Plasma Synthesis of Tungsten Carbide Nanopowder from Ammonium Paratungstate

A thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC_{1− x} ) powder, which sometimes contained a small amount of W₂C phase. The effects of reactant gas composition, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H₂) on the product composition and particle size were investigated. The produced tungsten carbide (WC_{1− x} ) powder was <20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC_{1− x} and W₂C phases to the WC phase as well as to remove excess carbon. Finally, WC powder of particle size <100 nm was obtained.     

In Situ Synthesis and Microstructures of Tungsten Carbide-Nanoparticle-Reinforced Silicon Nitride-Matrix Composites

A W₂C-nanoparticle-reinforced Si₃N₄-matrix composite was fabricated by sintering porous Si₃N₄ that had been infiltrated with a tungsten solution. During the sintering procedure, nanometer-sized W₂C particles grew in situ from the reaction between the tungsten and carbon sources considered to originate mainly from residual binder. The W₂C particles resided in the grain-boundary junctions of the Si₃N₄, had an average diameter of ∼60 nm, and were polyhedral in shape. Because the residual carbon, which normally would obstruct sintering, reacted with the tungsten to form W₂C particles in the composite, the sinterability of the Si₃N₄ was improved, and a W₂C–Si₃N₄ composite with almost full density was obtained. The flexural strength of the W₂C–Si₃N₄ composite was 1212 MPa, ∼34% higher than that of standard sintered Si₃N₄.     

Low-Temperature Chemical Vapor Deposition Tungsten Carbide Coatings for Wear/Erosion Resistance

A new chemical vapor deposition (CVD) process has been developed to deposit hard coatings, containing tungsten carbide, at temperatures below 500°C. These coatings, which have been applied to both ferrous and nonferrous alloys, exhibit excellent resistance to wear and erosion. The coatings comprise a mixture of tungsten and the tungsten carbide, the latter being present as W₂C, W₂C + W₃C, or W₃C. The coatings' composition and properties can be controlled by varying the CVD process parameters. The unique lamellar, fine-grained microstructures of these coatings contribute to their good tribological properties.     

Phase Equilibria in the System Tungsten—Carbon

Quenching from high temperatures, supplemented by differential thermal analysis, has shown that the tungsten-carbon binary is characterized by eutectic temperatures of 2710° and 2760°C between W and W₂C, and between W₂C and a new high-temperature phase (β-WC), respectively. Carbon solubility in excess of stoichiometric W₂C is evident only at 2525°C, the eutectoid temperature between W₂C and WC. W_2.35C melts congruently at 2795°C. A new fee phase (β-WC), stable only above 2525°, has been discovered between W₂C and α-WC. The cubic phase is formed by a periteci reaction at approximately 2785°C and has a broad homogeneity range ear the solidus. The phase, α-WC, decomposes into β-WC and C at 2755°C, approximately 25°C below the melting temperature.     

Synthesis-Fabrication of an Al2O3-Tungsten Carbide Composite

A composite of 70 vol% Al₂O₃ and 30 vol% tungsten carbide was formed by hot-pressing. Simultaneously carbon reacts with an intimate mixture of WO₃ and Al₂O₃ to form a dense body. The composite approached theoretical density; a 1- to -10-μm carbide phase was uniformly dispersed in a 2-μm Al₂O₃ matrix. Maximum density and fine-grained microstructure were obtained when pressure was applied during heating from 1200° to 1600°C and temperature and pressure were then maintained for 20 min. At an initial ratio of 2.8 and 2.9 mol C/mol WO₃, the tungsten appeared as free W, WC, and W₂C. For C/WO₃=3.0 to 3.6, mixtures of W₂C and WC were present, whereas for C/WO₃>3.6, free C appeared with WC. The effects of the hot-pressing parameters are discussed.     

