Materials Chemistry and Physics 92 (2005) 64–70

Fabrication of nanocomposite Ti(C,N)-based cermetby spark plasma sintering

Yong Zhenga,b,∗, Shengxiang Wangb, Min Youb, Hongyan Tana, Weihao Xiongc

a College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, Chinab College of Materials Science and Engineering, China Three Gorges University, Yichang 443002, China

c College of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China

Received 12 September 2004; received in revised form 17 December 2004; accepted 27 December 2004

Abstract

Several cermets with nano-TiC and TiN additions were prepared by spark plasma sintering (SPS). Their microstructures were studiedfurther by using SEM, TEM, EDX. Hardness (HRA, namely Rockwell Hardness A Scale) and transverse rupture strength (TRS) were alsomeasured. It was found that the cermets sintered by SPS contained many grains with the typical “core–rim” structures, which were believedt et with allt esides somel ld improvet n the rim oft the averages©

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o form by Ostwald ripening mechanism. The grain size was found to reduce with increasing nanoscale-sized additions. The cermhe ceramic powders being nanoscale-sized structure contained many small grains, the sizes of which were less than 100 nm, barger grains with about 200–300 nm size. In addition, when the cermets were sintered by SPS, the addition of nano-powders couhe mechanical properties of the cermets greatly. In the cermets with nano-additions, some nano-grains were found to be inlaid ihe larger ceramic grains and kept partial coherent with these larger ones. It was the firm joint between them and the reduction ofize of the grains that led to strengthening and toughening of the cermet with nano-additions.2005 Elsevier B.V. All rights reserved.

eywords:Nanocomposite; Microstructure; Ti(C,N)-based cermet; Spark plasma sintering

. Introduction

Recently, Ti(C,N)-based cermets attract much attentionrom researchers because of their excellent wear-resistance,igh hardness at high temperature, perfect chemical stability,ery low friction coefficient to metals, superior thermal de-ormation resistance[1–3]. However, cermets are less stronghan the commonly used WC–Co-based hardmetals. If theirtrength were improved, they would be a substitute for theommonly used WC–Co-based hardmetals. Efforts have beenade to produce cermets with a higher strength without los-

ng too much of their other properties.Nanocomposite ceramics have also been studied exten-

ively since the late 1980s when Japanese scholar Niiharaeported that the mechanical properties of ceramics coulde improved by adding nanometer additions to the ceramics

∗ Corresponding author. Tel.: +86 25 84594476; fax: +86 25 84594476.E-mail address:yzhengonly@263.net (Y. Zheng).

[4–8]. Nowadays it is widely accepted that nanomaterialsnanocomposite possess high mechanical properties.

Recently, interest has been growing in the use of splasma sintering (SPS) to fabricate metals, compositesespecially ceramics[9–12]. The SPS is a new process tprovides means by which ceramic powder can be sinvery rapidly to full density at lower temperatures and wshorter soaking times. It is similar to hot pressing, whiccarried out in a graphite die, but the heating is accompliby electric discharge in voids between particles. WithSPS method, raw powders in a carbon die are pressed uially and a d.c. pulse voltage is applied[9]. Thus, the biggesadvantage of SPS is that the dense sintering from acompact can be completed in a short time without signifigrain growth since the oxide layer on the particle surfacebe removed by spark plasma between the particles[13]. How-ever, the method has not been used in the field of prepcermets. Therefore, in the present study, we used SPS toTi(C,N)-based cermets with nano-TiC and TiN additions,

254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.matchemphys.2004.12.031

Y. Zheng et al. / Materials Chemistry and Physics 92 (2005) 64–70 65

Table 1Mean particle sizes and the oxygen contents of raw powders

Powders TiC (nm) TiC (�m) TiN (nm) TiN (�m) Ni Mo WC VC C

Particle size (�m) <0.1 2.88 <0.1 1.18 1.7 2.80 0.85 2.77 5.5O2 content (mass%) 2.47 0.37 7.51 0.33 0.30 0.10 0.21 0.16 –

investigated the microstructures and properties of the sinteredcompacts.

2. Experimental procedure

The mean particle sizes measured by the sedimentationmethod and the oxygen contents determined by TC-136oxygen–nitrogen analyzer are summarized inTable 1.

The compositions of all Ti(C,N)-based cermets inthe present study are 33 wt.% TiC–10 wt.% TiN–32 wt.%Ni–16 wt.% Mo–6.9 wt.% WC–1.5 wt.% C–0.6 wt.% Cr3C2.TiC and TiN are mixture of micro- and nano-powders, the de-tail percentages of which are shown inTable 2. With respectto A–D cermets, both of the weight ratios of nano- to micro-TiC and that of nano- to micro-TiN are 0%:100%, 20%:80%,60%:40%, 100%:0%, respectively.

