Nanograined WC-Co Composite Powders by Chemical Vapor Synthesis
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INTRODUCTION
AMONG the hard alloys and refractory carbides, cemented tungsten carbide (WC) has some of the widest applications in such areas as metal working, drilling, and mining industries under severe conditions of high pressure and high temperature and corrosive environments thanks to its outstanding properties of high hardness and good wear resistance. However, its performance has been limited by brittleness and relatively low toughness compared with metal alloys. Cobalt is added as a ductile matrix to improve the fracture toughness, which is a function of microstructural variables including metal contents, grain size, and contiguity of WC grains. The mechanical properties such as hardness, compressive strength, and transverse rupture strength depend on the composition and microstructural parameters such as the cobalt contents and the grain size of WC.[1–3]
TAEGONG RYU and KYU SUP HWANG, Graduate Students, H.Y. SOHN, Professor, and ZHIGANG Z. FANG, Associate Professor, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] GILSOO HAN, formerly with the Department of Metallurgical Engineering, University of Utah, is Associate Professor, School of Materials, Mechanical & Automation Engineering, Yanbian Univerisity of Science and Technology, Beishan St., Yanji City, Jilin Province, P.R. China 133000. YOUNG-UGK KIM, formerly with the Department of Metallurgical Engineering, University of Utah, is Senior Researcher, Samsung SDI Co. Ltd, Gyeonggi-do, Korea 447731. M. MENA, formerly with the Department of Metallurgical Engineering, University of Utah, is Process Engineer with FS Precision Tech, LLC, Rancho Dominguez, CA 90221. Manuscript submitted June 22, 2007. Article published online January 25, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
Nanograined powders have been produced by various methods, such as the thermochemical spray drying process,[4,5] mechanical alloying (MA),[6,7] and chemical vapor condensation (CVC).[8] The chemical vapor synthesis (CVS) process, which has been applied to the synthesis of metallic and intermetallic powders at the University of Utah, has several advantages in producing nanograined powders having composition uniformity, high purity, and small grain size. The CVS process is carried out by the reduction of vapor-phase reactant precursors and subsequent carburization by the addition of gaseous carburization sources such as hydrocarbon. Tungsten metal salts such as tungsten hexachloride (WCl6),[9,10] tungsten hexafluoride (WF6),[11] and tungsten hexacarbonyl (W(CO)6)[12] are generally favored as precursors because of their relatively low volatilization temperatures as well as the ease of reduction by hydrogen. Several carburizing agents have also been used such as propane (C3H8), acetylene (C2H2), and methane (CH4). Methane is the most commonly used carburizing agent because it is easy to control the amount of carbon reacted and stable up to a high temperature. The objective of this research was to opt
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