Ceramics International 42 (2016) 9382–9386

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Acrylamide route for the co-synthesis of tungsten carbide–cobaltnanopowders with additives

Marzie Mardali a, Rasoul Sarraf-Mamoory a,n, Behzad Sadeghi b, Babak Safarbali c

a Department of Materials Engineering, Tarbiat Modares University, P.O.Box: 14115-111, Tehran, Islamic Republic of Iranb Young Researchers and Elite club, Neyshabur Branch, Islamic Azad University, Khorasan Razavi, Islamic Republic of Iranc Department of Materials Engineering, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

a r t i c l e i n f o

Article history:Received 23 February 2016Accepted 23 February 2016Available online 26 February 2016

Keywords:Tungsten carbide-cobaltNanopowdersVanadium carbideHydrogelIn-situ carbon source

x.doi.org/10.1016/j.ceramint.2016.02.15242/& 2016 Elsevier Ltd and Techna Group S.r

esponding author.ail address: rsarrafm@modares.ac.ir (R. Sarraf-

a b s t r a c t

In this work, cemented tungsten carbide nano-particles were prepared by a chemical method calledacrylamide gel. In this process, first, a xerogel containing tungsten and cobalt oxide particles was syn-thesized. Then, it was carburized by a hydrogen reduction heating process.

Acrylamide and N, N-methylene-bis-acrylamide monomers were used as an in-situ carbon. Ammo-nium meta tungstate (NH4)6H2W12O40 � xH2O, and cobalt nitrate Co(NO3)2 �6H2O salts were used as theprecursor.

Both reduction and carburization reactions were carried out simultaneously and the formation of theintermediate phases of W2C, Co3W3C, and Co6W6C led to decrease in the activation barrier.

Transactions of reduction and carburization processes were studied by X-ray diffraction analysis atvarious temperatures. Accordingly, tungsten carbide phase formation was completed at 1100 °C. Theformation of W–C and V–C bonds was verified by Raman spectroscopy. SEM images showed the averagenano particle size of 50 nm.

& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

1. Introduction

Bulk cemented tungsten carbide is widely used in demandingindustrial applications such as cutting tools, mining and oil dril-ling, blasting, transportation, forestry tools, spray nozzles, sealinggrout pumps, etc [1]. High hardness, high toughness, and wearresistance are always desirable for these applications. Further-more, Nano-crystalline grain structure using nano-sized WC-Copowders promise dramatic improvement of these properties [2].

Currently, there are many methods which are used to carburizetungsten oxide (WO3) powder for the preparation of cementedtungsten carbide nanoparticles, such as, carbothermal hydrogenreduction [3], chemical vapor condensation by methane gas [4],self-propagating high-temperature synthesis (SHS or combustionsynthesis) [5,6]. The most important disadvantage of thesemethods was the particle agglomeration during carbonization inhigh temperatures. Earlier attempts to use in-situ carbon sourcestook place in former [6]. It is well known that additional carbonremaining in the products causes the suppression of grain growthinhibition and high concentration of tungsten dissolved in thebinder [7]. To avoid the presence of excess carbon in compounds,

.l. All rights reserved.

Mamoory).

in some procedures, organic compounds are used as carbonsources [8,9]. Here we try to use acrylamide gel as a source ofcarbon as well as network steric stabilizer for nano particles.

Acrylamide gel is a new method to the synthesis of Tungstencarbide-Cobalt powders.It has the advantages such as molecular-level combination that leads a homogeneous composition, notrequiring advanced apparatus, inexpensive salts and organic ma-terials as precursors compared with metal alcoxides, and goodcontrolling of particle size by the polymeric net [9–11]. We couldalso add vanadium carbide (VC) to WC–Co powders as a graingrowth inhibitor by this simple method. Therefore, homogeneityof the additive is given by this method that has more effect ongrain growth suppression. In fact, in the hydrogel method, poly-meric net plays a spatial precision stabilization role in nanoscaledimensions and is an in-situ carbon source for carburization. Thispolymeric gel is formed by co-polymerization of acrylamide (asmonomer) and N,N-methylene-bis-acrylamide (as cross linker).Polymerization reaction initiates by the addition of ammoniumpersulphate through the production of free radicals and reactionwith inactive monomers. The resulting gel is porous and size of itspores depends on polymerization conditions. The main differencebetween this method and sol-gel is in the use of an external agentas gully maker [9–11].

The purpose of the present work is to obtain both WC andcobalt metal nanopowders simultaneously using polymeric

M. Mardali et al. / Ceramics International 42 (2016) 9382–9386 9383

monomers as a carbon source.

