Porous WC Research Articles

Fabrication and microwave absorbing property of WO3@WC with a core-shell porous structure

Special microstructure can significantly improve the microwave absorption property of rare materials. In this paper, porous WC powders were successfully synthesized by spray granulation method. Then, WO3@WC materials with core-shell porous structure can be prepared after 410 °C heating treatment at different time to form the outer WO3 oxidation layer. In addition, the microstructure, morphology, phase analysis and electromagnetic property were fully studied by investigating the WC-based materials in different structures. For WO3@WC core-shell porous materials, when the coating thickness was 2.1 mm, the maximum reflection loss can reach −19.4 dB at 12.6 GHz, which shows quite good microwave absorbing effects. The core-shell porous structure enhances the original microwave absorption performance due to the multiple reflection reflections and polarizations.

Production and Characterization of Austenitic Stainless Steel Cast Parts Reinforced with WC Particles Fabricated by Ex Situ Technique.

In this work, austenitic stainless steel specimens were locally reinforced with WC particles. The reinforcements were fabricated via an ex situ technique based on powder technology. Mixtures of WC, Fe, and M0101 binder were cold-pressed to obtain powder compacts. After debinding and sintering, the porous WC–Fe inserts were fixed in a mold cavity, where they reacted with liquid metal. Microstructural analysis was conducted for characterization of the phases constituting the produced reinforcement zone and the bonding interface. The results revealed that the reinforcement is a graded material with compositional and microstructural gradients throughout its thickness. The zone nearest to the surface has a ferrous matrix with homogeneously distributed WC particles and (Fe,W,Cr)6C and (Fe,W,Cr)3C carbides, formed from the liquid metal reaction with the insert. This precipitation leads to austenite destabilization, which transforms into martensite during cooling. A vast dissolution of the WC particles occurred in the inner zones, resulting in more intense carbides formation. Cr-rich carbides ((Fe,Cr,W)7C3, and (Fe,Cr,W)23C6) formed in the interdendritic regions of austenite; this zone is characterized by coarse dendrites of austenite and a multi-phase interdendritic network composed of carbides. An interface free of discontinuities and porosities indicates good bonding of the reinforcement zone to stainless steel.

Complex-shaped, finely-featured ZrC/W composites via shape-preserving reactive melt infiltration of porous WC structures fabricated by 3D ink extrusion

Complex-shaped, finely-featured, ultra-high-melting ZrC/W composite structures were produced by coupling, for the first time, three-dimensional (3D) ink-extrusion printing with shape/size-preserving reactive melt infiltration (the Displacive Compensation of Porosity, DCP, process). Inks containing sub-micron WC powders were printed at ambient temperature into either fine-scale structures (sub-millimeter filaments) or into a larger-scale, finely-featured 3D structure (a centimeter-scale nozzle with a sub-millimeter-thick wall). After organic binder removal, the printed structures were sintered at 1650 °C for 1 h to achieve a porosity of 50%. The porous, rigid WC structures then underwent ambient pressure infiltration and reaction with Zr-Cu liquid at up to 1350 °C for 2 h. The reaction of WC to yield more voluminous ZrC and W products filled prior pores (DCP process) to yield shape-preserved, dense ZrC/W structures after Cu dissolution from external surfaces. This new integrated approach opens the door to ultra-high-melting ceramic/metal composite structures with complex 3D shapes and fine (millimeter-scale) features.

Comparing the performance of different extruders in the Robocasting of biopolymer-nanoparticle composites towards the fabrication of complex geometries of porous Tungsten Carbide

Porous tungsten carbide (WC) is a material that combines a high strength to weight ratio and electroconductivity, which makes it a candidate for the creation of lightweight structures with the application as structural energy components, or SECs. However, machining complex geometries out of porous WC can be challenging and can lead to a significant waste of material. Here we present the use of robocasting to print geometries using paste featuring a biopolymer and tungsten oxide nanoparticles. Once printed, these geometries can be heat treated in an inert environment to derive porous WC. The benefit of using an additive manufacturing approach towards the production of porous WC is the possibility of producing complex geometric samples for industrial applications that cannot be created through traditional machining techniques. While the robocasting approach we reported before succeeded in producing WC samples, such a process led to the poor print quality of the desired geometries due to inconsistent extrusion flow rate throughout the printing process. The consistent flow generated by the new robocasting system proposed here has resulted in reduced layer irregularities, leading to the improved print quality of the WC precursor when compared to our previous approach. The production of consistent geometries will now allow for conclusive experimentation on the drying and carbonization processes and serves as a robust step forward to unlocking the potential of manufacturing porous WC SECs using robocasting.

