Journal of Alloys and Compounds 547 (2013) 51–58

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Effects of the electric current on conductive Si3N4/TiN composites in sparkplasma sintering

Manyuan Zhou, Don Rodrigo, Yi-Bing Cheng ⇑Department of Materials Engineering, Monash University, Victoria 3800, Australia

a r t i c l e i n f o

Article history:Received 31 July 2012Received in revised form 22 August 2012Accepted 22 August 2012Available online 31 August 2012

Keywords:SinteringCompositesMechanical propertiesNitrides

0925-8388/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2012.08.091

⇑ Corresponding author. Tel.: +61 3 990 54930; faxE-mail address: yibing.cheng@monash.edu (Y.-B. C

a b s t r a c t

The effect of the electric current in spark plasma sintering (SPS) process on the densification, phase trans-formation, microstructure and mechanical properties of conductive Si3N4/TiN composites was studied incomparison with non-conductive monolithic a-Si3N4 and b-Si3N4. In SPS, the densification and a–b trans-formation of Si3N4 were enhanced by the presence of the highly conductive TiN phase, in contrast to itsopposite effect in Hot Pressing. The densification of the conductive Si3N4/TiN composites took place atabout 100 �C lower than that for the non-conductive monolithic Si3N4. Although TiN grains suppressedthe formation of elongated b-Si3N4 grains, the beneficial effect of TiN on densification resulted inSi3N4/TiN composites with improved fracture toughness and hardness. The experimental results pre-sented here combined with the literature data suggest that the effects of the presence of TiN on SPS sin-tered Si3N4 are derived from the increase in the electric current passing through the samples.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The spark plasma sintering (SPS) process uses a pressure-as-sisted furnace in which the heating is provided by a pulsed directcurrent supplied to the graphite die containing the powders tobe sintered. Compared to the traditional sintering techniques suchas pressureless sintering (PS), Hot Pressing (HP) or hot isostaticpressing (HIP), SPS allows much faster heating rates and shortersintering times, together with commonly lower sintering tempera-tures, which makes it suitable for sintering a variety of materialsincluding metals, ceramics, composites and polymers [1,2]. TheSPS technique can significantly enhance the sinterability of mostof the materials and extend the possibilities for developing newadvanced materials and tailoring their properties. Although thephenomena responsible for the enhanced sintering are uncertainin most cases, it is generally agreed that the Joule heating effectplays important roles.

Both electrically conductive and non-conductive powders canbe sintered by SPS. Conductive materials are heated by both Jouleheating (self-heating) caused by the direct current passing throughthe sample and thermal conduction from the container (usually agraphite die), whereas non-conductive materials are heated onlythrough thermal conduction. Therefore an obvious difference be-tween the SPS and traditional sintering methods, such as HP, isthe heating mode, because the materials sintered by HP are heatedexclusively by thermal conduction from the container irrespective

ll rights reserved.

: +61 3 9905 4940.heng).

of whether the material is electrically conductive or not. Thus,understanding the effect of the pulsed current in the SPS processis important for controlling the resulting microstructure and prop-erties of sintered products.

There have not many reports on direct measurements of thecurrent through a conductive specimen in SPS sintering. Tatsuyaet al. [3] observed the existence of an internal pulsed current thathad some effects on the sintering behaviour and progress of theSPS process, although several limitations during the current mea-surements may cast some doubt on the reliability of the findings.On the other hand, most researches have been focused on the indi-rect evaluation of the current flowing through a sample, such asthe temperature field [4] and temperature distribution [5] duringthe SPS process. Antonio et al. [6] proposed a methodology forinvestigating the effect of the pulsed current during the sinteringof conductive powders by SPS. Aluminium powder was used as acase study and 5% higher density was found in the sample withelectric current flowing through than the one without current flow.The influence of conductive nano-TiC on the microstructural evolu-tion of silicon nitride based nanocomposites in SPS was also stud-ied [7]. The Si3N4-based composite containing 5 wt.% nano-TiCxOyNz showed a larger average grain size and aspect ratio thanthe monolithic Si3N4-based ceramic possibly because of the Jouleheating during sintering caused by a leakage current hoppingacross the conductive titanium oxycarbo-nitride grains. However,the samples containing 10 and 20 wt.% nano-TiCxOyNz had smallergrain sizes and aspect ratios than the monolithic Si3N4 althoughthey had higher electrical conductivities. The authors attributedthis observation to a possible pinning effect of the nano-TiCxOyNz,

Table 1Electrical resistivity of samples and graphite die (unit: X cm).

