Pressure Effect on the Homogeneity of Spark Plasma-SinteredTungsten Carbide Powder

Salvatore Grasso,z,y Yoshio Sakka,*,w,z,y Giovanni Maizza,z and Chunfeng Huz

zWorld Premier International Research Center Initiative (WPI Initiative) on Materials Nanoarchitronics (MANA) andNano Ceramics Center, National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan

yGraduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305-0047, Japan

zDipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, I-10129 Torino, Italy

A combined experimental/numerical methodology was devel-oped to aid full densification of pure ultrafine tungsten carbidepowder by means of Spark Plasma Sintering (SPS) operating inCurrent Control mode. Applied pressure ranged from 5 to 80MPa while the current intensity was set and held constant at1400 A. The developed SPS model used a moving-mesh tech-nique to account for the electrothermal contact resistancechange during both shrinkage and punch sliding follow-up.The pressure dependence on the electrothermal contact resis-tance was also taken into account by the model. The experi-mental and numerical results showed the effects of pressure ongrain growth, residual porosity, and hardness observed alongthe sample radius. Upon increasing sintering pressure, completedensification was obtained by reducing the peak temperaturemeasured at the die surface. By combining experimental andmodeling results, a direct correlation between compact micro-structure homogeneity and sintering parameters (i.e., tempera-ture and applied pressure) was established.

I. Introduction

SEVERAL investigations1–4 have shown that Spark PlasmaSintering (SPS), unlike conventional techniques,5 is a suc-

cessful method to fully densify binderless ultrafine/nanometrictungsten carbide (WC) by overcoming severe difficulties relatedto the powder processing, such as spontaneous agglomeration,high reactivity, ease surface contamination, and grain coarsen-ing.1–4 Some patents6 have been granted concerning the pro-duction of single phase, high purity, binderless, and fully denseWC material having high hardness (i.e., Hv 27 GPa), highstrength, wear resistance,7 and anticorrosion resistance.8 More-over, SPS technology allowed the production of functionallygraded cemented carbides (i.e., WC/Ni or WC–Co)9 that com-bine wear resistance, weldability, and machine workability.10,11

In the past two decades, more than 2000 papers12 and about600 patents13 have been published worldwide on SPS. The in-creasing interest on such sintering technology is justified by theattainment of significantly improved compact properties.12

However, our fundamental understanding of the SPS processin terms of current effects and optimum operating parametersis quite limited. A number of experimental and modeling

works4,14–18 have recognized the intricate bulk and contact mul-tiphysics of SPS, which involves the coupling between electric,thermal, and stress/strain (i.e., shrinkage) fields not only in thebulk but also along the contact interfaces between constitutivegraphite elements (i.e., punches, dies, and spacers). According toour recent investigations,17 applied pressure played a significantrole on temperature distribution in the punch/sample/die as-sembly as it significantly affected the punch/die contact resis-tances as a result of induced Poisson deformation.17

The primary purpose of this study was to develop a combinedexperimental/numerical methodology that permitted us to (a)homogeneously densify ultrafine WC powder by SPS and (b)establish a direct relationship between the compact microstruc-ture and both the predicted sintering temperature inside thesample and the applied pressure. It is known that in the case ofelectric conductive powders, microstructure inhomogeneity (i.e.,grain growth and residual porosity) is particularly critical.4,14

Inhomogeneities inside the sample occur because of differentialcurrent/temperature across the punch/die/sample assembly asa result of (a) punch/die contact resistance14–18 and (b) radiationheat losses at the die surface.4,15,16

II. Experimental Procedure

Commercial ultrafine WC (WC02NR, A.L.M.T. Corp., Tokyo,Japan) was used. The specifications and morphology of thepowder are given in a previous work.4 The as-received WCpowder was filled into a cylindrical hollow graphite die. The die

Fig. 1. Voltage drop measured between the two rams as a function oftime for different applied pressures (i.e., 5, 20, 40, 60, and 80 MPa) andan imposed current of 1400 A.

A. Zavaliangos—contributing editor

This work was supported by World Premier International Research Center Initiative(WPI Initiative), MEXT, Japan.

*Member, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: sakka.yoshio@nims.

go.jp

Manuscript No. 26047. Received April 1, 2009; approved May 13, 2009.

Journal

J. Am. Ceram. Soc., 92 [10] 2418–2421 (2009)

DOI: 10.1111/j.1551-2916.2009.03211.x

r 2009 The American Ceramic Society

2418

and the punches were made of low-strength graphite (SyntexInc., Kawasaki, Japan). The outer and inner diameters andheight of the die were 40, 20, and 30 mm, respectively. The di-ameter and height of the punches were 19.95 and 20 mm, re-

spectively. Further details on experimental conditions are givenin the previous work.17

The imposed DC current intensity was assumed to be con-stant and equal to 1400 A in all the experiments. The applied

Fig. 2. Scanning field emission electron fracture surface micrographs of pure ultrafine tungsten carbide taken at the sample center (first row) and 8 mmapart from the center (second row) after sintering with 1400 A under different applied pressure values of (a, b) 5 MPa, (c, d) 20 MPa, (e, f ) 40 MPa, and(g, h) 80 MPa. Arrows indicate observed residual porosities in the microstructure.

October 2009 Communications of the American Ceramic Society 2419

pressure was assumed to be constant in each experiment, in therange of 5–80 MPa. From each experiment, the die surface tem-perature and voltage drop between the two water-cooled ramswere recorded. SPS experiments were carried out in a 100 kNSPS-1050 machine (SPS Syntex Inc.) operating in Current Con-trol mode. Current discharge was stopped when the recordedpunch displacement curve reached a steady state.

The diameter and thickness of the consolidated disk sampleswere about 20 and 2 mm, respectively. Microstructure and hard-ness were inspected at the mid-thickness cross section at threepoints along the sample radius (i.e., 0, 4, and 8 mm from thecenter). These points were denoted hereafter as I, II, and III,respectively. The fracture surface microstructure at these pointswas ultrasonically cleaned in acetone, and then examined by ascanning field emission electron microscope (FESEM, JEOLJSM-6500F, Tokyo, Japan). The hardness was measured on di-amond-polished cross sections using a Vickers hardness tester(AVK-A Akashi Corp., Yokohama, Japan) under an appliedload of 10 kg and dwell time of 15 s.

A finite element model was developed to predict the temper-ature distribution in the WC sample as a function of the appliedpressures (i.e., 5, 20, and 80MPa). Model details can be found inMaizza et al.4,15 and the pressure effects on contact resistancewith inherent model calibration are reported in Grasso et al.17

III. Experimental and Modeling Results

Figure 1 shows the recorded voltage drop between the water-cooled rams for various pressures. With increasing applied pres-sure from 5 to 20 MPa, a voltage decrease of approximately1.5 V was observed. With further increasing pressure, the volt-age decreased but less severely than at low pressures. Specifi-cally, with increasing pressure from 60 to 80 MPa, the voltagedrop was approximately 0.15 V. This behavior continued unal-tered during the entire heating cycle. A similar voltage drop be-havior was observed in a previous work,17 in the case of agraphite compact sample under identical operating conditions.The large changes in voltage profiles versus applied pressure at agiven imposed constant current reflected the significant changesof contact resistances. Such changes of electric contact resistancedeeply affected the current and temperature distribution17 and,consequently, the microstructure homogeneity.

Figure 2 shows the fracture surfaces of sintered microstruc-tures as observed by FESEM at points I and III for each appliedpressure. By progressively increasing applied pressure in therange of 5–80 MPa, we notice that (i) the grain growth at pointI was progressively reduced (first column of Fig. 2) and (ii) theresidual porosity shown by arrows at point III was progressivelysuppressed (second column of Fig. 2 and Table I).

However, because the SPS process did not allow the directmeasurement of the sintering temperature, computer modelingwas crucial to understand the sintering behavior of WC com-pacts and control the final microstructure and its properties.Figure 3 shows a plot of the computed temperature distributionalong the sample radius. The model was accurately calibratedand validated with respect to die surface temperature and volt-age drop, as detailed in Maizza and colleagues.4,15,17 The com-puted field temperature permitted a direct relationship betweenthe sintering temperature, microstructure, and hardness. Figure3 displays two main interrelated results: (i) the sintering tem-

perature decreased with applied pressure and (ii) the tempera-ture gradient across the sample decreased by increasing appliedpressure. These results were in agreement with the experimentalevidence on microstructure, voltage drop (Fig. 1), hardness, andtemperature (Figs. 3 and 4).

