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, Indiab The Pennsylvania State University, University Park, PA 16802, USA

⁎ Corresponding author. Tel.: +91 512 2597672; fax:E-mail address: anishu@iitk.ac.in (A. Upadhyaya).

0263-4368/$ – see front matter © 2010 Elsevier Ltd. Aldoi:10.1016/j.ijrmhm.2010.05.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 February 2010Accepted 5 May 2010

Keywords:Microwave sinteringTungstenMicrostructures

Microwave heating is recognized for its various advantages, such as time and energy saving, very rapidheating rates, considerably reduced processing cycle time and temperature, fine microstructures andimproved properties. The present paper investigates the feasibility of consolidating tungsten powdersthrough microwave sintering. A comparative analysis has also been attempted between the sinteringresponse of pure tungsten powder compact in a microwave and conventional furnace.

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1. Introduction

Tungsten belongs to Group VIB of periodic table. As a refractorymetal, tungsten is characterized by its very high melting point(3420 °C), high density (19.3 g/cm3), low coefficient of thermalexpansion (4.4 ppm/K at 20 °C) and superior mechanical propertiesat elevated temperatures, which render it highly suitable for manyengineering applications such as lighting filaments, heating source,aerospace, electronic devices, sports and military uses, etc. [1]. Owingto very high fusion point, the consolidation of a conventionalmicrocrystalline W powder is difficult and generally requires atemperature in excess of 1700 °C through solid-state sintering inelectrical resistance sintering furnace under hydrogen atmosphere.

Achievement of near theoretical sintered densities for pure tungstenat temperatures below1650 °C is typically not feasible [2]. Densificationof refractory metals, such as tungsten, can be enhanced greatly byactivating the sintering process wherein the sintering temperature isappreciably lowered. Activated sintering refers to combination ofprocessing approaches that reduce the activation energy for sintering.One such technique to activate sintering is by addition of small amounts(b1 wt.%) of GroupVIII transitionmetals [3–6]. It has been reported thatthe sintering temperature of tungsten can be brought down from2800 °C to 1400 °Cby less than 1 wt.% addition of transitionmetals, suchas palladium and nickel [6]. German and Munir [5,6] have extensivelyinvestigated the role of various transition metal additions in densifica-tion activationof tungstenpowder compacts. Fromtheir study, it is quiteevident that some transition metals (e.g. Pd, Ni) enhance densification,while others (e.g. Ag, Cr) have little influence. Densification enhance-ment in tungsten by transition metal additives was attributed to

enhanced solubility and mass transport kinetics. The additives whichremain segregated to theW–Wpowder interface and inwhich tungstenhas appreciable solubility, act as short-circuit diffusion pathways,thereby promoting densification.

Another important approach to activate sintering of tungsten isthrough selection of submicron or nano-sized precursor tungstenpowder. However, such powders are expensive and are prone tocontamination [7]. Many studies have shown that sintering temper-ature is related to the powder size, when the size is in nano-scale, thesintering temperature can be decreased up to several hundreds ofdegrees. The reduction of sintering temperature for nano tungsten hasbeen reported by several researchers [7–10]. Bose et al. [7] have alsoshown that pressure assisted process such as plasma pressurecompaction helps in the reduction of process temperature. Thereported sintering temperature of nano-sized tungsten produced byhigh energy mechanical milling was drastically decreased fromconventional temperature of 2500 °C to 1700 °C [8]. Other processessuch as hot-isostatic processing [10], and spark plasma sintering [11],too result in further reduction in the processing temperature.

Because of the characteristic feature and obvious advantages,application of microwave energy in consolidating particulate materi-als has become a preferred method over conventional (resistantheating) technique. The application of microwave energy in consol-idating tungsten powders was first studied by Jain et al. [12]. Acomprehensive study on microwave sintering of tungsten and itsalloys were also conducted by several researchers [13–21]. This paperreports the consolidation of nano-sized tungsten powder throughmicrowave and conventional sintering methods. To evaluate theinteraction of microwaves, tungsten powder with a median particlesize of 72 nm was subjected to 2.45 GHz microwaves in a multimodefurnace. For comparing the effect of heating mode, in a parallel set ofexperiments, tungsten powder compacts pressed to similar greendensity levels were consolidated in a conventional furnace.

598 A. Mondal et al. / Int. Journal of Refractory Metals and Hard Materials 28 (2010) 597–600

2. Experimental procedure

Tungsten of average crystallite size of 72 nm powders wassupplied by NEI Corporation, New Jersey, USA. The as-receivedpowders were pressed in a die of 1.6 cm inner diameter to make thegreen compacts of approximately 0.3 to 0.4 cm in height. The greendensity was around 49% of theoretical. To study the densificationbehavior the green (as-pressed) compacts were sintered usingconventional and microwave furnace. The conventional sintering ofgreen compacts was conducted in a MoSi2 heated horizontal tubularsintering furnace (model: OKAY 70T-7, supplier: Bysakh, Kolkata,India). Microwave sintering of the green compacts was carried outusing a multimode cavity 2.45 GHz, 6 kWmicrowave furnace. Furtherdetails of the microwave furnace and experimental arrangementshave been described elsewhere [19]. For each set of experiments(conventional and microwave sintering) four samples were investi-gated and the as-sintered samples were characterized for sintereddensity through both dimensional measurements as well as Archi-medes' density measurement techniques. To take into account theinfluence of the initial as-pressed density, the compact sinterabilitywas also expressed in terms of densification parameter which iscalculated as follows:

Densification parameter =sintered density−green densityð Þ

theoretical density−green densityð Þ ð1Þ

Metallographic techniques were employed on the sinteredsamples. The sintered samples were wet polished in a manualpolisher (model: Lunn Major, supplier: Struers, Denmark) using aseries of 6 µm, 3 µm and 1 µm diamond paste, followed by clothpolishing using a suspension of 0.04 µm colloidal SiO2 suspension.Murakami's reagent was used for etching the sintered tungstensamples. The scanning electron micrographs of as-polished sampleswere obtained by a scanning electron microscope (model: FEI quanta,Netherlands) in the secondary electron (SE) and back scattered (BSE)mode. Bulk hardness measurements were performed on polishedsurfaces of sintered cylindrical compacts at a load of 5 kg using Vickershardness tester (model: V100-C1, supplier: Leco, Japan).

3. Results and discussion

3.1. Effect of heating mode on sintering of tungsten

Fig. 1 compares the thermal profiles for tungsten powder compactsheated in conventional and microwave furnace. It is interesting tonote that W compact couples with microwaves and gets heated up

Fig. 1. Comparison of the thermal profiles of tungsten compact sintered in a radiatively-heated (conventional) and microwave furnace.

quite rapidly. Tungsten powders owing to their fine size aresusceptible to oxidation and hence need to be processed in reducing(hydrogen) atmosphere. Due to the poor thermal shock resistance ofthe alumina tube used, the heating rate of the conventional sinteringwas restricted to 5 °C/min. Furthermore, to ensure homogenization ofthe temperature, isothermal hold at intermediate sintering tempera-tures was provided. Unlike conventional sintering, in a microwavefurnace, the tungsten powder compact per se acts as a source of heatsince it couples directly with microwaves. Consequently, the overallheating rate achieved in microwave furnace was ∼25 °C/min for thetungsten compacts. Taking into consideration the lower heating rateand the intermittent isothermal holds in conventional furnacesintering, there is about 90% reduction in the overall processingtime during microwave sintering.

Unlike ceramic materials microwave interaction with metals isrestricted to its surface only. This depth of penetration in metals, alsoknown as skin depth (δ), is defined as the distance into the material atwhich the incident power drops to 1/e (36.8%) of the surface value.The skin depth is mathematically expressed as follows:

δ =1ffiffiffiffiffiffiffiffiffiffiffiffiπf μσ

p ð2Þ

where, f is the microwave frequency (2.45 GHz), μ is the magneticpermeability, σ is the electrical conductivity, and ρ is the electricalresistivity of the metals. From Eq. (2), it is evident that metals withhigher electrical conductivity have lower skin depths. For metals, asthe resistivity increases with increase in temperature, the skin depthtoo increases. Resistivity as a function of temperature has beenconsidered from the literature [22] and Fig. 2 plots the effect oftemperature on the skin depth of tungsten. For the sake of comparisonsimilar graphs have been plotted for various other conductive metalsas well. It is interesting to note as compared to some other metals, theskin depth of tungsten is relatively higher. Since the initial tungstenpowder size is lower hence the compacts undergo relativelyhomogenous heating despite the heating rate being so high. Sincethe objective of the present work was to restrict the temperature to1600 °C, owing to the small size of the initial powder the temperaturewas achieved in just applying 900 W of power. In comparison, to heatthe similar compact in a conventional furnace wherein the heattransfer was indirect and limited by the radiative losses, the inputpower was on an average 2.1 kW. While it is possible to increase theheating rate in the conventional sintering by increasing the powerthroughput, it will be at the expense of overall furnace life. One suchexperiment was tried; however, the as-pressed compact could notwithstand the thermal gradient and pulverized during processing.

Recently, Prabhu et al. [14] too have reported microwave sinteringof tungsten powders prepared by high energy milling. However, ascompared to our study, they could achieve 93% theoretical density at1800 °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. Forcomparison, the skin depths of some common metals have been plotted for varioustemperatures.

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 at100 MPa and sintered at 1600 °C in reducing atmosphere.

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of 16 kW. Furthermore, they did not compare their sintering resultswith the conventionally sintered W powder compacts.

3.2. Densification response of sintered tungsten

Fig. 3a compares the effect of heatingmode on the sintered densityand densification parameter of tungsten powder compact. It is worthnoticing that irrespective of the heating mode the compacts undergosignificant densification during sintering. This can be attributed to theultra-fine particle size of the powder used for this investigation.Elsewhere, Wang et al. [10] too reported similar sintering response ofultra-fine sized tungsten powder. The sintered density of the Wpowder compacts in microwave increased from 90% to 95% for puretungsten compacts when compared with conventional heating. Thedensification parameter is a normalized parameter which takes intoconsideration the initial variation of the green density on thedensification response. Since, the initial density of the W powdercompactwas kept the same hence the densification parameter followsa similar trend as the sintered density.

