Int. Journal of Refractory Metals and Hard Materials 43 (2014) 115–120

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Int. Journal of Refractory Metals and Hard Materials

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Fabrication of tungsten carbide foam through gel-casting process usingnontoxic sodium alginate

Ali Asghar Najafzadeh Khoee a,⁎, Ali Habibolahzadeh a, Fathallah Qods a, Hamidreza Baharvandi b

a Materials Engineering Department, Engineering Faculty, Semnan University, Semnan, Iranb School of Metallurgy and Materials Engineering, University of Tehran, Iran

⁎ Corresponding author. Tel.: +98 2313354119.E-mail address: a_najafzadeh@students.semnan.ac.ir (

0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rihttp://dx.doi.org/10.1016/j.ijrmhm.2013.11.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2013Accepted 20 November 2013

Keywords:SuspensionGel-castingSodium alginateTungsten carbideMicro-porous foam

In this study, nontoxic sodium alginate was utilized in gel-casting process to fabricate tungsten carbide (WC)micro-porous foam. Suspensions containing 20 and 25 vol.%WC and 1 wt.% sodium alginate were used. Calciumphosphate and sodium hexa-metaphoshphate were employed as solidifier agent and chelator, respectively. Thegreen bodies were dried at room temperature for 36 h and pre-sintered at 1450 °C for 4 h. The influence ofchelator and calcium salt on strength of dried green body was evaluated. Tungsten carbide foams with 50–60%porosity were successfully produced. SEMmicrographs of tungsten carbide foams show a uniformporousmicro-structure, with average size of 0.75 μm.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Macroporous ceramics have found many applications in variousindustrial processes, including thermal insulation, catalytic reactionsupporter, and filtration of molten metals, exhaust gases, and hotcorrosive gases [1]. These porousmediawere also employed to fabricatevarious types of metal matrix composites via liquid metal infiltrationmethod [2]. High melting point, high corrosion and wear resistancesare their outstanding properties, in combination with the featuresgained via replacement of solid material by voids in the components[1]. Such macroporous ceramics exhibit pore width larger than 50 nm,and are produced by various methods such as replica, sacrificial tem-plate and direct foaming. The sacrificial template technique usually con-sists of a continuous matrix of ceramic particles or ceramic precursorsand a dispersed sacrificial phase. The biphasic composite is preparedby forming a two phase suspension that is subsequently processed bywet colloidal routes [1].

Gel-casting is also widely applied to the fabrication of porous andcomplex-shaped ceramics (e.g., microbeads, rutile capacitor, thin-wallrutile tube, refractory nozzle). The gel-casting processwasfirst developedin the Metals and Ceramics Division-Ceramic Processing Group at OakRidge National Laboratory (ORN), Oak Ridge, USA by Omatete and Janneyduring the 1990s [3]. The in situ free radical polymerization of acrylamidemonomers, which originally developed for dense ceramics, was success-fully used for setting ceramic foams of various compositions by Binnerand Sepulveda [4,5], however, the neural toxicity of the acrylamidemonomers, which are commonly employed in the process, limits their

A.A. Najafzadeh Khoee).

ghts reserved.

applications [6]. Many researchers studied nontoxic or low toxicitypolymers in the process. The temperature or pH-induced gelation ofgelatin, ovalbumin, and bovine serum albumin, for instance, andtemperature-induced gelation of polysaccharides such as sucrose,agar, and carrageenan gum have been recently applied as nontoxic pro-cessing route to fabricate porous ceramics [1]. Alginate is a type of gellingpolysaccharide which is soluble in water at room temperature and gelsafter reaction with divalent metal ions [7]. The result is construction of athree dimensional (3D) network, suspending ceramic particles over thenetwork [8].

Several reports have been presented on gel-casting of alumina andsilicon carbide using sodium alginate [6–11], while fabrication of highdensity tungsten carbide via gel-casting route remains a seriouschallenge, as its density (15.8 g/cm3) [12] is 4–5 times of alumina(3.96 g/cm3) [7] and silicon carbide (3.22 g/cm3) [11] densities,and its suspension over a 3D network appears to be troublesome.In the present work, it is attempted to fabricate tungsten carbide foamvia a nontoxic gel-casting route, and a natural innoxious polymer-sodium alginate was then employed to coagulate the tungsten carbidesuspension. The resulted micro-porous green body was characterizedby SEM, TG/DTA and XRD tests and mechanical tests were carried outto evaluate its green strength.

2. Experimental procedure

2.1. Materials

Tungsten carbidewith purity of 99.6 wt.%, particle size range of 0.2–10 μm and mean particle size of 0.9 μm, was employed in the presentstudy. Themorphology and particle size distribution of tungsten carbide

Fig. 2. Particle size distribution of tungsten carbide powder.

116 A.A. Najafzadeh Khoee et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 115–120

powder were shown in Figs. 1 and 2, respectively. The particle size dis-tribution of the powder was determined by particle size analyzer(FRITSCH Particle Sizer Analysette 22).

2.2. Procedure

Controllable gel-casting and solidification of the suspensionsbecome possible through the use of appropriate amounts of sodiumalginate, the solidifier agent, the chelator and the hexanedioic acid,as listed in Table 1.

Sodium alginate undergoes chemical gelation to form a 3D networkin the presence of multivalent cations (e.g., calcium). The irreversiblegels can form very quickly through reaction between calcium ions andsodium alginate, as shown in Eq. (1) [13].

2Na−alginateþ Ca2þ⇄Ca−alginateþ 2Naþ: ð1Þ

Gelation occurs during casting with a fast gelation rate and thusresults in various cross-linking densities and heterogeneity within thegel [13]. The reaction rate of the gelation is controlled by chelator((NaPO3)6) and initiator (C6H10O4). The gelation reaction is delayedwith increasing chelator because the free calcium ions in the solutionwere limited due to the chelation [6]. A stable complex forms from thereaction between chelator and calcium salt. It allows chelated calciumions to be uniformly distributed in the alginate solution and preventsimmediate gelation, as shown in Eq. (2) [10]:

6PO−3 þ 3Ca2þ⇄Ca3 PO3ð Þ6: ð2Þ

Upon adding hexanedioic acid (C6H10O4) to the ceramic suspen-sions, the complex decomposes and gradually releases calcium ions, inwhich subsequently the ions react with sodium alginate, and gelationoccurs, according to the following equation [10]:

Ca3 PO3ð Þ6 þ 3C6H10O4⇄6HPO3 þ 3Ca2þ þ 3C6H8O2−4 : ð3Þ

Flow chart of the forming process is illustrated in Fig. 3. First; sodiumalginate was dissolved in the deionized water; then tungsten carbidepowder, dispersant (ammonium citrate tribasic — (NH4)3C6H5O7),calcium salt (Ca3 (PO4)2) and chelator (sodium hexa-metaphosphate— (NaPO3)6) were added to the solution, and the resulting suspension

Fig. 1. SEMmicrograph of WC powder.

was then ball milled for 24 h to break down the agglomerates and tomake a uniform slurry. After degassing the slurry in a rotary evaporatorunder vacuum, hexanedioic acid was introduced to the slurry, beforecasting it into a nonporous mold. After 24 h, the gelled wet greenbody were removed from the mold and dried at room temperature for36 h.

Slurries with different volume fractions of tungsten carbide (20 and25 vol.%) and 1 wt.% sodium alginate (water-based) were prepared andcast, as described in Fig. 3. The influence of chelator and calcium saltamounts on the dried green body strength was examined (Table 2).

Drying and sintering shrinkages of the final complex shapes de-crease by increasing loaded solid (i.e. tungsten carbide) in the slurry.However, an increase in loaded solid enhances viscosity of the slurryand reduces its fluidity to fill thin and complex molds [11]. Therefore,it is essential to precisely verify the slurry composition. The sufficientamount of dispersant (based on ceramic powder) decreases viscosityof the slurry due to an increase in the density of electron cloud on thesurface of ceramic particles and static repulsive forces [10]. Thus, thedispersant provides the opportunity to increase the solid loadingvolume fraction (i.e. WC) in the slurries and it decreases the shrink-age during drying and sintering. The utilized dispersant in this workwas 0.3 wt.% [14].

2.3. Measurements

The room temperature bending strength of dried green body sam-ples, which had been cut to a size of 20 × 10 × 3 mm3, was determinedby a three-point flexure test, taking into account ASTM C1341-95 stan-dard, with span sizes of 15 mm and 0.5 mm/min loading rate (ZWICK,Z050). Binder burnout behavior of the green bodies was determinedby thermogravimetric (TG) and differential thermal analyses (DTA)(NETZSCH, STA 409 PC Luxx) at the heating rate of 5 °C/min up to1069 °C under argon atmosphere. Phase analyses of the specimenswere achieved by X-ray diffractometer (GNR, MPD 3000). Fracture sur-faces of the green bodies (after flexural strength test) were pre-sinteredat 1450 °C for 4 h [15]. According to ASTM C373-88 standard, theArchimedes's method was applied to measure the bulk density of the

Table 1Additives of gel-casting suspensions.

Binder (gelation reagent) Sodium alginate (NaAlg)Dispersant Ammonium citrate tribasic, (NH4)3C6H5O7

Solidifier agent (calcium salt) Calcium phosphate tribasic, Ca3 (PO4)2Chelator Sodium hexameta phosphate, (NaPO3)6Acid Adipic acid, C6H10O4

Fig. 3. Flow chart of forming process.

Fig. 4. SEM micrograph of fracture surface of WC green body.

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pre-sintered specimens. The microstructures of both green bodies andpre-sintered specimens were observed by scanning electron micro-scope (SEM, VEGA\\TESCAN-LMU). The foam porosity size was mea-sured by CLEMEX image analysis software. Average of at least 10measurements was reported.

3. Results and discussion

3.1. Microstructure and strength of dried green body

Fig. 4 presents fracture surface of the resulted green bodywith a fairlyhomogenous pore distribution appearance. It demonstrates thatWC par-ticles has been almost uniformly dispersed in the suspension andmade a

Table 2Composition of different slurries and resulted green flexural strength.

Sample

1 2 3 4

NaAlg (ml) 20 20 20 20C6H10O4 (g) 0.15 0.57 0.15 0.57Ca3 (PO4)2 (g) 0.27 2.17 0.27 2.17(NH4)3C6H5O7 (g) 0.19 0.19 0.23 0.23WC (g) 63 63 78.5 78.5(NaPO3)6 (μl) 391.3 952.4 391.3 952.4Flextural strength (MPa) 1.5 3.2 1.6 3.3

CNaAlg = 1 wt.%; C(NaPO3)6 = 20 wt.%.ρWC = 15.8 g/cm3.

homogenous 3D porous framework. The flexural green strength of thesamples is also an important property for further processing steps. It issuggested [6] that the bonded strength of alginate molecular chains,attracted by calcium ions, determines the mechanical strength of thedried green body. The results of flexural green strength (Table 2) revealthat the strengths of samples 2 and 4 are equal, and are higher thanthose of samples 1 and 3. The difference in strength between samples 1and 2, as well as that of samples 3 and 4 resulted from the cross-linkingbetween polymer chains by calcium ions [6]. The amount of chelator de-pends on calcium salt content; also more chelator is needed to keep theslurry stable. The results reveal that the flexural strength is affected bycalcium salt/chelator ratio in the slurry (Table 2). It increases to3.3 MPa by increasing calcium salt/chelator ratio in the suspension. It in-dicates that more calcium ions attract more alginate molecular chainsand provide a higher green strength body. WC percent (up to 25 vol.%)apparently has no effect on final green strength.

3.2. Binder removal and pre-sintering of green bodies

The binders are often extracted by thermal treatment. The burnoutrate needs to be low in order to avoid cracking of the final body. Theresults of thermal analyses (Fig. 5) show an endothermic peak at151.1 °C which relates to vaporization of the residual moisture. Thenearly constant weight in the range of 600–1000 °C demonstrates thefact that decomposition of the additives mainly occurs prior to 600 °C.The observed weight loss was attributed to binder burnout. Therefore,all green bodies were dried and de-bounded up to 600 °C with heatingrate of 2 °C/min to prevent cracking, and then were kept at 600 °C for1 h to complete the drying step.

