ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2013, Vol. 87, No. 4, pp. 654–661. © Pleiades Publishing, Ltd., 2013. Original Russian Text © A.S. Kurlov, 2013, published in Zhurnal Fizicheskoi Khimii, 2013, Vol. 87, No. 4, pp. 664–671. 654 INTRODUCTION The nanocrystalline powders of tungsten carbide WC are considered to be the starting material for pre- paring nanocrystalline hardmetals with decreased sin- tering temperatures and improved mechanical proper- ties [1]. There are many methods for preparing the nanocrystalline WC powders [2, 3]. Among them, two procedures are of interest: high-energy grinding of coarse-grained powder of tungsten carbide WC (top- to-bottom nanotechnology) [4] and plasmochemical synthesis from tungsten oxide and hydrocarbon (bot- tom-to-top nanotechnology) [5]. Tungsten carbide WC powder is used in the produc- tion of widespread hardmetals of the WC–Co system. The maximum sintering temperature of these hard- metals from coarse powders by the traditional proce- dure in vacuum and a protective or inert atmosphere is 1380–1420°C. Studies of the oxidation of WC pow- ders showed that a decrease in the partial size led to a decrease in the temperature of the start of oxidation and growth of the oxidation rate [6]. Therefore, it is important to study the thermal stability of the size and composition of WC nanoparticles during annealing at temperatures of up to 1400°C. We investigated the effect of the vacuum annealing temperature on the chemical and phase compositions and the size of WC nanoparticles. EXPERIMENTAL Vacuum annealing was performed on three pow- ders of hexagonal WC (space group ) with differ- 6 2 P m ent dispersities stored in air: microcrystalline WC- coarse powder (Kirovgrad Hard Alloys Plant, OAO KZTS, Kirovgrad, Russia) with particles with a mean size of 9 μm; nanocrystalline WC-mill powder with particles with a mean size of 20 nm obtained by grind- ing WC-coarse powder in a Retsch PM 200 planetary ball mill; and nanocrystalline WC-plasm powder with particles with a mean size of 60 nm synthesized at OAO VNIIETO (Moscow) by the plasmochemical method from tungsten oxide WO 3 and propane C 3 H 8 in a low- temperature hydrogen plasma with subsequent ther- mochemical treatment (the synthesis procedure is described in [5]). The WC powder grinding conditions are described in [4, 7]. The mixture of WC-mill nan- opowder was also annealed with 2 wt % of carbon (MT-900 soot); the mixture is called below WC- mill+C. The freely poured WC powders (2–3 g) were annealed in a vacuum of 0.0013 Pa (10 –5 Torr). The annealing included slow (1–2 h depending on the maximum annealing temperature T ann ) heating to T ann , storage at T ann for 1 h, and cooling in the furnace. The temperatures were 400, 600, 800, 1000, 1200, and 1400°C. Only the starting WC powders were used in each annealing experiment. The phase composition of the WC powders before and after the annealing were studied by X-ray diffrac- tion in the range 2θ = 30°–125° at a step of 0.03° and scanning time 2 s per point (DRON-UM1 diffracto- meter, radiation). The X-ray diffraction pat- terns were numerically analyzed with the X’Pert Plus program (version 1.0, Philips Analytical B.V.). α 1,2 K Cu PHYSICAL CHEMISTRY OF NANOCLUSTERS AND NANOMATERIALS Effects of Vacuum Annealing on the Particle Size and Phase Composition of Nanocrystalline Tungsten Carbide Powders A. S. Kurlov Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia e-mail: kurlov@ihim.uran.ru Received March 7, 2012 Abstract—The effects of the vacuum annealing temperature (400–1400°C) on the phase and chemical com- position, particle size, and microstresses of the nanocrystalline powders of tungsten carbide WC with 20– 60nm particles were studied by X-ray diffraction and electron microscopy. Vacuum annealing of WC nano- powders at T ann ≤ 1400°C was accompanied by decarbonization, resulting from the interaction of carbon with the oxygen impurity. Changes in the chemical composition of the nanocrystalline powder of tungsten carbide led to changes in its phase composition. The annealing was accompanied by growth of powder particles due to the aggregation of nanoparticles and by a decrease of microstresses. Key words: tungsten carbide, nanocrystalline powders, phase composition, vacuum annealing, X-ray diffrac- tion, electron microscopy. DOI: 10.1134/S0036024413040158