Sintering Diamond/Cemented Carbides by the Pulse PlasmaSintering Method

Andrzej Michalskiw and Marcin Rosinski

Warsaw University of Technology, Faculty of Materials Science and Engineering, Warsaw, Poland

Under the conditions of thermodynamic instability, we produceddense sintered composites built of diamond particles (30 vol%)and a cemented carbide matrix. The composites were consoli-dated by high-current electric pulses at a temperature of about11001C and load of 75 MPa for 5 min. The diamond particlesare strongly bound with the cemented carbide matrix, by a tran-sition layer composed of a solid solution of carbon and tungstenin cobalt. No graphite precipitates were found in the sinteredcomposites, as examined by microstructure observations, exam-inations of the phase composition, and an analysis of the Ramanscattering spectra. The hardness of the diamond/cementedcarbide was 23 GPa.

I. Introduction

WC–CO cemented carbides have been widely used for fab-ricating cutting tools of various kinds and other machine

parts that are required to show a high resistance to frictionalwear. Their mechanical properties can be modified over a broadrange by changing the content of the binding Co phase and theWC grain size.1 An increase in the amount of the binding phaseincreases the fracture toughness of the material but decreases itshardness.2 A reduction of the WC grain size, on the other hand,increases the hardness and improves other mechanical proper-ties.3–5 Cemented carbides are usually produced by sinteringwith the participation of a liquid cobalt phase.6

The frictional wear resistance of cemented carbides canbe substantially increased by replacing part of the carbide phaseby diamond, which is several times harder. There is however atechnological difficulty in doing this, because, at the temperatureof sintering of cemented carbides (14001–15001C), diamond isa metastable phase and transforms into graphite. Under highvacuum (at low partial oxygen pressure) at temperatures upto 14001C, the graphitization proceeds slowly and only occurson the surface of the diamond particles, whereas above thistemperature, the transformation proceeds quickly and occursin the entire particle.7–9 In order to avoid graphitization, it istherefore necessary to conduct the sintering process in vacuum,quickly, and at a relatively low temperature.

This has been achieved by modern sintering techniques, suchas plasma-assisted sintering (PAS),10 spark plasma sintering(SPS),11–13 and field-assisted sintering (FAST).14,15 In these tech-niques, the sintering is carried out at lower temperatures and theprocess proceeds faster than conventional processes thanks tothe use of pulse current for heating the powders to be sintered.During a current pulse, spark discharges are ignited in the pores.These discharges remove adsorbed gases and oxides fromthe powder particle surfaces, thereby facilitating the formation

of active contacts between them. In effect, the process time maybe shortened and the sintering temperature may be reduced.

Moriguchi et al.16 sintered diamond/cemented carbide com-posites by the SPS method at a temperature of 13001C under apressure of 41 MPa for 3 min. To avoid the graphitization ofdiamond during the sintering, they covered the diamond particleswith a SiC layer. Shi et al.,17 who conducted the SPS sintering ofdiamond/cemented carbide at a temperature between 10001 and12801C, covered the diamond particles with a tungsten layer.

Pulse plasma sintering (PPS) is a new method in which the ma-terial to be sintered is heated by periodically repeated high-currentelectric pulses generated by discharging a capacitor battery. Thepulsed way of supplying the high capacitor energy (of the order ofseveral kJ) during several hundred microseconds creates specificsintering conditions. The PPS method has been used for sintering awide variety of materials, such as Cu/diamond composites,18 nano-crystalline sinters of various materials,19–22 and, with the partici-pation of the SHS reaction, high melting ceramics.23–25

The paper presents the results of examinations of thediamond/cemented carbide composites sintered by PPS underthe conditions of thermodynamic instability of diamond.

II. Experimental Procedure

Diamond/cemented carbide containing 30 vol% of diamondparticles was produced using a mixture of a 6 wt% Co-addedWC powder, with a WC particle size of 0.8 mm (Fig. 1(a)) and adiamond powder with a particle size ranging from 40 to 60 mm(Fig. 1(b)).

The diamond powder was dry-mixed with the WC6Co pow-der, using a horizontal mill. Figure 2 shows a scanning electronmicroscopic (SEM) image of the diamond/WC6Comixture aftermilling. As can be seen, most of the diamond particles are sur-rounded by agglomerates of the WC6Co powder. The powdermixture was sintered in a graphite die to form samples 3 mm inthickness and 20 mm in diameter. The sintering process wasconducted in a PPS apparatus, shown schematically in Fig. 3.

Figure 4 shows the plots of the temperature and load varia-tion during PPS sintering of the diamond/cemented carbidecomposites. Before the sintering, the chamber was pumped outto a pressure of 5� 10�3 Pa. Then, under a load of 30 MPa, thesample was heated to a temperature of 7001C for 5 min so as toremove the gases adsorbed on the powder particle surfaces.After degassing, the sample was further heated to reach therequired sintering temperature of 11001C and was maintained atthis temperature for 5 min. At the beginning of this stage, theload was increased to 75 MPa. The final stage included coolingthe sample to room temperature, still under a load of 75 MPa.All the operations were performed in a vacuum of 5� 10�3 Pa.

During sintering, the temperature of the sample and the heat-ing rate were controlled by controlling the pulse repetitionfrequency and the pulse discharge energy (E5U2C/2 whereU is the voltage, and C is the capacitance of the capacitor bat-tery). Table I gives the parameters of the sintering process.

The temperature on the surface of the graphite die during thesintering process was measured with an Ahlborn IR AMIR7838-51 (Holzkirchen, Germany) temperature sensor.

A. Zangvil–contributing editor

This study, conducted within the framework of the research project No. N507 017 32/0586, was financed from the funds allotted to research works during the years 2007–2010.

wAuthor to whom correspondence should be addressed. e-mail: mihalski@inmat.pw.edu.pl

Manuscript No. 24560. Received April 20, 2008; approved August 21, 2008.

Journal

J. Am. Ceram. Soc., 91 [11] 3560–3565 (2008)

DOI: 10.1111/j.1551-2916.2008.02738.x

r 2008 The American Ceramic Society

3560

The phase composition of the sintered materials was deter-mined with a Philips PW 1140 (Eindhoven, the Netherlands)X-ray diffractometer equipped with a PW 1050 goniometer us-ing CoKa radiation. A Raman micro-probe coupled with aDilor XY-800 (Lille, France) three-net Raman spectrometerwere used for examining the sintered diamond/cemented carbidecomposites for the presence of graphite.

The microstructure and the chemical composition wereexamined in a Hitachi S3500N (Hitachinaka-shi, Japan) scan-ning electron microscope equipped with a Noran Vantage EDS-

Thermo system (Madison, WI) designed for chemical analyses.The hardness was determined using a ZWICK (Ulm, Germany)hardness meter under a load of 1 kG.

III. Results

Figure 5 shows an SEM micrograph of the surface of thesintered diamond/cemented carbide composite. The diamondparticles distributed uniformly within the cemented carbidematrix do not form a skeleton because they only constitute30% of the matrix volume. They are, however, well bonded withthe matrix, and no pores and graphite precipitates occur aroundthem. Table II compares the hardness of the diamond/cementedcarbide composites produced by PPS and their cemented carbidematrix with the hardness of the composites and their matrixproduced by the SPS method as reported by Moriguchi et al.16

The hardness of the diamond/WC6Co composite consolidatedby PPS is higher by about 3 GPa than that of the cemented car-bide without diamond particles, and it is also higher than thehardness of the diamond/carbide sintered by Moriguchi et al.,16

Fig. 2. Scanning electron microscopy image of the diamond/WC6Copowder mixture.

Fig. 3. Schematic representation of the pulse plasma sintering apparatus.

Fig. 1. Scanning electron microscopy image of (a) the WC-6 wt%Co powder, (b) the diamond powder.

700 C

1100 C

75 MPa

30 MPa

Time

Leve

l

5 min 5 min

Fig. 4. Plots of the temperature and load variation during pulse plasmasintering.

