International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209

www.elsevier.com/locate/ijrmhm

One step synthesis and densification of ultra-fine WCby high-frequency induction combustion

Hwan-Cheol Kim a, In-Jin Shon a,*, Jin-Kook Yoon b, Sang-Kwon Lee c, Z.A. Munir d

a Department of Advanced Materials Engineering, Research Center of Advanced Material Development, Engineering Research Institute,

Chonbuk National University, Chonbuk 560-756, South Koreab Metal Processing Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, South Korea

c Eltek Co. 90-8, Namgung building, Yangjae-dong, Seocho-gu, Seoul 137-890, South Koread Facility for Advanced Combustion Synthesis, Department of Chemical Engineering and Materials Science, University of California,

Davis, CA 95616, USA

Received 23 February 2005; accepted 18 March 2005

Abstract

Dense WC with grain size of 0.43 lm was synthesized by high-frequency induction combustion synthesis from milled elemental

powders of W and C. The milled W powders had a grain size in the range 45–73 nm. Dense product (98.5%) could be obtained

within 2 min under a pressure of 60 MPa. Due to loss of carbon (by interaction with surface oxides), products made from stoichio-

metric powders (W:C = 1:1) contained the sub-carbide W2C. With excess carbon, products containing the WC phase only were

obtained. The effect of initial grain size (of W) and the W:C stoichiometry on the grain size of the product WC was investigated.

The grain size of WC increased with an increase in the amount of excess carbon. The maximum values for fracture toughness

and hardness obtained for the dense WC were 4.8 MPam1/2 and 2708 kg/mm2, respectively.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Ultra-fine WC; Dense hard materials; Combustion synthesis; Hardness; Fracture toughness

1. Introduction

Tungsten carbide–cobalt (WC–Co) hard materials

are widely used in tools for machining, cutting, drilling,

and other applications. Morphologically, they consist of

a high volume fraction of the hard hexagonal WC phase

embedded within a relatively soft and tough Co binderphase [1]. Such materials are typically densified by liquid

phase sintering with the mechanical properties of the

dense product depending on composition and micro-

structure (especially on the grain size of the carbide

phase [2]). Thus, the control of grain size of the carbide

phase during liquid phase sintering is an important goal.

0263-4368/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmhm.2005.04.004

* Corresponding author. Tel.: +82 63 270 2381; fax: +82 63 270 2386.

E-mail address: ijshon@moak.chonbuk.ac.kr (I.-J. Shon).

In general, decreasing the carbide grain size increases

such properties as hardness, wear resistance, and trans-

verse rupture strength of the cemented carbide compos-

ites. Furthermore, increasing the volume fraction of Co

increases the fracture toughness but at the expense of

hardness and wear resistance [3,4].

Cemented carbides are usually prepared by consoli-dating WC powders with the cobalt binder by conven-

tional sintering techniques at temperatures near the

melting point of cobalt. In view of its high-melting

point, WC is difficult to sinter without the addition of

Co or another low-melting binder. The binder facilitates

sintering by the presence of a liquid phase [5]. However,

the advantage of the addition of the binder (gained in

the sintering process) is counteracted by deleterious ef-fects on the cemented carbides. The binder phases are

inferior to the carbide phase in chemical characteristics.

H.-C. Kim et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209 203

Corrosion and oxidation attacks occur in the binder

phase [6]. To overcome this, effort was made to develop

binderless cemented carbides, i.e., through the addition

of other ceramic phases to WC. Such carbides, exempli-

fied by WC–TiC–TaC, have been utilized in mechanical

seals and sliding parts because of their corrosion resis-tance [7]. TiC has been used as a carbide binder because

it forms WC–TiC solid solution phase [8]. However, in

such binderless cemented carbides, segregation of car-

bon at the grain boundaries between WC and TiC grains

has been observed, resulting in a decrease in the wear

resistance and toughness of materials [9,10].

In all of the reported studies on the preparation of

dense bulk WC, the process includes two steps: the syn-thesis of the carbide phase and the subsequent consoli-

dation with or without the metallic additive. Recently

it was shown that the method of high-frequency induc-

tion combustion could be successfully employed to syn-

thesize and densify in one step, and in a relatively short

time. The method has been used to synthesize a variety

of ceramics and composites, including WSi2 and MoSi2and their composites, and WC–Co hard materials [11–14]. These materials, which are generally characterized

by low adiabatic combustion temperatures, cannot be

synthesized directly by the self-propagating high-tem-

perature synthesis (SHS) method. And when formed

by the SHS method through thermal activation, WC,

for example, had low relative density and contained

W2C as a second phase [12,13].

