chlorine (< 3 at.%), and aluminum (< 1 at.%). The as-deposited films were nanocrystalline and converted to the non-stoichiometric β-WC1-x phase above 600 °C. Film densities of the as-deposited films according to XRR were 50-60% of bulk values, which may explain the relatively high film resistivities (1140 mΩ·cm at 300 °C) despite the high film purity.

Thermal atomic layer deposition of tungsten carbide films from WCl6 and AlMe3

The atomic layer deposition (ALD) of tungsten carbide films is described with a thermal process employing WCl6 and AlMe3 as precursors. WCl6 has not been previously reported as an ALD precursor. Furthermore, WCl6 has favorable precursor characteristics and advantages over the widely used WF6, which is corrosive and toxic. The process displays an ALD window between 275 and 350 °C and a high growth rate of 1.5–1.8 Å/cycle within the ALD window. High purity tungsten carbide films with low Al and Cl contaminants were deposited at 300 °C. Film resistivities decreased with increasing growth temperature, reaching a minimum value of 1500 μΩ cm at 375 °C. The as-deposited films are amorphous, which could indicate superior performance as a Cu diffusion barrier over previously studied polycrystalline tungsten carbide barrier layers.

Vapor-Deposited Tungsten Carbide Nano-Dendrites as Sulfur-Tolerant Electrocatalysts for Quantum Dot-Sensitized Solar Cells

Recent advances in optoelectronic properties of quantum dots (QDs) have led to significant improvement in QD-sensitized solar cells (QDSCs); however, for practical utilization of these devices, performance of the constituent electrocatalytic counter electrodes (CEs) needs to be further enhanced. Pt CEs are prone to severe sulfur poisoning by polysulfide redox electrolytes in QDSCs, and Cu2S CEs with state-of-the-art activity are vulnerable to light-induced degradations. In this study, for the first time, tungsten carbide (W2C) films were used as CEs for QDSCs. Instead of the conventional methods of carbide nanomaterial synthesis that involve thermal treatments in toxic/explosive atmospheres, room-temperature vapor deposition was employed for the preparation of W2C electrodes, and dendritic nanostructures with large surface areas were obtained. Although the electronic structures of Pt and W2C are highly similar, W2C was completely inert to sulfur poisoning. This led to a substantial improvement in the electrocatalytic performance for polysulfide reduction, and ∼27% enhancement in power conversion efficiency was achieved when Pt CEs were replaced with W2C CEs in QDSCs. Moreover, QDSCs comprising W2C CEs manifested excellent photostability, and they showed performances superior to those of QDSCs comprising state-of-the-art Cu2S electrodes within 40 min of operation, without any sign of drop in efficiency.

Deposition and Characterization of Reactive Magnetron Sputtered Tungsten Carbide Films

Tungsten carbide thin films were deposited on silicon (100) substrates by DC reactive magnetron sputtering using CH4 as a carbon source. The microstructure, compressive stress, hardness and tribological behaviors showed great dependences on the rates of CH4 flow (FCH4). Increasing the FCH4 from 2 to 5 sccm, the film exhibited a phase transition from hexagonal-W2C to cubic-WC1-x. Further increasing the FCH4 larger than 10sccm, the film presented amorphous state. As the FCH4 increased, the Raman revealed that the films showed a graphitization trend, meanwhile, the surface of the films became smoother and smoother. The hardness of tungsten carbide films first increased, and then decreased after reaching the maximum 38.5GPa (FCH4=10 sccm). While the sample deposited at 15 sccm obtained the lowest wear rate (2.17×10-6 mm3/Nm) and low coefficient of friction (CoF, 0.24) and still maintained a high hardness of 32.1 GPa. The lowest wear rate could be ascribed to the highest ratio of H3/E2.

Electrochemical synthesis of nanoporous tungsten carbide and its application as electrocatalysts for photoelectrochemical cells.

