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Materials Science and Engineering A 434 (2006) 23–29

Effect of ball milling on simultaneous spark plasma synthesis anddensification of TiC–TiB2 composites

Antonio M. Locci a, Roberto Orru a,b, Giacomo Cao a,b,∗, Zuhair A. Munir c

a Dipartimento di Ingegneria Chimica e Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA),Unita di Ricerca del Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali (INSTM),

Universita degli Studi di Cagliari, Piazza d’Armi, 09123 Cagliari, Italyb PROMEA Scarl, c/o Dipartimento di Fisica, Cittadella Universitaria di Monserrato, S.S. 554 bivio per Sestu,

09042 Monserrato (CA), Italyc Facility for Advanced Combustion Synthesis, Department of Chemical Engineering and Materials Science,

University of California, Davis, CA 95616, USA

Received 22 November 2005; received in revised form 6 June 2006; accepted 29 June 2006

bstract

The effect of ball milling pre-treatment of the starting Ti, B4C and C powders mixture on the synthesis of dense TiC/TiB2 composite wasnvestigated through the combined use of a SPEX mill and the spark plasma sintering (SPS) apparatus.

When unmilled reactants were used, amorphous carbon was converted to TiC through a combustion reaction with elemental titanium, whilehe complete conversion is achieved through a relatively slow reaction between the residual Ti and B4C. In contrast, powders ball milled for 24 heacted completely under combustion regime.

SPS product density was observed to increase from about 97% to 100% of the theoretical value, when the original powders were milled for 6 h,

hile further increase of the milling time led to a slight decrease in product density.Crystallite size of both boride and carbide phases decreased with milling down to about 50 nm after 24 h of mechanical treatment. The latter one

lso allowed for the formation of a material with relatively finer microstructure and more homogeneous phase distribution.2006 Elsevier B.V. All rights reserved.

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eywords: Ball milling; Spark plasma sintering (SPS); Synthesis; Composites;

. Introduction

Ball milling (BM) is a solid-state powder process involv-ng welding and fracturing of powder particles in a high-energyall mill [1]. Originally developed to produce oxide-dispersiontrengthened nickel and iron-base superalloys [2], BM has beenhown to be capable of synthesizing a variety of equilibriumnd non-equilibrium alloy phases starting from blended ele-ental or pre-alloyed powders. The non-equilibrium phases

ynthesized include solid solutions, metastable crystalline anduasi-crystalline phases, nanostructures and amorphous alloys

1]. In addition, it has been found that BM may increase powderseactivity [3,4]. This feature is associated to reactants interfaceormation, increase of internal and surface energy as well as

∗ Corresponding author. Tel.: +39 070 6755058; fax: +39 070 6755057.E-mail address: cao@visnu.dicm.unica.it (G. Cao).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.06.131

es; Carbides

urface area, which all contribute to the so-called mechanicalctivation (MA) of powders [1,5].

In many cases, powders produced by taking advantage ofigh-energy mechanical milling are used directly as obtained forchieving desirable functions in several application fields suchs electronics, cosmetics, optics, coatings and paints fabrications6,7]. On the other hand, bulk nanostructured materials with tai-ored mechanical, physical, and chemical properties are neededor structural applications or as components in magnetic andlectrical devices. These materials have been, for example, pre-ared by consolidation of mechanically activated or ball-milledowders [8]. Several techniques, such as hot-pressing [9,10],hock consolidation [11–13], sintering with the application ofc currents [14–16] or pulsed electric current [17–21], have been

mployed for the sintering/densification step. In particular, whenanometric powders obtained by mechanical milling need to beensified in order to produce bulk bodies with desirable anduperior properties, it can be very challenging to identify the

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ppropriate conditions during the consolidation process whichre able to preserve the nanostructure.

The above considerations are particularly valid for producingense nanostructured refractory materials such as borides andarbides of transition metals. These materials have been rec-gnized as candidate for advanced structural applications dueo their exceptional hardness and stability at high temperature.urthermore, the use of these ceramics in composites offers thedvantages of enhanced fracture toughness [22,23]. However,ecause of their extremely high melting points, consolidationf these materials requires high temperatures and long hold-ng times. These conditions adversely affect both microstruc-ure, i.e. grain growth accompanies the consolidation, and costffectiveness of the production process. Therefore, intensiveesearch is needed for developing and optimizing processesimed to the production of nanostructured dense refractoryaterial, specifically when the approach consisting of the com-

ination of mechanical milling and subsequent consolidation isdopted.