In this paper we report on the simultaneous synthesisand properties of dense ultra-fine WC using elemental

reactants of W and C.

Al2O3 Block

GraphitePunch

Graphite Die

PowderMaterials(W+C)

Pressure Application

High-frequencyInduction Coil

Fig. 1. Schematic diagram of the high-frequency induction combus-

tion apparatus.

2. Experimental procedure

Powders of 99.9% pure tungsten (with average sizes

of 0.4 lm and 4.3 lm measured by FSSS, Korea Tung-sten Co., Teagu, South Korea) and 99.9% pure activated

(amorphous) carbon (<20 lm, Kojundo Chemical Co.

Osaka, Japan) were used as starting materials. The ini-

tial particle size of tungsten is according to the specifica-

tion by the vendor. The tungsten and carbon ratio was

varied from 1:1 to 1:2 for the case of 0.4 lm tungsten

and from 1:1 to 1:1.3 for the case of 4.3 lm tungsten

to investigate the effect of stoichiometry on the micro-structure and mechanical properties of the WC product.

Tungsten and carbon powder mixtures were first milled

in a high-energy ball mill, Pulverisette-5 planetary mill.

Tungsten carbide balls (5 mm in diameter) were used

in a sealed cylindrical steel vial under argon atmosphere.

The weight ratio of ball-to-powder was 30:1 and the

powders were milled for 10 h. Milling resulted in a sig-

nificant reduction of grain size. The grain size and theinternal stress are calculated by Strokes and Wilsol�s for-mula [15],

b ¼ bd þ be ¼ kk=ðd cos hÞ þ 4e tan h ð1Þwhere b is the full width at half-maximum (FWHM) of

the diffraction peak after instrument correction; bd andbe are FWHM caused by small grain size and internal

stress, respectively; K is constant as 0.9; k is wavelength

of the X-ray radiation; d and e are grain size and internal

stress, respectively; and h is the Bragg angle. b and bsfollow Cauchy form with the relationship: B0 = b + bs,where B0 and bs are FWHM of broadened Bragg peaks

and the standard sample�s Bragg peaks, respectively.

The average grain size measured by Stoke–Wilson equa-tion was about 45 nm and for the powder with an initial

size of 0.4 lm and 73 nm for the 4.3 lm tungsten. Thus

reference to grain size will make for milled powders, i.e.,

sizes of 45 and 73 nm.

The milled powders were placed in a graphite die

(outside diameter, 45 mm; inside diameter, 20 mm;

height, 40 mm) and then introduced into the high-

frequency induction system (Eltek Co., Seoul, SouthKorea). A schematic of the system is shown in Fig. 1.

The system was first evacuated and a uniaxial pressure

of 60 MPa was applied. An induced current (frequency

of about 50 kHz) was then activated and maintained un-

til densification was observed, indicating the occurrence

of the reaction and the concomitant shrinkage of the

sample. Sample shrinkage is measured by a linear gauge,

measuring the vertical displacement. The choice of thelevel of the induced current was based on preliminary

experiments aimed at determining the highest heating

rate (highest output of total power capacity). Tempera-

tures were measured by a pyrometer focused on the sur-

face of the graphite die. At the end of the process, the

induced current was turned off and the sample was al-

lowed to cool to room temperature. The entire process

of densification using this technique consists of four

204 H.-C. Kim et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209

major control stages. These are chamber evacuation,

pressure application, power application, and cool down.

Typical parameters for the process are presented in Ta-

ble 1. The process was carried out under a vacuum of

4 · 10�2 Torr.

The relative density of the synthesized sample wasmeasured by the Archimedes method. Microstructural

information was obtained from product samples which

had been fractured or etched, using a Murakami�s re-

agent for 1–2 min at room temperature. Compositional

and microstructural analyses of the products were made

through X-ray diffraction (XRD) and scanning electron

microscopy (SEM) with energy dispersive spectroscopy

(EDS). Vickers hardness was measured by performingindentations at a load of 10 kg and a dwell time of

15 s. The carbide grain size dwc was obtained by the lin-

ear intercept method [16,17]. The carbon content of the

synthesized WC was determined analytically (Leco CS-

400, St. Joseph, MI).

