Spark Plasma Sintering of Diamond Binderless WC Composites

Salvatore Grasso,‡ Chunfeng Hu,§ Giovanni Maizza,¶ and Yoshio Sakkak,†,*

‡Nanoforce Technology Ltd and School of Engineering and Materials Science, Queen Mary University of London,London E1 4NS, U.K.

§Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

¶Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24,I-10129 Torino, Italy

kAdvanced Materials Processing Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen,Tsukuba Ibaraki 305-0047, Japan

A new method was developed to fully consolidate binderless

tungsten carbide and diamond powders by means of sparkplasma sintering (SPS) in current-control mode (CCm). Below

900°C, the 2 cm diameter sample was slowly heated by a dc

current of 1000 A. Above 900°C the imposed current was sud-denly raised to 4000 A. The combination of the relatively high

heating rate of 2000°C/min and the relatively short holding

time of 1.5 min (above 1300°C) was successful to fabricate

fully dense binderless WC/diamond composite. No graphitiza-tion of diamond was detected after ultrafast sintering as con-

firmed by optical and SEM microstructure observations, XRD

and Raman analysis. The sample showed very high wear resis-

tance in comparison to fully dense monolithic binderless WCcompacts. The developed method, unlike previously published

works, did not require any diamond coating to prevent diamond

graphitization.

I. Introduction

DIAMOND, owing to its excellent high hardness, has beenwidely applied as a wear-resistant material in a multi-

tude of engineering applications ranging from micromachin-ing to heavy duty mining. Many processing techniques havebeen assessed for incorporating diamond intimately in a cera-mic matrix and to obtain a dense body. The main inconve-nience encountered is the thermal metastability of diamondat high temperatures and the required low sintering pressures(�100 MPa). High pressure methods have so far been con-sidered necessary to obtain good results. The methods dis-closed in Refs. [1–3] required a high pressure and hightemperature apparatus. Using an ultra high pressure vessel,composites containing diamond particles were densified athigh temperature (i.e. 1400°C–2400°C) and high pressure (i.e.5.5–10 GPa). However, the production of these diamondmaterials requires expensive facilities such as ultra high pres-sure vessels. Table III summarizes the methods to sinterdiamond-cemented carbide composites under thermodynami-cally metastable pressure and temperature conditions fordiamond. Methods for the production of diamond-cemented

carbide based on electric current activate/assisted sintering(ECAS)13 are disclosed in a number of patents.

In 1935, Thomson Houston Co. Ltd. patented4 a sinter-bonding method to produce grinding wheels consisting of anabrading outer ring bonding to a metallic core. The outerring was composed of a powder mixture containing diamond(10 � 20 wt%) embedded in WC–Co (3 � 20 wt%). In1937, Willey et al.5 patented an ECAS method based on thereactive sintering at high temperature of diamond with tung-sten powders. In 1978, Bakul et al.6 adopted a high-fre-quency induced current source as an indirect heating sourceto enhance heating rates up to 104°C/min. The high heatingrate prevented diamond graphitization and ensured a uni-form temperature distribution across WC-6 Co/diamondcompact during sintering at 1800°C and 2 �3 s holding time.

More recently Moriguchi et al,7 starting from diamond-coated powders, developed a direct resistance heating methodto obtain diamond–tungsten carbide with cobalt (i.e., of vol-ume fraction higher than 10%).

Similarly Shi et al.8 sintered tungsten-coated diamondpowder by SPS and showed that tungsten layer partially pre-vented diamond graphitization and improved its bondingstrength with the matrix. Michalski et al.9 obtained nearlyfull dense diamond cemented carbide by employing pulseplasma sintering (PPS) method. The PPS sintering cycle con-sists of electric current pulse trains each of which severalhundred microseconds wide and several tens of kA ampli-tude.

As shown in Table I, unlike previous developed methods,the present investigation is directed to the production of bin-der-free diamond tungsten carbide composite. The directresistance heating and pressurized sintering enables rapidtemperature rise and short time sintering, resulting in dia-mond cemented carbide while inhibiting transformation ofdiamond to graphite.

