Oxidation of tungsten and tungsten carbide in dry and humid atmospheres

Oxidation experiments were performed on pure tungsten and hot-pressed tungsten carbide. The chemical state and thickness of the oxide products were determined by ESCA. The oxidation of W and WC in dry atmosphere was performed in oxygen at temperatures ranging from 20 to 500 °C. The oxide formed is WO₃. The thickness of the oxide layer increases slowly up to 200 °C, after which the oxide growth is rapid.

The oxidation behaviour of W and WC in humid atmospheres was studied at room temperature in air at relative humidities of 60 and 95%. It was found that the thickness of the oxide layer increases with increased humidity. No formation of hydroxide was observed. Exposing W to water for one week results in a thick layer of WO₂, WO₃ and hydroxide. In the case of WC no oxide at all was visible after exposure to water. Furthermore, WC is resistant to further oxidation after exposure.     

Sinterbonding cobalt-cemented tungsten carbide to tungsten heavy alloys

Cobalt-cemented tungsten carbide (WC–Co) powder was sinterbonded to nickel–iron tungsten heavy alloy (WHA) for use in high-temperature tooling applications. Sinterbonding was performed under various conditions, including changes to the sintering conditions and initial WHA material forms, to determine various processing conditions that yield a consolidated interface indicative of a high degree of bonding. Sinterbonding WC–Co to fully dense WHA bar stock yielded a consolidated interface comprised primarily of complex η-carbides. Defects at the interface, including voids, microcracking, and porosity were the result of other sinterbonding processing conditions explored in this work. Co-rich η-carbides were found to form at the interface in every condition examined. A thermodynamic evaluation of η-carbides as a function of carbon activity determined that Co-rich η-carbides formed preferentially in regions of low carbon activity. The predicted thermodynamic trends are in agreement with interfacial microstructural observations.     

Synthesis of high-purity ultrafine tungsten and tungsten carbide powders

High-purity ultrafine W or WC powder was prepared via a two-step process composed of the carbothermic pre-reduction of WO_2.9 and the following deep reduction with H₂ or carbonization with CH₄+H₂ mixed gases. The effects of C/WO_2.9 molar ratio and temperature on phase composition, morphology, particle size, and impurity content of products were investigated. The results revealed that when the C/WO_2.9 ratio was in the range from 2.1:1 to 2.5:1, the carbothermic pre-reduction products consisted of W and a small amount of WO₂. With changing C/WO_2.9 ratio from 2.1:1 to 2.5:1, the particle sizes were gradually decreased. In order to prepare ultrafine W or WC powder, a relatively high C/WO_2.9 ratio and a lower reaction temperature at this stage were preferred. After the second reaction, the final products of ultrafine W and WC powders with a high purity could be obtained, respectively.     

Picosecond laser machining of tungsten carbide

Hard materials such as tungsten carbide (WC) are extensively used in cutting tools in high-value manufacturing, and the machining of these materials with sufficient speed and quality is essential to exploit their full potential. Over the last two decades, short (nanosecond (ns)) and ultra-short (picosecond (ps); femtosecond (fs)) pulse laser machining has been evaluated by various researchers and proposed as an alternative to the current state-of-the-art machining techniques for advanced materials like WC, which include mechanical grinding and electrical discharge machining. However, most of the established/existing research on this topic is based on low power lasers, which may not be adopted in industrial production environments due to its low material removal rate. This paper presents the results of a fundamental study, on using a 300 W picosecond laser for the deep machining of tungsten carbide. The influence of various laser parameters on the geometric precision and quality (surface and sub-surface) of the ablated area was analysed, and the ablation mechanism is discussed in detail. Laser pulse frequency and scanning speed have minimal effect on ablation rate at high power levels. The surface roughness of the ablated area increases with the ablation depth. At optimal conditions, no significant thermal defects such as a recast layer, micro crack or heat affected zone were observed, even at a high average power of 300 W. The material removal rate (MRR) seems to be proportional to the average power of the laser, and a removal rate of around 45 mm³ per minute can be achieved at 300 W power level. Edge wall taper appears to be a significant issue that needs to be resolved to enable industrial exploitation of high power ultra-short pulse lasers.     

On the contiguity of carbide phase in WC–Co hardmetals

In this paper the analysis of dependence of carbide phase contiguity in WC–Co hardmetals on the variation coefficient of WC grain size distribution is carried out. The relationship proposed is

C=1−V_Co^0.644exp(0.391V)

Here C is the contiguity, V_Co is the volume fraction of binder and V is the coefficient of variation. This relation is obtained on the basis of experimental data available in the literature. The basic meaning of this model is that with its help one more microstructural parameter V is introduced explicitly. This parameter essentially affects the mechanical behaviour of WC–Co hardmetals.     

