Microstructure and hardness scaling in laser-processed B4C-TiB2 eutectic ceramics

Surface layers of the pseudo-binary eutectic comprised of boron carbide (B₄C) and titanium diboride (TiB₂) were directionally solidified via direct laser irradiation in an argon atmosphere. The resulting surface eutectic layers had highly oriented lamellar microstructures, whose scale (i.e. interlamellar spacing) was controlled directly by the laser scan rate, following an inverse square root dependence for lower solidification velocities. Higher velocities (>∼4.2 mm/s) departed from this relationship, although well-ordered microstructures were still achieved. A concomitant increase in the Vickers hardness with decreasing interlamellar spacing was observed, although the trend did not correspond to traditional Hall-Petch behavior. The hardness of the eutectic composites became load-independent at indenter loads greater than 9.81 N, indicating a potential transition from plastic to fractural deformation during indentation. A Vickers hardness of 32 GPa was achieved in the highest solidification velocity samples (42 mm/s) which had interlamellar spacings of 180 nm.     

Study of processing routes for WC-MgO composites with varying MgO contents consolidated by FAST/SPS

Two different preparation routes were applied to process WC-MgO composites with varying MgO contents (4.1 wt.% and 5.9 wt.% MgO). WC-MgO powder mixtures were synthesized by a milling process at 600 rpm for 6 h of partially oxidized WC (WC + WO₃), Mg₃N₂ and C. Alternatively, WC and MgO as initial powders were used. For consolidation of the powder mixtures the field-assisted sintering technology (FAST) was used. X-ray diffraction shows that samples out of different powder mixtures and sintered between 1600 °C and 1750 °C exhibited WC, MgO and the W₂C phase independent of the preparation route of the powder mixtures. A higher density and better mechanical properties (hardness and indentation fracture toughness) of WC-MgO were achieved of pure WC and MgO as initial powders were consolidated by FAST. It was found that a lower MgO content results in higher hardness values and in a slightly decreased indentation fracture toughness.     

Mechanical properties and deformation mechanisms of B4C–TiB2 eutectic composites

Samples of B₄C–TiB₂ eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5 N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1 μm is about 31 GPa, and the indentation fracture toughness (5–10 N indenter load) of the eutectics is 2.47–4.76 MPa m^1/2. Indentation-induced cracks are deflected by TiB₂ lamellae, and indentation-induced spallation is reduced in the B₄C–TiB₂ eutectic compared to monolithic B₄C. Indentation-induced amorphization in monolithic B₄C and the B₄C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B₄C and B₄C–TiB₂ eutectics. Dislocations are observed in the TiB₂ phase of eutectics with an interlamellar spacing of 1.9 μm.     

Eutectoid WxC embedded WS2 nanosheets as a hybrid composite anode for lithium-ion batteries

A novel eutectoid structure, W_xC-embedded WS₂ nanosheets hybrids composite, was developed by hydrothermal reaction followed by a carbonization process. The fabricated WS₂–W_xC hybrid nanosheets electrode was used for lithium-ion batteries as an anode material, and demonstrated the specific capacity of 272 mA h·g^?1 at 0.01 A g^?1 with enhanced rate competence and cycling behavior when compared with individual WS₂ and W₂C electrode. While the large interlayer spacing in WS₂ facilitates rapid Li⁺ transport, the extremely high electronic conductivity of W_xC provides a highly conductive electron transfer pathway, which facilitates fast and reversible (de)lithiation reactions during charging and discharging. Further, these outcomes point the way for developing future eutectoid hybrid systems for advanced energy-storage applications.     

cBN–TiC and WC–Co Composites: Optimization of Co‐Extruding Conditions

In this study, double‐layer tubes with inner‐layer cubic boron nitride–titanium carbide (cBN–TiC) and outer‐layer tungsten carbide–cobalt (WC–Co) were fabricated by co‐extrusion. The Taguchi method was used to determine the optimal parameters, including extrusion ratio, half‐die angle, extrusion speed, and binder amount. The double‐layer tube made using these optimal parameters had an inner‐layer hardness of 3825 Hv and an outer‐layer hardness of 1849 Hv. The porosity of the double‐layer tube was 10.4%. Furthermore, the crystalline structures obtained from the inner and outer layers were cBN, TiC, TiB₂, and BNi and WC and Co₂C phases.     

