Effect of Heating Mode on Sintering of Tungsten

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Int. Journal of Refractory Metals and Hard Materials 28 (2010) 597–600

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Int. Journal of Refractory Metals and Hard Materials

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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.

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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 of polishing using a suspension of 0.04 µm colloidal SiO2 suspension. temperature on the skin depth of tungsten. For the sake of comparison Murakami's reagent was used for etching the sintered tungsten similar graphs have been plotted for various other conductive metals samples. The scanning electron micrographs of as-polished samples as well. It is interesting to note as compared to some other metals, the were obtained by a scanning electron microscope (model: FEI quanta, skin depth of tungsten is relatively higher. Since the initial tungsten Netherlands) in the secondary electron (SE) and back scattered (BSE) powder size is lower hence the compacts undergo relatively mode. Bulk hardness measurements were performed on polished homogenous heating despite the heating rate being so high. Since surfaces of sintered cylindrical compacts at a load of 5 kg using Vickers the objective of the present work was to restrict the temperature to hardness tester (model: V100-C1, supplier: Leco, Japan). 1600 °C, owing to the small size of the initial powder the temperature was achieved in just applying 900 W of power. In comparison, to heat 3. Results and discussion the similar compact in a conventional furnace wherein the heat transfer was indirect and limited by the radiative losses, the input 3.1. Effect of heating mode on sintering of tungsten power was on an average 2.1 kW. While it is possible to increase the heating rate in the conventional sintering by increasing the power Fig. 1 compares the thermal proﬁles for tungsten powder compacts throughput, it will be at the expense of overall furnace life. One such heated in conventional and microwave furnace. It is interesting to experiment was tried; however, the as-pressed compact could not note that W compact couples with microwaves and gets heated up withstand the thermal gradient and pulverized during processing. Recently, Prabhu et al. [14] too have reported microwave sintering of tungsten powders prepared by high energy milling. However, as compared to our study, they could achieve 93% theoretical density at 1800 °C with a heating rate of 7.8 °C/min and at a power throughput

Fig. 2. The variation in the skin depth of tungsten subjected to 2.45 GHz microwave. For Fig. 1. Comparison of the thermal proﬁles of tungsten compact sintered in a radiatively- comparison, the skin depths of some common metals have been plotted for various heated (conventional) and microwave furnace. temperatures. Author's personal copy

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Fig. 3. Effect of heating mode on (a) the sintered density and densification parameter and (b) the axial and radial shrinkage of tungsten compact. All compacts were pressed at 100 MPa and sintered at 1600 °C in reducing atmosphere. of 16 kW. Furthermore, they did not compare their sintering results particle sizes the onset of densification occurs at lower temperatures with the conventionally sintered W powder compacts. of 0.2–0.4Tm (Tm — melting temperature in K) as compared to 0.5– 0.8Tm for micron-sized powders [23]. Moreover, it has been 3.2. Densification response of sintered tungsten established that for nano-structured powders, grain boundary sliding and rotation and viscous flow significantly contribute to densification Fig. 3a compares the effect of heating mode on the sintered density enhancement [24–27]. In the present study, due to the chemical and densification parameter of tungsten powder compact. It is worth processing route, the tungsten powders were prepared in nano-scale noticing that irrespective of the heating mode the compacts undergo in strain free condition. The sintering temperature selected for the significant densification during sintering. This can be attributed to the present study corresponds to 0.46Tm. ultra-fine particle size of the powder used for this investigation. Jain et al. [12] also reported about 10 to 12% higher density in case Elsewhere, Wang et al. [10] too reported similar sintering response of of microwave sintered compacts over the conventional heating. More ultra-fine sized tungsten powder. The sintered density of the W recently, Prabhu et al. [14] have also investigated microwave sintering powder compacts in microwave increased from 90% to 95% for pure of high energy milled tungsten powder compacts. However, in their tungsten compacts when compared with conventional heating. The experiments the compacts were sintered under nitrogen atmosphere densification parameter is a normalized parameter which takes into and at a high temperature (1800 °C). Despite consolidating at such a consideration the initial variation of the green density on the high temperature, the sintered density was less than that achieved in densification response. Since, the initial density of the W powder this study. It can therefore be inferred that for effective utilization of compact was kept the same hence the densification parameter follows the microwave heating, the powder compacts must be sintered in a similar trend as the sintered density. reducing atmosphere. Usually, the sintering characteristics of the tungsten powders have Fig. 3b shows the axial and radial shrinkage of the cylindrical been investigated in micron-sized powders [10]. For achieving small tungsten compacts sintered in a conventional and microwave furnace. sizes, these powders were usually milled [8]. Due to the contamina- Due to the low green density (49%), the compacts heated in both the tion from the milling media and high defect structure within the modes undergo substantial shrinkage. In case of conventional crystallites, proper evaluation of the sintering response of the sintering, however, the radial shrinkage is more than that observed tungsten powder is usually difficult. However, the general consensus in the longitudinal (axial) direction. In contrast, the shrinkage in both in the literature has converged to the fact that for submicron/nano the directions is nearly similar for microwave sintered compacts

Fig. 4. SEM photomicrograph of (a) the as-pressed tungsten powder and the powder compact sintered at 1600 °C for 30 min in (b) conventional and (c) microwave furnace. Author's personal copy

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Table 1 Acknowledgement Average grain size and bulk hardness of conventionally and microwave sintered tungsten powder compacts. The authors would like to acknowledge NEI Corporation, New

Heating mode Grain size, µm Hardness, HV5 Jersey, USA for supplying the powder used for this investigation. This Conventional 2.9±1.1 412±12 study was also partially supported by the Indo-US Science and Microwave 2.6±1.0 441±9 Technology Forum (IUSSTF), New Delhi and the Institute for Plasma Research (IPR), Ahmedabad. which indicates more isotropic shrinkage behavior. This further References confirms that the compact heating in microwaves is more uniform [1] Lassner E, Schubert WD. Tungsten: properties, chemistry, technology of the despite the high heating rate. This is attributed to the volumetric elements, alloys, and chemical compounds. New York, NY, USA: Kluwer Academic/ heating microwave provides. Plenum Publishers; 1999. [2] German RM. Critical developments in tungsten heavy alloys. In: Bose A, Dowding RJ, editors. Tungsten and tungsten alloys — 1992. Princeton, NJ, USA: Metal 3.3. Effect of heating mode on microstructure and hardness Powder Industries Federation; 1992. p. 3–13. 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