Fracture resistant super hard materials and hardmetals composite with functionally designed microstructure

Polycrystalline diamond and other hard materials are widely used in earth boring, mining, and construction tool applications. Chipping and fracture resistance is often improved by various means at the expense of hardness and wear resistance. This trade-off between wear resistance and chipping resistance hinders the development of hard and super hard materials for many industrial applications. A new approach, characterized as “hard materials composites with functionally designed microstructure” including polycrystalline diamond and cemented tungsten carbide, is discussed. The functionally designed microstructure offers enhanced chipping resistance and toughness without significantly sacrificing wear resistance.     

Tensile behavior of hybrid tungsten composites with zirconium carbide nanoparticles and tungsten fibers

Hybrid tungsten composites reinforced with zirconium carbide (ZrC) nanoparticles and tungsten fibers were developed by the conventional powder metallurgy process (ball-mill mixing of powders and fibers followed by spark plasma sintering). The synergistic and mutual influences of the fibers and nanoparticles on tungsten were investigated in tensile behavior and fracture-energy tests. Aided by the ZrC nanoparticles, up to 30% of the fibers could be embedded in the tungsten matrix. The fracture energy was maximized by co-introducing 0.2 wt% ZrC particles with 20 wt% short tungsten fibers. The fracture-energy enhancement of the short fibers is contributed by pseudo-toughness from the fiber–matrix interface, inherent toughness from the fibers themselves, and grain refinement (by 50%) of the tungsten matrix. The fracture energy of the composite is very sensitive to the ZrC content, because the two-way action of ZrC weakens the pseudo-toughness of the interface energy.     

Preparation and electrocatalytic properties of tungsten carbide electrocatalysts

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Characteristics of high speed micro-cutting of tungsten carbide

In this study, experiments are carried out to evaluate the characteristics of high speed cutting of tungsten carbide material using a Makino V55 high speed machine tool with cubic boron nitride (CBN) tool inserts. The cutting forces were measured using a three-component dynamometer, the surface roughness of the machined workpiece was measured using a Mitutoyo SURFTEST 301, and the machined workpiece surfaces and the chip formation were examined using a scanning electron microscope (SEM). Experimental results indicate that the radial force F_x is much larger than the tangential force F_z and the axial force F_y. Two types of surfaces of the machined workpiece are achieved: ductile cutting surface and fracture surface. Continuous chips and discontinuous chips are formed under different cutting conditions. Depth of cut and feed rate almost have no significant effect on the surface roughness of the machined workpiece. The SEM observations on the machined workpiece surfaces and chip formation indicate that the ductile mode cutting is mainly determined by the undeformed chip thickness when the tool cutting edge radius is fixed. Ductile cutting can be achieved when the undeformed chip thickness is less than a critical value.     

Properties and rapid consolidation of ultra-hard tungsten carbide

Extremely dense WC with a relative density of up to 99% was obtained within 3 min under a pressure of 80 MPa using the high frequency induction heating sintering method (HFIHS) method. The average grain size of the WC was about 87 nm. The advantage of this process is not only rapid densification to obtain a near theoretical density but also the prohibition of grain growth in nanostructured materials. The hardness and fracture toughness of the dense WC produced by the HFIHS were investigated.     

Direct solid-state synthesis of tungsten carbide nanoparticles from mechanically activated tungsten oxide and graphite

Solid-state carbothermic reduction of tungsten oxide (WO₃) to nano-sized tungsten carbide (WC) particles was achieved by calcining mechanically activated mixtures of WO₃ and graphite at 1215 °C under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, WO_2.72, WO₂ and metallic tungsten (W), were observed at 741 °C, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.     

Electro-oxidation behavior of tungsten carbide electrode in different electrolytes

The electrochemical activity and stability of tungsten carbide gas diffusion electrode in different electrolytes were determined by galvanostatic charge method. It is shown that WC exhibits good electrocatalytic activity and stability for hydrogen oxidation in acidic solutions when the electrode potential is below about 800 mV (vs DHE), WC is firstly oxidized to an unstable blue tungsten oxides at 800-900mV which are closed to a composite stoichiometry of W2O5 in H2 SO4 solution and W8O23 in HCl solution calculated by charge consumed. Furthermore,the generated intermediate tungsten oxides can be further oxidized into WO3 at higher potentials. While in alkali solution, WC can not be used as anodic catalyst for its poor stability and catalytic activity due to the fact that WC will be directly oxidized into WO3.     

