Ultrafine WC-10Co cemented carbides fabricated by electric-discharge compaction
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This research investigates the microstructure and mechanical properties of ultrafine WC-10Co cemented carbides fabricated by an electric-discharge compaction (EDC) process, from powder synthesized by a spray-conversion process (SCP). Due to a short holding time during EDC, a grain size as small as 120 nm can be achieved. We also found that dispersion of pores in WC-Co cemented carbides may contribute to fracture toughness, besides the bridging ligament mechanism.
I. INTRODUCTION
WC-Co cemented carbides are widely used for cutting tools, rock drill tips, and other wear-resistant materials, due to a combination of desirable high hardness and moderate fracture toughness. Recently, nanocrystalline ceramics sintered at lower temperatures than that of conventional coarse-grained counterpart are found to reveal higher hardness and good fracture toughness.1 Based on this knowledge, it is of great interest to explore optimizing fabrication methods for ultrafine and even nanocrystalline WC-Co cemented carbides with excellent mechanical properties. It is expected that researchers will find more extensive industrial applications. Nowadays, nanocrystalline WC-Co powders have been synthesized by mechanical alloying and thermochemical process.2 However, the WC grain size in sintered WC-Co cemented carbides becomes much larger than that in pre-sintered powders during conventional liquid-phase sintering.3,4 The grain growth in WC-Co cemented carbides is controlled by a 2-D nucleation mechanism,5–7 which induces abnormal grain growth.8 Furthermore, grain growth of nanocrystalline WC-Co powders can occur at 1073 K,9 below the eutectic temperature of W-C-Co system. So, in liquid-phase sintering of fine WC-Co powders, large-grained WC can form at solid-state sintering region. Larger WC can be obtained during subsequent liquid-phase sintering.10 Great importance should be attached to kinetically inhibited grain growth induced by conventional consolidation of nanocrystalline WC-Co powders. Some attempts have been reported, such as spark-plasma sintering (SPS) and using grain-growth inhibitors.11–14 If the heating rate is a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0324 2240
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 8, Aug 2004 Downloaded: 15 Mar 2015
high enough to avoid solid-state sintering, and if the cooling rate is high enough to keep holding time as short as possible, then the grain growth of WC may be restricted. We think that electric-discharge compaction (EDC) is a promising route. In this EDC process, an insulating tube is filled with the powder to be pressed. Consolidation of powders under external pressure is achieved by using a high-voltage and high-density current-pulse treatment.15–19 Therefore, the ultra-short discharge time restricts the growth of grains; meanwhile, it is long enough for powder compaction under the external pressure. Although this method has been used to compact metallic and intermetallic powders into the corresponding nanocrystalline
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