IV. Discussion

According to modeling results (Fig. 3), 5 MPa applied pressureled to a decrease in sintering temperature along the radius (i.e,TI/TIII5 2264/21541C). As shown in Fig. 2(a), abnormal graingrowth occurred at the sample center. The visible elongatedgrain up to 2 mm in length could be attributed to the physicaland chemical characteristics of the powder used and to anisot-ropy of surface energy of WC grains.19 At a 5 MPa appliedpressure, some voids and flaws exist in the sintered specimens.Similar defects were reported by Jayaseelan et al.20 in the sinte-ring of alumina powders under a 5.5 MPa applied pressure.Moreover, the application of low pressure (i.e., 5 MPa) led tosample distortion induced by differential shrinkage. As a result,internal stresses originated in the sample because prior densifi-cation of the inner (hotter) part compared with the outer(colder) one. Abnormal grain growth together with the presenceof voids, flaws, and residual stresses could explain the low hard-ness values measured from the sample center toward the sampleedge (Fig. 4).

For 20 MPa applied pressure, the sintering temperature waslowered in comparison to the case of 5 MPa (Fig. 3), and graingrowth was less pronounced (Figs. 2(a) and (c)). The change inhardness along the radius could be attributed to incompletedensification at point III, as confirmed by microstructure ob-servation, which did show a consistent residual porosity(Fig. 2(d)). Indeed, the final temperature at point III in thecase of 20 MPa was only 18541C. This value appeared to beinsufficient for the full densification of pure WC within a short

Table I. Summary of Experimental Measurements

Pressure (MPa) 5 20 40 60 80

Discharge Time (s) 274 374 419 371 308Peak T (1C) 1802 1622 1655 1619 1579Relative density (%) 99 96 98.4 98.6 99.3Homogeneity� Poor Poor Fair Good Very goodGrain growth Abnormal Significant Limited Limited Limited�Homogeneity was qualitatively evaluated by microstructure observations and hardness measurements.

Fig. 3. Computed temperature profiles after sintering for 5, 20, and 80MPa applied pressures and a 1400 A imposed current. Inspection pointswere taken at the mid-thickness of the sample, along its radial direction:x, center (I); 1, outer point (III).

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sintering time. The latter was consistent with model results4 andexperimental observations.3

By increasing pressure from 20 to 60MPa, the microstructureat the sample center exhibited large and fine grains, following abimodal distribution (Figs. 2(c) and (e)). The size of large grainswas progressively reduced from 1.5 mm (Fig. 2(c)) for 20 MPa toabout 0.5 mm (not shown) for 60 MPa. The microstructural in-vestigation showed that also residual porosity progressively re-duced (Figs. 2(d) and (f )). This was demonstrated by thedecrease of the thermal gradient inside the sample (Fig. 3),which also contributed to an increase in hardness at point III(Fig. 4).

For 80 MPa applied pressure, the temperature gradient be-tween points I and III (Fig. 3) was 531C. Such thermal gradienttogether with intrinsic pressure effects led to a uniform micro-structure as well as to homogeneous hardness profile across thesample. The average measured grain size at points I and III wasin the range of 250–300 nm (Figs. 2(g) and (h)). The hardnessvalues at points I, II and III compared favorably with those inShimojima and colleagues.1–4 Such microstructures could dis-play a hardness of about 2700Hv, provided that they are fullysintered. All above considerations confirmed that microstruc-ture inhomogeneities along the radius occurred in WC compactswhen different pressures are applied.

V. Conclusion

The role of external pressure on sintering was reported in anumber of hot pressing and SPS papers. In conventional hotpressing,21 pressure enhanced the densification driving force,thereby promoting densification kinetics. Grain growth was notdirectly affected by an external pressure.21 As the densificationrate increased with pressure, the sintering temperature and timecould be reduced and grain growth suppressed further.21

The results in this work showed that external pressure in SPS(a) played an intrinsic role comparable to conventional hot-pressing process and (b) significantly influenced the overall bulkelectrical and thermal fields, including internal heat generation.

Specifically, in the case of WC powder, for a lower pressurerange (i.e., 5–20 MPa), a high sintering temperature and abnor-mal grain growth with a nonhomogeneous microstructure and

nonuniform hardness profile were observed. In the case of alarger pressure range (i.e., 60–80 MPa), the vertical punch/dieinterface contact resistance depressed the thermal gradient alongthe radius and promoted homogeneously dense samples. Thevertical punch/die contact resistance directly affected the radialthermal gradients in the sample/die assembly,14 and in turn itseffect increased with decreasing pressure.

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Fig. 4. Measured hardness profiles along the radial direction of thesample for 1400 A imposed current and different applied pressures.

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