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Materials Science and Engineering A 527 (2010) 584–589

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

ulsed electric current sintering and characterization of ultrafinel2O3–WC composites

huigen Huang, Kim Vanmeensel, Omer Van der Biest, Jef Vleugels ∗

epartment of Metallurgy and Materials Engineering (MTM), Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium

r t i c l e i n f o

rticle history:eceived 30 June 2009eceived in revised form 7 August 2009ccepted 13 August 2009

a b s t r a c t

Ultrafine Al2O3 powder mixtures with up to 80 vol% ultrafine WC were prepared and consolidatedby pulsed electric current sintering (PECS) for 4 min at 1250–1750 ◦C under a mechanical pressure of60 MPa. Fully dense composites with ≤40 vol% WC could be achieved when PECS at 1450 ◦C for 4 min,whereas 1650 ◦C was needed for the WC matrix composites. The microstructure, electrical conductivity

eywords:eramic compositesintering behaviourrain growth

and mechanical properties of the composites were assessed as a function of the WC content. Microstruc-tural analysis showed that Al2O3 and WC grain growth was significantly suppressed due to secondaryphase addition, resulting in ultrafine (<500 nm) microstructures. Excellent mechanical properties, com-bining a Vickers hardness of 25 GPa, fracture toughness of 4.8 MPa m1/2 and flexural strength of 1000 MPa,as well as electrical conductivity of 1.09E+6 �−1 m−1 were obtained for the Al2O3–60 vol% WC composite.

echanical propertiesulsed electric current sintering

. Introduction

As one of the most commonly applied engineering ceramics,l2O3 has great potential to be used in many applications where

ow density, high hardness, chemical inertness and good high-emperature properties are required [1,2]. The intrinsic low fractureoughness and modest flexural strength of bulk Al2O3 ceramicsowever limits the applicability of pure alumina under severe con-itions such as for high-speed cutting tools. The incorporation ofecondary phase particulates into a ceramic matrix on the otherand can largely improve the mechanical properties of ceram-

cs. The flexural strength and fracture toughness of Al2O3 matrixeramics can be enhanced by incorporating secondary phase par-icles such as WC [3–6], TiC [7–9], NbC [10] or TiB2 [11–13].l2O3 reinforced with hard ceramic particles represents a newlass of materials with improved mechanical properties, hard-ess and wear resistance when compared to monolithic aluminaeramics [3–13]. Al2O3 reinforced with WC has been investigatedor metalworking applications, combining resistance to high ser-ice temperatures and improved toughness [3–6]. The presencef dispersed WC particles is reported to enhance the hardness

nd thermal shock resistance of Al2O3 ceramics [14]. Anotherdvantage of Al2O3–WC composites is the thermo-mechanicallyompatibility of Al2O3 and WC due to their similar thermal expan-ion coefficients, i.e., 4–9 × 10−6/◦C for Al2O3 and 5–7 × 10−6/◦C

∗ Corresponding author. Tel.: +32 16 321244; fax: +32 16 321992.E-mail address: Jozef.Vleugels@mtm.kuleuven.be (J. Vleugels).

921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2009.08.035

© 2009 Elsevier B.V. All rights reserved.

for WC, limiting thermal residual stresses during thermalcycling.

One possible route for the production of nanometric WC dis-persed Al2O3 composite is high-energy reactive milling of WO3,Al and C powder mixtures and subsequent sintering at ele-vated temperature [15,16]. WC, W2C and Al2O3 were concurrentlyformed during the reduction and carbonisation processes [15,16].The sintered composite obtained from in situ synthesized mix-tures however exhibits an uneven WC grain size distributionfrom 1 to 10 �m [15]. Another approach is to sinter Al2O3–WCparticulate mixtures by means of pressureless sintering or hotpressing. Fully dense Al2O3-composites with up to 10 vol% WCwere obtained by pressureless sintering with or without Y2O3sintering additive [3–6]. When forming a percolating WC grainstructure around 30 vol% WC, densification of Al2O3–WC com-posites was hard to achieve without applying external pressure,resulting in a modest hardness and flexural strength. Densificationof such composites to closed porosity is only possible by pressure-assisted sintering or with the use of liquid phase forming sinteringadditives.

