Materials Research Bulletin 47 (2012) 3283–3286

In situ synthesis of WC–Co nanocomposite powder via core–shell structureformation

Hua Lin a,b,*, Bowan Tao a, Qing Li b, Yanrong Li a,**a State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR Chinab School of Materials Science and Engineering, Southwest University, Chongqing 400715, PR China

A R T I C L E I N F O

Article history:

Received 27 January 2012

Received in revised form 24 April 2012

Accepted 28 July 2012

Available online 8 August 2012

Keywords:

A. Carbides

A. Nanostructures

B. Chemical synthesis

C. X-ray diffraction

A B S T R A C T

Cemented carbide WC–Co nanocomposite powders were synthesized through in situ reduction and

carbonization of a core/shell precursor in vacuum. Samples were characterized by X-ray diffraction

(XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy

(HRTEM). The results revealed that WC–Co composite powders can be obtained at 950 8C for 1 h and the

particle size is in the range from 30 to 50 nm with good dispersion. The formation mechanism of the WC–

Co composite by in situ reduction and carbonization reactions was proposed. The preparation process

could be divided into three steps: first, the reagents were dissolved and mixed to an aqueous solution;

second step is to synthesize a carbon encapsulated core/shell nanostructure precursor using

hydrothermal route, and finally, in situ reduction and carbonization of the precursor to the desired

nanocomposite powders in vacuum.

� 2012 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

With inimitable properties, such as excellent wear resistance,high hardness and toughness, WC–Co has been widely used inmilitary, aerospace, automotive, electronics, mining and machin-ing [1–3]. Furthermore, the hardness and the strength of cementedcarbides increase remarkably when the grain size is reduced to arange of nanometer, as well as the toughness improves greatly [4–7]. In the previous twenty years, research and preparation ofnanocrystalline WC–Co cemented carbides has become one of thehot issues in the field of high-performance hard materials [8,9].

The conventional methods involve production of WC and Coseparately, and forming the cement powder by mixing the twoconstituents [10]. The results have shown that the conventionalmethods are difficult to prepare nanostructured alloys. A crucial step inthe preparation of nano grade cemented carbides is to directly obtainhomogeneous nanocrystalline cemented carbide powder of WC–Co.Nowadays, many methods have been developed to synthesize WC–Conanocomposite powders, such as mechanical alloying [11], co-precipitation [12], spray conversion process [13,14], and high energyball milling [15,16]. Most of the methods mentioned above involve

* Corresponding author at: State Key Laboratory of Electronic Thin Films and

Integrated Devices, University of Electronic Science and Technology of China,

Chengdu 610054, PR China. Tel.: +86 28 83201232; fax: +86 28 83201232.

** Corresponding author. Tel.: +86 28 83201232; fax: +86 28 83201232.

E-mail addresses: lh2004@swu.edu.cn (H. Lin), yrli@uestc.edu.cn (Y. Li).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.07.038

many disadvantages [17–19], such as the complicated processingprocedures, the use of some special equipment, uncontrollablereaction gas atmosphere, and high carbonization temperature. Toobtain high-performance hard materials, it is necessary to develop aninexpensive and facile method which uses simply process, user-friendly reagents and easy-operating equipments to synthesize purephase configuration and uniformity of different components.

To directly prepare nanocrystalline powder of WC–Co, anessential step in the process is to prepare a precursor mixing twoor more compounds [5]. Core/shell structured nanoparticles areconstructed of cores and shells of different chemical compositions,which are now attracting more and more interest to be investigated[20,21]. Presently, core/shell structured nanoparticles with a carbonshell have stimulated great interest [22]. A series of works have beendone on the synthesis of metal@C, oxides@C or compound@C [23–25]. Particularly, the carbonaceous sheath is penetrable for smallmolecules such as NH3, CO or CO2, which is in favor of the reactioncarried out smoothly. So this idea can be used to synthesize core/shell nanoparticles in which the core is a compound of tungsten andcobalt, and the shell is carbon. Then it can be converted to uniformnanostructured WC–Co powders by in situ reduction and carburi-zation without expensive reductive gas and complex equipment.However, up until now, this method has not been developed as thepost synthesis methods, while relative reports are rare.