Microstructure and properties of flame sprayed tungsten carbide coatings

This article reports on feasibility experiments carried out with oxy-acetylene spray system with various oxygen to fuel ratios using two different tungsten carbide powders and powder feeding methods, to evaluate the newly developed fused WC, synthesised by transferred arc thermal plasma method. Transferred arc thermal plasma method is more economical and less energy intensive than the conventional arc method and results in a fused carbide powder with higher hardness. The microstructure and phase composition of powders and coatings were analysed by optical and scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Carbon content of the powders and coatings were determined to study the decarburisation of the material during spraying process. Coatings were also characterised by their hardness and abrasive wear. The effects of metallurgical transformation and phase content are related to wear performance. The results demonstrate that the powders exhibit various degree of phase transformation during the spray process depending on the type of powder, powder feeding and spray parameters. The carbon loss during the spray process in excess of 45% resulted in reduced hardness and wear resistance of the coatings. Coatings with high amount of WC and W₂C along with FeW₃C showed higher wear resistance. Thus, coatings of high wear resistance can be produced using fused tungsten carbide powder with WC and W₂C phases, which can be economically synthesised by thermal plasma transferred arc method.     

Ablation resistance of tungsten carbide cermets under extreme conditions

A cobalt-free tungsten carbide cermet (WC-FeNi) has been subjected to oxyacetylene flame tests to simulate extreme operating conditions such as a worst-case fusion reactor accident. In such an accident, air-ingress to the reactor may impinge on components operating at surface temperatures in excess of 1000 °C, leading to tungsten oxide formation and its subsequent hazardous volatilisation. Here, the most challenging accident stage has been simulated, where the initial air-ingress could lead to extremely rapid air-flow rates. These conditions were simulated using an oxidising oxyacetylene flame. The separation between flame nozzle and sample was varied to permit peak surface temperatures of ~950–1400 °C. When the peak temperature was below 1300 °C, the cermet gained mass due to the dominance of oxide scale formation. Above 1300 °C, the samples transitioned into a mass loss regime. The mass loss regime was dominated by liquid-phase ablation of the scale rather than its volatilisation, which was confirmed by performing a systematic thermogravimetric kinetic analysis. The result was unexpected as in other candidate shielding materials, e.g. metallic tungsten, volatilisation is considered the primary dispersion mechanism. The unusual behaviour of the cermet scale is explained by its relatively low melting point and by the lower volatility of its FeWO₄ scale compared to tungsten's WO₃ scale. The substantially lower volatility of the WC cermet scale compared to metallic W scales indicates it may have a superior accident tolerance.     

Electrochemical investigation on hydride electrode with tungsten carbide additive

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Sintering of tungsten carbide particles sputter-deposited with stainless steel

The purpose of this work was to study the sintering process of WC–stainless steel AISI 304 composite powders prepared by an innovative process, which consists in the use of a magnetron sputtering to coat WC powder particles with the stainless steel elements. The sintering of pressed compacts was performed in a conventional vacuum furnace using a heating rate of 5 °C min⁻¹ until the selected maximum temperature, a holding time of 50–173 min and a sintering pressure of 2–20 Pa. For comparison, a conventional prepared WC powder with 6.5 wt.% of stainless steel AISI 304 was also studied.During the sintering of the coated powders, three different sintering stages were identified: an initial one due to solid state matter transport until ∼1150 °C, followed by two other stages where liquid phase may be already present. Very high weight losses occurred during the sintering of coated powder which was diminished by the shortening of the holding time, the increase of the pressure in the sintering furnace and the appropriate control of the sintering atmosphere. Despite the high values of weight loss, 96% of densification can be obtained at a relatively low sintering temperature, T=1325 °C, for an initial content of ∼10 wt.% of binder phase.     

亚微米SiC粉体的氧化过程

从热力学和动力学角度研究了亚微米级SiC粉体的氧化过程 ,结果表明 :当温度低于 80 0℃ ,亚微米级的SiC粉体很难在空气中氧化 ;但在较高温度下 (90 0～ 12 0 0℃ )极易氧化 ,且服从抛物线速度方程 ,受氧气通过SiO2 氧化膜的内扩散控制 ,反应的平均表观活化能为 143.4kJ/mol。     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.