Material Temperature (�C)

25 100 200 300 400 500

Graphite diea 0.1 � 10�3 – – – – –100% Si3N4

b Approx. 1014 – – – – 2 � 1013

70% Si3N4 + 30% TiNc >4 � 104 – – – >4 � 104 –50% a-Si3N4 + 50% TiN 4.6 � 10�3 4.2 � 10�3 3.9 � 10�3 3.4 � 10�3 3.0 � 10�3 –TiNd 20 ± 10 � 10�6 – – – – –

a According to the data from the graphite die manufacturer.b http://www.siliconfareast.com/sio2si3n4.htm [20].c This value is the upper limit of the instrument used. The resistivity of this composition should be less than that for 100% silicon nitride.d Ref.[21].

Fig. 1. Relative densities of Si3N4/TiN composites spark plasma sintered at differenttemperatures: the starting powders were (a) a-Si3N4 and (b) b-Si3N4 respectively.

52 M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58

when present in larger quantities, limiting the growth of Si3N4

grains.Titanium nitride (TiN) has good physical properties and a high

electrical conductivity. It has been widely used in Si3N4 based com-posites as a promising additive in cutting tools and in making elec-trical discharged machining of ceramic composites. The toughnessand hardness of silicon nitride have been reported to be enhancedwith the incorporation of TiN particles [8–10]. It has also been re-ported that conductive TiN–Si3N4 [10–15] and TiN–SiAlON [16,17]

Table 2Comparison of the relative densities (%) of conductive and non-conductive samples sinter

50% a-Si3N4 + 50% TiN 100% a-Si3N4

Hot Pressing SPS Hot Pressing SPS

88.5 ± 0.1 91.3 ± 0.1 97.0 ± 0.1 84.7 ± 0.1

composites were successfully fabricated by SPS. However, the ef-fect of the current through the samples in SPS of such conductivematerials has not been well understood. In this paper, we will focuson the effect of the current in SPS process on the densification,phase transformation, microstructure and mechanical propertiesof conductive TiN–Si3N4 composites and non-conductive mono-lithic Si3N4.

2. Experimental procedure

The starting powders in this study were commercial a-Si3N4 containing 90.2%a-phase and 9.8% b-phase (HC Starck, grade M11) and b-Si3N4 containing 20.3%a-phase and 79.7% b-phase (DENKI KAGAKU KOGYO KABUSHIKI KAISHA). The aver-age particle size of the silicon nitride powder was 0.6 lm. The TiN powder (AldrichChemical) with particles smaller than 10 lm was used as a conductive phase. Thecompositions chosen contained 0, 30 and 50 wt.% conductive TiN phase in thenon-conductive matrix of silicon nitride. 5 wt.% Y2O3, 3 wt.% Al2O3 and 2 wt.% AlNwere also added as sintering additives in all the samples. The powders were mixedin 100 g batches and ball milled for 48 h using Si3N4 milling-balls and 200 ml of iso-propanol. The milled mixtures were dried in an oven at 80 �C for 12 h and then re-mixed for 10 min using a porcelain mortar and pestle in order to ensurehomogeneity of the powder mixtures. The sample size used in SPS was 20 mm indiameter and approximately 5 mm in height. Approximately 5 g of powder mixturewas packed into a graphite die with a height of 50 mm and inner and outer diam-eters of 20 and 50 mm respectively. The green powder was separated from the diebody and punches by 0.2 mm thick graphite sheets between them. The graphite diewas covered with a heat insulating carbon fibre mat to avoid heat wastage from theexternal surface of the die. After the chamber was evacuated to a pressure less than6 Pa, the samples were first heated to 600 �C in 4 min and then heated to the sinter-ing temperature (1300–1600 �C) at 100 �C/min in a flowing high-purity nitrogenatmosphere and held for 10 min under a uniaxial pressure of 20 MPa in a SPS unit(Dr. Sinter 950, SPS Syntex Inc., Japan). The cooling rate was about 200 �C/min. Thefinal sintered sample was a 20 mm diameter disc with a thickness of about 5 mm.