Reaction Sintering of HfC/W Cermets with High Strength and Toughness

Hafnium carbide/tungsten (HfC/W) cermets were prepared by an in situ reaction sintering process, using hafnium oxide (HfO₂) and tungsten carbide (WC) as the raw materials. The reaction path, densification behavior, microstructure development, and mechanical properties of the cermets were comprehensively investigated. It was found that WC decomposed to tungsten semicarbide (W₂C) and tungsten (W) in sequence, and meanwhile HfC was formed by carbothermal reduction between HfO₂ and as‐released carbon from the dissociation of WC. The solid solution formation between HfC and W during sintering was also studied. The obtained cermets (>98% TD) have a Vickers' hardness of 8.16 GPa, a fracture toughness of 14.45 MPa m^1/2, and a high flexural strength of 1211 MPa.     

Surface and cross–section characteristics and friction–wear properties of high velocity oxy fuel sprayed WC–12Co coating

A WC–12Co coating was sprayed on H13 hot work mould steel using a high velocity oxy fuel (HVOF). The surface and cross–section morphologies, chemical compositions, and phases of obtained coatings were analyzed using a field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS), and X–ray diffraction (XRD), respectively. The friction–wear properties were investigated using a wear test, the wear mechanism of WC–12Co coating was also discussed. The results show that the WC–12Co coating primarily is composed of WC hard phase with high hardness and Co as a binder, which is evenly distributed on the coating surface, no atom–rich zones. There is no W₃O phase appearing in the HVOF spraying, showing that the WC–12Co coating has high oxidation resistance, the new phases of W₂C and C are produced due to the decarburization of WC. The coating thickness is ~200 μm, which is combined the substrate with the mechanical binding and local micro–metallurgical bonding. The average coefficient of friction (COF) of WC–12Co coating is 0.272, showing good friction performance, the wear mechanism is primarily abrasive wear, accompanied with fatigue wear.     

Mechanical properties of (TixW1−x)C–Co cermet prepared by carbothermal reduction of high energy ball milled TiO2–WO3–C

A series of solid–solution carbides, (Ti_xW_1−x)C (x=0.9, 0.8, 0.7, 0.6), was prepared by the high-energy milling of TiO₂–WO₃–C mixtures via subsequent carbothermal reduction. With high-energy milling, only the size reduction of the constituent powders was apparent without any chemical reaction. The milled mixture powder was transformed to a single–phase (Ti_xW_1−x)C solid solution by heat treatment in a vacuum at 1200 °C. (Ti_xW_1−x)C–Co cermets were consolidated by isothermal sintering at 1300, 1400, and 1500 °C. The powders were fully densified by liquid-phase sintering at 1500 °C because the Co melted at 1430 °C. The mechanical properties of the (Ti_xW_1−x)C–Co cermet (H_v: ~24 GPa) were significantly better than those of the conventional WC–Co (H_v: ~13 GPa) or TiC–Co cermets (H_v: ~16 GPa). The use of a solid–solution carbide instead of conventional WC almost doubled its hardness values without a loss of toughness. It is indicated that the improved hardness of the (Ti_xW_1−x)C–Co cermet originates from the high hardness of (Ti_xW_1−x)C, and the solid–solution carbide would be a valuable substitute for conventional carbide cermets.     

Effect of graphite content on properties of B4C‐W2B5 ceramic composites by in situ reaction of B‐Gr‐WC

Strip‐shaped W₂B₅ reinforced B₄C ceramic composites were prepared via in situ reaction of boron(B)‐graphite(Gr)‐WC system by powder metallurgy (P/M). In order to study the effect of the graphite content on the properties of the as‐fabricated ceramic composites, the powder mixture of B‐Gr‐WC with various amounts of Gr powder were blended and consolidated by spark plasma sintering (SPS). The sintering parameters were shown as following: sintering pressure was set as 30 MPa; The three‐step sintering temperature was 1100‐1550‐1700°C and the duration time was set as 5‐5‐6 minutes, respectively. In situ formed strip‐shaped W₂B₅ particles were dispersed homogeneously in B₄C matrix, which resulted in a remarkable improvement on the fracture toughness and mechanical properties. Appropriate 5vol% residual Gr in the composite shows positive effect on the mechanical properties which achieved an optimal counter‐balance of fracture toughness and hardness, the relative density was 99.8%, the Vickers hardness can reach 30.2 GPa, and the fracture toughness was 11.9 MPa·m^1/2 when the sintering temperature was set at 1700°C.     