Recently, with the aim of synthesizing dense nanostructurediC/TiB2 composites, the mechanical activation of elementalTi, C, and B) reactants followed by the field-activated pressure-ssisted synthesis (FAPAS), which combines high ac currentith uniaxial pressure, has been proposed [15].Along these lines, in the present investigation the simulta-

eous synthesis and densification of TiC/TiB2 using the sparklasma sintering (SPS) apparatus was investigated starting fromall milled reactants (Ti, B4C and C). While considering theame ceramic composite, i.e. TiC/TiB2 with a molar ratio equalo 1, as that one investigated by Lee et al. [15], it is worth not-ng that in this work the elemental boron is replaced by theelatively cheaper boron carbide. In addition, the SPS appara-us, which is characterized by a pulsed electric current insteadf the ac current as in FAPAS, is employed in the proposedtudy.

Specifically, the effect of the milling time on the charac-eristics of powder reactants, SPS process dynamics, reactantsonversion, relative density, and crystallite sizes of both ceramiconstituents of the final product, will be systematically investi-ated. The results are compared with those recently reported inhe literature [24], which were obtained by the same method butn unmilled powders.

. Experimental materials and methods

Characteristics of the starting powders used in the presentnvestigation are reported in Table 1. Powders were milledogether (co-milled) in a stoichiometric ratio corresponding to

able 1tarting powders used in the present investigation

eactant Vendor Particle size Purity (%)

itanium (Ti) Sigma–Aldrich (366994) −325 mesh 99.98oron carbide (B4C) Alfa Aesar (40504) 1–7 �m 99.4arbon black (C) Alfa Aesar (39724) 0.042 �m

(average)99.9+

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Engineering A 434 (2006) 23–29

he following reaction:

Ti + B4C + C → 2TiC + 2TiB2 (1)

echanical activation experiments were conducted in a SPEX000 shaker mill (SPEX CertPrep, USA) by using stainless steelars having an internal volume equal to about 65.8 cm3 (internaliameter, 3.8 cm; internal height, 5.8 cm). The jars were loadedith about 8 g of powders mixture and one stainless steel ball

weight, 8 g; diameter, 1 cm) so that the charge ratio, CR (ball toowders mass ratio) equals to about 1. In order to minimize oxi-ation, all powder handling and loading were performed inside alove box filled with argon. The jars were then sealed and trans-erred to the mill. After each run, powders were removed andials and balls were cleaned by using a slurry of silica and ace-one with the aim to eliminate all residual powders of previous

illing experiments.The range of ball milling time investigated, hereafter indi-

ated as tBM, was 0–24 h.A spark plasma sintering (SPS) system (mod. 515S, Sumit-

mo Coal Mining Co. Ltd., Japan) was used to synthesize denseiC/TiB2 ceramic composites. Details related to SPS apparatusre reported elsewhere [24]. The adopted pulse cycle consistedf 12 pulses on and 2 pulses off, for a total pulse cycle durationqual to about 46.2 ms, with the characteristic time of a singleulse equal to about 3.3 ms.

About 4 g of mechanically activated powder mixture was firstold-compacted inside a graphite die. With the aim to minimizeeat losses by thermal radiation, the die was covered with amm thick layer of graphite felt (Atal s.r.l., Italy). The die was

hen placed inside the reaction chamber of the SPS apparatusnd two cylindrical graphite blocks (diameter, 80 and 30 mmnd both 40 mm high) were placed between the upper plungernd the upper electrode, as well as the lower plunger and theower electrode. The blocks were made of AT101 graphite (Atal.r.l., Italy) while the die and plungers were all made of E-940raphite, supplied by Electrodes Inc. (USA). Additional exper-mental details can be found elsewhere [24].

The experiment is initiated with the application of a con-tant value of the integral mean electric current (I) equal to100 A and a mechanical pressure (P) of 20 MPa for 4 min (tSPS).emperatures were measured during synthesis by pyrometerrcon Mirage OR 15-990 (Ircon, USA) and C-type thermo-ouple (Omega Engineering Inc., USA), which was insertednside a small hole in the lateral surface of the graphite die.emperature, applied current, voltage, mechanical load, and theertical displacement of the lower electrode were measured ineal time and recorded. The latter parameter can be regarded ashe degree of powder compact densification. However, also ther-

al expansion of the sample as well as that of both electrodes,raphite blocks, spacers and plungers, contribute to the varia-ion of this parameter. Such a contribution has been evaluatedy performing a “blank test”, which consists in the application

f same SPS conditions (current, mechanical load, time, etc.) tohe ensemble die/plungers when powder compact is not present.t is worth noting that following this procedure, thermal expan-ion of processing powders is not considered. Moreover, it has

e and Engineering A 434 (2006) 23–29 25

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A.M. Locci et al. / Materials Scienc

een experimentally verified that temperature time-profile didot significantly change in absence of powder compact. Thus,y subtracting the resulting displacement from that recorded inresence of powder sample, the real sample shrinkage (δ) isbtained.