Table 1

Processing parameters of high-frequency induction combustion syn-

thesis and densification of WC

Parameter Applied value

Vacuum level 40 mTorr

Applied pressure 60 MPa

Induction frequency 50 kHz

Total power capacity 15 kW

Output of total power 80% and 90%

Duration 2 min

Heating 1200 �C/min

Cooling rate 600 �C/min

0 50 100

500

600

700

800

900

1000

1100

1200

1300

Time (

Tem

pera

ture

(o C)

Fig. 2. Variations of temperature and shrinkage displacement with heating t

total power, W:C = 1:1).

3. Results and discussion

The variations of shrinkage displacement and tem-

perature of the graphite die surface with heating time

during the processing of tungsten and carbon systems

with ratio of 1:1 under 60 MPa pressure and 80% outputof total power capacity are shown in Fig. 2. The heating

rate of the die was about 1000 �C/min. As the induced

current was applied, the shrinkage displacement

increased gradually with temperature up to about

940 �C, and then abruptly increased at about 1050 �C.When the reactant mixture of W + C was heated under

60 MPa pressure to 940 �C, no reaction took place and

no significant shrinkage displacement was observed, asjudged by subsequent XRD and SEM analyses. Fig. 3

shows the SEM (secondary electron) image of (a) the

milled reactant powder, (b) a sample heated to 940 �C,and (c) a sample heated to 1250 �C. Fig. 3(a) and (b)

shows the presence of the reactants as separate phases.

X-ray diffraction results, shown in Fig. 4(a) and (b), re-

veal peaks pertaining to the reactant W only (the carbon

used was amorphous). However, when the temperaturewas raised to 1250 �C, the powders reacted producing

porous and non-faceted products. SEM (secondary elec-

tron) images of an etched surface of the samples heated

to 1250 �C under a pressure of 60 MPa is shown in Fig.

3(c). It was determined from an EDS analysis that an

incomplete reaction between the reactants had taken

place under these conditions. This was supported by

X-ray diffraction analyses which showed peaks of prod-uct phases, WC and W2C phase, Fig. 4(c).

When synthesis was carried out with a reactant stoi-

chiometric ratio of W:C = 1:1, the product contained

significant amounts of W2C due to loss of carbon by

150 200 250

sec)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

Shrin

kage

dis

plac

emen

t (m

m)

ime during synthesis and densification in WC (60 MPa, 80% output of

Fig. 3. Scanning electron microscope images of W + C system

(60 MPa, 80% output of total power, W:C = 1:1): (a) after milling,

(b) before combustion synthesis, (c) after combustion synthesis.

20 30 40 50 60 70 80

Inte

nsity

2 Theta

W+C - after milling W+C - before synthesis W+C - after synthesis

: W2C: WW: WCWC

a

b

c

(a)(b)(c)

Fig. 4. XRD patterns of W + C (60 MPa, 80% output of total power,

W:C = 1:1): (a) after milling, (b) before combustion synthesis, (c) after

combustion synthesis.

0 10 20 30 40 50 60 70

500

600

700

800

900

1000

1100

1200

1300

1400

Time (sec)

Tem

pera

ture

(o C)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

Shrin

kage

dis

plac

emen

t (m

m)

Fig. 5. Variations of temperature and shrinkage displacement with

heating time synthesis and densification of WC (60 MPa, 90% output

of total power, W:C = 1:1.3).

H.-C. Kim et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209 205

interaction with the surface oxide on the tungsten parti-

cles. Because of the narrow compositional range of WC,

the loss of even small amounts of carbon contributes to

this problem. That the loss is related to the presence of

surface oxide has been indirectly shown by studies ontungsten with different particle size. Those with smaller

size experienced greater carbon loss [18]. Furthermore,

the relative density of the W:C = 1:1 samples was low,

about 75%. To compensate for the loss of carbon, sam-

ples were prepared with W:C ratios ranging from 1:1 to

1:2, and synthesized under 90% output of total power

capacity. Fig. 5 shows the variations of shrinkage dis-

placement and temperature with heating time during

the processing of W + 1.3C sample using the 45 nm

(milled) tungsten. In contrast to the case of the

W:C = 1:1 ratio samples (Fig. 2), the higher rate of

shrinkage begins at a much shorter time, a consequenceof the higher power.