The sample size used in Hot Pressing was 25 mm in diameter and about 5 mmin thickness. Sample pellets were sintered in a graphite die which was coated withboron nitride to avoid contact between the graphite and Si3N4 raw powders. A smallamount of boron nitride was also used as packing powder above and under the pel-let so as to separate the pellet and the graphite rams. Hot Pressing was performed ina Thermal Technology Group 1400 Laboratory Hot Press under a flowing high-pur-ity nitrogen atmosphere. Samples were heated to designed temperature (1300–1600 �C) at 20 �C/min and held for 1 h under a uniaxial pressure of 20 MPa. Thetemperature was measured using a pyrometer.

After sintering, the samples were first ground to remove the thin graphite layeradhered onto the outer surface. Density of the cleaned samples was determined bythe Archimedes method using water as the buoyant medium. Samples were firstsoaked in water for at least 48 h to ensure that the surface pores were filled withwater. To determine the electrical resistivity of the samples at different tempera-tures, metal electrodes were applied by vacuum evaporation of sliver onto the flatsurfaces of the samples. An oven was used for heating up the samples. The four-probe method and a micro-X metre were used for the electrical resistivitymeasurement.

ed by Hot Pressing and SPS at 1500 �C.

70% b-Si3N4 + 30% TiN 100% b-Si3N4

Hot Pressing SPS Hot Pressing SPS

79.7 ± 0.1 88.7 ± 0.1 79.5 ± 0.1 84.3 ± 0.1

Fig. 2. Shrinkage curves for Si3N4/TiN composites SPSed at 1600 �C: the starting powders were (a) a-Si3N4 and (b) b-Si3N4, respectively.

M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58 53

Samples for microstructural analysis of the cross-section were prepared bygrinding and polishing down to 1 lm finish followed by the application of a thingold coating by sputtering. The microstructural characterization was carried outusing a JEOL 7001F scanning electron microscope. X-ray diffraction (XRD) was un-der utilized to determine the crystalline phases present including polymorphicforms of Si3N4 in both the sintered samples and original powders. The samplesfor XRD were prepared by removing the surface layer and then sectioning to ensurethat the bulk material of the specimen was investigated. XRD analysis was doneusing a Philips PW 1140/90 X-ray Diffractometer with a Cu filament operating at40 kV and 25 mA. Standard X-ray diffraction scans for phase identification weredone for the range of 2h angles from 10� to 80� at a scan rate of 2�/min with a stepsize of 0.02�. The b0/(a + b0) ratio of the phases in sintered silicon nitride sampleswas determined by XRD peak-area method of Kall, which was deemed appropriatedue to the chemical and crystallite size similarity of the compounds [18]:

b0=ðaþ b0Þ ¼ 100� Ib0 ð101ÞIb0 ð101Þ þ KIað102Þ ð1Þ

where Iað1 02Þ and Ib0 ð102Þ are the peak integrated intensities (peak areas) of thea(102) and b0(101) reflections for the a and b0 phases respectively. K is the normal-izing parameter, which was taken to be 1.652. The a(102) and b0(101) peaks werechosen because of their high intensities and minimal overlap with peaks from otherphases that were present in the samples. The areas of the two peaks were deter-mined by obtaining the XRD patterns for the 2h range from 30� to 40� with a slowscanning rate of 0.5�/min and a small step size of 0.01�.