Photoelectrochemical (PEC) cells are promising tools for renewable and sustainable solar energy conversion. Currently, their inadequate performance and high cost of the noble metals used in the electrocatalytic counter electrode have postponed the practical use of PEC cells. In this study, we report the electrochemical synthesis of nanoporous tungsten carbide and its application as a reduction catalyst in PEC cells, namely, dye-sensitized solar cells (DSCs) and PEC water splitting cells, for the first time. The method employed in this study involves the anodization of tungsten foil followed by post heat treatment in a CO atmosphere to produce highly crystalline tungsten carbide film with an interconnected nanostructure. This exhibited high catalytic activity for the reduction of cobalt bipyridine species, which represent state-of-the-art redox couples for DSCs. The performance of tungsten carbide even surpassed that of Pt, and a substantial increase (∼25%) in energy conversion efficiency was achieved when Pt was substituted by tungsten carbide film as the counter electrode. In addition, tungsten carbide displayed decent activity as a catalyst for the hydrogen evolution reaction, suggesting the high feasibility for its utilization as a cathode material for PEC water splitting cells, which was also verified in a two-electrode water photoelectrolyzer.

Fabrication of Tungsten Carbide Films by Filtered Pulse Arc Deposition with Cemented Tungsten Carbide Cathodes

Tungsten carbide films (W-C films) were fabricated on silicon substrates by using the filtered pulse arc deposition (FPAD) method. Two types of cemented tungsten carbide (WC) were used as cathode, one containing Co and the other Ti, which were used as binders for forming the cathode shape. The films were fabricated by varying the pulse arc current and substrate bias voltage. The discharge, deposition and film properties were investigated under these deposition conditions. The cathode wear amount when using WC-Co (WC cathode containing Co) was found to be smaller than that measured when WC-Ti (WC cathode containing Ti) was used. The W-C film thickness was approximately 30 - 40 nm under all conditions, except when the pulse arc current was 50 A and the film thickness, was approximately 10 nm. Compared to the WC-Ti, the consumption of cathode material is suppressed in the WC-Co, indicating that the efficiency for film preparation of the latter is good. From the X-ray diffraction analysis, the crystalline phase of W-C films fabricated using WC-Co and WC-Ti were observed as W2C and WC1-x, respectively, indicating that different crystalline phases could be fabricated using different cathodes. From the X-ray photoelectron spectroscopy analysis, the oxidation layer formed by air exposure was observed to exclusively exist on the W-C film surface. Moreover, almost all oxygen in the oxidation layer bonded with tungsten.

Magnetic field imaging of a tungsten carbide film by scanning nano-SQUID microscope

We present the results of magnetic field imaging by scanning nano-superconducting quantum interference device (SQUID) microscopy on a tungsten carbide (W-C) film fabricated using focused-ion-beam chemical vapor deposition. We have investigated magnetic field change by a W-C film in an external magnetic field using a scanning nano-SQUID microscope system. We have found that the reduction of the magnetic field above the W-C film was 0.9%, indicating the penetration of vortices in the W-C at an external magnetic field of 0.171 mT.

WC-based thin films obtained by reactive radio-frequency magnetron sputtering using W target and methane gas

Deposition of tungsten carbide (WC) films was investigated by radio-frequency reactive sputtering using a tungsten target and methane gas. The effect of some processing parameters (pressure, power, CH4-to-Ar gas flow ratio) upon the chemical and structural properties of the films has been investigated. The evolution of the chemical composition has been analyzed by photoemission, the microstructure has been studied through electron microscopy techniques and the crystallographic structure was investigated by X-ray diffraction as well as Raman spectroscopy. This study demonstrates that the formation of tungsten carbide is highly dependent on the deposition conditions: thin films are composed of either metallic tungsten or carbon for a total gas pressure of 0.5Pa while for higher total gas pressure (5Pa), tungsten carbide films are deposited for a large power range (50–180W) but a narrow range of methane flow rate (2–8%). In this latter case, no changes are observed in microstructure and crystallography of tungsten carbide films with processing parameters: all films present a columnar growth and are mainly formed of cubic sub-stoichiometric WC1−x nanocrystallites embedded in amorphous carbon. However, as function of process parameters, a strong variation in chemical composition of the films is revealed and is attributed to the defective structure of cubic sub-stoichiometric WC1−x phase.