2. Experimental procedure

The procedure used to prepare WC-Co with VC additive na-nocomposite particles can be divided into 3 subsequent stages:solution, gelation, and calcination. First acrylamide (AM)(C2H3CoNH2, Merck, Germany) and N,N′-methylene-bis-acryla-mide (MBAM) (CH2CH3CONH)CH2, Merck, Germany) monomerswere easily dissolved in the mixture of aqueous solutions of am-monium meta-tungstate (AMT) ((NH4)6H2W12O40 � xH2O, Sigma-Aldrich), cobalt nitrate (Co(NO3)2 �6H2O, Merck, Germany), andammonium meta-vanadate (NH4VO3, Merck, Germany). The con-centration of monomers solution was 1.2 M. Then, the resultingsolution was stirred completely for 45 min and gradually heated ina water bath at 75 °C to obtain a transparent pink sol. When thetemperature reached about 75 °C, 2 mL of 0.2 M ammonium per-sulphate aqueous solution ((NH4)2S2O8(APS), 10 wt%, Merck, Ger-many) was added to the mixture as the initiator. An orange semi-transparent gel was rapidly produced. The gel was dried at 100 °Cfor 24 h to yield a xerogel. At the next stage, the obtained xerogelwas grinded to form a homogenous powder. The powders werecalcined at 900 and 1000 ̊C under a hydrogen atmosphere for 1 h.To analyze the thermal decomposition and reduction process, athermogravimetric (TG)/differential thermal analysis (DTA) ex-periment was carried out in hydrogen gas. The flow rate for eachgas was 50 mL/min, the heating rate was 10 ̊C/ min, and thetemperature was varied from room temperature to 1100 °C.

The structure of the powder was examined at room tempera-ture by X-ray diffraction (XRD) with Cu-Kα radiation (AXS-D8,Holland). The morphology and particle size of the powder wereexamined using FE-SEM (F4160, Hitachi, Japan). The formations ofV–C and W–C bonds were proved by Raman Spectroscopy (secondharmonic of Nd:Yag laser with λ¼532 nm).

3. Results and discussion

Fig. 1(a) shows the Raman Spectroscopy surveys for V–C andW–C bonds formation in the prepared powder. As it can be seenthe W–C bond position is at 337 cm�1. Raman shift of W¼C bondis also indicated at 937 cm�1. According to Fig. 1(a) as a reference,D peak of W–C bond is at 1340 cm�1 and moreover D peak of V–C

Fig. 1. (a) Reference Raman spectra for some carbides [12

bond is at 1360 cm�1 [12]. The asymmetric Raman shift at1353 cm�1 is assigned to the combination of D peaks of V–C andW–C bonds in Fig. 1(b). The unsymmetrical shift at 1581 cm�1 isalso referred to the blend of G peaks of W–C and V–C bonds.Decomposition of these peaks is difficult due to their proximity.These results show that the reduction process is done completelybecause there was no oxide compounds shift.

Also, X-ray diffraction pattern was used to characterzation ofthe phase transformation of the WC-10% Co nanocompositepowder during the reduction processing at 900 and 1100 °C(Fig. 2). It exhibits characterizing peaks of tungsten carbide, theintermediate phases and Cobalt at 900 °C. It seems that WC phaseis a dominant phase. By increasing temperature to 1100 °C, thepeaks of intermediate phases are eliminated and only WC and Cophase remains with a small amount of W2C phase. The averagecrystalline size is estimated from the Scherrer equation. This valueis 32 nm at 1100 °C.

Based on diffraction thermal analysis in Fig. 3, the first peak isrelated to the water evaporation at 100 °C. The weight loss in rangof 250–750 °C accompanied by the release of heat that is Due toexit of structural water and organic compounds from the system.The drastic change of thermal curve at 280 °C is the shift of processtype from endothermic to exothermic. It seems that in addition ofthe crystallization of amorphous phases, ignition has taken place.This claim has been proven in the thermos gravimeteri analysisdiagram (Fig. 3a). It shows the maximum weight change at 280 °C.Oxygen for combustion provided of organic compounds and mi-neral salts sources.