Hardness and fracture toughness values of samples were determined using aVickers hardness indentation tester. A weight of 10 kg was applied to the surfaceof the sample via the Vickers diamond indenter for �10 s.

Fig. 3. XRD patterns of Si3N4/TiN composites spark plasma sintered at 1600 �C.

3. Results and discussion

In this study, a-Si3N4 and b-Si3N4 with different weight percent-ages of TiN (0%, 30% and 50%) were sintered by SPS under the sameconditions. The electrical conductivity data of the a-Si3N4/TiNcomposite samples, which were obtained by the 4-probe methodat different temperatures, are given in Table 1. According to theresults, the 100% Si3N4 sample is non-conductive and the

70%Si3N4–30%TiN sample is moderately conductive, whereas, the50%Si3N4–50%TiN sample is almost as conductive as the graphitedie (the same order of magnitude), which means that a large DCpulse current would pass through the 50%Si3N4–50%TiN sampleas well as the graphite die during the SPS process. The electrical

Fig. 4. b-Fraction of Si3N4 in Si3N4/TiN composites spark plasma sintered atdifferent temperatures: (a) a-Si3N4 and (b) b-Si3N4.

54 M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58

conductivity data of the samples at the temperature higher than500 �C are not given here due to the experiment limitation. Be-cause electrical conductivity of Si3N4 increases with the tempera-ture from about 10�15 (X cm)�1 at room temperature to about10�7 (X cm)�1 at 1500 �C [19] (refer to the Appendix A), the elec-trical conductivity of the Si3N4/TiN composite at the sintering tem-peratures (1300–1600 �C) will be higher than that at roomtemperature so that the large current may pass through the samplemore than the graphite die during the SPS process.

3.1. Densification

The relative densities of a-Si3N4 and b-Si3N4 with 0%, 30% and50% TiN, sintered at different temperatures, are shown in Fig. 1.At 1600 �C, both a-Si3N4 and b-Si3N4 based composites are almostfully densified after being sintered for 10 min. Even monolithic sil-icon nitrides are densified to a similar extent at this temperature.The results also indicate that the relative densities of the conduc-tive samples (50% Si3N4 + 50% TiN) are clearly higher than thoseof the non-conductive ones (a-Si3N4 and b-Si3N4 with no addedTiN) after sintering at all temperatures, especially at 1400–1500 �C. In addition, the samples with 30% TiN, which are moder-ately conductive, also have higher relative densities than thosewithout any TiN. One possible reason for the observed trends inthe densification of these materials is the effect of the variationin conductivity on the DC pulse current passing through the spec-imen during the SPS sintering. The other is a chemical effect of thevarying amounts of TiN added. To separate the effect of the DC cur-rent that is passing through the specimen in SPS from the chemicaleffects resulting from the addition of TiN on the densification ofSi3N4/TiN composites, monolithic Si3N4 and Si3N4/TiN compositesamples were sintered by both Hot Pressing and SPS at 1500 �C.The relative densities of the sintered samples are given in Table2. These results show that, in the case of hot pressed samples,the presence of TiN suppressed the densification of a-Si3N4 andproduced no noticeable effect on the densification of b-Si3N4 be-cause the TiN grains hinder the elongation of b-Si3N4 further thea–b transformation of a-Si3N4. But in the case of SPSed samples,the introduction of the conductive TiN phase improved the densi-fication of both a-Si3N4 and b-Si3N4 samples because the beneficialeffect of TiN on the SPS densification of Si3N4 arises from the in-crease in the Joule heating effect due to the large current passingthrough the conductive samples. Antonio’s research [6] alsoshowed that the final density of samples (aluminium powder) withelectric current flowing through is about 5% higher than that ofones without electric current passing.