Despite numerous reports on the synthesis and characteriza-tion of Al2O3–WC composites, little information is available on finegrained composites with a substantial WC content. In the presentstudy, Al2O3–WC composites with up to 80 vol% WC were densified

by pulsed electric current sintering (PECS), also known as sparkplasma sintering (SPS). This sintering method has the advantageof higher heating rates and shorter dwelling times in comparisonwith conventional sintering techniques like hot pressing, pressure-less sintering and hot isostatic pressing [17,18]. Higher densities,

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maximum shrinkage rate at 1450 C. Full density was reached after1 min at 1650 ◦C under a pressure of 60 MPa. With increasing WCcontent, no apparent differences in the onset of densification andmaximum shrinkage rate were detected in the WC matrix A80Wcomposite. In contrast to the A40W ceramic, densification contin-

S. Huang et al. / Materials Science

efined microstructures, clean grain boundaries, surface impuritylimination as well as an overall improvement in materials per-ormance have been reported for several PECS ceramic materials18–20].

. Experimental procedure

.1. Materials preparation

Al2O3–WC composites were made from Al2O3 (TM-DAR, 0.2 �m,aimei Chemicals Co. Ltd, Nagano, Japan) and WC (CRC-015, 0.5 �m,olfram Bergbau und Hütten GmbH, Austria) powders. Composite

rades A20W, A40W, A60W and A80W, with 20, 40, 60 and 80 vol%C, respectively were prepared by low energy multidirectionalixing (Turbula T2A, WAB, Switzerland) in ethanol for 24 h usingl2O3 milling balls (� = 10 mm, grade AL9, Cerlim, France). Afterixing, the suspension was dried in a rotating evaporator at 65 ◦C.

ECS (Type HP D 25/1, FCT Systeme, Rauenstein, Germany) was per-ormed in a vacuum of 4 Pa. A pulsed electric current was appliedith a pulse duration of 10 ms and pause time of 5 ms throughout all

he experiments. The powder mixture was poured into a cylindricalraphite die with an inner diameter of 30 mm and outer diameterf 56 mm and sintered for 4 min at 1250–1650 ◦C, under a maxi-um pressure of 60 MPa, with a heating and initial cooling rate of

00 ◦C/min. During PECS, the mechanical pressure was increasedontinuously from 7 to 30 MPa during heating from 450 to 1050 ◦Cnd from 30 to 60 MPa within 1 min at 1050 ◦C. The maximumressure was retained until completion of a 4 min dwell sectiont the maximum temperature. Graphite paper was used to sepa-ate the graphite die/punch/powder set-up. A 10 mm thick porousarbon felt insulation was placed around the graphite die to obtainhomogeneous temperature distribution in the powder compact.he sintering temperature was measured by a two-colour pyrome-er (400–2300 ◦C, Impac, Chesterfield, UK), focussed at the bottomf a central borehole in the upper punch, only 2 mm away fromhe top surface of the sample. The actual set-up and temperature

onitoring procedure is described in detail elsewhere [21].

.2. Characterization

After PECS and sand blasting, the sintered discs had a thicknessf 4 mm and diameter of 30 mm. The samples were subsequentlyross-sectioned and polished to a 1 �m finish. The bulk density ofhe sintered composites was measured in ethanol. Phase identifi-ation was conducted by a �–� X-ray diffractometer (XRD, Seifert,hrensburg, Germany) using Cu K� radiation (40 kV, 40 mA). Theicrostructure of the starting powders and sintered compositesere examined by scanning electron microscopy (SEM, XL30-FEG,

EI, Eindhoven, The Netherlands). The Vickers hardness, HV10, waseasured (Model FV-700, Future-Tech Corp., Tokyo, Japan) with

n indentation load of 98.1 N. The fracture toughness, KIC, was cal-ulated from the length of the radial cracks of the indentationsccording to the formula proposed by Anstis et al. [22]

IC = 0.016(

E

H

)1/2 (P

c3/2

), (1)

here KIC is the indentation toughness (MPa m1/2), E the Young’sodulus (GPa), H the Vickers hardness (GPa), P the indentation

oad (N) and c the half-length of the radial crack (m). The flexuraltrength at room temperature was measured in a three-point bend-ng test (Series IX Automated Materials Testing System 1.29, Instron

orporation) with a span width of 20 mm and a crosshead displace-ent of 0.1 mm/min on rectangular (25 mm × 3 mm × 2 mm) bars,hich were cut from the PECS discs by electrical discharge machin-

ng. All surfaces were ground (grinding wheel type D46SW-50-X2,echnodiamant, The Netherlands) on a Jung grinding machine

gineering A 527 (2010) 584–589 585

(JF415DS, Göppingen, Germany). The edges of the rectangular barswere not chamfered. The elastic modulus, E, of the Al2O3–WCcomposites was measured on rectangular bars by the resonancefrequency method [23]. The resonance frequency was measuredby the impulse excitation technique (Grindo-Sonic, Lemmens N.V.,Leuven, Belgium). The reported hardness, toughness and flexuralstrength values are the mean and standard deviation of five mea-surements. The electrical resistance of the samples was measuredaccording to the 4-point contact method using a Resistomat (TYP2302 Burster, Gernsbach, Germany).