Table I. Process Parameters used in the Sintering ofDiamond/Cemented Carbide Composites

Parameters I stage II stage

Discharge energy (kJ) 3.75 5.4Pulse repetition frequency (s) 1 0.6Voltage (kV) 5 6.1Heating rate (1C/s) 9.2 6.5Annealing time (min) 5 5Temperature (1C) 700 1100Pressure (MPa) 30 75Vacuum (Pa) 5� 10�3 5� 10�3

November 2008 Sintering Diamond/Cemented Carbides by the Pulse Plasma Sintering Method 3561

who used the SPS method. The lower hardness values of thesintered composites produced by Moriguchi et al. may beexplained in terms of the smaller diamond content and the lowerhardness of the WC10Co matrix. It is however worth notingthat, in order to avoid graphitization, Moriguchi et al. also pre-covered the composite with a thin SiC film before sintering.

Figure 6 shows a diffraction pattern obtained for the diamond/cemented carbide sintered by PPS. The phases identified in thesintered composite were tungsten carbide, diamond, and a cobaltphase.

Figure 7 shows a Raman spectrum measured on the surfaceof a fracture of the composite. The only sharp peak occurring inthe spectrum appears at 1331.8 cm�1; it corresponds to the sp3

bond of diamond. Diamond has a single Raman active mode at1332 cm�1, which is a zone-center mode of the T2g symmetry.

00-005-0727 (N) - Cobalt - Co - Y: 2.43 % - d x by: 1. - WL: 1.78897 - Hexagonal - a 2.50310 - b 2.50310 - c 4.06050 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63/mmc (194) - 2 - 22.032

03-065-0537 (C) - Diamond, syn - C - Y: 16.67 % - d x by: 1. - WL: 1.78897 - Cubic - a 3.56691 - b 3.56691 - c 3.56691 - alpha 90.000 - beta 90.000 - gamma 90.000 - Face-centered - Fd-3m (227) - 8 - 45.

00-051-0939 (*) - Unnamed mineral, syn [NR] - WC - Y: 100.00 % - d x by: 1. - WL: 1.78897 - Hexagonal - a 2.90631 - b 2.90631 - c 2.83754 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P-6

WCND-diam - File: WCND-diamv1.raw - Type: 2Th/Th locked - Start: 30.000 ° - End: 105.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 30.000 ° - Theta: 15.000 Operations: Background 0.081,1.000 | Import

Sqr

t (C

ount

s)

0

10

100

1000

200

300

400500600

2000

3000

2-Theta - Scale30 40 50 60 70 80 90 100

WC

Diamond

Co

Fig. 5. X-ray diffraction pattern obtained for cemented carbide combined with diamond particles.

Table II. Properties of the Sintered Composites Producedby Pulse Plasma Sintering (PPS) and Spark Plasma

Sintering (SPS)

Method

Temperature

[1C]

Time

[min] Composition

Hardness

[GPa] Reference

PPS 1100 5 30 vol% diamond/WC-6 wt% Co

23 Thiswork

PPS 1100 5 WC-6 wt% Co 20 Thiswork

SPS 1400 3 20 vol% diamond/WC-10 wt% Co

18 17

SPS 1400 3 WC-10 wt% Co 17 17

Fig. 6. Scanning electron microscopy image of the diamond/WC6Cosinter surface.

Fig. 7. Raman scattering spectrum obtained for cemented carbide re-inforced with diamond particles.

3562 Journal of the American Ceramic Society—Michalski and Rosinski Vol. 91, No. 11

Single-crystal graphite, on the other hand, has a single Ramanactive mode at 1580 cm�1, and it is a zone-center mode of theE2g symmetry known as the mode ‘‘G’’.26

Figure 8(a) shows an SEM micrograph of a fracture ofthe diamond/cemented carbide composite. We can see that thediamond particles are strongly bonded with the cementedcarbide matrix because only few of them have been torn outfrom the matrix, whereas most have cleaved on fracture in atrans-crystalline way. The surface of the diamond particlesbecomes more rough (Fig. 8(b)), which indicates that chemicalreactions took place between the diamond particles and thecemented carbide matrix.