Synthesis of Compositionally Homogeneous Sr(CuxZn1−x)1/2W1/2O3

A complex perovskite of Sr(Cu _x Zn_{1- x} )_1/2 W_1/2O₃ (SCZW) is synthesized by a new combination of wet and dry processess. Mixed oxides containing Cu²⁺ and Zn²⁺ (CZ) are prepared by the wet process (coprecipitate method). SCZW is obtained by the dry process (mixed-oxide method) from a mixture of CZ, SrCO₃, and WO₃. SCZW has practically no compositional, unlike solid solutions prepared by the conventional dry method. The wet–dry process method is useful because the wet process is applied to only B-site cations having the same valence.     

Mechanism and Kinetics for the Formation of Uranium Mononitride by the Reaction of Uranium Dioxide with Carbon and Nitrogen

The mechanism and kinetics of UN formation by reaction of a pellet of mixed UO₂ and C with N₂ were studied for temperatures of 1420° to 1750°C. The reaction followed the first-order rate equation; the activation energy was 83 kcal/mol. Only UN_1−xC _x was produced. The lattice parameter variation of UN_1−xC _x had a minimum and a maximum during reaction; at the maximum, UN_1−xC _x + C + N₂ were in equilibrium. The overall reaction was divided into four stages: (1) formation of UN_1−xC _x from UO₂, (2) decarburization of UN_1−xC _x , (3) formation of UN_1−xC _x with the equilibrium composition, and (4) pure UN formation. The lowest reaction rate was in stage (4).     

Hardness of Polycrystalline Tungsten and Molybdenum Oxides at Elevated Temperatures

Vickers hardness of WO₃, W₁₈O₄₉, and MoO₂ is reported for temperatures up to 800°C. Polycrystalline samples of the oxides were prepared by hot-pressing, and hardness was determined using a Vickers hardness tester modified for high-temperature applications. The hardness of a heavily deformed tungsten rod was also measured as a reference.     

Solid-State Diffusion Bonding of Carbon–Carbon Composites with Borides and Carbides

Solid-state diffusion bonding of carbon–carbon (C─C) composites by using boride and carbide interlayers has been investigated. The interlayer materials used in this study were single-phase borides (TiB₂ or ZrB₂), eutectic mixtures of borides and carbides (ZrB₂+ ZrC or TiB₂+ B₄C), and mixtures of TiB₂+ SiC + B₄C produced in situ by chemical reactions between B₄C, Ti, and Si or between TiC, Si, and B. The double-notch shear strengths of the joints produced by solid-state reaction sintering of B₄C + Ti + Si interlayers were much higher than those of joints produced with other interlayers. The maximum strength was achieved for C─C specimens bonded at 2000°C with a 2:1:1 mole ratio of Ti, Si, and B₄C powders. The reaction products identified in the interlayers, after joining, were TiB₂, SiC, and TiC. The joint shear strength increased with the test temperature, from 8.99 MPa at room temperature to an average value of 14.51 MPa at 2000°C.     

Synthesis, Microstructure, and Mechanical Properties of Al3BC3

In situ synthesis of bulk Al₃BC₃ was achieved via a reactive hot-pressing method using Al, B₄C, and graphite powders at 1800°C for 2 h. The reaction path for synthesizing Al₃BC₃ was investigated. It was found that Al₃BC₃ formed via the reaction of C, B₄C, and Al₄C₃ above 1180°C. Dense Al₃BC₃ was prepared with a little B₄C and graphite remained. Microstructure observations revealed the plate-like morphology of Al₃BC₃ grains. Moreover, the mechanical properties of Al₃BC₃ were characterized (Vickers hardness of 11.1 GPa, bending strength of 185 MPa, fracture toughness of 2.3 MPa·m^1/2, and Young's modulus of 163 GPa). Young's modulus decreased slowly with increasing temperature, and at 1600°C remained 79% of that at ambient temperature. These results show that Al₃BC₃ is a promising lightweight high temperature structural material.     