The shrinkage curves for a-Si3N4 and b-Si3N4 based compositeswith 0%, 30% and 50% TiN sintered by SPS at 1600 �C are shown inFig. 2a and b respectively. It should be noted that the dimensionalchanges of the samples are not directly comparable due to the dif-ferences in the particle size and the amount of the raw powderused in sample preparations. However, the temperature at whichthe shrinkage due to sintering exceeds the thermal expansion ofthe assembly (the lowest point on the displacement curve) canbe easily identified for each composition. The results show that thishappened at a temperature of the die wall (measured by thepyrometer) about 120 �C lower for the electrically conductingspecimen with 50% TiN added to a-Si3N4 than for the non-conduct-ing a-Si3N4 specimen without any addition of TiN. Similarly, thecorresponding temperature was about 80 �C lower for the moder-ately conducting specimen with 30% TiN added to b-Si3N4, thanfor the non-conducting b-Si3N4 specimen without any addition ofTiN. In the absence of any beneficial chemical effect of TiN on thedensification, as shown by the densification data for hot pressedcomposites, the commencement of sintering of composites in SPSat apparently a lower temperature than for monolithic Si3N4 can

be attributed to a physical effect arising from the presence of con-ductive TiN, which is the temperature difference between the dieand the core of the sample. For the non-conducting monolithicSi3N4 sample, the core of the sample is at a lower temperature thanthat measured on the surface of the die, whereas, for the conduct-ing composites, which can be self-heated to some extent by Jouleheating, the temperature of the core of the sample can be higherthan that measured on the surface of the die. This is also knownas the overshooting effect which is a very important behaviourduring SPS process. Previous studies have presented some evidenceto support this overshooting effect [4,5,22–24]. The temperaturedifference between the measured temperature (on the surface ofthe die) and the actual temperature (on the surface of the sample)is about 140 and �170 �C for non-conducting silicon nitride andconducting tungsten carbide [5]. According to Wang Yucheng’s re-search, the temperature difference between the measured temper-ature and the actual temperature (the centre of the sample) forconducting material (TiB2 and BN) can be even up to �345 �C [4].

3.2. Phase transformation and microstructure

The XRD patterns of Si3N4/TiN mixtures with different percent-ages of TiN, SPSed at 1600 �C, are compared in Fig. 3. The resultsshow that all three specimens have the same crystalline phases.The b-fraction (b/(a + b)) of Si3N4 in composites with different per-centages of TiN, sintered by SPS at different temperatures, is givenin Fig. 4. It can be seen that the b-fraction of the conductive samplebased on a-Si3N4 is obviously higher than that of the non-conduc-tive one (100% a-Si3N4) after sintering at 1500–1600 �C when a–btransformation occurs. In addition, the samples with 30% TiN,which are moderately conductive, also have higher b-fractionsthan those with no TiN if the starting material was based on a-Si3N4. This means that the a–b transformation of silicon nitride is

Fig. 5. SEM micrographs of Si3N4–TiN composites SPSed at 1600 �C (a) 100% a-Si3N4, (b) 70% a-Si3N4 + 30% TiN, (c) 50% a-Si3N4 + 50% TiN, (d) 100% b-Si3N4 and (e) 70%b-Si3N4 + 30% TiN.

M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58 55

promoted by the presence of TiN in the spark plasma sinteredsamples. This can be a result of the conductive samples experienc-ing a higher internal temperature than the non-conductive sam-ples at the same temperature measured on the surface of the die,due to the Joule heating effect caused by the large pulse currentthrough the conductive samples, promoting the solution-precipita-tion process of a–b transformation. This is consistent with the re-sults of density data given in Fig. 1. Jan Räthel’s research alsoshows that the b/a ratio of silicon nitride compositions can be usedto monitor the real temperature distribution during the SPS pro-cess where the b/a ratio is higher in the higher temperature region(the out surface of the sample) than that in lower one (the centre ofthe sample) [5].

For the samples based on b-Si3N4, the Si3N4/TiN composite sam-ples have slightly lower b-fractions than monolithic Si3N4 samplesafter sintering at 1400 and 1500 �C. These differences are not sig-nificant since they are well within the errors of measurementsand there is very little a–b transformation occurring in predomi-nantly b samples at these low sintering temperatures. On the

contrary, after sintering at 1600 �C, the Si3N4/TiN composite sam-ples have also higher b-fractions than monolithic Si3N4 ones. Thiscan also be attributed to the enhanced Joule heating effect in con-ductive composites containing TiN.