Figure 9 shows X-ray spectra of the elements present in thediamond particle and cemented carbide matrix. The right-sidediagram shows the results of an electron probe microanalysisacross the diamond particle/cemented carbide interface. Both theX-ray examination and the electron probe microanalysis indicatethat cobalt and tungsten are present on the surface of the dia-mond particle. This suggests that, during the sintering process,the diamond particle and tungsten carbide dissolve in cobalt.

IV. Discussion

The solubility of carbon in liquid cobalt is about 3 wt% and thatof tungsten carbide is about 45 wt%.6 Although the PPS sinte-

ring process is conducted at 11001C whereas the meltingtemperature of cobalt is 14951C, the dissolution of diamondand tungsten carbide in cobalt may occur because of the specificway of heating of the material to be sintered.

The PPS process differs essentially from conventional sinte-ring. In PPS, the material to be sintered is heated via the Jouleheat and by the spark discharges ignited during the periodicallyrepeated current pulses with an amplitude of about 30 kA and aduration of about 0.6 ms (Fig. 10). Because of the very shortpulse duration compared with the time intervals between theconsecutive pulses (500 ms), the sintering process acquires aquasi-adiabatic character.

During the flow of electric current through the compositebuilt of dispersed, electrically non-conducting diamond particlesand conducting cemented carbide matrix, the instantaneoustemperature on the diamond particle surface can even reachseveral thousands degrees on the Celsius scale. This happensbecause, in the surroundings of the non-conducting diamondparticles, the current density increases, thereby increasing theamount of generated Joule heat. In effect, during the process ofsintering with high-current electric pulses, the instantaneoustemperature substantially exceeds the melting temperature ofcobalt; as a result the diamond and tungsten carbide dissolve inthe liquid cobalt. On the other hand, the rapid cooling to 11001Cof the sintered material during the interpulse intervals hampersthe precipitation of the dissolved carbon in the form of graphite.

Fig. 8. Surface of a fracture of the diamond/WC6Co composite: (a) trans-crystalline way of fracture of diamond particles, (b) rough (deployment)surface of the diamond particle.

Fig. 9. X-ray image of (a) carbon, (b) cobalt, and (c) tungsten; (d) shows the microstructure of the sample; (e) shows the results of an electron probeanalysis across the diamond particle/cemented carbide interface.

November 2008 Sintering Diamond/Cemented Carbides by the Pulse Plasma Sintering Method 3563

Figure 11 shows schematically the temperature variation duringhigh-current pulse sintering.

A similar morphology of the diamond particles with strongsurfaces unsure of meaning was also observed by Shi et al.,17

who sintered diamond/cemented carbide by SPS at a tempera-ture between 11001 and 12801C. Moreover, they observedgraphite precipitates on the diamond particle surfaces, andattributed the degradation of the diamond particle surface andits graphitization to the high instantaneous temperature. Theysuggest that, during the SPS sintering process, the temperatureof the diamond particle surface during a high-current pulsemay reach the value of the order of ten thousand degrees onthe Celsius scale. Because in both SPS and PPS processes thematerial to be sintered is heated by the Joule heat and bythe heat delivered by the spark discharges, we may supposethat the absence of graphite precipitates in the diamond/WC6Cocomposites sintered by PPS is associated with the differences inthe method of generation of the high-current pulses. Table IIIcompares the electric parameters used for heating in the PPS andSPS processes.

As seen in Table III, the electric parameters of the PPS pro-cesses differ substantially from those used in the SPS processes.The differences include the electric current intensity, electricvoltage, pulse duration, and the pulse repetition frequency. InPPS, the instantaneous electric current (I) is several times higherthan in SPS, whereas the voltage (U) is several hundred times ashigh, so that the instantaneous electric power in PPS is about180 MVA (U � I) compared with 50 KVA in SPS. The PPS pulseduration is also about 60 times shorter and the inter-pulse in-terval is about 80 times as long as those in SPS. We may supposethat the most essential factors that determine whether graphiteprecipitates in the diamond/WC6Co composites sintered by PPSare the high power of the current pulse and the short pulse du-ration, because these promote the dissolution of cobalt in car-bon on the diamond surface. Another advantage is the long

interpulse interval, considerably longer in PPS compared withthat in SPS, which hampers the precipitation of the dissolvedcarbon from cobalt.

V. Conclusions

Dense diamond/cemented carbide was consolidated by the PPSmethod, under the conditions of thermodynamic instability ofdiamond. The specific conditions created when the material isheated by high-current electric pulses permit avoiding graphiteprecipitation in the sintered composites. The diamond particlesare strongly bonded with the cemented carbide matrix by atransition layer built of a solid solution of carbon and tungstenin cobalt.

References

1B. K. Kim, G. H. Ha, and D. W. Lee, ‘‘Sintering and Microstructure of Nano-phase WC/Co Hardmetals,’’ J. Mater. Proc. Technol., 63, 317–21 (1997).

2W. D. Schubert, H. Neumeister, G. Kinger, and B. Lux, ‘‘Hardness to Tough-ness Relationship of Fine-grained WC–Co Hardmetals,’’ Int. J. Refract.Met. Hard Mater., 16, 133–42 (1998).

3V. Richter and M. V. Ruthendorf, ‘‘On Hardness of Ultrafine NanocrystallineHard Materials,’’ Int. J. Refractory Met. Hard Mater., 17, 141–52 (1999).

4T. Sailer, M. Herr, H-G. Sockel, R. Schulte, H. Feld, and L. J. Praksh,‘‘Microstructure and Properties of Ultrafine gradient Hardmetals,’’ Int. J. Refract.Met. Hard Mater., 19, 553–9 (2001).

5C. Allen, M. Sheen, J. Williams, and V. A. Pugsley, ‘‘The Wear UltrafineWC–Co Hard Metals,’’ Wear, 250, 604–10 (2001).

6G. S. Upadhyaya, Cemented Tungsten Carbides Production, Properties, andTesting. Noyes Publications, Westwood, NJ, U.S.A., 1998.

7K. C. Uspenskaya, U. N. Tomashev, and D. V. Fedoceev, ‘‘Oxidization andGraphitization of Diamond Condition of Low Pressure Atmosphere,’’ J. Phys.Chem., 56 [2] 495–6 (1982) (Russian).

8D. V. Fedoseev, V. L. Buhovest, and S. P. Vnukov, ‘‘Surfacial Graphitizationof Diamond at High Temperatures,’’ Surface, 1, 92–9 (1980) (Russian).

9W. Z. Shao, V. V. Ivanov, L. Zhen, Y. S. Cui, and Y. A. Wang, ‘‘A Study onGraphitization of Diamond in Cooper-Diamond Composite Materials,’’ Mater.Lett., 58, 146–9 (2003) 146–49.

10S. H. Risbud and C. H. Shan, ‘‘Fast Consolidation of Ceramics Powders,’’Mater. Sci. Eng., A204, 146–51 (1995).

11S. I. Cha, S. H. Hong, and B. K. Kim, ‘‘Microstructure and MechanicalProperties of Nanocrystalline WC-10Co Cemented Carbides,’’ Scripta Mater., 44,1535–9 (2001).

12P. Feng, W. Xiong, L. Yu, Y. Zheng, and Y. Xia, ‘‘Phase Evolution and Mi-crostructure Characteristics of Ultrafine Ti(C,N)-Based Cermets by Spark PlasmaSintering,’’ Int. J. Refract. Met. Hard Mater., 22, 133–8 (2004).

13L. H. Zhu, Q. W. Huang, and H. F. Zhao, ‘‘Preparation of NanocrystallineWC-10Co-0.8VC by Spark Plasma Sintering,’’ J. Mater. Sci. Lett., 22, 1631–3(2003).

14A. Groza and J. R. Zavaliangos, ‘‘Nanostructured Bulk Solids by FieldActivated Sintering,’’ Adv. Mater. Sci., 5, 24–33 (2003).

15J. R. Groza and A. Zavaliangos, ‘‘Sintering Activation by External ElectricalField,’’ Mater. Sci. Eng., A287, 171–7 (2000).

16H. Moriguchi, K. Tsuduki, A. Ikegaya, Y. Miyamoto, and Y. Morisada,‘‘Sintering Behavior and Properties of Diamond/Cemented Carbides,’’ Int. J.Refract. Met. Hard Mater., 25, 237–43 (2007).