Usually, the sintering characteristics of the tungsten powders havebeen investigated in micron-sized powders [10]. For achieving smallsizes, these powders were usually milled [8]. Due to the contamina-tion from the milling media and high defect structure within thecrystallites, proper evaluation of the sintering response of thetungsten powder is usually difficult. However, the general consensusin the literature has converged to the fact that for submicron/nano

Fig. 4. SEM photomicrograph of (a) the as-pressed tungsten powder and the powder com

particle sizes the onset of densification occurs at lower temperaturesof 0.2–0.4Tm (Tm — melting temperature in K) as compared to 0.5–0.8Tm for micron-sized powders [23]. Moreover, it has beenestablished that for nano-structured powders, grain boundary slidingand rotation and viscous flow significantly contribute to densificationenhancement [24–27]. In the present study, due to the chemicalprocessing route, the tungsten powders were prepared in nano-scalein strain free condition. The sintering temperature selected for thepresent study corresponds to 0.46Tm.

Jain et al. [12] also reported about 10 to 12% higher density in caseof microwave sintered compacts over the conventional heating. Morerecently, Prabhu et al. [14] have also investigatedmicrowave sinteringof high energy milled tungsten powder compacts. However, in theirexperiments the compacts were sintered under nitrogen atmosphereand at a high temperature (1800 °C). Despite consolidating at such ahigh temperature, the sintered density was less than that achieved inthis study. It can therefore be inferred that for effective utilization ofthe microwave heating, the powder compacts must be sintered inreducing atmosphere.

Fig. 3b shows the axial and radial shrinkage of the cylindricaltungsten compacts sintered in a conventional andmicrowave furnace.Due to the low green density (49%), the compacts heated in both themodes undergo substantial shrinkage. In case of conventionalsintering, however, the radial shrinkage is more than that observedin the longitudinal (axial) direction. In contrast, the shrinkage in boththe directions is nearly similar for microwave sintered compacts

pact sintered at 1600 °C for 30 min in (b) conventional and (c) microwave furnace.

Table 1Average grain size and bulk hardness of conventionally and microwave sinteredtungsten powder compacts.

Heating mode Grain size, µm Hardness, HV5

Conventional 2.9±1.1 412±12Microwave 2.6±1.0 441±9

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which indicates more isotropic shrinkage behavior. This furtherconfirms that the compact heating in microwaves is more uniformdespite the high heating rate. This is attributed to the volumetricheating microwave provides.

3.3. Effect of heating mode on microstructure and hardness

To investigate the effect of heating mode on the microstructuralcoarsening and its homogeneity, the as-sintered compacts weremetallographically prepared. Fig. 4a shows the SEMmicrograph of theas-pressed tungsten powder. Fig. 4b and c shows the photomicro-graphs of compacts sintered in conventional and microwave furnace,respectively. It is worth noticing that as owing to the initial low as-pressed density, sintering results in significant microstructuralcoarsening in tungsten powder compacts. For both the heatingmodes, the as-sintered compacts attain well-developed polyhedralgrains. The average tungsten grain size was measured using interceptmethod and is summarized in Table 1. From Fig. 4 and Table 1, it isevident that varying heating modes have not significantly influencedthe microstructures.

Table 1 also compares the effect of heating mode on the bulkhardness of the sintered tungsten compacts. As expected, themicrowave sintered compacts have higher hardness. This can beattributed to the higher density and a more refined microstructure ofthe tungsten grains in case of microwave sintering. The hardness ofWcompact achieved throughmicrowave sintering in this study is higherthan those reported earlier by Upadhyaya et al. [15] and Kim et al.[28]. The hardness results achieved in microwave sintered Wcompacts in the present study is 30% higher than the best values(303VHN) achieved by Prabhu et al. [14] by microwave sinteringtungsten compact at 1800 °C.

4. Conclusions

In this study, tungsten powder compact was successfully consol-idated through microwave sintering. It is interesting to note thattungsten compact strongly couples with microwaves and gets heatedvery rapidly. This result in the drastic reduction in the sintering cycletime over the conventional process: up to as high as 90%. Despitebeing heated at high heating rates, none of the tungsten powdercompacts displayed any distortion or cracking during microwaveheating. This underscores the volumetric heating aspects of micro-waves. Moreover, due to the low thermal mass, microwave heatingwas less energy (power) consuming as compared to the equivalentvolume of material being consolidated conventionally. Anothercommon observation is that the microwave sintered compactsundergo more densification as compared to their conventionallyprocessed counterparts. Because of the rapid heating rates achieved inmicrowave furnace, the microstructure coarsening was relativelylower. Consequently, as compared to conventional sintering, themechanical properties of the microwave sintered compacts werehigher.

Acknowledgement

The authors would like to acknowledge NEI Corporation, NewJersey, USA for supplying the powder used for this investigation. Thisstudy was also partially supported by the Indo-US Science andTechnology Forum (IUSSTF), New Delhi and the Institute for PlasmaResearch (IPR), Ahmedabad.

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