The microstructures of the a-Si3N4 and b-Si3N4 based compos-ites sintered by SPS at 1600 �C are shown in Fig. 5. The results showthat the porosity of Si3N4/TiN composites (Fig. 5b, c and e) isslightly less than that of monolithic Si3N4 (Fig. 5a and d). TheSEM micrographs of the samples SPSed at 1300 �C, given inFig. 6, also show that the Si3N4/TiN composite samples (Fig. 6b, cand e) have higher densities than the monolithic Si3N4 ones(Fig. 6a and d) irrespective of whether the starting material isbased on a-Si3N4 or b-Si3N4. This observation is in agreement withthe density data in Section 3.1 in which the introduction of theconductive phase, TiN, promotes the densification of Si3N4 duringspark plasma sintering because of the improved Joule heating ef-fect in the presence of conducting TiN. The average grain sizeand aspect ratio of the Si3N4/TiN composites sintered by SPS at1600 �C are listed in Table 3. We can see that the average grain size

Fig. 6. SEM micrographs of – Si3N4/TiN composites SPSed at 1300 �C (a)100% a-Si3N4, (b)70% a-Si3N4 + 30% TiN, (c)50% a-Si3N4 + 50% TiN, (d)100% b-Si3N4 and (e)70%b-Si3N4 + 30% TiN.

56 M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58

of Si3N4/TiN composites is bigger than that of monolithic Si3N4inboth a-Si3N4 and b-Si3N4, but the aspect ratio Si3N4/TiN compositesare smaller than that of monolithic Si3N4. It indicates that the dif-ference between the actual temperature at the centre of conduc-tive Si3N4/TiN composite samples and the nominal temperaturemeasured on the surface of the, due to the Joule heating effect ofthe conductive samples, the solution-precipitation process involv-ing the a–b phase transformation and the b-Si3N4 grain growth inthe Si3N4/TiN composites is promoted, and hence, the b-fraction inthe Si3N4/TiN composites is higher than that in monolithic Si3N4

(Fig. 4). However, the presence of TiN suppresses the elongationof b-Si3N4 grains, which leads to the smaller aspect ratio of b-Si3N4-

in Si3N4/TiN composites. It can also be found that the aspect ratiofor a-Si3N4based samples is bigger than that of b-Si3N4based onesbecause there is fewer a phase and less a–b phase transformationduring SPS in b-Si3N4 based samples.

3.3. Mechanical properties

The hardness and fracture toughness of monolithic Si3N4 andSi3N4/TiN composites are given in Tables 4 and 5 respectively. Toseparate the effect of the current passing through the specimenin SPS from the chemical effects resulting from the addition ofTiN, the monolithic Si3N4 and Si3N4/TiN composite samples weresintered by both Hot Pressing and SPS at 1600 �C. The results showthat the hardness and fracture toughness of both a-Si3N4/TiN andb-Si3N4/TiN composite samples containing either 30% TiN or 50%TiN, sintered by SPS, are higher than those of the correspondingones sintered by HP. The 50% TiN sample sintered by SPS has thehighest fracture toughness and the biggest difference in hardnesscompared by hot pressed one, which means that the improvementin mechanical properties of the SPSed Si3N4/TiN composite materi-als increases with the increase of conductivity. This can also be

Table 3Average grain size and aspect ratio of theSi3N4/TiN composites sintered by SPS at 1600 �C.

100% a-Si3N4 + 0% TiN 70% a-Si3N4 + 30% TiN 50% a-Si3N4 + 50% TiN 100% b-Si3N4 + 0% TiN 70% b-Si3N4 + 30% TiN

Grain size (lm) 0.3 0.5 0.6 0.4 0.6Aspect ratio 9 3 2 5 2

Table 4Vickers hardness of Si3N4 with different fractions of TiN sintered by Hot Pressing and SPS at 1600 �C (kg/mm2, load = 10 Kg).