17X. L. Shi, G. O. Shao, X. L. Duan, Z. Xiong, and H. Yang, ‘‘The Effect ofTungsten Buffer Layer on the Stability of Diamond with Tungsten Carbide–Cobalt Nanocomposite Powder During Spark Plasma Sintering,’’ Diamond Relat.Mater., 15, 1643–9 (2006).

18T. Schubert, y. Ciupinski, W. Zielinski, A. Michalski, T. WeiXgarbera, and B.Kieback, ‘‘Interfacial Characterization of Cu/diamond Composites Prepared byPowder Metallurgy for Heat Sink Applications,’’ Scripta Mater., 58, 263–6 (2008).

19A. Michalski and D. Siemaszko, ‘‘Nanocrystalline Cemented Carbides Sin-tered by the Pulse Plasma Method,’’ Int. J. Refract. Met. Hard Mater., 25, 153–8(2007).

Cur

rent

[kA

]

Time [ s]0 100 200 300 400

30

20

10

0

Fig. 10. Example of the electric current waveform during the pulseplasma sintering process.

Tem

pera

ture

[a.u

]

Time [a.u.]0.6 ms

Average temperature of heated materials (measured)

500 ms

Temperature during the electric pulse

Fig. 11. Temperature variations during high-current pulse sintering ofdiamond/cemented carbide.

Table III. Electric Parameters of the Pulse Plasma Sintering(PPS) and Spark Plasma Sintering (SPS) Processes

Parameter PPS SPS27

Current (kA) 30 5Voltage (V) 6000 10Pulse duration (ms) 0.6 36�

Pulse repetition frequency (ms) 500 6�

�Assuming that the default pulse pattern is 12:2, and each pulse has the same

period of about 3 ms. Thus a pattern has sequence of 12 pulses (3 ms each) ‘‘on’’

and 2� 3 pulses (6 ms) ‘‘off’’ (with no current).

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20A. Michalski, J. Jaroszewicz, M. Rosinski, D. Siemaszko, and K. J.Kurzyd"owski, ‘‘Nanocrystalline Cu-Al2O3 Composites Sintered by the PulsePlasma Technique,’’ Solid State Phenomena, 114, 227–32 (2006).

21A. Michalski, M. Rosinski, D. Siemaszko, J. Jaroszewicz, and K. J.Kurzyd"owski, ‘‘Pulse Plasma Sintering of Nano-Crystalline Cu Powder,’’ SolidState Phenomena, 114, 239–44 (2006).

22M. Rosinski and A. Michalski, ‘‘Nanocrystalline NiAl–TiC Composites Sin-tered by the Pulse Plasma Method,’’ Solid State Phenomena, 114, 233–8 (2006).

23A. Michalski, J. Jaroszewicz, M. Rosinski, and D. Siemaszko, ‘‘NiAl–Al2O3

Composites Produced by Pulse Plasma Sintering with the Participation of the SHSReaction,’’ Intermetallics, 14, 603–6 (2006).

24A. Michalski, J. Jaroszewicz, and M. Rosinski, ‘‘The Synthesis of NiAl Usingthe Pulse Plasma Method with the Participation of the SHS Reaction,’’ Int.J. SHS., 12, 237–46 (2003).

25J. Jaroszewicz and A. Michalski, ‘‘Pulse Plasma Sintering Combined with aCombustion Synthesis of a TiB2 Composite with a Nickel Matrix,’’ J. Eur. Ceram.Soc., 26, 247–30 (2006).

26S. A. Solin and A. K. Ramdas, ‘‘Raman Spectrum of Diamond,’’ Phys Rev. B,1, 1687–98 (1970).

27W. Chen, U. Anselmi-Tamburini, J. E. Garay, J. R. Groza, and Z. A. Munir,‘‘Fundamental Investigations on theSpark Plasma Sintering/Synthesis Pprocess I.Effect of DC Pulsing on Reactivity,’’ Mater. Sci. Eng., A394, 132–8 (2005). &

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