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Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM Effect of heating mode on sintering of tungsten Avijit Mondal a, Anish Upadhyaya a,⁎, Dinesh Agrawal b a Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India b The Pennsylvania State University, University Park, PA 16802, USA article info abstract Article history: Microwave heating is recognized for its various advantages, such as time and energy saving, very rapid Received 4 February 2010 heating rates, considerably reduced processing cycle time and temperature, fine microstructures and Accepted 5 May 2010 improved properties. The present paper investigates the feasibility of consolidating tungsten powders through microwave sintering. A comparative analysis has also been attempted between the sintering Keywords: response of pure tungsten powder compact in a microwave and conventional furnace. Microwave sintering © 2010 Elsevier Ltd. All rights reserved. Tungsten Microstructures 1. Introduction enhanced solubility and mass transport kinetics. The additives which remain segregated to the W–W powder interface and in which tungsten Tungsten belongs to Group VIB of periodic table. As a refractory has appreciable solubility, act as short-circuit diffusion pathways, metal, tungsten is characterized by its very high melting point thereby promoting densification. 3 (3420 °C), high density (19.3 g/cm ), low coefficient of thermal Another important approach to activate sintering of tungsten is expansion (4.4 ppm/K at 20 °C) and superior mechanical properties through selection of submicron or nano-sized precursor tungsten at elevated temperatures, which render it highly suitable for many powder. However, such powders are expensive and are prone to engineering applications such as lighting filaments, heating source, contamination [7]. Many studies have shown that sintering temper- aerospace, electronic devices, sports and military uses, etc. [1]. Owing ature is related to the powder size, when the size is in nano-scale, the to very high fusion point, the consolidation of a conventional sintering temperature can be decreased up to several hundreds of microcrystalline W powder is difficult and generally requires a degrees. The reduction of sintering temperature for nano tungsten has temperature in excess of 1700 °C through solid-state sintering in been reported by several researchers [7–10]. Bose et al. [7] have also electrical resistance sintering furnace under hydrogen atmosphere. shown that pressure assisted process such as plasma pressure Achievement of near theoretical sintered densities for pure tungsten compaction helps in the reduction of process temperature. The at temperatures below 1650 °C is typically not feasible [2].Densification reported sintering temperature of nano-sized tungsten produced by of refractory metals, such as tungsten, can be enhanced greatly by high energy mechanical milling was drastically decreased from activating the sintering process wherein the sintering temperature is conventional temperature of 2500 °C to 1700 °C [8]. Other processes appreciably lowered. Activated sintering refers to combination of such as hot-isostatic processing [10], and spark plasma sintering [11], processing approaches that reduce the activation energy for sintering. too result in further reduction in the processing temperature. One such technique to activate sintering is by addition of small amounts Because of the characteristic feature and obvious advantages, (b1 wt.%) of Group VIII transition metals [3–6]. It has been reported that application of microwave energy in consolidating particulate materi- the sintering temperature of tungsten can be brought down from als has become a preferred method over conventional (resistant 2800 °C to 1400 °C by less than 1 wt.% addition of transition metals, such heating) technique. The application of microwave energy in consol- as palladium and nickel [6]. German and Munir [5,6] have extensively idating tungsten powders was first studied by Jain et al. [12].A investigated the role of various transition metal additions in densifica- comprehensive study on microwave sintering of tungsten and its tion activation of tungsten powder compacts. From their study, it is quite alloys were also conducted by several researchers [13–21]. This paper evident that some transition metals (e.g. Pd, Ni) enhance densification, reports the consolidation of nano-sized tungsten powder through while others (e.g. Ag, Cr) have little influence. Densification enhance- microwave and conventional sintering methods. To evaluate the ment in tungsten by transition metal additives was attributed to interaction of microwaves, tungsten powder with a median particle size of 72 nm was subjected to 2.45 GHz microwaves in a multimode furnace. For comparing the effect of heating mode, in a parallel set of ⁎ Corresponding author. Tel.: +91 512 2597672; fax: +2597505. experiments, tungsten powder compacts pressed to similar green E-mail address: [email protected] (A. Upadhyaya). density levels were consolidated in a conventional furnace. 0263-4368/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2010.05.002 Author's personal copy 598 A. Mondal et al. / Int. Journal of Refractory Metals and Hard Materials 28 (2010) 597–600 2. Experimental procedure quite rapidly. Tungsten powders owing to their fine size are susceptible to oxidation and hence need to be processed in reducing Tungsten of average crystallite size of 72 nm powders was (hydrogen) atmosphere. Due to the poor thermal shock resistance of supplied by NEI Corporation, New Jersey, USA. The as-received the alumina tube used, the heating rate of the conventional sintering powders were pressed in a die of 1.6 cm inner diameter to make the was restricted to 5 °C/min. Furthermore, to ensure homogenization of green compacts of approximately 0.3 to 0.4 cm in height. The green the temperature, isothermal hold at intermediate sintering tempera- density was around 49% of theoretical. To study the densification tures was provided. Unlike conventional sintering, in a microwave behavior the green (as-pressed) compacts were sintered using furnace, the tungsten powder compact per se acts as a source of heat conventional and microwave furnace. The conventional sintering of since it couples directly with microwaves. Consequently, the overall green compacts was conducted in a MoSi2 heated horizontal tubular heating rate achieved in microwave furnace was ∼25 °C/min for the sintering furnace (model: OKAY 70T-7, supplier: Bysakh, Kolkata, tungsten compacts. Taking into consideration the lower heating rate India). Microwave sintering of the green compacts was carried out and the intermittent isothermal holds in conventional furnace using a multimode cavity 2.45 GHz, 6 kW microwave furnace. Further sintering, there is about 90% reduction in the overall processing details of the microwave furnace and experimental arrangements time during microwave sintering. have been described elsewhere [19]. For each set of experiments Unlike ceramic materials microwave interaction with metals is (conventional and microwave sintering) four samples were investi- restricted to its surface only. This depth of penetration in metals, also gated and the as-sintered samples were characterized for sintered known as skin depth (δ), is defined as the distance into the material at density through both dimensional measurements as well as Archi- which the incident power drops to 1/e (36.8%) of the surface value. medes' density measurement techniques. To take into account the The skin depth is mathematically expressed as follows: influence of the initial as-pressed density, the compact sinterability fi 1 was also expressed in terms of densi cation parameter which is δ = pffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ calculated as follows: πf μσ ðÞsintered density−green density μ Densification parameter = ð1Þ where, f is the microwave frequency (2.45 GHz), is the magnetic ðÞtheoretical density−green density permeability, σ is the electrical conductivity, and ρ is the electrical resistivity of the metals. From Eq. (2), it is evident that metals with Metallographic techniques were employed on the sintered higher electrical conductivity have lower skin depths. For metals, as samples. The sintered samples were wet polished in a manual the resistivity increases with increase in temperature, the skin depth polisher (model: Lunn Major, supplier: Struers, Denmark) using a too increases. Resistivity as a function of temperature has been series of 6 µm, 3 µm and 1 µm diamond paste, followed by cloth considered from the literature [22] and Fig. 2 plots the effect