Origin of the distinction in hardness of lower tungsten carbides: An experimental and ab initio theoretical study

The so-called lower tungsten carbides W₂C and Fe₃W₃C often appear in tungsten carbide–reinforced iron matrix (WC–Fe) composites. The effect of their presence on the mechanical properties of the material, such as hardness, is not well understood. In this study, we extensively measured the hardness distribution in the WC–Fe composites and also performed hundreds of microhardness measurements to determine the hardness values for the lower carbides. The hardness values calculated by ab initio were compared, where the theoretical values of Fe₃W₃C had little difference from the experimental values. However, the experimentally obtained hardness data for W₂C were significantly smaller than the reported theoretical data. Moreover, the experimental hardness values for W₂C reported in the literatures are very different. To understand the origin of the discrepancy, the focused-ion-beam–transmission-electron-microscopy technique was used to obtain the high-resolution images of W₂C, which revealed a high density of planar defects. As a further comparison, ab initio molecular dynamics simulations illustrate that the complex interactions between different atomic pairs in Fe₃W₃C make it difficult for this type of crystal defect to occur. The lattice of W₂C can adjust—resulting in shuffle defects and stacking faults—to a certain degree affecting its hardness, which was not revealed in previous studies.     

Nanoindentation of WC–Co hardmetals

WC–Co cemented carbide has been investigated using instrumented indentation with maximum applied loads from 0.1 to 10 mN. The hardness and indentation modulus of individual phases and the influence of crystallographic orientation of WC on the hardness and indentation modulus have been studied. The hardness of the Co binder was approximately 10 GPa and that of WC grains up to 50 GPa with relatively large scatter under the indentation load of 1 mN. Investigation of the role of crystallographic orientation of WC grains on hardness at 10 mN load revealed average values of H_ITbasal = 40.4 GPa (E_ITbasal = 674 GPa) and H_ITprismatic = 32.8 GPa (E_Itprismatic = 542 GPa), respectively. The scatter in the measured values at low indentation loads is caused by the effects of surface and sub-surface characteristics (residual stress, damaged region) and at higher loads by “mix-phase” volume below the indenter.     

Synthesis of Boron Carbide by Reactive‐Pulsed Electric Current Sintering in the Presence of Tungsten Boride

The method of pulsed electric current sintering (PECS) has been used to obtain dense boron carbide (B₄C) and B₄C‐based composite materials containing tungsten boride (W₂B₅). To elucidate the role of the sintering additives and the mechanism of reactive densification, three types of materials have been obtained by PECS at 1850°C and 1900°C: “pure” B₄C, B₄C doped with 10 wt% W₂B₅, and B₄C doped with 10 wt% tungsten carbide (WC). X‐ray diffraction and X‐ray photoelectron spectroscopy have been used to determine crystallite size, phase changes, and the peculiarities of the chemical bonds of the densified materials. Structural and mechanical properties of the materials have been investigated using scanning electron microscopy, optical microscopy, ultrasound velocity measurements, and hardness tests. The electrochemical impedance spectra have been used to investigate the electrical properties of the PECS‐ed materials.     