Nano-powders were dispersed by using an ultrasonicdevice and then powder mixtures consisting of micro-, nano-powders were mixed in a planetary ball-mill in ethanol to-gether with cemented carbides balls for 24 h at a speed of200 rpm. The slurries were dried at 353 K in an infrared stove,then sieved and pelletised. After that, the powders were com-pacted by SPS, which was carried out in a vacuum chamberusing an SPS apparatus (Dr. Sinter 1050, Sumimoto CoalM 25-m at ah f2 inter-i n ofa mina , thes 73 K

within 2–3 min. During the sintering, the on/off time ratioof the pulsed current was set to 12/2 in each run. The max-imum current reached approximately 3500 A. The sinteredsamples were approximately 25 mm in diameter and 5 mmthick. All the sintered disks were cut and ground into bars witha dimension of 4 mm× 3 mm× 18 mm for strength measure-ments. The bars were polished with 600-grit SiC on one sidein order to eliminate edge flaws for strength testing. Den-sity measurements were made using the Archimedes tech-nique. The hardness and transverse rupture strength (TRS)at room temperature were measured. There were 4 bars ineach group for measuring TRS. The fracture surfaces of thetested specimens were observed by a Hitachi X650 scanningelectron microscopy (SEM), and the lower and higher magni-fication microstructures by a JSM-5600LV SEM and HitachiS4800 SEM in backscattered electron (BSE) mode, respec-tively. Thin slices of the specimens were polished to 20�mthickness. The disks for the TEM investigation were thinnedto electron transparency by ion-milling in a Gatan691 miller.A JEM-2010 transmission electron microscopy (TEM) wasused to study the microstructure in more detail, and a INCAenergy dispersive X-ray analysis (EDX) in combination withTEM to fulfill element microanalysis.

3

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C TiN

A 10 .5B 8 5C 4 5D 0 .5

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C T m

A 553B 794C 74D 879

ining, Japan). The powders were inner placed into am-diameter graphite die and then heated to 1623 Keating rate of approximately 300 K min−1. A pressure o0 MPa was applied from the start and retained to the s

ng temperature and released during the cooling portiocomplete sintering cycle. The sample was held for 3

t the sintering temperature. Immediately after holdingintered sample was cooled to a temperature below 9

able 2omposition design of the experimental materials (wt.%)

ermet TiC (nm) TiC (�m) TiN (nm)

0 33 06.6 26.4 2

19.8 13.2 633 0 10

able 3echanical properties and density of cermets with different nano-TiC

ermet Weight ratio of nano- to micro-powder

0%:100%20%:80%60%:40%100%:0%

. Results and discussion

.1. Mechanical properties and density

The hardness and transverse rupture strength (TRoom temperature and density of the cermets with diffeano-TiC and TiN additions sintered by SPS are summa

n Table 3.

(�m) Ni Mo WC VC C

32 16 6.9 0.6 132 16 6.9 0.6 1.32 16 6.9 0.6 1.32 16 6.9 0.6 1

N additions

RS (MPa) Hardness (HRA) Density (g c−3)

.3+22.4−34.6 91.0 6.40

.4+35.3−27.8 90.8 6.44

2.8+30.4−23.5 89.5 6.52

.5+42.8−36.2 89.8 6.48

66 Y. Zheng et al. / Materials Chemistry and Physics 92 (2005) 64–70

Fig. 1. Fracture appearance of (a) cermet A; (b) cermet B; (c) cermet C; (d) cermet D (1: tearing ridge; 2: dimple; 3: concavity; 4: grain).

Cermet D with all the ceramic powders being nano-sizestructure showed the highest TRS. On the contrary, cermet Awith no nano-ceramic powders added showed very low TRS.The TRS of cermets B and C were also higher than that ofcermet A. In addition, the hardness of the cermets with somenano-ceramic additions decreased slightly.

Table 1shows that the oxygen contamination of nano-TiCand TiN powders is much higher than that of other micro-powders. When the amount of the added nanoscale-sizedpowders was too much, the oxygen contamination might notbe reduced completely during sintering, resulting in the de-crease of the surface energy of solidγSV [14]. Thereby theresidual oxygen on the surface of the ceramic grains wouldincrease the wetting angle and accordingly reduced the wet-ting of the ceramic grains by liquid metal during sintering,resulting in low density and low TRS of the sintered cer-mets, which was consistent with the experimental results un-der vacuum sintering[15]. However, when the compacts weresintered by SPS in the present study, the densities of the cer-mets didn’t reduce with increasing nano-powder additions,which expresses that the oxygen contamination was reducedcompletely and accordingly the wettability still kept goodduring SPS process. It was reported that the oxide layer onthe particle surface can be removed by spark plasma betweenthe particles[13], which was also verified indirectly by thep

