Synthesizing of nanocomposite WC/MgO powders by mechanical solid-state reduction and subsequent plasma-activated sinteri
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I. INTRODUCTION
SINCE 1970, many materials scientists and metallurgists have been intrigued by the development of the mechanical alloying (MA)[1] method for preparing numerous advanced materials with unique properties and structures.[2,3] Amorphous alloys,[4–7] nanostructured metal nitrides,[8] metal hydrides,[9] metal carbides,[10] and, more recently, nanocomposite materials[11] are some typical examples of the advanced engineering materials that are obtained by MA, using ball-milling and/or rod-milling[12] techniques. The MA method has been also employed for reducing some metal oxides,[13] using a technique known as mechanical solidstate reduction (MSSR).[14] Among hard alloys, WC alloys find wide industrial applications as tips for cutting tools and wear-resistant parts. The intrinsic resistance to oxidation and corrosion at high temperatures also makes them desirable as a protective coating for devices at elevated temperatures. In the industrial scale of production, WC is prepared by direct alloying of M. SHERIF EL-ESKANDARANY, formerly Visiting Professor, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan, is Associate Professor of Materials Science and Physical Metallurgy, Mining and Petroleum Engineering Department, Faculty of Engineering, Al Azhar University, Nasr City 11371, Cairo, Egypt. M. OMORI, Research Associate, T. HIRAI, Professor, Department of High-Temperature Materials Science, T.J. KONNO, Research Associate, and K. SUMIYAMA, Associate Professor, Department of Chemical Physics of Non-Crystalline Materials, are with the Institute for Materials Research. K. SUZUKI, Professor, formerly Dean, Institute for Materials Research, Tohoku University, is Executive Senior Advisor, Sumitomo Metal Industries Ltd., Advanced Materials Research, Amagasaki 660-0891, Japan. Manuscript submitted March 18, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the elements at temperatures from 1673 to 1873 K.[15] Accordingly, the high cost of preparation is a disadvantage of this process. It has been shown by El-Eskandarany et al.[14] that nanocrystalline WC can be obtained by reducing WO3 through ball milling this material in the presence of Mg and graphite. The Mg reduces the oxide, and the resulting metallic W reacts with graphite to form WC. The oxide phase of MgO is simply removed by selective leaching. Mi et al. have reported the formation of fine-grained WC by ball milling the elemental powders of W and C.[16] They have also shown the possibility of synthesizing composite WC-Co by milling their previously reacted WC particles with elemental Co powder. However, because the hexagonal phase of the WC alloy possesses a high hardness (23 GPa),[14] it is rarely used in industrial applications in its native form. This is due to its relatively poor shock resistance, high values of elastic moduli and bulk density, and low fracture toughness. Thus, a binding material (usually Co) is often added to WC during sintering in order to improve the mechanical properties. Co is desirable, because it will wet
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