II. Experimental Procedure

Commercial tungsten carbide of nanometer grain size (nWC)was supplied by KS Trading Co., Ltd., Osaka. The powderhad a BET of 5.05 m2/g. Ninety-six percent of the powderhad an average particle size below 0.1 lm (turbidimeter mea-surement). The composite powders, consisting of 22 vol%diamond and 78% nWC, were dry-mixed for 24 h in a rotaryV mixing and then poured into a cylindrical hollow graphitedie. The die and the punches were made of high strengthgraphite (Syntex Inc., Japan). The outer and inner diametersand height of the die were 50, 20.5 and 40 mm, respectively.

The diameter and height of punches were both 20 mm. Agraphite paper (0.2 mm thick) was interposed between the

E. Olevsky—contributing editor

Manuscript No. 30311. Received September 12, 2011; approved November 15,2011.

Paper Presented at the Sintering 2011 Conference.*Member, The American Ceramic Society.†Author to whom correspondence should be addressed. e-mail: sakka.yoshio@nims.

go.jp

1

J. Am. Ceram. Soc., 1–6 (2011)

DOI: 10.1111/j.1551-2916.2011.05009.x

© 2011 The American Ceramic Society

Journal

Table

I.TheMost

RelevantPapersandPatents

ConcerningtheProductionofWC

CoDiamondMaterial.TheMethodsEmployElectricCurrentto

Activate

theSinteringProcess

andPressure

TypicallyisBelow100MPa.TheMost

RelevantSinteringConditionsare

AlsoListed

Reference

Process

name

Tem

perature

(°C),

Holdingtime(s)†

Pressure

WC

powder

Diamond(vol%

),Average

particle

size

(lm)

Diamondpowder

treatm

ent

Diamondgraphitization

ThomsonLtd.4

Resistance

sintering

1000°C

;<30s

(notgiven)

WC

and3�

20%

wtCo

10-20%

wt,�5

0lm

Uncoated

Preventedbylow

sintering

temperature

andshort

holdingtime<30s

WilleyH.F5

Resistance

sintering

�1350,“A

few

second”

70MPa

WC

6wt%

Co

(notgiven),<800

Uncoated

Reactivesinteringofdiamond

withtungsten

Bakulet

al.6

Dualheating

modeSPS

<1800

(heatingrate

104°C

/min),

2–3

s

10�

20MPa

WC

6wt%

Co

�20,500�

630

Uncoated

Fullypreventedbyhighheating

rate

and2–3

sholdingtime

Moriguchi

etal.7

SPS

1300‡,180

41MPa

WC

10wt%

Coball-m

illed

20�

30,10�

50

SiC

coated

FullypreventedbytheSiC

protectivelayer

Shiet

al.8

SPS

1280,180

30MPa

WC

10wt%

Coball-m

illed

(notgiven),250

Tungsten

by

vacuum

vapor

deposition

FullypreventedbytheW

protectivelayer

Michalsky

etal.9

PPS

1100,300s

75MPa

WC

6wt%

Co

Average

particle

size

0.8

lm

30%

vol,�6

0Uncoated

Preventedbyim

pulsivedischarge

Eganand

Flynn10

EDC

Estim

ated

1200,<50ms

currentdensity

>1kA/cm

2

400MPa

WC

10wt%

Co

20�

30,500�

700

Uncoated

Fullypreventedbytheultra

rapid

compaction

Grassoet

al.11

SPS

1600,30s

120MPa

WC

10wt%

Conanosized

20–3

0%

,60

Uncoated

Fullypreventedbythe

rapid

sintering

EDC,electric

dischargecompaction;SPS,spark

plasm

asintering;PPS,pulseplasm

asintering.

†Holdingtimeatmaxim

um

temperature.

‡Measuredbythermocouple

insidethedie.

2 Journal of the American Ceramic Society—Grasso et al.

graphite punches and the inner die wall. The graphite diewas surrounded with a 2 cm thick graphite felt to minimizethe heat loss by radiation. SPS experiments were carried outwith a 100 kN SPS-1050 machine (SPS Syntex Inc., Japan)operated in current-control mode (CCm). The consolidateddisks were about 20 and 4 mm in diameter and thickness,respectively.