Fig. 1shows the fracture images of the four cermets, all ofwhich consisted of ceramic grains, concavities formed by theremoval of ceramic grains, tearing ridges formed by the tear-ing of the metal phase, dimples along the expanding paths ofthe tearing ridges, which also included some small concavi-ties with ceramic grains as the core formed during fracture,and a few ceramic grains, resulting from cleavage fracture.The removal of ceramic grains resulted from weak interfacecoalescence between the grains and the metal phase. Moretearing ridges, less intergranular fracture, more dimples, andgreater plastic deformation occurred in cermets B–D, with aamount of nano-TiC and TiN added, than in cermet A, with nonano-addition. Obviously, the nano-additions strengthenedand improved the TRS of the cermets. In addition, there ex-isted an obvious trend for the TRS as a function of nano-additions. It can be determined further that when the cermetswere sintered by SPS, the addition of nano-powders couldimprove the mechanical properties of the cermet greatly.

3.2. Microstructure

The microstructure was observed by a JSM-5600LV scan-ning electron microscopy (SEM) using backscattered elec-trons, as shown inFig. 2. It was found that the grain sizewas affected by the amount of nano-additions. Nano-additionc ore,

resent experiments. ould reduce the grain size of the cermet greatly. Furtherm

Y. Zheng et al. / Materials Chemistry and Physics 92 (2005) 64–70 67

Fig. 2. SEM micrographs of the microstructures of (a) cermet A; (b) cermet B; (c) cermet C; (d) cermet D.

the grains of the cermet D with all the added ceramic powdersbeing nanoscale-sized were the smallest of the four cermets.In addition, there existed three kinds of grains in the fourcermets: one had “black core–white rim” structure, which iswell known as the typical structure of conventional cermets,the second had “white core–black rim” structure and the thirdhad no obvious rim phase.

Cermet A with no nano-TiC and TiN additions had the mi-crostructures mainly consisting of large ceramic grains withno obvious rim phase besides a few small ceramic grains with“white core–black rim” and a few with “black core–whiterim”.

However, in the cermets B and C with nano-TiC andTiN additions, the amount of the small grains with “whitecore–black rim” increased greatly while the amount of thelarge grains decreased. In cermet D with all the added ce-ramic powders being nanoscale-sized, the amount of thesmall grains was very much so that it was difficult to observethe microstructure in detail by JSM-5600LV SEM. To eluci-date the characteristics of the microstructure of the cermetswith nano-additions in detail, high resolution observation of

the microstructure of cermet D was performed again by aHitachi S4800 SEM, as shown inFig. 3. The cermet showeda microstructure consisting of some larger grains, the sizesof which were about 200–300 nm, and some small grains,the sizes of which were less than 100 nm. Some grains had“core–rim” structures, and others had no obvious rim. In ad-dition, a part of smaller grains were inlaid in the rim of thelarger grains.

The larger grains with “core–rim” structures of cermet Dwere observed by a TEM, as shown inFig. 4, and a selectedgrain was analysed by an EDX in combination with the TEM.Main metal contents of different phases were determined andsummarized inTable 4.

Fig. 4shows that the microstructure of Ti(C,N)-based cer-mets compacted by SPS was characterized by carbonitrides,exhibiting a “core–rim” structure, bonded with a metallicphase. The inner rim and outer rim could be distinguished,and the inner rim was very thin and unevenly distributedaround the cores. The quantitative results summarized inTable 4indicates that Mo, W, Ni and V always existed richlyin the rim phase, and the inner rim had a substantially higher

structu .

Fig. 3. SEM micrographs in higher magnification of the micro res of cermet D: (a) magnification is 100 K; (b) magnification is 200 K

68 Y. Zheng et al. / Materials Chemistry and Physics 92 (2005) 64–70

Fig. 4. TEM micrograph in bright field of cermet D sintered by SPS.

Mo and W content compared with the outer rim. So the vol-ume fraction of the binder phase was lower than expecteddue to the distribution of a part of the metal elements Mo andNi in the rim phase. In addition, after sintering was finished,it was found that there was metal volatile on the surface ofthe sintering vessel, which indicated that the volatilizationunder SPS condition was heavier. It was believed that thiswas another cause resulting in the lower volume fraction ofthe binder phase.