In this work, we will show a facile method to preparenanometer WC–Co composite powders. Here, the hydrothermalmethod was adopted to prepare the carbon-shelled core–shellprecursor to which the elemental W, Co and C are tightly mixed

Fig. 1. The schematic illustration of preparation mechanism of WC–Co nanocomposite powders.

Fig. 3. HRTEM images of the as-prepared precursor.

H. Lin et al. / Materials Research Bulletin 47 (2012) 3283–32863284

together. The WC–Co nanocomposite powders were obtainedthrough in situ reduction–carbonization of the precursor at lowertemperature and shorter time in vacuum. The formation mecha-nism of the WC–Co composite powders was explored.

2. Experimental details

Commercial ammonium metatungstate [AMT,((NH4)6(H2W12O40)�4H2O)], cobalt chloride (CoCl2�6H2O) andsoluble starch (C6H10O5)n (molecular weight 342.29) were usedas the source of W, Co and C, respectively. All of the reagents usedwere of analytical grade, which were purchased from ChengduKelong Chemical plant. In order to form a final product of WC–10 wt%Co, CoCl2�6H2O (0.4 g) and (C6H10O5)n (0.69 g) weretogether dissolved in 40 ml deionized water to form a transparentpurple solution. AMT (1.17 g) was dissolved in 20 ml deionizedwater under vigorous stirring at 50 8C. After being completelydissolved, the solution was mixed with the above mentionedsolution and added into a 100 ml capacity Teflon-lined autoclave.The autoclave was sealed into a stainless steel tank and maintainedat 200 8C for 8 h without shaking or stirring. When the autoclavehad been naturally cooled to room temperature, the products weretaken out and diluted by adding 100 ml deionized water to formsuspension. The puce precursor powders were obtained by spray-drying the suspension with hot air at 250 8C with a solution feedingrate of 25 ml min�1. Finally, the target products were obtained byheating the precursor powders in a vacuum furnace at 950 8C for1 h, with the temperature rate increase of 10 K min�1.

Phase identification was performed by an X-ray diffractometer(XD-3, Purkinje, Beijing) using Cu Ka radiation (l = 0.15406 nm) ata scanning rate of 0.028/s in the 2u range of 30–888. Thetransmission electron microscopy (TEM) images were taken witha Hitachi H-800 transmission electron microscope, using anaccelerating voltage of 200 kV. High-resolution transmissionelectron microscopy (HRTEM) images were operated at a FEITecnai-G2F20 using an accelerating voltage of 200 kV.

Fig. 2. The function of the amount of residual mental ions versus hydrothermal time.

3. Results and discussion

Convenient strategy of WC–Co nanocomposite is schematicallyshown in Fig. 1. The whole progress involved three primary steps.The first step is to prepare and mix aqueous solutions of theprecursor compounds. The second step is to prepare thenanostructured core/shell precursor using hydrothermal and spraydrying. The last step is in situ reduction and carbonization to obtainthe desired nanocrystalline powder.

Fig. 4. XRD patterns of the samples prepared at: (a) 850 8C, 1 h; (b) 900 8C, 1 h; (c)

950 8C, 1 h; (d) 1000 8C, 1 h; (e) 1000 8C, 4 h.

H. Lin et al. / Materials Research Bulletin 47 (2012) 3283–3286 3285

In the hydrothermal process, as the solution was sealed inautoclaves and heated to 200 8C, the metallic compound willdecompose and form nanoparticle nucleates. The temperature andtime were the critical factors to influence the final structure in thisprocess. If the temperature was lower than 180 8C, the precipitatewas less after 12 h, indicating that the nanoparticle nucleationrates were very slow. When the temperature was higher than220 8C, the carbonization was violent and some graphite sphereswould be generated. Thus we chose 200 8C as a model and foundthat the amount of metal ions to form core could be manipulated

Fig. 5. TEM images of the samples prepared at: (a)

by adjusting the time. The amount of metal ions which were notdeposited to form cores was checked through chemical analysis.The function of the amount of residual metal ions versushydrothermal time is shown in Fig. 2. It can be seen that theamount of residual metal ions decreased with prolonging thehydrothermal time. As the dwell time prolonged to 8, 10 and 12 h,the amount of residual metal ions decreased from 5.9, 1.1 to 0.8%.The core/shell structure formation mechanism was complicated,and needs more in-depth specialty research; however, a littleresidual metal ions could not affect the preparation of composite

950 8C, 1 h; (b) 1000 8C, 1 h; (c) 1000 8C, 4 h.