3. Results and discussion

3.1. Densification behaviour

The densification during PECS, i.e. the shrinkage and shrink-age rate, of the Al2O3–WC composites was automatically recordedfrom the displacement of the upper piston of the equipment. PureAl2O3 and WC powders were sintered as reference revealing thatmonolithic Al2O3 could be fully densified within 4 min at 1250 ◦C,whereas pure WC could only be densified at 1900 ◦C for 2–4 min at60 MPa. The shrinkage behaviour of the A40W and A80W compos-ite powders, PECS for 4 min at 1650 ◦C and 60 MPa, are compared inFig. 1. Due to the high heating and cooling rate, the overall durationof the thermal cycle was less than 20 min. The onset of shrinkageat 1050 ◦C is due to the increased mechanical pressure from 30 to60 MPa. For the Al2O3 matrix A40W composite (Fig. 1a), rapid den-sification was achieved during heating from 1150 to 1600 ◦C, with a

◦

Fig. 1. Sintering behaviour of Al2O3 composites with 40 (a) and 80 (b) vol% WCduring PECS for 4 min at 1650 ◦C and 60 MPa.

586 S. Huang et al. / Materials Science and Engineering A 527 (2010) 584–589

Table 1PECS parameters and properties of Al2O3 and Al2O3–WC composites.

WC content(vol%)

Temperature(◦C)

Time(min)

Pressure(MPa)

Density(g/cm3)

Relativedensity (%)

E-modulus(GPa)

Hardness(HV10) (GPa)

Toughness (KIC)(MPa m1/2)

Bending strength(MPa)

0 1250 4 60 3.98 100 390 20.0 ± 0.5 2.1 ± 0.1 530 ± 690 1450 4 60 3.97 99.8 387 19.4 ± 0.4 2.5 ± 0.2 622 ± 56

20 1450 4 60 6.26 100 427 20.4 ± 0.2 3.3 ± 0.1 775 ± 14740 1450 4 60 8.54 99.2 474 23.8 ± 0.3 3.6 ± 0.2 1164 ± 3040 1650 4 60 8.58 99.4 480 24.0 ± 0.4 4.3 ± 0.4 1187 ±11760 1650 4 60 10.91 99.5 529 25.1 ± 0.3 4.8 ± 0.6 1009 ± 20080 1650 4 60 13.07 98.1 585 25.3 ± 0.5 5.3 ± 0.4 892 ± 211

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The Young’s modulus (E), Vickers hardness (HV10) and fracturetoughness (KIC) of the PECS composites are compared with those ofpure alumina and WC in Table 1. The measured and calculated E-moduli of the fully densified composites are graphically presentedas a function of the WC content in Fig. 4. The E-moduli were esti-

80 1750 4 60 13.21 99100 [27] 1900 2 60 15.66 99

ed during the dwelling stage of 4 min. The sintering behaviour ofl2O3–WC composites is comparable to that of Al2O3–SiC compos-

tes, where a higher sintering temperature is needed to densify theigher SiC content composites [24].

The variation of the relative density with PECS temperature andC content is summarised in Table 1, indicating that the 20, 40 and

0 vol% WC composites could be densified up to more than 99% ofhe theoretical density when PECS for 4 min at 1450 and 1650 ◦C,espectively, whereas 1750 ◦C was needed for the A80W grade.

For the Al2O3 matrix composites, the slower densification ratepon adding WC should be attributed to the covalent nature andoncomitant lower lattice diffusivity of WC, especially when annterconnected WC structure is formed above 35 vol% WC. Therain boundary diffusion path of Al2O3 grains became longer,nd the diffusion of atoms/vacancies along the interface alsoecreased due to the presence of secondary particles [25]. More-ver, plastic deformation of the Al2O3 phase is significantlyuppressed by the presence of secondary harder phase particles26]. Therefore, higher temperatures are required for densifica-ion to compensate for the decreased diffusivity, grain mobilitynd plastic deformation, compared to pure Al2O3. On the otherand, the incorporation of 20–40 vol% Al2O3 significantly reduceshe sintering temperature required for achieving full densifica-ion from 1900 ◦C for binderless WC [27] to 1750 ◦C for A60W and80W.