The decomposition of organic phases starts in the range of250–300 °C. Despite the removal of some of the organic com-pounds in the process of restoration, the carbon content is enoughto carburizing of the powder. It seems that in this stage, stack ofactive carbon remains in the adjacency of tungsten and cobalt ionsthrough the pores of xerogel. It makes the carburizing at highertemperatures. In the range of 450–550 °C a relatively wide exo-thermic peak can be seen that is due to reduction of Cobalt Oxide.At 750 °C with exit of remaining organic compounds from thesystem, an endothermic peak is observed in the graph. This pro-cess can also be found from the weight changes in TG diagram. Inthe DTA diagram, a relatively wide exothermic peak is observed inthe temperature range of 900–1100 °C that shows the carburiza-tion and reduction of products. In practice, due to two simulta-neous reduction and carburization process, showing a knowndistance between them is difficult.

] (b) Raman spectrum of sample sintered at 1000 °C.

Fig. 2. X-ray diffraction pattern of WC- Co reduced at (a) 900 °C and (b)1100 °C.

Fig. 3. (a) Thermo gravimeteri analysis diagram and (b) Differential thermal ana-lysis diagram.

Fig. 4. Scanning electron microscope image of the sample calcined in the magni-fication of 1000.

M. Mardali et al. / Ceramics International 42 (2016) 9382–93869384

As the results showed, In the temperature range of 300–750 °Corganic phases removed from system to form of CO, CO2 and H2Ogases. Product has a very high specific surface area due to gasemissions from raw gel resulted in a highly porous network. Theincreased surface area allows reduction and carburization at alower temperature and therefore obtained much finer particles.Fig. 4 shows a SEM image of calcined sample with porous network.

The Micrographs of the final powders calcined at 900 and1100 °C are shown in Fig. 5 wherein morphology of the powders isregular and the particle size distribution was homogenous. InFig. 5(a), the average particle size is about 50 nm.

The EDX diagram of the sample reduced at 1100 °C is indicatedin Fig. 6. The absence of any oxygen element and the presence ofcarbon in Table1 show that reduction processes are done suc-cessfully. Low weight percentage of carbon element is due to theabsorption of its characterized X-ray beam by the detector.

The mechanism of polymeric cell formation in polyacrylamidegel is demonstrated in Fig. 7. Then, ammonium meta-tungstateand cobalt nitrate are dissolved into the water by the followingreactions [13]:

(NH4)6H2W12O40þH2O-H2W12O40�6þ6NH4

þ (1)

H2W12O40�6þH2O-H2W6O20

�6 (2)

H2W6O20�6þH2O-6WO4

�2þ6Hþ (3)

Co(NO3)2þH2O -Co(NO3)þþNO3– (4)

Co(NO3)þþH2O-Coþ2þNO3� (5)

During the studies, electronegativity of Cobalt ions is more thanhydrogen ions. So it seems that the particles in the initial cellcontain a mixture of cobalt and tungsten ions.

The structure of polyacrylamide gel is a net like and the visc-osity of the solution containing WO4

�2 and Coþ2 ions is relativelyhigh. Therefore, the velocity of the ions is limited, and they areconfined in the polymeric cells. These mechanisms cause the

Fig. 5. FE-SEM photographs of WC–Co nano powders reduced at (a) 900 °C, (b) 1100 °C.

Fig. 6. EDX diagram of the sample reduced at 1100 °C.

Table1Elemental characterization of calcined sample at 1100 °C.

Elt Lin e Int Error K Kr wt%

C Ka 16.3 11.3554 0.0199 0.0178 7.69Co Ka 98.8 0.6230 0.0533 0.0477 4.02W La 255.3 0.6230 0.6383 0.5712 60.66Au La 71.4 0.6230 0.2885 0.2581 27.62

1.000 0.8948 100.00

Fig. 7. Mechanism of polymeric cell formation in polyacrylamide gel [10].

M. Mardali et al. / Ceramics International 42 (2016) 9382–9386 9385

decrease of agglomeration particles during drying and calcinationprocess.

The subsequent carburization sequence appeared asW-Co6W6C-Co3W3C-W2C-WC. The proposed route for thecarburization of W was according to the following equations:

6Wþ6 CoþC¼Co6W6C (6)

Co6W6CþC¼2Co3W3C (7)

2Co3W3CþC¼3 W2Cþ6Co (8)

As evident, cobalt plays the role of a catalyst for this process [14].

4. Conclusions

In this study, the hydrogel new method for WC-10wt% Co na-nocomposite powders was reported. Controlling the size, reduc-tion, and carburization was carried out in one step by this method.During the process, acrylamide gel was an in-situ carbon sourceand carburization reaction finished at 1100 °C with 32 nm meancrystallite size, and the average nanoparticles size of WC-10wt% Cois about 50 nm. The monomer/salt ratio of 3/1 provided the carbonrequired to carbonization process and no free carbon wasremained.

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