H. Lin et al. / Materials Research Bulletin 47 (2012) 3283–32863286

powders. In the spray drying, the residual metal ions would beevenly deposited on the surface of the core/shell particles to obtainthe uniform precursor. Fig. 3 shows the HRTEM image of theprecursor particles prepared at 200 8C for 10 h. From the picture,one can see that the particles are typical core/shell structuresencapsulated in carbonaceous shells, and take on spherical shapesof about 15–30 nm in diameter and disperse evenly.

The X-ray diffraction patterns of the products prepared underdifferent conditions are shown in Fig. 4. WC could be synthesizedin the temperature range of 850–1000 8C, Co6W6C would beformed at lower temperatures (Fig. 4(a)). By heating at 900 8C, allpeaks could be indexed as the closely packed hexagonal structureWC (JCPDS 89-2727), and the peaks of Co6W6C were vanished, onecould not find the diffraction peaks of cobalt (Fig. 4(b)). Increasingthe reaction temperature to 950 8C, the intensity of the diffractionpeaks increases, and the peaks of cobalt appeared. As shown inFig. 4(c), all sharp peaks could be attributed to WC and Co (JCPDS89-4307), without the unwanted h phase like W2C, Co6W6C,Co2W4C, and so on. The reason why Co is amorphous at 850 8C butit is crystallized at 900 8C may be attributed to that Co act as acatalyst for the WC formation [19]. After the reaction, the catalystwould recover its original character. It means that the reaction ofthis process was over. In order to investigate the stability of theproducts, the sample prepared at 1000 8C for 1 h (Fig. 4(d)) and thesample prepared at 1000 8C for 4 h (Fig. 4(e)) were investigated,respectively. Compared with the corresponding lines of Fig. 4(c),one can find that the peaks becoming sharper without anychemical change. It indicated that the particles will become greaterwith increasing temperature and prolonging dwell time, but thereis no change in the phase of products without metallic tungsten orother carbon-deficient tungsten carbides generated. This result isnot consistent with the reports which prepare products in CH4–H2

atmosphere [26]. This may be because the gases will react with WCat higher temperature to form some volatile carbon-containingspecies. In this experiment, however, the products can existstabilized in the vacuum condition. It must be pointed out that theunwanted Co6W6C, which generated at lower temperatures, couldnot be eliminated through prolonging the dwell time or increasingthe temperature. However, Co6W6C would not be generatedthrough directly increasing the temperature.

Fig. 5 shows TEM images of the as-prepared WC–Co nano-composite powders under different conditions. From the picture,one can see that the particles present spherical or ellipticalmorphologies, with uniform size distribution. The as-preparedsample at 950 8C (Fig. 5(a)) exhibits a particle size range from 15 to30 nm. At 1000 8C, the sample (Fig. 5(b)) exhibits a particle sizerange from 20 to 60 nm. With the prolonging of reaction time to4 h, the particles increase rapidly and the size is in range from 50 to80 nm (Fig. 5(c)).

To form the nanostructured WC–Co powder, a series ofdecomposition, reduction and carburization treatments werecarried out. When the precursor powders were put into thevacuum furnace, along with the increase of temperature, the gasesor impurities adsorbed at the powder surfaces would be released orvolatilized, and the gases generated from the decomposition andreduction would be timely expelled by the vacuum devices; hence,

the powders are cleaned in the vacuum system. This ensures thepurity and activity of the reactant powder. Because the elementalW, Co and C mixed tightly, carbon could easily reduce the metalcompounds and carburize with tungsten without long distancediffusion. Therefore, the whole reaction would be completed at therelatively lower temperature and shorter time, which can inhibitcoarsening of grain. Thus fine particles and uniform particle sizedistribution can be achieved.