Microstructure and formation mechanism in a surface carburized tungsten heavy alloy

A 95W–3.5Ni–1.0Fe–0.5Co heavy alloy was surface modified by pack carburization. A new carburized layer consisting of outside surface layer and inner surface layer was formed. The double layered microstructure showed porous WC aggregates in the outside surface layer and W grains coated by shell-like (Ni, Co, Fe)6W6C carbides in the inner surface layer. There were two paths for C atoms diffusion and reaction through W grains and matrix phase. Co changed the formation orders of carbides and acted as a medium for promoting C atoms reaction with Ni, Fe, Co and W in the matrix to form shell-like (Ni, Co, Fe)6W6C carbides in the inner surface layer. Meanwhile, C atoms diffused in the W grains and reacted with W in an order of W→W2C→WC. WC nucleated and grew in carburized W grains and micropores produced in the growing WC crystals due to the lattice expansion from W→W2C→WC. As the carburization process continued, the shell-like (Ni, Co, Fe)6W6C carbide was also carburized to WC, and finally the whole W grain was carburized into porous WC aggregates in the outside surface layer.

Evaluation of a worn out WC–Co–TiAlN cutting tool used in industry

In this work, a cutting tool with premature wear was characterized by Rockwell hardness test, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). In addition, electrochemical impedance spectroscopy (EIS) measurements were carried out in order to investigate the characteristic properties of the wear coating (TiAlN) of tool (WC–Co substrate). The substrate hardness was 90 HRA, which is in accordance with the observed microstructure of a cobalt matrix with tungsten carbides. SEM-EDS analysis confirmed the presence of a TiAlN coating and of typical features of an adhesive wear mechanism. EIS results revealed typical behavior of a porous WC–C substrate. Hence, premature wear of cutting tool could be explained in terms of an inadequate performance of the defects of coating deposited on the WC-Co substrate, as suggested by porosity revealed in the SEM images and the EIS measurements.

Microstructure and tribological properties of carburized 95W–3.5Ni–1.0Fe–0.5Co heavy alloy

Tungsten heavy alloys (WHAs) produced by powder technology are widely used for the mechanical manufacturing, electronic and defense components, etc. Tribological properties of these alloys need to be improved to meet the severe service conditions demanded. Carburization is a promising way to resolve this problem. In this work, microstructure and tribological properties of the carburized 95W–3.5Ni–1.0Fe–0.5Co heavy alloy were investigated in comparison with those of the untreated alloy. Results show that the carburized layer consists of a porous, outer WC layer and a modified W grain layer surrounded by Fe6W6C and Co6W6C at 970 °C, regardless of the carburizing time. The depth of the carburized layer linearly increases in a relatively short time and slightly increases during the subsequent period. Surface roughness increases with carburizing time. Carburization can stabilize friction coefficient and effectively improve the wear resistance of the tungsten heavy alloy due to its significantly increased hardness and non-deformability, but the porous structure in the WC layer has a negative influence on its wear resistance. The carburized layer is damaged in the porous WC layer in the form of the spalling of WC particles where there are some microcracks and micropores, accompanied with peeling due to the solid tribofilm being pushed away.

Effect of carburization on microstructure and rolling contact fatigue property of 95W–3.4Ni–1.6Fe heavy alloy

95W–3.4Ni–1.6Fe heavy alloy was carburized by pack carburization. Microstructure and hardness of the carburized alloy were investigated by SEM, EDS XRD. Effect of carburization on the rolling contact fatigue (RCF) property of the alloy was studied. The results showed that the carburized layer was composed of the outer, porous WC layer and the modified subsurface layer with each W grain surrounded by a WC shell. Carburization not only decreased the RCF performance of the alloy but also aggravated the wear of the counter balls. The untreated alloy was damaged by two modes of spalling and delamination under RCF condition. The subsurface main crack of the untreated alloy initiated where the maximum shear stress existed and preferentially propagated along the W–W interfaces. Spalling was the main failure mode of the carburized alloys, and the crumbling WC particles intensified the abrasion of the carburized surface.