Mechanical properties up to 1900 K of Al2O3/Er3Al5O12/ZrO2 eutectic ceramics grown by the laser floating zone method

Directionally solidified Al₂O₃–Er₃Al₅O₁₂–ZrO₂ eutectic rods were processed using the laser floating zone method at growth rates of 25, 350 and 750 mm/h to obtain microstructures with different domain size. The mechanical properties were investigated as a function of the processing rate. The hardness, ∼15.6 GPa, and the fracture toughness, ∼4 MPa m^1/2, obtained from Vickers indentation at room temperature were practically independent of the size of the eutectic phases. However, the flexural strength increased as the domain size decreased, reaching outstanding strength values close to 3 GPa in the samples grown at 750 mm/h. A high retention of the flexural strength was observed up to 1500 K in the materials processed at 25 and 350 mm/h, while superplastic behaviour was observed at 1700 K in the eutectic rods solidified at the highest rate of 750 mm/h.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 ternary eutectic materials

Directionally solidified fibers and rods have been grown from the ternary Al₂O₃/Y₃Al₅O₁₂/ZrO₂ system using micro-pulling-down method. Fiber diameter could be varied 0.3 mm–2 mm at pull-rates ranging 6–900 mm/h and 500 mm in length. The ternary eutectic fibers had homogeneous colony patterned eutectic microstructures. The interlamellar spacing λ exhibited an inverse-square-root dependence on the growth speed v according to λ = 8 × v^−1/2, where λ has the dimension of μm and v is in μm/s. The tensile strength was recorded 1730 MPa at 25 °C and 1100 MPa at 1200 °C for a fiber crystals grown at a growth speed of 900 mm/h. Eutectic rods having 5 mm of diameter and up to 80 mm in length were also successfully grown by the micro-pulling-down method. The eutectic rods showed 1400 MPa of mechanical strength by compressive mode at 1500 °C with homogeneous colony microstructures.     

Effects of carbothermal prereduction temperature and Co content on mechanical properties of WC–Co cemented carbides

WC–Co cemented carbides were prepared via an in situ synthesis method, including the carbothermal prereduction of WO₃ and Co₂O₃ to remove all oxygen and a subsequent carbonization-vacuum sintering process. The experimental results revealed that as the prereduction temperature increased from 1000 to 1200°C, the grain sizes of WC in WC–6Co and WC–12Co cemented carbides increased from .91 to 1.09 and .97 to 1.19 μm, respectively. Further, the fracture toughness of the sintered WC–6Co and WC–12Co cemented carbides increased from 9.97 to 10.83 and 11.11 to 18.30 MPa m^1/2, respectively. In contrast, the hardness of the WC–6Co and WC–12Co cemented carbides decreased from 1477 to 1368 and 1351 to 1184 HV₃₀, respectively. For a given prereduction temperature, an increase in Co content can improve the fracture toughness while lowering the hardness. In addition, an increase in the prereduction temperature or Co content led to an increase in the grain size of WC, which resulted in a transgranular fracture as the dominant mode.     

Microstructural refinement and mechanical response of Al2O3–ZrO2 eutectics fabricated by a novel pulse discharge plasma assisted melting method

In the present work, microstructural refinement and mechanical response of Al₂O₃–ZrO₂ eutectics fabricated by a pulse discharge plasma assisted melting (PDPAM) method were investigated. The solidified microstructure evolves from polygonal eutectic colonies into irregular cellular colonies with increasing the superheating temperature of the melt from 1820 °C to 1900 °C. The average eutectic spacing inside the colonies decreases from 1.80 ± 0.10 μm to 0.25 ± 0.06 μm, and the coarse inter-colonial structure is refined, which is attributed to the increase in undercooling temperature. High-temperature microstructural stability of Al₂O₃–ZrO₂ eutectics is improved significantly as contrasted with the as-sintered ceramics. Besides, the load dependence of Vickers hardness for Al₂O₃–ZrO₂ eutectics is investigated.     

Microstructures and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic by laser floating zone melting with high temperature gradient

Directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic rods with high densities and low solidification defects are prepared by laser floating zone melting at solidification rate from 2 to 200 μm/s. The microstructure evolution, eutectic growth behavior and mechanical properties are investigated. At low solidification rates (<30 μm/s), the eutectic rods present a homogeneous irregular eutectic microstructure, whereas cellular microstructure containing regular lamella/rod structure is developed at higher solidification rates. The relationship is established between the eutectic interphase spacing and solidification rate, which follows the Magnin-Kurz eutectic model. The Vickers hardness (15.9–17.3 GPa) increases slightly with decreasing interphase spacing, but the fracture toughness (4.08 MPa m^1/2) shows little dependence with the solidification rate. Different crack propagation mechanisms are revealed among the indentation cracks. The flexural strength at ambient temperature reaches up to 1.14 GPa for the eutectic grown at 100 μm/s. The fracture surface analysis indicates that the surface defects are the main crack source.     