The on-line measurements are shown in Fig. 2. Theobtained fracture surfaces were ultrasonically cleaned in ace-tone and subsequently examined using scanning field emis-sion electron microscope (JEOL JSM-6500F, Japan). Thesintered samples were first polished with a 50 lm automaticgrinding wheel (20 000 rpm) and successively with diamondslurries. In addition, the polished surfaces were inspectedusing X-ray diffraction (XRD) (Rint 2000, Rigaku, Tokyo,Japan) and a micro-Raman spectrometer NR-1800 (JASCOCorp., Tokyo, Japan). The XRD profiles were recorded usingCuKa radiation under 40 KV and 300 mA at room tempera-ture. The micro-Raman spectrometer was used to assess theeventual presence of graphite in the sintered diamond/WCcomposite due to diamond graphitization.

The fabricated diamond/WC composite was tribologicallytested (ball-on disk tribometer CSM Instruments, SA Swit-zerland) and the results were compared with those attainedfrom a monolithic binderless nWC composite under the samedry conditions (JIS R 1613 standard). A 6 mm SiC ball slid-ed against the polished samples at a constant linear speed of0.10 m/s with an applied load of 10 N. The radius of theimprint was 3 mm and the test was carried for 30 000 revolu-tions. All the tests were performed at 25°C with a relativehumidity of ~22%.

III. Results and Discussion

The SEM image of the WC powder is shown in Fig. 1(a). Ascan be seen, the WC grains exhibited severe initial agglomer-ation. The optical micrograph of the diamond particles isshown in Fig. 1(b). The average particle size wasaround 60 lm. The SEM image of the milled powders mix-ture is shown in Fig. 1(c). As can be seen, the diamond parti-cles are fully surrounded by agglomerates of the WCnanopowder.

After the mixing process the powder mixture was sinteredby SPS under the following two-step process conditions:1 kA current was imposed; subsequently, when the tempera-ture reached 900°C the current intensity was suddenly raisedto 4–5 kA while the heating rate was as high as 2000°C/min.As shown in Fig. 2, after shutting down the electric power,both the measured temperature (at the die surface) and theshrinkage continued to increase. A constant pressure of140 MPa was applied for the entire duration of the sinteringprocess.14,15 The current supply was interrupted as soon asthe die surface attained a temperature above 1650°C. Fromeach experiment the die surface temperature, the voltagedrop between the two water-cooled rams and lower ram dis-placement were recorded. During the cooling, the similarthermal expansion coefficient of the WC and the diamondparticles introduced minimal residual stresses. For instance,at 1000°C, both the WC and the diamond thermal expansioncoefficients are identical (i.e. 5 9 10�6°C�1).16 The relativedensity of the sintered body was as high as 96%.

As reported by Moriguchi et al. (see fig. 5 of Ref. 12)upon employing low heating rate SPS (i.e. 10°C/min), thediamond particles converted into graphite. Figure 3 showsthe FESEM images of the (a) polished and (b) the fracturesurface of the WC/diamond composite. The diamond parti-cles were homogeneously distributed within the cementedcarbide matrix.

Figures 3 and 4 suggest that the diamond particles arestrongly bonded to the WC matrix. The diamond during thefracture was not torn out from the matrix, whereas mostfractured by trans-crystalline cleavage. The clean interface

between the diamond particles [Fig. 4(b)] indicates theabsence of diamond graphitization.

Figure 5 shows the XRD pattern of the WC/diamondcomposite. As can be seen, no graphite was formed and no

Fig. 1. (a) SEM image of the as received nanosized WC powder;(b) Optical micrograph of diamond particles; (c) SEM image of thediamond WC powders mixture after 24 h rotary V mixing. Insetshows a diamond particle fully covered by nanosized WC particles.

Fig. 2. Sintering parameter profiles for the diamond/cementcarbides compact as a function of time: die surface temperature,lower ram displacement and voltage drop between upper and lowerram.

Spark Plasma Sintering of WC Composites 3

reaction between WC and diamond could be detected.Figure 6 shows a Raman spectrum of the polished surface ofthe composite, the beam was focused at the interface betweenthe diamond and the WC. The only sharp peak in the spec-trum occurred at 1333 cm�1 which corresponds to the sp3

bond of diamond as diamond has a single Raman active

mode at 1332 cm�1 and no graphite peak at around1581 cm�1 was detected.