There is also an exothermic peak at 1029 °C (Fig. 5), to evaluate itspresence; two green bodies were prepared from the same slurry bath.One sample was dried at 600 °C with 2 °C/min heating rate and theother was pre-sintered at 1100 °C under similar conditions. Then, theywere cooled to room temperature as quickly as possible by purgingargon gas. XRD patterns for initial powder and 600 °C pre-sinteredbody indicate that WC is the major phase in the samples (Fig. 6a and b),while increasing temperature to 1100 °C leads to appearance of someadditional peaks which relates to elemental tungsten, beside themajor phase of WC (Fig. 6c). Therefore, the exothermic peak at1029 °C (Fig. 5) might relate to formation of elemental tungsten. To ex-plain the presence of elemental tungsten, it should be considered thatthe WC powder is expected to be surrounded by a superficial oxide

Fig. 5. TG/DTA plots for green body under argon atmosphere.

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layer at ambient temperature. The surface oxide could be reduced bycarbon content of WC at elevated temperatures. If carbon is consumeddue to reduction of the surface oxide, there should be a lack of carbonto maintain WC phase [16], i.e. decarburization of WC. The result is for-mation of W2C phase according to Eq. (4) during pre-sintering at hightemperatures [12]. The XRD pattern of pre-sinteredWC at 1450 °C con-firms the presence of W2C phase, as shown in Fig. 6d.

WCþW⇄W2C: ð4Þ

No detectable phases containing Na, Ca or Pwere acquired in the XRDpatterns as shown in Fig. 6, due to their very low content, suggestingalmost complete drying step.

Table 3 reports the porosity, P, and bulk density, B, of tungsten car-bide foams after pre-sintering at 1450 °C. Furthermore, the relativedensity, ρr (ratio of bulk density to theoretical density)was determined.The porosity of samples 2 and 4 are higher than those of correspondingsamples, i.e. samples 1 and 3, respectively. The reason could attribute tothe amount of the additives (gelling agents) which burnt during the

Fig. 6. XRD patterns of (a) initial powder, dried green body at (b) 6

pre-sintering process. The results indicate that the porosity of finaltungsten carbide foams was in the range of 50–60%.

Considering the fact that the density of WC is higher than those ofAl2O3 and SiC, stabilizing of the ceramic slurry and homogenous dis-tributing of WC powder within a 3D network, using sodium alginateand its additives, is rather difficult. Fig. 7 illustrates the fracture sur-faces of the green bodies after pre-sintering. The well-distributedporosities imply that binders and ceramic powder were distributeduniformly in the process. However, pre-sintering of WC at 1450 °C,despite its high melting point (2785 °C) [17], leads the fusing of theWC particles together and provides a compact ceramic skeleton withproper strength for further treatments. A highly porous structure is ob-tained during the binder burnout and low-temperature pre-sinteringsteps. Low-temperature pre-sintering also prevents shrinking or elimi-nating of small pores.

Thus, the homogenously pore distribution could be resulting fromthe use of sufficient amount of the chelator and calcium salt in the sus-pension, leading to a uniform 3D network, with an average porositysize of 0.75 μm.

00 °C and pre-sintered bodies at (c) 1100 °C, and (d) 1450 °C.

Table 3Porosity (P), bulk (B) and relative (ρr) densities of tungsten carbide foams.

Sample P (%) B (g/cm3) ρr (%)

1 54.03 7.218 45.972 59.30 6.39 40.703 48.96 8.014 51.044 59.37 6.38 40.63

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4. Conclusion

Gel-casting processwas utilized to fabricate tungsten carbidemicro-porous foam. The following are the results:

(1) Sodium alginate, a natural innoxious polymer, was successfullyutilized to uniformly suspend the high-density WC particles ona 3D network via gel-casting process.

(2) The bending strength of the green bodies increases to 3.3 MPa byincreasing calcium salt/chelator ratio in the suspension.

(3) The dried green bodies exhibit a homogenous microstructure, inrespect of particle and pore distributions.

Fig. 7. SEM micrographs of the fracture surface of WC pre-sintered bo

(4) The pre-sintered bodies illustrate uniform distribution of porosi-ties with an average size of 0.75 μm. The porosity offinal tungstencarbide foams was in the range of 50–60%.

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