Fig. 6 shows the XRD patterns of products produced

under 60 MPa pressure and 90% output of total power

capacity with various tungsten–carbon ratios. In the

case of W:C = 1:1.3 samples, only WC peaks are ob-

served, as can be seen from Fig. 6. With higher carbon

content (W:C > 1:1.5), extra carbon is observed in the

20 30 40 50 60 70 80

Rel

ativ

e In

tens

ity

2 Theta

: WC: W2C

1:2.0

1:1.5

1:1.3

1:1

Fig. 6. XRD patterns of WC after combustion synthesis with various

W:C ratios (W: 45 nm, 60 MPa, 90% output of total power).

20 30 40 50 60 70 80

1:1.5

1:1.3

1:1.2

1:1.1

1:1

Rel

ativ

e in

tens

ity

2 Theta

: WC: W2C

Fig. 8. XRD patterns of WC products after combustion synthesis with

various W:C ratios (W: 73 nm, 60 MPa, 90% output of total power).

206 H.-C. Kim et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209

product. The fracture surface images of the WC prod-

ucts obtained with various W:C ratios are shown inFig. 7. In samples with W:C of 1:2.0, carbon is present

in the product, as can be seen in Fig. 7(d). Furthermore,

the W:C ratio seems to have an effect on the grain size of

the WC phase. As the ratio increased from 1:1.3 to 1:2.0,

the grain size of WC increased significantly, with the

W:C = 1:2 sample showing abnormal grain growth.

The average grain sizes of samples, determined by the

linear intercept method, were about 0.43, 1.8, and4.5 lm for the cases of W:C ratios of 1:1.3, 1:1.5, and

1:20, respectively. The abnormal grain growth in the

presence of carbon has been observed previously but re-

Fig. 7. Fracture surface images of WC products with various W:C ratios (a) 1

total power).

mains not well understood [18]. It has been proposed

that carbon lowers the activation energy for two-dimen-

sional nucleation on singular grain boundary surfaces of

WC [18].

Examining Fig. 7(a), the fracture surface of WC with

ratio of W:C = 1:1, shows porous regions. These appar-ent pores are the results of pull-outs of the brittle W2C

phase during fracture. When the carbon ratios of 1:1.5

and 1:2.0 were used, excess carbon was seen in the

boundaries, as confirmed by X-ray mapping analysis.

As indicated above, when a ratio of 1:1.3 was used,

:1, (b) 1:1.3, (c) 1:1.5, and (d) 1:2.0 (W: 45 nm, 60 MPa, 90% output of

0.9 1.0 1.1 1.2 1.3 1.4 1.5 2.03.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Carbon contentin stoichiometric WC

Car

bon

cont

ents

(wt.%

)

Tungsten-Carbon ratio

0.4 micron tungsten 4.0 micron tungsten

Fig. 9. Carbon content of sintered WC from 45 and 73 nm powders

with various tungsten–carbon ratios.

H.-C. Kim et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209 207

the product contained WC only, with a grain size of

about 0.43 lm. It is interesting to note that the induc-

tion power level had a significant effect on the density

of the product. The relative density of samples with

W:C = 1:1 increased from 75% to 98% as the power level

was increased from 80% to 90%. Thus an increase in thepower increases the relative density, regardless of carbon

content. The presence of W2C, thus, does not in itself

contribute to the porosity.

To investigate the effect of tungsten particle size on

synthesis, milled tungsten powders with a grain size of

73 nm were used. Fig. 8 shows the XRD patterns of

WC after synthesis with various W:C ratios under

60 MPa pressure and 90% output of total power capac-ity. When W:C ratios of 1:1 and 1:1.1 were used, the

phase W2C was present in the product. With an excess

carbon content of 20% (W:C = 1:1.2) the sub-carbide

phase was absent. This is in contrast to the case of the

finer grain size W (45 nm) where the sub-carbide phase

persisted until a higher W:C ratio (1:1.3) was used, an

observation that supports the role of the surface oxide

in the loss of carbon and is in agreement with previousreports [18]. Fig. 9 shows the results of carbon analyses

of products made with differing W:C ratios. In all cases,

the carbon content is higher for the cases where the lar-

ger particle size of W was used. The loss of carbon is sig-

nificant in both cases, being as high as about 40 mol.%

for the case of the 45 nm tungsten powder at W:C =

1:1. The analyses reveal that the W:C ratios which

resulted in only WC are relatively close to the stoichiom-etric value shown in the figure.

