Crystallization and characterization of substoichiometric compound (W 0.5 Al 0.5 )C 0.5 obtained by solid-state reaction
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INTRODUCTION
TUNGSTEN carbide (WC), as one of the hard materials, is now widely used in masonry drill bits, saw blades, cutting discs, sanding blocks, files, metal cutting tools, mining tools, and other hand tools. Moreover, in recent years, WC has replaced hard chromium as a coating material for aircraft landing gear, hydraulic actuators, jet engine bearings and bearing housings, turbine shafts, and even some dynamic components such as helicopter drive trains and propeller assemblies.[1] However, the high density of WC (15.63 g cm#3),[2] to some extent, is a weak point when used on the above-mentioned aviation engine and airframe components. Aluminum is lighter than tungsten metal, so the dissolution of Al into the lattice of WC to form a solid solution is expected to reduce its density. In addition, aluminum is less expensive than tungsten, and thus the material cost of ternary carbide Al-W-C is less than that of WC. In our previous work,[3,4] (W1-xAlx)C solid solution has been synthesized. (W1-xAlx)C crystallizes in the hexagonal structure of WC type. Al atoms partially substitute the lattice sites of W and are stable in the hexagonal structure at high temperature. The most noteworthy point is that the density of WC has been effectively decreased by the dissolution of Al. Unfortunately, the hardness of WC is decreased by the addition of Al—for example, the Vickers microhardness JUNMIN YAN, HUAGUO TANG, CHANGJUN ZHU, and SHUGUANG CAI, Ph.D. Students, XIANFENG MA, Professor, and WEI ZHAO, Engineer, are with the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted October 14, 2005. 1692—VOLUME 37A, MAY 2006
of (W0.5Al0.5)C is only about 1500 kg mm#2,[5] which is much lower than that of WC (2200 to 2500 kg mm#2).[6,7] Therefore, it is a challenge to increase the hardness of ternary carbide Al-W-C. It is reported[8,9] that vacancies are beneficial to increase the hardness of materials. For instance, the hardness of NbCx and TaCx increases consistently as the carbon vacancy concentration increases about 12 pct. Moreover, other researchers indicate that vacancies can increase the hardness of iron aluminides, where hardness is proportional to the square root of vacancy concentration.[10] In this sense, we began to work on the solid solution of Al in WC with many carbon vacancies, and expected that these vacancies would endow this type of material with good mechanical properties such as high hardness and wear resistance as comparable to WC. In this paper, we describe the synthesis of the pure (W0.5Al0.5)C0.5 substoichiometric compound, which has about 50 pct carbon vacancies as compared with WC, by the following two steps: (1) mechanical milling (MM) W0.5Al0.5 with an adequate quantity of carbon in an appropriate atmosphere for a certain time and (2) reactive sintering of the ball-milled powders at high pressure. W0.5Al0.5 was chosen as the precursor for the solid-state reaction with carbon because it has
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