.2. Phase constitution and microstructural analysis

XRD analysis of the WC starting powder revealed mainly hexag-nal WC with a small amount of W2C. Since PECS was carriedut in a reducing graphite environment, it can be expected thathe W2C phase would transform to WC due to carbon diffusion,t least on the as-sintered sample surface. As compared in Fig. 2,he XRD patterns of the as-sintered and cross-sectioned surfaces ofhe A40W composite indeed confirmed that the carbon-deficient

2C phase diminished on the as-sintered sample surface, whereas2C remained on the polished cross-sectioned surface, implying

arbon interdiffusion from the graphite punches. Since no reac-ion phases could be identified even at PECS temperatures up to750 ◦C, the Al2O3 and WC phases can be considered as chemicallytable.

Representative microstructures of the ceramics with 0–80 vol%C, PECS at 1250–1750 ◦C, are compared in Fig. 3. The dark and

right atomic number contrast phases correspond to Al2O3 andC, respectively. In the composites, the WC grains were homo-

eneously distributed in the Al2O3 matrix and vice versa. The WC

hase in the Al2O3–20 vol% WC composite is present as individualarticles or small agglomerates with a rounded surface, similar tohat of the WC starting powder.

The grain size in the pure Al2O3 ceramic, shown in Fig. 3a and, was found to have grown rapidly even when PECS at 1250 ◦C,

588 24.8 ± 0.4 5.3 ± 0.3 964 ± 150660 27.4 ± 0.1 4.4 ± 0.2 980 ± 120

whereas the Al2O3–WC composites exhibited limited Al2O3 graingrowth. Since the alumina grain size in the composites is smallerthan for the pure Al2O3 ceramic PECS at 1250 ◦C, it is clear thatthe presence of WC inhibits Al2O3 grain boundary migration, limit-ing grain growth. The grain size of the pure Al2O3 ceramics can beestimated from the fracture surface (Fig. 3a and b) due to the inter-granular fracture mode. The average Al2O3 grain size in the Al2O3ceramic PECS at 1250 and 1450 ◦C was 0.7 and 2 �m, respectively,whereas the Al2O3 grain size in A20W PECS at 1450 ◦C was <1 �m.The average Al2O3 grain size further decreased below 0.3 �m inthe 40–80 vol% WC composites, due to alumina grain isolation. Theevolution of the Al2O3 grain size in the Al2O3–WC composites isconsistent with that reported for Al2O3–SiC composites [24]. TheWC grain size on the other hand slightly increased with increasingWC content and concomitantly higher densification temperature.Since both Al2O3 and WC grain sizes in the composites with 40,60 and 80 vol% WC are below 500 nm, these composites can beclassified as having an ultrafine microstructure.

3.3. Mechanical properties

Fig. 2. XRD patterns of polished cross-section (1) and PECS surface (2) of an Al2O3

composite with 40 vol% WC.

S. Huang et al. / Materials Science and Engineering A 527 (2010) 584–589 587

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ig. 3. Fracture surfaces of Al2O3, PECS at 1250 (a) and 1450 ◦C (b), and Al2O3–WCC.

ated according to the Voigt and Reuss rule of mixtures [28] usingstiffness of 660 and 390 GPa for pure WC and Al2O3, respectively.he E-moduli of the Al2O3–WC composites increased with increas-ng WC content due to the higher intrinsic E-modulus of pure WC.

ig. 4. E-modulus of PECS Al2O3–WC composites as a function of the WC content.

osites with 20 (c, 1450 ◦C), 40 (d, 1650 ◦C), 60 (e, 1650 ◦C) and 80 (f, 1650 ◦C) vol%

The experimental data are in close agreement with the lower boundReuss value.

The hardness (HV10) and fracture toughness (KIC) evolution ofthe Al2O3–WC composites is presented in Fig. 5. Only the near fullydense ceramics with minimum grain growth are compared (see

Fig. 5. Hardness and fracture toughness of PECS Al2O3–WC composites as a functionof the WC content.