Low-Temperature Densification of TiN–TiB2 Composites Through Reactive Hot Pressing with Excess Ti Additions

Reactive hot pressing of Ti and BN powder mixtures is used to produce dense TiN _x –TiB₂ composites. The effect of excess Ti along with a small addition, ∼1 wt% Ni, on the reaction and densification of the composite was investigated. A composite of ∼99.9% relative density (RD) was produced at 1200°C at 40 MPa for 30 min with 1 wt% Ni, whereas composites produced without Ni are porous and contain residual reactants. The microstructural studies on composite samples with excess Ti produced at short durations indicate the presence of a transient (Ni–Ti) phase from which Ti is finally removed to form substoichiometric TiN _x . The hardness of the dense TiN _x –TiB₂ composite is ∼22 GPa. The densification mechanism in this system is contrasted with the role of nonstoichiometry in the Zr–B₄C system.     

Preparation of Tungsten Cobalt Carbides and Oxide via an Organometallic Heterobimetallic Complex as a Single-Source Precursor

Tungsten cobalt carbides and oxides can be obtained via a single-step pyrolysis of an organometallic single-source precursor (eta⁵-C₅H₅)(CO)₃WCo(CO)₄ (1). Pyrolysis of 1 in an oxygen atmosphere produced WCoO₄ at 600°C. In a nitrogen atmosphere W₆Co₆C was obtained when 1 was heated at 700°C. However, under vacuum, the pyrolysis of 1 produced the other phase-W₃Co₃C-at 700°C. Both carbides were contaminated with graphitic carbon, as indicated by their ESCA spectra. Powders that were obtained by using these procedures had particle sizes of up to 100 µm. Micrography showed that the particles were porous, which indicated outgassing during pyrolysis.     

The Effect of Stoichiometry on Mechanical Properties of Boron Carbide

The mechanical properties of chemically vapor-deposited boron carbides (B₄C) with varied B/C ratios were investigated as a function of composition. The maximum hardness, H, and fracture toughness, K_1c, were observed at an almost stoichiometric composition. For nonstoichiometric B₄C (B/C>4), H and K_1c decreased with increasing B content, suggesting that excess B diminishes the bond strength in the B₄C structure. The decrease in H and K_1C at B /C <4 was attributed to free C in the microstructure.     

Elevated-Temperature Toughness and Hardness of a Hot-Pressed Al2O3—WC—Co Composite

The fracture toughness and hardness of an Al₂O₃80WC10Co composite were investigated in air at elevated temperatures. The primary phases in the composite were WC, α-Al₂O₃, and Co₃W₃C, but small amounts of Co and C (graphite) appeared at elevated temperatures, related to decomposition of the Co₃W₃C phase. The fracture toughness of the composite was constant with increasing temperature up to 330°C and then increased in the range 400° to 550°C. A transition of brittle to ductile behavior occurred at about 700°C. The enhancement of fracture toughness at elevated temperature is attributed to the decomposition of Co₃W₃C to Co and C, and enhanced crack deflection and bridging. Decreases in hardness with increasing temperature are attributed to the softening of WC matrix and decomposition of Co₃W₃C.     

Al-B-C Phase Development and Effects on Mechanical Properties of B4C/Al-Derived Composites

B₄C/A1 offers a family of engineering materials in which a range of properties can be developed by postdensiflcation heat treatment. In applications where hardness and high modulus are required, heat treatment above 600°C provides a multiphase ceramic material containing only a small amount of residual metal. Heat treatment between 600° and 700°C produces mainly A1B₂; 700° and 900°C results in a mixture of A1B₂ and A1₄BC; 900° and 980°C produces primarily A1₄BC; and 1000° to 1050°C results in A1B₂₄C₄ with small amounts of A1₄C₃ if the heating does not exceed 5 h. Deleterious A1₄C₃ is avoided by processing below 1000°C. All of these phases tend to form large clusters of grains and result in lower strength regardless of which phase forms. Toughness is also reduced; the least determinal phase is A1B₂. The highest hardness (88 Rockwell A) and Young's modulus (310 GPa) are obtained in Al₄BC-rich samples. AlB₂-containing samples exhibit lower hardness and Young's modulus but higher fracture toughness. While the modulus, Poisson's ratio, and hardness of multiphase B₄C/A1 composites containing 5–10 vol% free metal are comparable to ceramics, the unique advantage of this family of materials is low density (>2.7 g/cm³) and higher than 7 MPa-m^1/2 fracture toughness.     