The “core–rim” structure within the larger grains is thetypical microstructure of cermets sintered under vacuum[16].In the present study, all cermets were sintered by SPS. Theirmicrostructure characteristics were different from that of thecermets sintered under vacuum to some extent. It was re-ported that the rim phase (Ti,Mo)(C,N), which is in equilib-rium with the liquid, precipitate on Ti(C,N) grains due to thedissolution of small grains and reprecipitation on large ones(Ostwald ripening) during sintering[17]. There are furtherthermodynamic calculations and experiments indicating thatthe composition of the rim can be determined by N activity.Lower N activity can result in a higher Mo and W content ofthe rim and vice versa[18]. The inner rim with a higher Moand W content is formed during solid state sintering at whichthere is an open porosity and a lower N activity. Contrary tothe above, the outer rim with a lower Mo and W content isf r Na typi-c cess

TM

R

CIOB

the microstructure of the cermet with nano-addition was stillformed by Ostwald ripening mechanism.

On the other hand, during spark plasma sintering, thedischarges are generated by an instantaneous pulsed directcurrent. Due to these discharges, the particle surface is in-stantaneously activated and purified, and concurrently self-heating phenomena are generated among these particles,leading to heat- and mass-transfer to be completed in an ex-tremely high speed[19,20]. In addition, it was reported thatthe formation of core–rim structure was concerned with thediffusion of elements[17,21]. So it is reasonable that the for-mation of core–rim structure could be finished within veryshort time in the present study.

Other TEM observations of the microstructure of cermet Dwere also performed, as shown inFig. 5. It was observed thatthere existed smaller grains (grain 2 inFig. 5) inlaid in the rimphases of the larger grains (grain 1 inFig. 5) for a certainty,which was consistent with the above SEM observation.

Grains 1 and 2 chosen by the diaphragm of TEM, selectingarea diffraction analysis was performed, as shown inFig. 6.

Fig. 6 shows the orientation relationship between grainsof cermet D. Some smaller grains inlaid in the rim of thelarger grains kept partial coherence with the larger ones. Thestrong joint between them led to the cracks deflecting duringthe fracture and a limitation of their extension. Moreover, thes dis-l oupsc alsoh As ar rmetw

d’ss withd ndt th in-c f the

F allerg

ormed during liquid sintering at which there is a highectivity due to enclosed porosity. The existence of theal “core–rim” structure expresses that during SPS pro

able 4etal contents of different phases of cermet D sintered SPS

egion Content (wt.%)

Ti W Mo Ni V

ore 69.81 4.67 20.30 2.78 1.69nner rim 47.86 8.74 35.76 3.48 1.44uter rim 58.51 4.52 18.01 16.95 1.30inder 4.12 0.31 4.26 90.50 0.51

maller grains existing in the rim phase resulted in someocation groups inside the larger ones. The dislocation grould be pinned by the smaller grains themselves, whichindered the cracks extension and led to their deflection.esult, the fracture energy was improved and thus the ceith nano-additions was strengthened.In addition, it could be determined from Gurlan

trength theory that the strength of the cermet increasedecreasing grain size[18]. Fig. 2shows that there was a tre

hat the average grain sizes of the cermets reduced wireasing nano-additions under SPS. Clearly, the fining o

ig. 5. TEM micrograph of the region consisting of larger grains and smrains of cermet D sintered by SPS.

Y. Zheng et al. / Materials Chemistry and Physics 92 (2005) 64–70 69

Fig. 6. Diffraction pattern showing the orientation relationship between larger grains and smaller grains of cermet D: (a) diffraction pattern; (b)determinationof diffraction pattern.

grains in the cermets with nano-additions would improve thestrength of the cermet. So cermet D with the smallest averagegrain size showed the highest TRS.

However,Table 2shows that the hardness of the cermetsslightly decreased with additions of nano-ceramic. This phe-nomenon could be due to microstructural variation. Hardnessis affected by grain size, the volume fraction of the cores andrim phase, the hardening extent of the binder, etc. The highervolume fraction of rim phase in the nanocomposite cermetmight result in the above hardness behaviour because thehardness of the rim phase (Ti,Mo)(C,N) is lower than that ofTi(C,N) core.

4. Conclusions

(1) The microstructure of the cermets sintered by SPSshowed that many grains had the typical “core–rim”structures, which expressed that during SPS process themicrostructures of the cermets with nano-additions werestill formed by Ostwald ripening mechanism.

(2) Even if all of the added TiC and TiN powders werenanoscale-sized, the cermet could still be densified bySPS, which was different from the experimental resultsunder vacuum sintering at all. In addition, nano-additions

( ano-be-

con-erebout

( ionsnd

s thef the

average size of grains that led to the strengthening of thecermet.

Acknowledgements

This research was financially supported by the NationalNatural Science Foundation of China under project no.50104006, Natural Science Fund for Distinguished Schol-ars of Hubei Province of China under project no. 2003-31and China Postdoctoral Science Foundation under project no.20040350169.

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