After the synthesis process, the sample was first allowed toool and then removed from the die. The relative density ofhe product was determined by the Archimedes method usingistilled water as wetting liquid.

Phase identification and crystallite size determination of bothilled powders and SPS products were made by using XRD

nalysis (mod. PW1830, Philips Analytical B.V., The Nether-ands) with Cu K� radiation and Ni filter. In particular, thenalysis of SPS samples was performed on powdered samplen diffraction angle (2θ) range 20–120◦ with a scan step and acan time equal to 0.01◦ and 10 s, respectively. Crystallite sizeas determined by means of the Halder–Wagner method [25]

rom the line broadening of XRD peaks.The microstructure of end products was examined by scan-

ing electron microscopy (SEM) (mod. S4000, Hitachi, Japan)nd local phase composition was determined by energy disper-ive X-rays spectroscopy (EDXS).

Indentation method using a Zwick 3212 Hardness testerachine (Zwick & Co. GmbH, Germany) was employed to

etermine Vickers hardness of the SPS obtained products. Thepplied load was equal to 196.2 N while the dwell time was 18 s.ased on the Vickers indentation and by assuming a Palmqvistracks geometry, fracture toughness (KIC) was also estimatedhrough the following formula [26]:

IC = 0.0319P

al1/2 (2)

here P is the applied load, a the mean indentation half-diagonalength and l the crack length.

. Results and discussion

.1. Powder mixture

Fig. 1 shows the variation of the XRD patterns of the powderixture with milling time, tBM, for the conditions indicated in

he Section 2. It should be noted that, since carbon was presentn amorphous form, only Ti and B4C were identified among theeactants by this analysis. With increasing milling time, the ini-ially sharp diffraction peaks become broader and their absolutentensity decreases. This is due to powder crystallite size refine-

ent and internal strain increase induced by the mechanicalreatment. In addition, these XRD results indicate that, withinhe detection limit of this analysis, additional phases are notormed at tBM up to 6 h, including any possible contaminantsrom the milling operation.

First evidence of product formation was observed when theilling time was 12 h. However, progressive grain refinement

nd increase of internal strain make the diffraction peaks broadernd less intense with consequent overlapping. Beside the start-ng reactants, the TiC phase only was discernable but, for theeason above, it is not possible to exclude the formation of addi-

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ig. 1. XRD patterns of blended 4Ti + B4C + C powders as a function of millingime.

ional phases as a consequence of the milling. The TiC contentlearly increased as tBM was increased up to 24 h. The apparentormation during milling of TiC before TiB2 is consistent withhe higher diffusivity of carbon into Ti as compared to that oforon [27].

On the basis of these results, it is clear that within the experi-ental conditions considered in this work, no complete conver-

ion of the starting reactants to the desired composite takes placeuring milling. This result was, on the other hand, achieved byee et al. [15] after 8 h milling of a Ti, B and C powders mix-

ure by using a Fritsch planetary mill and a significantly higherharge ratio (20).

.2. SPS process dynamics and reactive system behavior

The milled powders were processed in the SPS apparatusnder optimized operating conditions (I = 1100 A, P = 20 MPa,

SPS = 4 min), which arise from a previous investigation ofhe simultaneous synthesis and densification of the compos-te TiC/TiB2 by SPS of commercial, unmilled powders [24].pecifically, under these conditions the complete conversion ofeactants to the desired TiC/TiB2 composite with about 98%ensity was achieved. In Fig. 2, the XRD pattern of SPS productbtained from powders milled for 24 h is reported. It can be seenhat the final product consists of the desired TiC and TiB2 phasesnly.

We now consider the SPS process dynamics when unmilledr co-milled powders were used. The sample shrinkage and tem-erature time profiles recorded during the process for the casehen I = 1100 A, P = 20 MPa and tSPS = 4 min, and correspond-

ng to unmilled (tBM = 0 h) and milled (tBM = 24 h) powders, arehown in Fig. 3.

First of all, it is clearly seen that milling does not have aarked effect on the temperature evolution during the process.

n the other hand, the system behavior is significantly differentith respect to shrinkage (δ) variation with time. In particular,

he shrinkage profile related to the unmilled powders shows noignificant changes during the first 15 s. An approximately linear

26 A.M. Locci et al. / Materials Science and

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ig. 2. XRD patterns of SPS reacted powders milled for 24 h (I = 1100 A,= 20 MPa, tSPS = 4 min).

ncrease (to about 1.1–1.2 mm) followed by a steep increase (to.2 mm) takes place in the time range of 15–70 s. The shrinkagehen increases gradually to reach its maximum value (3.4 mm)t 240 s.

A completely different behavior was observed when the start-ng powders were mechanically treated for 24 h. Fig. 3 clearlyhows that an abrupt variation of the shrinkage up to approx-mately 3.4 mm takes place only after about 20 s from theeginning of the application of the pulsed electric current. Thehrinkage then gradually increases up to 5.5 mm at about 190 s.t should be noted that the total variation of the sample shrink-ge obtained in the case of milled powders is considerably largerhan that observed in the case of unmilled powders.

The evolution of the reaction synthesis given in Eq. (1) dur-ng SPS process was investigated in our previous work [24],sing unmilled powders and I = 800 A. It was found that theost rapid and significant sample shrinkage corresponded to

ig. 3. Sample shrinkage and temperature time profiles of SPS outputs forhe case of unmilled and milled (tBM = 24 h) powders (I = 1100 A, P = 20 MPa,

SPS = 4 min). (The asterisks indicate the time instants at which the electric cur-ent application was interrupted for kinetic mechanism investigations).

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Engineering A 434 (2006) 23–29

he carburization of Ti to form TiC, while TiB2 formation wasompleted subsequently. Since the current level considered inhe present work is different, this result has been verified bynalyzing the product obtained when unmilled powders are sub-ected to I = 1100 A, P = 20 MPa, for about 70 s, i.e. just feweconds after the time instant at which the rapid sample shrink-ge takes place. The corresponding XRD analysis is reported inig. 4(a). It is seen that, a relatively high degree of conversion oftarting reactants into one of the desired products, i.e. TiC, haseen achieved. However, only small amounts of TiB2 have beenbtained, while secondary phases such as TiB and Ti3B4, alongith small amounts of reactants are present in the final sample.herefore, as reported in a previous work [24], the pulsed elec-

ric current needs to be applied longer, i.e. up to 4 min, in ordero obtain the pure product.

Different results arise from the XRD analysis performed onhe SPS sample of powders milled for 24 h. Specifically, inig. 4(b) the composition of sample obtained by interrupting

he application of the current after only 20 s, i.e. immediatelyfter the abrupt shrinkage variation occurs, is reported. It ispparent that complete conversion of the starting reactants tohe desired phases TiC and TiB2 is achieved in this case. There-ore, the change of δ represents in this case an indication of theompleteness of the synthesis reaction (1). It is interesting toote that the temperature of the die corresponding to the abrupthrinkage is very low, i.e. equal to about 150 ◦C, when pow-ers milled for 24 h are processed, while it is about 850 ◦C, ifnmilled reactants are used as starting materials.

Typically, rapid changes of the parameter δ during SPS pro-esses may be associated to melting of reactants or products,r to the occurrence of combustion reactions, because theyre both accompanied by a considerable sample shrinkage in

resence of mechanical load. Since the recorded temperaturesorresponding to the time at which the abrupt shrinkage takeslace are in both cases (unmilled and mechanically treated pow-

ig. 4. XRD patterns of the SPS samples (I = 1100 A, P = 20 MPa) when start-ng from: (a) unmilled powders (tSPS = 70 s); (b) milled powders (tBM = 24 h,

SPS = 30 s).

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A.M. Locci et al. / Materials Scienc

ers) much lower than the melting point of Ti (1639 ◦C), i.e. theow-melting-point-component, the presence of molten phasesan be excluded. On the other hand, combustion reactions areikely to take place. It should be mentioned that an analogousehavior (fast reaction and abrupt shrinkage occurring at lowemperature) was already observed for instance during the syn-hesis and simultaneous densification of MoSi2 by SPS, whentarting from mechanically activated elemental reactants [18].

Some considerations related to the combustion behavior dis-layed by the system under investigation during the SPS processan be examined by taking advantage of recent results obtainedhen starting from unmilled powders [24]. It was suggested that

he formation of the composite is due to the following reactionath:

i + C → TiC (3)

Ti + B4C → 4TiB + TiC (4)

6TiB + B4C → 5Ti3B4 + TiC (5)

Ti3B4 + B4C → 8TiB2 + TiC (6)

ased on the experimental results reported in the cited work24] and in the present investigation when considering unmilledowders, it is postulated that the direct carburization of elementali by amorphous carbon, i.e. Eq. (3), is the first step taking placeuring the spark plasma synthesis of TiC/TiB2, and correspondso the most considerable shrinkage change. This step occursresumably as a combustion reaction because the temperatureeached at this point was relatively low and, thus, the formationf molten phases can be excluded. In addition, Eqs. (4)–(6) arexpected to be more likely responsible of the subsequent TiB2ormation, as well as of the intermediate phases, i.e. TiB andi3B4, detected before the synthesis reaction was completed. It

s worth noting that, during the occurrence of reactions (4)–(6),o drastic variation of the parameter δ are observed (cf. Fig. 3),hus they most probably did not evolve in the combustion modeut through a gradual solid–solid diffusion mechanism.

On the other hand, when reactants mechanically activated for4 h were used, a different reacting system behavior is observed.n fact, the examination of sample shrinkage (cf. Fig. 3) in paral-el with product composition (cf. Fig. 4), allows us to concludehat the entire reaction path (3)–(6) evolves in this case underombustion regime, thus giving rise to complete conversion ofeactants to the desired composite.

Thus far, the effect of mechanical activation on the dynamicehavior of SPS process is examined only for powders milledor 24 h. For relatively shorter time intervals, i.e. 6 and 12 h, thewo synthesis mechanism discussed above are both randomlybserved regardless of the same MA and SPS conditions. Sincehis dual behavior was never displayed either when starting fromnmilled powders or when powders were mechanically treatedor 24 h, the existence of a transition zone, where the two kineticechanisms are possible, may be postulated. Although further

nvestigations are needed to better clarify this issue, this behavioray be related to the heterogeneous nature of the ball milling

reatment. Milling consists of a series of collision events, eachf them involving only a portion of the total powder mixture

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Engineering A 434 (2006) 23–29 27

ontained into the jar. Thus, it is expected that when relativelyhorter milling times are considered, the resulting powders hadot reached a uniform activation level and thus, may also behaveike unmilled powders.

It also worth noting that, in spite of the possibility of theccurrence of the two observed regimes in the transition zone ofhe milling time interval, SPS products obtained after 4 min andelated to powders milled for the same time interval, not only doot display differences in their composition, but also final densi-ies and crystallite sizes characteristics are very close, regardlessf the dynamic behavior observed during the process. This resultaybe related to the fact that the sample, after the rapid shrink-

ge changes, is still subjected to the sintering conditions forelatively long time, during which the final characteristics of theroduct are defined.

.3. Product characteristics

Fig. 5 reports the effect of milling time on the density ofnal product obtained by SPS. All experiments refers to theame SPS operating conditions, i.e. I = 1000 A, P = 20 MPa,SPS = 4 min. It may be seen that as milling time, tBM is increased,he density first increases, reaches a maximum value and thenlightly decreases. In particular, final product density increasesrom about 97% to about 100% of the theoretical density4.687 g/cm3) when tBM was 6 h. A further increase of thearameter tBM led to a slight decrease in product density, whichecomes about 99.5% when tBM = 24 h.

As milling time increases, interfacial area between reactantsncreases and powder size decreases. These phenomena inducedy mechanical treatment enhance mass transport by diffusionnd, consequently, synthesis and sintering processes. However,s seen in Fig. 1, some products are also formed as a consequencef mechanical treatment. The presence of such refractory phases,

ig. 5. Effect of milling time on the density of the obtained SPS samplesI = 1100 A, P = 20 MPa, tSPS = 4 min, theoretical density = 4.678 g/cm3).

28 A.M. Locci et al. / Materials Science and Engineering A 434 (2006) 23–29

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ig. 6. Effect of milling time on the crystallite sizes of TiC and TiB2 in thebtained SPS samples (I = 1100 A, P = 20 MPa, tSPS = 4 min).

ossible that increased milling contributed to the formation ofgglomerates, which make the sintering more difficult.

