The sintering behavior of nanosized tungsten powder prepared by a plasma process Taegong Ryu, Kyu Sup Hwang, Young Joon Choi, Hong Yong Sohn * Department of Metallurgical Engineering, University of Utah, 135 South 1460 East Room 412, Salt Lake City, Utah 84112, USA article info Article history: Received 27 August 2008 Accepted 12 November 2008 Keywords: Plasma Mechanical milling Powder processing Sintering abstract The sintering behavior of nanosized tungsten powder (25 nm average size) synthesized by a thermal plasma process, in which ammonium paratungstate (APT) was reduced by hydrogen was investigated, compared with submicron W powder (0.5 lm average size) and nanosized W powder (23 nm average size) produced from the latter by high energy milling. The hardness of the compact of the plasma-synthe- sized powder (315 VHN) sintered at 1400 °C was higher than that of submicron-sized powder (193 VHN) and was similar to that of the milled powder (309 VHN). The plasma-synthesized powder, however, yielded a compact with much lower tendency to form cracks than the milled powder. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Tungsten is used in the production of numerous end use items. It is widely used for high temperature applications such as fila- ments in light bulbs, cathodes in high power lamps, and rocket nozzles in space crafts. Tungsten is also used in the manufacture of superalloys and catalysts (hydrodesulfurization and hydrodeni- trogenation) [1–4]. In addition, its considerable applications in industry as a refractory metal owing to its outstanding hardness and wear resistance in the form of tungsten carbide are well known [5]. The preparation of tungsten metal powder is a critical step in the production of tungsten metal and alloys in consolidated forms, since the powder properties significantly affect the subsequent operations, such as pressing and sintering. Nanoscaled tungsten powder promises to yield very hard, strong, and wear resistant materials via the press-sinter route. A small particle size requires lower temperatures and shorter times of sintering to attain dense but small grain size structures [6,7]. Nanoparticles in particular display the onset of sintering at a significantly lower temperature than conventional microcrystalline powders [8,9]. In this work, a thermal plasma process was applied to produce nanosized tungsten powder using ammonium paratungstate (APT) as the precursor, which is an intermediate product obtained from high grade concentrates of wolframite or scheelite ore. Then, its sintering behavior at 1400 °C with different dwelling times (1, 30, and 60 min) was investigated in comparison with nanosized tungsten powder produced by high energy milling and commercial submicron tungsten powder. 2. Experimental Nanosized tungsten powder was synthesized in a thermal plas- ma reactor, in which ammonium paratungstate (99.9%, Alldyne Powder Technologies Inc.) was used as the precursor and hydrogen (99.9%) was used as the reducing agent [10]. The experimental re- sults on the synthesis of nanosized tungsten powder are described in Ref. [10]. In the previous work, the effects of hydrogen concen- tration, plasma torch power, and the flow rate of plasma gas on the product composition and particle size were investigated. The par- ticle size of synthesized W powder was less than 30 nm in all cases tested. The thermal plasma process performed in the previous work has confirmed its potential as an efficient technique com- pared with other conventional multi-step processes, owing to the high temperature generated by the plasma flame to rapidly volatil- ize APT powder and the rapid quenching of the product to directly yield nanosized W powder. This paper presents the results of sintering behavior of the plas- ma-synthesized W powder, compared with submicron-sized W powder and nanosized W powder produced from it by milling. For the comparison of its sintering behavior, submicron (0.5 lm) tungsten powder (99.95% purity) was obtained from Osram Sylva- nia Inc., which was also used for the preparation of nanosized tungsten powder by high energy milling. The synthesized tungsten powder (hereafter referred to as ‘‘PW”) and submicron tungsten powder (hereafter referred to as ‘‘SW”) were each mixed with 2 wt.% of wax. The wax was used to decrease friction between the particles as well as between the powder and the press die in the subsequent compaction process. In order to prepare nanosized tungsten powder (hereafter referred to as ‘‘MW”), the submicron tungsten powder mixed with 2 wt.% of wax was milled using a High Energy-Dual Planetary Mill (HE-DPM) 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.004 * Corresponding author. Tel.: +1 801 581 5491; fax: +1 801 581 4937. E-mail address: h.y.sohn@utah.edu (H.Y. Sohn). Int. Journal of Refractory Metals & Hard Materials 27 (2009) 701–704 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM