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Abstract: Functionally graded tungsten carbide–cobalt (FG WC–Co) composites were fabricated using a carburization process that preferentially segregated cobalt away from the free surface, establishing a compositional gradient into the bulk of the material. The resulting surface, which is harder due to increased WC content, has potentially broad application in tools for rock drilling and metal machining. The microstructure and mechanical properties of FG WC-10wt.% Co and FG WC-16wt.% Co samples were compared to conventional WC–Co for hardness, wear, fracture toughness, transverse rupture strength (TRS), impact, compression strength, and compressive fatigue. The increased surface hardness of the functionally graded materials resulted in substantially improved wear resistance, from 40% to 80%, compared to their conventional homogeneous WC–Co counterparts at equivalent levels of hardness, strength, and fracture toughness. FG WC–Co also exhibited improved impact and compressive fatigue resistance in a cutting element with a dome-topped geometric shape. [Copyright &y& Elsevier]

Abstract: The sintering behavior of nanosized tungsten powder (25nm average size) synthesized by a thermal plasma process, in which ammonium paratungstate (APT) was reduced by hydrogen was investigated, compared with submicron W powder (0.5μm average size) and nanosized W powder (23nm average size) produced from the latter by high energy milling. The hardness of the compact of the plasma-synthesized powder (315 VHN) sintered at 1400°C was higher than that of submicron-sized powder (193 VHN) and was similar to that of the milled powder (309 VHN). The plasma-synthesized powder, however, yielded a compact with much lower tendency to form cracks than the milled powder. [Copyright &y& Elsevier]

Functionally graded WC-Co (FG WC-Co) offers enhanced property combinations and superior engineering performance compared to conventional homogeneous WC-Co. However, production methods face challenges in creating the graded microstructure, cost-effectiveness, and scalability. A versatile carburizing technique has been developed to address these challenges and to apply FG WC-Co to various WC-Co grades and applications. This paper reviews the carburizing technique's underlying principles and mechanisms of a one-step to a two-step process for treating off-the-shelf homogeneous products. Key two-step process parameters such as temperature, time, and carbon potential, which influence the control of the Co gradient, free carbon depth, and Co build-up, are highlighted. Both laboratory findings and field test data suggest that by optimizing these parameters, the carburizing technique can produce desired results, thus broadening the potential industrial applications of both the process itself and the FG WC-Co. • A viable carburizing heat treatment process for manufacturing functionally graded WC-Co (FG WC-Co) is described. • Co gradient structure can be effectively tailored by optimizing the carburizing process parameters. • The potential industrial applications of FG WC-Co materials were demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation, nanocrystalline tungsten powders prepared by either chemical method or high energy mechanical milling were sintered to study the size-dependence of the densification of tungsten powders. Results show that nanocrystalline tungsten powders produced via an ultra-high energy milling process have significantly enhanced sinterability compared to conventional submicron- or micron-sized powders. This study for the first time shows nanocrystalline tungsten powder can be sintered via a pressureless process to near-full densification at a temperature as low as 1100°C. [Copyright &y& Elsevier]

Abstract: Nanosized tungsten carbide–cobalt composite powder was synthesized by a thermal plasma process, in which ammonium paratungstate (APT) and cobalt oxide were reacted with a gas mixture containing CH4, H2, and Ar. The reduction and carburization of vaporized precursors produced nanosized tungsten carbide (WC1−x )–cobalt composite powder, which contained small amounts of W2C and/or W phase. The particle size of synthesized tungsten carbide powder was less than 20nm. The synthesized composite powders were also subjected to a hydrogen heat treatment to fully carburize WC1−x , W2C, and W phases to the WC phase as well as to remove excess carbon in the product. Finally nanosized WC–Co composite powder of particle size less than 100nm was obtained. [Copyright &y& Elsevier]

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 (WC1− 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 (WC1− x) powder was <20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC1− 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. [ABSTRACT FROM AUTHOR]

Abstract: Nanosized tungsten powder was synthesized by a thermal plasma process using ammonium paratungstate (APT) as the precursor. The injected precursor was vaporized in the plasma flame and the subsequent reduction of the vaporized precursor produced nanosized tungsten powder. The effects of H2 concentration, plasma torch power, and the flow rate of plasma gas on the product properties were investigated and particle size was determined. From the results, nanosized tungsten powder of particle size less than 50nm was obtained by the thermal plasma process. [Copyright &y& Elsevier]

Abstract: Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen contents. In this work, the hydrogen release and uptake properties of Mg/5%Ti nanopowder mixture prepared by a chemical vapor synthesis (CVS) process were investigated. Samples were made in a CVS reactor, in which reactant powders were fed into evaporator placed inside the reactor by means of specially designed powder feeders. The produced Mg/5%Ti was hydrogenated in an autoclave under 10.3MPa pressure and 150°C for 12h. Thermogravimetric analysis (TGA) showed that 5.2wt% of hydrogen began to be released from 190°C while temperature was increased at a heating rate of 5°C/min up to 350°C under an argon flow. This onset temperature of Mg–Ti nanopowder dehydrogenation was much lower than that of MgH2 alone, which is 381°C. In addition, the activation energy of dehydrogenation was 104kJ/mol, which is much smaller than that of as-received MgH2 (153kJ/mol). [Copyright &y& Elsevier]

Abstract: Nanocrystalline WC–Co materials have been the subject of interests and focus of research programs around the world for the past two decades owing to the expectations that the mechanical behavior of the material may improve significantly when grain sizes reduce to nanometer scale. However, although numerous technologies are available for making nanosized tungsten carbide powders, obtaining true nanocrystalline WC–Co (average WC grain size <100nm) has been a great challenge due to the difficulties of controlling grain growth during sintering. Evaluation of the mechanical properties of nanocrystalline WC–Co materials is also difficult because there is little published data that are based on specimens with truly nanoscale grain sizes. In this review, the challenges and results of sintering nanocrystalline WC–Co powders will be examined as well as the various technologies for producing nanosized tungsten carbide powders. It will be discussed that the key challenge to the production of bulk nanocrystalline cemented tungsten carbide materials is to control the rapid grain growth during the early stage of sintering. The current understanding on the mechanical properties of cemented tungsten carbide made from nanoscaled WC–Co powders will also reviewed. [Copyright &y& Elsevier]