Infiltration Behavior Of Mechanical Alloyed 75 wt% Cu-25 wt% WC Powders Into Porous WC Compacts

AbstractIn this work infiltration behavior of mechanical alloyed 75 wt% Cu – 25 wt% WC powders into porous WC compacts were studied. Owing to their ductile nature, initial Cu powders were directly added to mechanical alloying batch. On the other hand initial WC powders were high energy milled prior to mechanical alloying. Contact infiltration method was selected for densification and compacts prepared from processed powders were infiltrated into porous WC bodies. After infiltration, samples were characterized via X-Ray diffraction studies and microstructural evaluation of the samples was carried out via scanning electron microscopy observations. Based on the lack of solubility between WC and Cu it was possible to keep fine WC particles in Cu melt since solution reprecipitation controlled densification is hindered. Also microstructural characterizations via scanning electron microscopy confirmed that the transport of fine WC fraction from infiltrant to porous WC skeleton can be carried out via Cu melt flow during infiltration.

Production of net-shaped dense metal-carbide-based composites through an infiltration process at intermediate temperature

Tungsten composites are interesting materials for high temperature applications. These composites are conventionally produced by solid state reaction, where elevated temperatures and long sintering processes are inevitable, and generally produce volatile components that increase defects and impair the properties of the composites for high temperature applications. In the present paper, synthesis of dense near net-shaped transition metal–metal carbide by reaction casting is studied, which creates a potential application for the composite at 2000°C. Since the pores of the ceramic preform are filled with reaction products, this technique is referred to as displacive compensation porosity (DCP). The effects of sintering and infiltration parameters on composition of ZrC/W composites produced by DCP were studied. For this purpose, porous WC preforms were first uniaxially pressed and then sintered at 1400°C for 2h leading to preforms with various porosities (54 to 57%). Unidirectional and spontaneous infiltration of the sintered preform by ZrCu liquid was performed at 1200°C for 1, 2 and 3h. The resulting specimens were then analyzed via X-ray diffraction. Scanning electron microscopy (SEM) was used to examine the microstructure and diffusion of molten phase into porous preforms. The microstructures show that grains of the WC are being surrounded with infiltrated eutectic melt, thereby promoting reaction between the melts and the preform components. Furthermore, the extent of incorporation of ZrCu melt into the preform was investigated as a function of time. It was concluded that infiltration time has a limited influence on residual unreacted reagent and the percentage of the resulting product. On the other hand, improvement in wettability/contact angle with reduction in either particle size or presence of additive can increase the efficiency of the infiltration process.

Microstructure and mechanical properties of ZrC[sbnd]W matrix composite prepared by reactive infiltration at 1300 °C

Fully dense ZrCW matrix composite was fabricated by reactive infiltration technique, where molten Zr2Cu alloy infiltrated into a porous WC preform at 1300°C in vacuum. The ZrCW composite exhibits a core-shell structure feature: gray WC cores were enveloped by bright tungsten shells, which are homogeneously distributed in the ZrC matrix. X-ray diffraction and transmission electron microscopy investigation show that nano-sized ZrxCuyCz particles were formed in the ZrC matrix. The Vicker's hardness, flexural strength and fracture toughness of the ZrCW composite are measured as 14.5GPa, 615MPa and 11.7MPam1/2, respectively.

Porous Tungsten Carbide Nanoplates Derived from Tungsten Trioxide Nanoplates

Porous WC nanoplates (PWCPs) were synthesized by a nitridization‐carbonization process using WO3 nanoplates as the precursor for morphological control. The WO3 nanoplates were derived from tungstate‐based inorganic‐organic hybrid belts. The synthesis of PWCPs involved a nitridization reaction of WO3 nanoplates in NH3 at 650°C for 2 h, followed by a carbonization reaction in a CO/CO2 (10:1, v/v) mixture at 750°C for 10 h. The as‐obtained product is a pure hexagonal WC phase with a plate‐like and porous morphology. The aggregates of the PWCPs with dimensions of (100–300) × (10–30) nm form a house‐of‐card structure with open channels and high‐specific surface areas, and can act as potential supports of electrocatalysts for high‐performance fuel cells.