4. Conclusion

WC–Co nanocomposite powders with a diameter of about 15–30 nm were successfully prepared through a hydrothermal routeto synthesize the core/shell precursor and in situ reduction andcarburization in vacuum. Based on the experiment results, thephase transformation from reactant to resultant is completed inshorter time and at a lower temperature, which would beattributed to depressing the atomic diffusion energy with theuniform mixed precursor. Such a synthetic method could easilyadjust the stoichiometry of WC–Co and add grain growthinhibitors (such as VC, Cr3C2, NbC, and TaC). So it may providean opportunity to obtain high performance cemented carbides.

Acknowledgment

This work was supported by the Fundamental Research Fundsfor the Central Universities (No. XDJK2010C009).

References

[1] D. Kim, A. Accary, Mater. Sci. Res. 13 (1980) 235.[2] R. Koc, S.K. Kodambake, J. Eur. Ceram. Soc. 20 (2000) 1859.[3] M. Koopman, K.K. Chanla, C. Coffin, B.R. Patterson, X. Deng, B.V. Patel, Adv. Eng.

Mater. 4 (2002) 37.[4] S. Berger, R. Porat, R. Rosen, Prog. Mater. Sci. 42 (1997) 311.[5] Z.G. Ban, L.L. Shaw, J. Mater. Sci. 37 (2002) 3397.[6] G.Q. Shao, B.L. Wu, X.L. Duan, J.R. Xie, M.K. Wei, R.Z. Yuan, Ceram. Trans. 15 (2000)

375.[7] S. Wahlberg, I. Grenthe, M. Muhammed, Nanostruct. Mater. 9 (1997) 105.[8] B.K. Kim, G.H. Ha, G.G. Lee, D.W. Lee, Nanostruct. Mater. 9 (1997) 233.[9] G.R. Goren-Muginstein, S. Berger, A. Rosen, Nanostruct. Mater. 5 (1998) 795.

[10] S.W.H. Yin, C.T. Wang, Tungsten: Sources, Metallurgy, Properties andApplications, Plenum Press, New York, NY, 1979.

[11] M.A. Xueming, J.I. Gang, J. Alloys Compd. 245 (1996) 30.[12] Z.Y. Zhang, S. Wahlberg, M.S. Wang, M. Muhammed, Nanostruct. Mater. 12 (1999)

163.[13] B.H. Kear, L.E. McCandlish, Nanostruct. Mater. 3 (1993) 19.[14] Z. Xiong, G.Q. Shao, X.L. Shi, X.L. Duan, L. Yan, Int. J. Refract. Met. Hard Mater. 26

(2008) 242.[15] F.L. Zhang, C.Y. Wang, M. Zhu, Scripta Mater. 49 (2003) 1123.[16] M. Mahmoodan, H. Aliakbarzadeh, R. Gholamipour, Int. J. Refract. Met. Hard

Mater. 27 (2009) 801.[17] J. Sun, F. Zhang, J. Shen, Mater. Lett. 57 (2003) 3140.[18] R. Koc, S.K. Kodambaka, J. Eur. Ceram. Soc. 20 (2000) 1859.[19] M.F. Zawrah, Ceram. Int. 33 (2007) 155.[20] F. Caruso, Adv. Mater. 13 (2001) 11.[21] W. Schartl, Adv. Mater. 12 (2000) 1899.[22] X.M. Sun, J.F. Liu, Y.D. Li, Chem. Mater. 18 (2006) 3486.[23] K.T. Lee, Y.S. Jung, S.M. Oh, J. Am. Chem. Soc. 125 (2003) 5652.[24] S. Han, Y. Yun, K.W. Par, Y.E. Sung, T. Hyeon, Adv. Mater. 15 (2003) 1922.[25] X.L. Dong, Z.D. Zhang, S.R. Jin, B.K. Kim, J. Appl. Phys. 86 (1999) 6701.[26] M. Fitzsimmons, V.K. Sarin, Surf. Coat. Technol. 76–77 (1995) 250.