Figure 7 shows X-ray spectra of the elements present inthe diamond particle and cemented carbide matrix. The dia-gram on the right shows the results of an X-ray energy-dis-persive spectrometer (EDS) analysis across the interfacebetween the diamond particle and the cemented carbidegrain. Both the EDX map and the trace along the interfacesindicate that tungsten is located on the surface of the dia-mond particle. The thickness of the interlayer between dia-mond and WC grain is around 5 lm. By inspecting Fig. 7(e),it is observed that the gradual composition changes along theWC/diamond interface which suggests that during the sinter-ing process the diamond particle and the tungsten carbidegrain are chemically bonded through the formation of inter-mediate thin layer. As proved by Figs. 7(a) and (e) there wasno graphite at the boundary.

After the ball-on-disk wear test on bulk samples, the slid-ing wear rate was calculated from the measurement of thewear track area. Figures 8 and 9 show the SEM image of thesurfaces of the WC/diamond composite and that of themonolithic binderless WC compact, respectively after3 9 104 revolutions. The magnified image (inset Fig. 8),shows very few scratches for the composite whereas themonolithic binderless WC sample (Fig. 9) exhibits manyapparent engraved tracks caused by the SiC ball. The resultsof the wear tests are summarized in Tables II and III. Dur-ing the revolutions neither the WC grains nor the diamond

Fig. 4. FESEM image of a fracture surface of the sintered WC/diamond composite at (a) low magnification and (b) highmagnification. The diamond particles are strongly bonded to thematrix as no diamond particles were torn out from the matrix,whereas all cleaved diamond particles fractured in a trans-crystallineway.

Fig. 5. XRD pattern of the sintered WC/diamond composite.

Fig. 6. Raman scattering spectrum of sintered WC/diamondcomposite.

Fig. 3. (a) Polished surface and (b) fracture surface of the sinteredWC/diamond composite.

4 Journal of the American Ceramic Society—Grasso et al.

particles were damaged, however, the SiC ball was heavilyabraded by the diamond particles (Table III). The continualcontact with diamond particles during wear tests allowedassessment of a low friction against them. The measureddynamic friction coefficient of the WC/diamond compositewas measured as low as 0.117 ± 0.019, to be compared with0.328 ± 0.022 of the monolithic binderless WC.

Thus, the developed diamond/WC composite has superiorwear properties as compared to conventional cemented car-bides and diamond compacts. They could be used as newcompetitive wear-resistant materials for uses such as center-less blades, and high performance grinding tools as their costcould set between that of the cemented carbides and the dia-mond compacts. However, as diamond particles in the dia-mond/cemented carbides are not bonded directly to eachother, they would not be suitable for applications requiringhard and long-lasting cutting edges.

IV. Conclusion

A new method was developed to fully consolidate diamond/cemented carbide powder by means of spark plasma sintering

Fig. 7. (a) microstructure region over which EDS mapping was performed: (b) carbon and tungsten map across the interface; (c) carbon, and(d) tungsten maps; (e) profiles from the electron probe analysis across the WC/diamond composite interface.

Fig. 8. SEM image and magnification of the sintered body surfaceafter 30 000 revolutions of the SiC balls.

Fig. 9. Optical images of monolithic binderless WC surface after30 000 revolutions of the SiC balls.

Table II. Specific Wear Resistance Rate Measured

Binderless WC and WC Diamond Composite

Specific wear rate m3 (N·m)�1

WC binderless 7.470 9 10�17

WC diamond Not possible to measure

Table III. Specific Wear of the SiC Ball Used in the Test

Specific wear rate SiC ball m3 (N·m)�1

WC binderless 1.16 9 10�15

WC diamond 43.73 9 10�15

Spark Plasma Sintering of WC Composites 5

(SPS) operating in current-control mode (CCm). The combi-nation of the ultrafast heating rate of about 2000°C/min andshort holding time was successful to fabricate diamond/cemented carbides. Nevertheless the sintering temperatureexceeded 1650°C, no graphitization of diamond was observedin the nearly full dense samples. The present method, unlikepreviously published works, did not require any diamondcoating or expensive high pressure devices to prevent dia-mond graphitization. The sintered material exhibits highwear resistance.

References

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6 Journal of the American Ceramic Society—Grasso et al.