Preparation and Characterization of Ir Coating on WC Ceramic by Double Glow Plasma

Dense and adherent Ir coating was deposited on porous WC ceramic by double glow plasma (DGP). There were two cathodes in the vacuum deposition chamber. The bias voltage of Ir target and WC substrate were −900 and −350 V, respectively. The Ir coating was characterized using scanning electron microscopy, x-ray diffraction, nanoindentation instrument, and scratch tester to examine the microstructure and the mechanical properties. The results indicated that the deposition rate by DGP was up to 5-6 μm/h which was faster than that by metal-organic chemical vapor deposition and magnetron sputtering. The Ir coating had a preferential growth orientation of (220) crystal face. The hardness was 800 HV. The elastic modulus was 644 GPa. The excellent mechanical properties were attributed to the preferential growth, the large compressive stress, and the shrinkage of the lattice parameters. The adhesive force was up to 51 N. The strong adhesion was attributed to the mechanical locking and chemical reaction between the Ir coating and the porous WC substrate.

Reactive wetting and infiltration of polycrystalline WC by molten Zr 2Cu alloy

Good wettability with equilibrium contact angles of 41–28° was achieved for molten Zr2Cu alloy on dense polycrystalline WC substrates at 1050–1150 °C in an Ar atmosphere, favoring pressureless infiltration of the molten Zr2Cu alloy into a porous WC preform. Interfacial microstructure examination revealed that the Zr2Cu alloy reacted with WC, yielding W and ZrC at the interface through a solution–precipitation mechanism.

Near net-shape/net-dimension ZrC/W-based composites with complex geometries via rapid prototyping and Displacive Compensation of Porosity

ZrC/W-based composites with complex shapes have been fabricated by combining rapid prototyping methods for synthesizing porous WC preforms with the shape/dimension-preserving, reactive infiltration-based Displacive Compensation of Porosity (DCP) process. Two automated rapid prototyping methods were examined: (i) computer-numerical-controlled machining of porous WC powder compacts, and (ii) 3D printing of WC powder. After binder removal and partial sintering (to neck the WC particles), the shaped, porous, and rigid preforms were exposed to molten Zr 2Cu at 1150–1300 °C and ambient pressure. Upon infiltration, the Zr in the melt underwent a displacement reaction with WC to yield more voluminous ZrC and W products that filled prior pores (reaction-induced densification). The resulting ZrC/W-based composites retained the shapes and dimensions (to within 1%) of the WC preforms. This work demonstrates, for the first time, that rapid preform prototyping can be integrated with the DCP process to generate dense, ultrahigh-melting carbide/refractory metal composites with tailorable near net-shapes and -dimensions.

Near net-shape, ultra-high melting, recession-resistant ZrC/W-based rocket nozzle liners via the displacive compensation of porosity (DCP) method

Dense, near net-shaped ZrC/W-based composites have been fabricated at modest temperatures and at ambient pressure by a reactive infiltration process known as the Displacive Compensation of Porosity (DCP) method. Porous WC preforms with hourglass shapes (for rocket nozzle liners) were produced by gel casting, whereas simple bar-shaped preforms were produced by uniaxial pressing. The porous preforms were exposed to molten Zr2Cu at 1200–1300°C and ambient pressure. The Zr2Cu liquid rapidly infiltrated into the preforms and underwent a displacement reaction with the WC to yield a more voluminous mixture of solid products, ZrC and W. This displacement reaction-induced increase in internal solid volume filled the prior pore spaces of the preforms (“displacive compensation of porosity”) to yield dense, ZrC/W-based composites. Because the preforms remained rigid during reactive infiltration, the final composites retained the external shapes and dimensions of the starting preforms. A DCP-derived, ZrC/W-based nozzle insert was found to be resistant to the severe thermal shock and erosive conditions of a Pi-K rocket motor test. The DCP process enables dense, ceramic/refractory metal composites to be fabricated in complex and near net shapes without the need for high-temperature or high-pressure densification or for extensive machining (i.e., relatively expensive processing steps are avoided).

Formation of micronised WC from scheelite (CaWO 4)

A stoichiometric mixture of graphite and scheelite have been mechanically milled together for 40 h. The resultant powder, and an unmilled powder, have been examined by thermal analysis, isothermal annealing and X-ray diffraction to determine the effect of milling on the reduction of scheelite to tungsten carbide. The milled powder underwent a rapid reduction reaction at 800°C, 300°C lower than the unmilled powder. The reduction sequence was shown to be multistage with Ca 3WO 6, W 2C, and W all formed as intermediate phases prior to WC. The milled powder showed complete reduction to WC in 1 h at 1000°C, whereas the unmilled powder was incompletely reduced, even after 1 h at 1200°C. Unoptimised acid leaching of the reduced powder showed that separation of the final phases was readily achievable, leaving highly porous WC particles which were composed of grains around 200 nm in size.