Sintering method 100% a-Si3N4 + 0% TiN 70% a-Si3N4 + 30% TiN 50% a-Si3N4 + 50% TiN 100% b-Si3N4 + 0% TiN 70% b-Si3N4 + 30% TiN

HP 1741 ± 29 1589 ± 13 1311 ± 15 1623 ± 11 1438 ± 11SPS 1735 ± 9 1602 ± 40 1509 ± 28 1591 ± 34 1509 ± 21

Table 5Fracture toughness of Si3N4 with different fractions of TiN sintered by Hot Pressing and SPS at 1600 �C (MPa m1/2).

Sintering method 100% a-Si3N4 + 0% TiN 70% a-Si3N4 + 30% TiN 50% a-Si3N4 + 50% TiN 100% b-Si3N4 + 0% TiN 70% b-Si3N4 + 30% TiN

HP 4.6 5.2 7.4 4.8 5.7SPS 4.7 7.7 8.1 4.2 6.0

M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58 57

explained by improved Joule heating due to the higher currentspassing through more conductive samples.

4. Conclusions

The effects of the electric current that is passing through thesample during spark plasma sintering on the densification, phasetransformation, microstructure and mechanical properties of con-ductive Si3N4/TiN composites and non-conductive monolithic a-Si3N4 and b-Si3N4 were studied. The introduction of TiN into siliconnitride increases the electrical conductivity of the composite mate-rials and consequently affects their sintering behaviour in SPS andthe microstructure and properties of the resultant densified ceram-ics. The densification and a–b transformation of Si3N4 are signifi-

Fig. A1. Electrical conductivity of our best insulating materials [19].

cantly improved by the passage of a higher current through thesample during SPS. Although TiN suppresses the formation of elon-gated b-Si3N4 grains, the beneficial effect of TiN on densification re-sults in Si3N4/TiN composites with improved fracture toughnessand hardness. This study shows that increasing the current thatis passing through a sample is a possible means of improving thesintering efficiency of the SPS process.

Acknowledgements

M. Zhou acknowledges the Monash scholarship. The Authorsacknowledge the use of facilities within the Monash Centre forElectron Microscopy. Useful discussion with Dr. Aaron Seeber isacknowledged.

Appendix A

See Fig. A1.

References

[1] Z.A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, The effect of electric field andpressure on the synthesis and consolidation of materials: a review of the sparkplasma sintering method, J. Mater. Sci. 41 (2006) 763–777.

[2] R. Orru, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Consolidation/synthesis ofmaterials by electric current activated/assisted sintering, Mater. Sci. Eng. R 63(2009) 127–287.

[3] Tatsuya Misawa, Noboru Shikatani, Yuji Kawakami, Takashi Enjoji, YasunoriOhtsu, Hiroharu Fujita, Observation of internal pulsed current flow through theZnO specimen in the spark plasma sintering method, J. Mater. Sci. 44 (2009)1641–1651.

[4] Wang Yucheng, Fu Zhengyi, Study of temperature field in spark plasmasintering, Mater. Sci. Eng. B 90 (2002) 34–37.

[5] Jan Räthel, Mathias Herrmann, Wieland Beckert, Temperature distribution forelectrically conductive and non-conductive materials during field assistedsintering (FAST), J. Eur. Ceram. Soc. 29 (2009) 419–425.

[6] Antonio Mario Locci, Alberto Cincotti, Sara Todde, Roberto Orru, Giacomo Cao,A methodology to investigate the intrinsic effect of the pulsed electric currentduring the spark plasma sintering of electrically conductive powders, Sci.Technol. Adv. Mater. 11 (2010) 045005. 13pp.

[7] Ching-Huan Lee, Horng-Hwa Lu, Chang-An Wang, Pramoda K. Nayak, Jow-LayHuang, Influence of conductive nano-TiC on microstructural evolution ofSi3N4-based nanocomposites in spark plasma sintering, J. Am. Ceram. Soc. 94(2011) 959–967.