Structurally different interfaces between electrospark-deposited titanium carbonitride and tungsten carbide films on steel

Protective hard coatings on steel that are produced by electrospark deposition (ESD) methods that do not require vacuum conditions are compared, and the interfaces formed are interrogated by a combination of analytical methods. A titanium carbonitride (TiCN) coating is produced and compared to a tungsten carbide (WC) ESD coating. Following deposition onto a 4140 grade steel substrate, the coatings were compared by X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, atomic force microscopy, X-ray diffraction, and nanoindentation test to determine Young's modulus as an indicator of mechanical strength. It was found that the coatings produced void- and impurity-free interfaces but that the interfaces are drastically different for the two coatings investigated and some of the differences can be explained based on the different melting points of the two materials that affect the process of ESD.

Effect of interfacial interactions on the thermal conductivity and interfacial thermal conductance in tungsten–graphene layered structure

Graphene film was deposited by microwave plasma assisted deposition on polished oxygen free high conductivity copper foils. Tungsten–graphene layered film was formed by deposition of tungsten film by magnetron sputtering on the graphene covered copper foils. Tungsten film was also deposited directly on copper foil without graphene as the intermediate film. The tungsten–graphene–copper samples were heated at different temperatures up to 900 °C in argon atmosphere to form an interfacial tungsten carbide film. Tungsten film deposited on thicker graphene platelets dispersed on silicon wafer was also heated at 900 °C to identify the formation of tungsten carbide film by reaction of tungsten with graphene platelets. The films were characterized by scanning electron microscopy, Raman spectroscopy, and x-ray diffraction. It was found that tungsten carbide film formed at the interface upon heating only above 650 °C. Transient thermoreflectance signal from the tungsten film surface on the samples was collected and modeled using one-dimensional heat equation. The experimental and modeled results showed that the presence of graphene at the interface reduced the cross-plane effective thermal conductivity and the interfacial thermal conductance of the layer structure. Heating at 650 and 900 °C in argon further reduced the cross-plane thermal conductivity and interface thermal conductance as a result of formation nanocrystalline tungsten carbide at the interface leading to separation and formation of voids. The present results emphasize that interfacial interactions between graphene and carbide forming bcc and hcp elements will reduce the cross-plane effective thermal conductivity in composites.

Evaluation of Deformation, Mass Loss, and Roughness of Different Metal Burs After Osteotomy for Osseointegrated Implants

This study used bovine ribs to comparatively assess the deformation, roughness, and mass loss for 3 different types of surface treatments with burs, used in osteotomies, for the installation of osseointegrated implants. The study used 25 bovine ribs and 3 types of helical burs (2.0 mm and 3.0 mm) for osteotomies during implant placement (a steel bur [G1], a bur with tungsten carbide film coating in a carbon matrix [G2], and a zirconia bur [G3]), which were subdivided into 5 subgroups: 1, 2, 3, 4, and 5, corresponding to 0, 10, 20, 30, and 40 perforations, respectively. The surface roughness (mean roughness [Ra], partial roughness, and maximum roughness) and mass (in grams) of all the burs were measured, and the burs were analyzed in a scanning electron microscope before and after use. Data were tabulated and statistically analyzed by use of the Kruskal-Wallis test, and when a statistically significant difference was found, the Dunn test was used. There was a loss of mass in all groups (G1, G2, and G3), and this loss was gradual, according to the number of perforations made (1, 2, 3, 4, and 5). However, this difference was not statistically significant (P < .05). Regarding the roughness, G3 presented an increase in Ra, partial roughness, and maximum roughness (P < .05) compared with G2 and an increase in Ra compared with G1. There was no statistically significant difference (P > .05) between G1 and G2. The scanning electron microscopy analysis found areas of deformation in all the 2.0-mm samples, with loss of substrates, and this characteristic was more frequent in G3. The 2.0-mm zirconia burs had a greater loss of substrates and abrasive wear in the cutting area. They also presented an increased roughness when compared with the steel and the tungsten carbide coating film in carbon matrix. There was no statistically significant difference (P < .05) between G1 and G2 in any mechanical test carried out.

Light Scattering Techniques Applied to Materials Science

Light scattering techniques provide powerful methods to investigate a variety of different materials properties. Brillouin scattering is used to the study the elastic properties of bulk solids and thin supported films at ambient and high temperatures. Work on iron pyrite and tungsten carbide films are described as illustrations with theoretical methods using surface Green’s functions being used in the analysis. Raman scattering is used to study corrosion and passivation of iron in an alkaline solution and stress patterns in diamond plastically deformed at high temperature.