The changes of average crystallite sizes of product phasesormed during SPS with milling time are shown in Fig. 6. Withncreasing milling time, crystallite sizes markedly decrease ini-ially and then decrease at a lower rate down to 50 nm after

echanically treating the starting powders for 24 h.The observed gradual crystallite size refinement in dense

roduct obtained by SPS as the ball milling time of reactantsowders increases, is consistent with the results reported in theiterature for analogous systematic investigations [13,15,19].ndeed, ball milling leads to grain refinements in the start-ng powders and the formation of interfaces between reactants.hese features may induce a relative increase of nucleation sites,s compared to the case of unmilled powders, thus favoringucleation with respect to grain growth. Moreover, by consider-ng the starting mixture to be processed by SPS, the presence ofanostructured reaction products formed during milling, whosemount increases as mechanical treatment proceeds, also con-ributes to the formation of a material with grain size in theanoscale range. This behavior was clearly observed in the lit-rature during the synthesis of dense nanometric MoSi2 throughechanical and field activation [18].The effect of milling on the product microstructure is shown

n Fig. 7(a) and (b) where, respectively, the SEM back-scatteredicrographs of SPS samples obtained from unmilled powders

nd milled powders (tBM = 6 h) are shown. It may be seen thatechanical activation resulted in a finer microstructure and aore homogeneous phase distribution.Vickers hardness and fracture toughness were determined for

amples obtained starting from unmilled powders and those syn-hesized by using 6 h milled reactants, which led to fully denseamples (cf. Fig. 5). It was found that Vickers hardness values for

he samples obtained starting from unmilled and 6 h milled pow-ers were 18.5 ± 1.0 and 19.5 ± 0.8 GPa, respectively. For theame samples, fracture toughness values were 6.94 ± 0.49 and.73 ± 0.44 MPa m1/2, respectively. Thus, mechanical activation

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ig. 7. SEM back-scattered micrograph of SPS end-product when starting from:a) unmilled; (b) ball milled (tBM = 24 h) powders (I = 1100 A, P = 20 MPa,

SPS = 4 min).

eems to improve mechanical properties of SPS product, espe-ially regarding fracture toughness. It can probably due to higherensity, microstructure homogeneity as well as finer crystalliteize obtained when starting from milled reactants powders.

It should be noted that the values obtained here are very sim-lar to, and in some cases higher, than the best results reportedn the literature for TiC/TiB2 dense composites having the sametoichiometry. In particular, hardness and fracture toughness ofhe synthesized SPS material are significantly higher than thealues reported by Lee et al. [15] and Zhang et al. [28], whenomparing samples obtained starting from unmilled powders.he observed differences may be explained on the basis of theigher product density, i.e. greater than 98% of theoretical value,chieved during the present investigation. The density of theamples reported by Lee et al. [15] and Zhang et al. [28] are lesshan 96.5% and about 91%, respectively. On the other hand, Yuant al. [29] have produced TiC/TiB2 composites characterized

y very high density (99.8%) and hardness (19.6 GPa). How-ver, fracture toughness was 5.53 MPa m1/2, i.e. significantlyower than the value obtained in this work. The comparison sug-ests that through the SPS method is possible to obtain dense

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iC/TiB2 composite characterized by superior fracture tough-ess properties.

By comparing TiC/TiB2 composite sample synthesized start-ng from milled powders, Lee et al. [15] obtained hardness valuelose to 21 GPa for milling time equal to 10 h. However, whenBM was set to 6 h, the authors reported about 18 GPa as hard-ess, which is lower than the one achieved in this work for theame milling time. Since, Lee et al. [15] did not report fractureoughness data, comparison about this mechanical property isot possible. It also worth noting that the mechanical pressurepplied in this work is lower than those ones considered by otheruthors. In addition, except for Zhang et al. [28], who adopted aery fast technique, the synthesis time (4 min) needed to obtainy SPS the results reported in this work, is shorter than the oneequired by competitive methods proposed by other authors.

cknowledgements

The financial support of MIUR-PRIN (2002) and PRISMA-NSTM (2003), Italy as well as NAMAMET project (NMP3-CT-004-001470), EU is gratefully acknowledged. The contributioniven by Mrs. Gloria Arangino when performing some experi-ental trials is also gratefully acknowledged. The support of thisork by the Army Research Office (ARO) to one of us (Z.A.M)

s acknowledged.

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[

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