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able 1). In general, the hardness of the composites increases dras-ically from 18.5 GPa for pure Al2O3 PECS at 1250 ◦C, to 25.1 GPa forhe 60 vol% WC composite PECS at 1650 ◦C. The increased hardnesspon adding WC can be directly attributed to the incorporation ofarder WC particles as well as the Al2O3 grain refinement due torain boundary pinning. This observation is consistent with earlierndings for hot-pressed Al2O3–WC composites with up to 20 wt%C [5]. Upon further increasing the WC content up to 80 vol%, the

ardness levels off due to incomplete densification. Although theensity of the 80 vol% WC composite can be increased when PECS at750 ◦C, the hardness is lower due to grain growth, as summarised

n Table 1.The fracture toughness increases with increasing WC content

rom 2.5 ± 0.2 MPa m1/2 for pure alumina up to a maximum of.3 ± 0.4 MPa m1/2 at 80 vol% WC. Investigating the crack propa-ation of the radial cracks originating at the corners of Vickersndentations revealed that the cracks mainly propagate intergran-larly in all PECS Al2O3–WC composites, which is confirmed by theicrostructures of the fracture surfaces in Fig. 3. The predominant

oughening mechanism in the Al2O3–WC composites is thereforerack deflection, strongly promoted by the large difference in E-odulus of both phases and their chemical stability.In comparison to other reported Al2O3–WC composites made by

ot pressing or pressureless sintering [4–6], the PECS Al2O3–WComposites exhibit improved mechanical properties, especiallyardness and flexural strength due to the microstructural refine-ent. As shown in Table 1, the flexural strength of the Al2O3 matrix

omposites increases gradually from 600 MPa for pure alumina upo a maximum of 1160 MPa at 40 vol% WC. At higher WC contents,he strength decreases due to grain growth or incomplete densifi-ation. The improved mechanical properties obtained in this workre attributed to the low residual porosity and the homogeneousltrafine microstructure obtained by PECS. Optimum mechanicalroperties were obtained for the PECS Al2O3–WC composites with0 or 60 vol% WC, combining an excellent hardness of 24 GPa withquite acceptable fracture toughness of 4.5 MPa m1/2 and a highexural strength of 1000–1200 MPa. These properties are substan-ially higher than for submicrometer sized Al2O3 with a hardness of0 GPa, toughness of 2.1 MPa m1/2 and flexural strength of 530 MPa4–6].

.4. Electrical conductivity

The measured electrical resistivity and conductivity of the fullense Al2O3–WC composites are graphically presented in Fig. 6.

ig. 6. Electrical resistivity and conductivity of the Al2O3–WC composites as a func-ion of the WC content.

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gineering A 527 (2010) 584–589

Since WC has a very high electrical conductivity and Al2O3 isan insulator, the electrical resistivity of the composites decreaseswith increasing WC content and drops rapidly as the WC contentincreased from 20 to 30 vol%, indicating that the WC grains form apercolating path. The electrical resistivity of the composites con-taining 20 and 30 vol% WC is 0.115 and 2.15 E−5 � m, respectively.Once the percolated WC structure is formed, the electrical con-ductivity increased gradually with the WC content. Assuming athreshold of 1–3 � m for electrical discharge machine [29] suggeststhat the 40 and 60 vol% WC composites with excellent mechanicalproperties can be electrical discharge machined, allowing to realisecomplex shaped components.

4. Conclusions

Al2O3–WC composite powders with up to 80 vol% WC couldbe fully densified by means of pulsed electric current sinteringwithin 4 min in the 1250–1750 ◦C range under a pressure of 60 MPa.The higher the WC content, the higher the sintering temperatureneeded to achieve fully dense composites.

The dispersion of 20–60 vol% WC grains significantly suppressedAl2O3 grain growth, resulting in ultrafine grained composites withhigh stiffness. The hardness of the Al2O3–WC composites linearlyincreased from 20.0 GPa for pure Al2O3 to 24.0 GPa for the 40 vol%and 25.1 GPa for the 60 vol% WC composites. The PECS Al2O3–WCcomposites with 40 or 60 vol% WC combine an excellent hardnessof 24 GPa with a quite acceptable fracture toughness of 4 MPa m1/2

and a high flexural strength of 1000–1200 MPa. The electrical con-ductivity of these composites is high enough for the materials to besuitable for electrical discharging machining.

Acknowledgements

This work was performed within the framework of the ResearchFund of K.U.Leuven under project GOA/08/007. The authors alsoacknowledge the Belgian Federal Science Policy Office (BELSPO) forfinancing the NACER project (contract P2/00/07) and the Fund forScientific Research Flanders under grant number G.0305.07. K. Van-meensel thanks the Fund for Scientific Research Flanders (FWO) forhis post-doctoral fellowship.

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