The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites

Carbon nanofiber (CNF)-dispersed B₄C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B₄C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σ_b) of ∼710 MPa, a Vickers hardness ( H _v) of ∼36 GPa, a fracture toughness ( K _{I C} ) of ∼7.9 MPa m^1/2, and high-temperature σ_b of 590 MPa at 1600°C in N₂. Interfaces between the CNF and the B₄C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy.     

BxO: Phases Present at High Pressure and Temperature

Boron suboxide compounds are of interest because of their low densities coupled with high hardness. In the present study, we have attempted to determine the nature of the B _x O phases that occur in the field defined by pressures of zero to 1.5 GPa, temperature between 1200° and 1700°C, and the compositional range 2/3 x 24. Amorphous boron powder and boric acid B₂O₃ were the starting reactants for all the runs. The processing of the specimens was carried out in a controlled atmosphere furnace, a hot-pressing assembly, and in a piston—cylinder high-pressure apparatus within quasi-hydrostatic and inductively heated cell assemblies. After processing at elevated temperature and pressure, for compositions over the range 2/3 x 6, B₂O₃ (identical to the hexagonal starting material) and B₆O ( R 3¯ m ) were the dominant phases present. For the compositions 6 x 24, B₆O and rhombohedral B were the primary phases identified. In general, the hardness of the processed composites was dominated by the occurrence of B₆O (approximately equivalent to B₄C). However, there is some suggestion of particularly high values of hardness on a very localized scale in specimens near the B₂₂O composition.     

Fabrication and Mechanical Properties of Al2O3-WC-Co Composites by Vacuum Hot Pressing

Al₂O₃-WC-Co composites were fabricated by vacuum hot-pressing mixtures of Al₂O₃, WC, and cobalt powders. The phases formed with WC additions of up to 40 wt% were α-Al₂O₃, WC, Co₃W₃C, and small amounts of f-Co (face-centered cubic cobalt) and carbon (graphite); no cobalt or carbon phases formed at >40 wt% WC. A more-uniformly distributed and connected WC matrix formed as the WC content increased. The 10Al₂O₃-80WC-10Co (in wt%) composite exhibited high bending strength (1250 MPa), fracture toughness (9 MPam^1/2), and hardness (20.6 ± 0.5 GPa) simultaneously. The high bending strength was mainly attributed to fewer fracture origins due to the uniformly distributed and connected WC matrix together with a lower porosity. Increased fracture toughness was caused mainly by crack deflection and crack bridging in a uniformly connected WC matrix. High hardness resulted from finer WC metallic compounds and Co₃W₃C precipitation in almost all ranges.     

Synthesis of (Mg,Si)Al2O4 Spinel from Aluminum Dross

The spinel (Mg,Si)Al₂O₄ was synthesized from aluminum dross using an induction synthesis method. X-ray diffraction analyses on products formed at different temperatures provided an understanding of the formation mechanism of the spinel. After removal of soluble components, the induction heating of the dross resulted first in the oxidation of some of the AlN component and the subsequent formation of the spinel by the following reaction: x SiO₂+ (1− x )MgO + [1−( x /3)]Al₂O₃+ (2 x /3)AlN = (Mg_{1− x} ,Si _x )Al₂O₄+ ( x /3)N₂( g ).     

Densification and Mechanical Properties of B4C with Al2O3 as a Sintering Aid

The densification behavior and mechanical properties of B₄C hot-pressed at 2000°C for 1 h with additions of Al₂O₃ up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al₂O₃. The improvement was attributed to the enhanced mobility of elements through the Al₂O₃ near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al₂O₃ exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B₄C and Al₂O₃.