[8] R.G. Duan, G. Roebben, J. Vleugels, O. VanderBiest, Optimization ofmicrostructure and properties of in situ formed beta-O-sialon-TiNcomposite, Mater. Sci. Eng. A 427 (1–2) (2006) 195–202.

58 M. Zhou et al. / Journal of Alloys and Compounds 547 (2013) 51–58

[9] R.G. Duan, G. Roebben, J. Vleugels, O. VanderBiest, Effect of TiX (X = C, N, O)additives on microstructure and properties of silicon nitride based ceramics,Scripta Mater. 53 (2005) 669–673.

[10] Norhayati Ahmad, Hidekazu Sueyoshi, Microstructure and mechanicalproperties of silicon nitride–titanium nitride composites prepared by sparkplasma sintering, Mater. Res. Bull. 46 (2011) 460–463.

[11] S. Kawano, J. Takahashi, S. Shimada, The preparation and spark plasmasintering of silicon nitride-based materials coated with nano-sized TiN, J. Eur.Ceram. Soc. 24 (2004) 309–312.

[12] Ching-Huan Lee, Horng-Hwa Lu, Chang-An Wang, Pramoda K. Nayak, Jow-LayHuang, Microstructure and mechanical properties of TiN/Si3N4

nanocomposites by spark plasma sintering (SPS), J. Alloys Comp. 508 (2010)540–545.

[13] Shuichi Kawano, Junichi Takahashi, Shiro Shimada, Fabrication of TiN/Si3N4

ceramics by spark plasma sintering of Si3N4 particles coated with nanosizedTiN Prepared by controlled hydrolysis of Ti(O-i-C3H7)4, J. Am. Ceram. Soc. 86(2003) 701–705.

[14] Norhayati Ahmad, Hidekazu Sueyoshi, Properties of Si3N4–TiN compositesfabricated by spark plasma sintering by using a mixture of Si3N4 and Tipowders, Ceram. Int. 36 (2010) 491–496.

[15] Zhiquan Guo, Gurdial Blugan, Rene Kirchner, Mike Reece, Thomas Graule,Jakob Kuebler, Microstructure and electrical properties of Si3N4–TiN

composites sintered by hot pressing and spark plasma sintering, Ceram. Int.33 (2007) 1223–1229.

[16] E. Ayas, A. Kara, F. kara, A novel approach for preparing electrically conductivea/b SiAlON–TiN composites by spark plasma sintering, J. Ceram. Soc. Jpn. 116(7) (2008) 812–814.

[17] S. Shimada, K. Kato, Coating and spark plasma sintering of nano-sized TiN onY-alpha-sialon, Mater. Sci. Eng. A 443 (1–2) (2007) 47–53.

[18] Per-Olov Kall, Quantitative phase analysis of Si3N4-based materials, Chem.Scripta 28 (1988) 439–446.

[19] W. David Kingery, H.K. Bowen, Donald R. Uhlmann, Introduction to Ceramics,second ed., Wiley, Interscience, 1976.

[20] <http://www.siliconfareast.com/sio2si3n4.htm, 2012>.[21] H.O. Pierson, Handbook of Refractory Carbides and Nitrides, William Andrew

Publishing, Noyes, 1996 (Chapter 11, pp. 187).[22] M. Suganuma, Y. Kitagawa, S. Wada, N. Murayama, Pulsed electric current

sintering of silicon nitride, J. Am. Ceram. Soc. 86 (3) (2003) 387–394.[23] Z.J. Shen, Z. Zhao, H. Peng, M. Nygren, Formation of tough interlocking

microstructures in silicon nitride ceramics by dynamic ripening, Nature 417(6886) (2002) 266–269.

[24] D. Salamon, Z. Shen, P. Sajgalık, Rapid formation of alpha-sialon during sparkplasma sintering: its origin and implications, J. Eur. Ceram. Soc. 27 (6) (2007)2541–2547.