Fabrication of in situ elongated β-Sialon grains bonded to tungsten carbide via two-step spark plasma sintering

In order to reduce the difficulty of preparing binder-less cemented carbide and further broaden its application prospects, tungsten carbide toughened by in situ elongated β-Sialon grains was developed via sintering ball-milled WC and α-Si₃N₄ powders using Al₂O₃–ZrO₂ as a sintering aid and transformation additive. The two-step spark plasma sintering of the mixture at 1650 °C with dwelling at 1500 °C for 10 min was conducted under 30 MPa uniaxial pressure, and the densification behaviors, phase transformations, mechanical properties, and microstructures of the produced composites were investigated. The addition of Al₂O₃–ZrO₂ reduced the initial temperature of the densification process by approximately 100 °C and its final temperature by 200 °C (compared with the densification temperatures of pure WC and Si₃N₄ materials) and fully transformed α-Si₃N₄ to Sialon (Si–Al–O–N) phases. Microstructural characterization data showed that the WC matrix contained homogeneously distributed equiaxed and elongated β-Si₅AlON₇ grains. The WC composites containing in situ elongated β-Sialon grains exhibited an optimal hardness of 18.93 ± 0.03 GPa and enhanced fracture toughness of 10.43 ± 0.27 MPa m^1/2. The toughening mechanism of the β-Sialon phase involved the pull-out of elongated grains and crack bridging.     

Processing and Testing of RE2Si2O7 Fiber–Matrix Interphases for SiC–SiC Composites

Rare‐earth disilicates (RE₂Si₂O₇) are investigated for use as oxidation‐resistant alternatives to carbon or BN fiber–matrix interphases in ceramic matrix composites (CMC). Dense α, β, γ‐Y₂Si₂O₇, and γ‐Ho₂Si₂O₇ pellets were formed at 64 MPa and 1050°C–1200°C for 1 h using the field‐assisted sintering technique (FAST). Pellet modulus was measured using nanoindentation, and Vickers hardness was measured at loads of 100, 500, and 1000 g. The sliding stress of SCS‐0 SiC fibers incorporated in α‐, β‐, and γ‐RE₂Si₂O₇ matrices were measured by fiber push‐out. Deformation of RE₂Si₂O₇ after indentation and after fiber push‐out was characterized by TEM. Implications of the results for use of RE₂Si₂O₇ as a fiber–matrix interphase in CMCs are discussed.     

Effect of Co addition on microstructural and mechanical properties of WC–B4C–SiC composites

In this study, the effect of Co addition on microstructural and mechanical properties of WC-B₄C–SiC composites sintered by spark plasma sintering (SPS) method was investigated. For this purpose, three batches of WC-B₄C–SiC with different contents of Co (10 vol%, 15 vol%, and 20 Vol %) were sintered at 1400 °C. The results of X-ray diffraction (XRD) analysis of the samples indicated the formation of W₂B₅, W₃CoB₃ as well as the remained C phases and unreacted SiC phase. It was observed that by increasing the Co content, the amount of W₂B₅ phase reduces and W₃CoB₃ and C contents increase. Therefore, W₂B₅ peaks were not detected in the sample containing 20vol% Co. Relative density values above 97% were obtained for all the composites. However, a decrease was observed in relative density by increasing the Co content in the composites. The highest flexural strength (510 ± 42 MPa), fracture toughness (10.34 ± 0.82 MPa m^1/2), and hardness (20.63 ± 0.75 GPa) were also obtained for the sample containing 10vol% Co compared to the other samples. In addition, Transgranular fracture of SiC as well as pulling out of W₃CoB₃ and W₂B₅ particles were observed in the fracture surface micrographs of the samples. The presence of micro-cracks in the SiC grains, fracture of W₃CoB₃ grains, and crack deflection was reported as dominant toughening mechanisms.