Fig. 10. Fracture surface images of WC products with various W:C ratios: (a

of total power).

The fracture surface images of the products with dif-

ferent W:C ratios for samples with 45 nm tungsten are

shown in Fig. 10. An increase of amount of excess car-

bon resulted in an increase in the grain size of the tung-sten carbide. The pores present in samples with

W:C = 1:1 are believed to be the consequence of W2C

pull-outs during fracturing, as indicated above. The

grain size of WC for the case where it is the only phase

in the product (i.e., W:C = 1:1.2) was determined by the

linear intercept method to be about 0.6 lm for the

samples with an initial grain size of 73 nm. Table 2

shows values of the density, sample volume, and volumechange at different stages in the synthesis and densification

) 1:1, (b) 1:1.1, (c) 1:1.2, and (d) 1:1.3 (W: 73 nm, 60 MPa, 90% output

Table 2

Density, volume, and volume change during high-frequency induction combustion synthesis of WC

Property Initial sample Before ignition Reactant (Theo.) Product

Exp. Theo.

Density (g/cm3) 8.38 10.00 13.88 15.48 15.70

Sample volume (cm3) 1.79 1.50 1.08 0.97 0.96

Pore volume (cm3) 0.71 0.42 0.00 0.01 0.00

Volume change (%) 0.00 16.20 39.66 45.81 46.37

Incremental volume change (%) 0.00 16.20 23.46 6.15 0.56

208 H.-C. Kim et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 202–209

of WC under 60 MPa pressure and 90% output of total

power capacity using 45 nm tungsten and a ratio of

1:1.3. The table shows that 16% of the total volume

shrinkage occurred prior to ignition and 46% occurred

during the synthesis and consolidation stage.

Vickers hardness measurements were made on pol-

ished sections of the WC product using a 10 kgf load

and a 15 s dwell time. The calculated hardness valuesof the materials using the 45 and 73 nm tungsten were

2708 and 2552 kg/mm2, respectively, for the ratios 1:1.3

for the 45 nm and 1:1.2 for the 73 nm sizes. These values

represent averages of eight measurements. Indentations

with large enough loads produced radial cracks emanat-

ing from the corners of the indent. The length of these

cracks permits an estimation of the fracture toughness

of the material by means of Anstis expression [19]:

KIC ¼ 0.016ðE=HÞ1=2P=C3=2 ð2Þwhere E is Young�s modulus, H the indentation hard-ness, P the indentation load, and C is the trace length

of the crack measured from the center of the indenta-

tion. The calculated fracture toughness values of the

WC using the 45 and 73 nm powders were 4.4 and

4.8 MPam1/2, respectively, for the ratios 1:1.3 for the

smaller W powder and 1:1.2 for the larger powder. As

in the case of hardness values, each of the toughness val-

ues is the average of eight measurements. It should berecalled that the grain sizes of the dense samples made

from the powders indicated above are 0.43 and

0.6 lm, respectively. With such similar values, the values

of fracture toughness as well as the hardness are rela-

tively similar. The former changing by <10% and the lat-

ter by about 6%.

4. Summary

Using high-frequency induction combustion, the

simultaneous synthesis and densification of binderless

WC hard materials was accomplished using milled ele-

mental powders of W and C. The process was achieved

within 2 min. When a stoichiometric W:C ratio (1:1) was

used, the product contained the sub-carbide, W2C. Thisis attributed to the loss of carbon through interaction

with surface oxides. With the use of excess carbon, prod-

ucts containing the WC phase only can be obtained. The

final product had a relative density of 98.5% and a grain

size of 0.43–0.6 lm, when synthesized under an applied

pressure of 60 MPa. The fracture toughness and hard-

ness values for the dense WC are 4.4–4.8 MPam1/2 and

2552–2708 kg/mm2, respectively.

Acknowledgement

This work was supported by KISTEP (Korean Insti-

tute of Science and Technology Evaluation and Plan-

ning) through a National R&D Project for Nano-

Science and Technology under the contract #

M10212430003-04M0343-00210 (2004). The support to

one of us (ZAM) by the US Army Research Office

(ARO) is acknowledged.

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