Sintering and Mechanical Properties of ZrB 2 -TaSi 2 and HfB 2 -TaSi 2 Ceramic Composites

Sintering and Mechanical Properties of ZrB2–TaSi2 and HfB2–TaSi2Ceramic Composites

Diletta Sciti,w Laura Silvestroni, Giancarlo Celotti, Cesare Melandri, and Stefano Guicciardi

CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy

Fully dense fine-grained ZrB2- and HfB2-based composites con-taning 15 vol% TaSi2 were produced by hot pressing at 18501–19001C. Gas formation and mass loss, which occurred duringsintering in both systems, were in agreement with thermo-dynamic predictions. In both composites, the presence of a solidsolution formed by the diffusion of tantalum into the boridematrix was observed. The HfB2-based composite was harder (22GPa), stiffer (528 GPa), and tougher (4.1M Pa .m

1/2) than the

ZrB2-based composite. Although the room-temperature flexuralstrength of the ZrB2-based composite (830 MPa) was higherthan that of the HfB2-based composite (700 MPa), the oppositewas true at 12001 and 15001C. Contrary to the significantstrength decrease observed for the ZrB2-based materials atelevated temperature, the HfB2 composite retained B86% ofits room temperature strength up to 15001C (B600 MPa).

1. Introduction

ZIRCONIUM diboride and hafnium diboride belong to the classof ceramic materials known ultra-high-temperature-ceram-

ics (UHTCs). These materials are of particular interest becauseof the unique combination of properties they possess such ashigh refractoriness, high electrical and thermal conductivity,chemical inertness against molten metals, or nonbasic slags,and good oxidation resistance.1–10 Applications that take ad-vantage of these properties include refractory linings, electrodes,microelectronics, and cutting tools. Potential applications ofzirconium and hafnium diboride also include aerospace manu-facturing, for example, the sharp leading edge parts on hyper-sonic atmospheric reentry vehicles.1–10

Much effort has been devoted in recent years to improve thedensification and microstructure of these ultra refractory com-pounds.10–17 Previous studies have demonstrated that the addi-tion of silicides such as MoSi2

17–22 has positive effects on thedensification and mechanical properties of borides, even at tem-peratures as high as 15001C. In addition, silicides, as silica-form-ing compounds, can offer significant improvement to theoxidation behavior.

In this work the effect of TaSi2 addition on densification andproperties of the borides is studied. So far, this silicide has beenemployed for thin film applications in the field of electronics. Tothe best of the authors’ knowledge, few data are available in theliterature on the properties of polycrystalline TaSi2 ceramics. TaSi2has a melting point of 22001C, bulk density of 9.14 g/cm3, hardnessof 15.6 GPa, and is electrically conductive (electrical resistivity 50–55 mO � cm).23 The elastic properties of TaSi2 single crystals wereinvestigated by Chu et al.24 who reported a room temperatureYoung’s modulus of 360 GPa and Poisson’s ratio of 0.189. Pastorand Meyer22 first studied the effect of addition of TaSi2 and othersilicides on the densification and oxidation resistance of ZrB2,

assessing the formation of boride-silicide solid solutions and im-provement of oxidation resistance in air up to 12001C. Recently,Talmy et al.25 have studied ceramics in the system ZrB2–Ta5Si3 andhave reported a significant improvement in the densification, whichwas fully accomplished at 19001C and of the oxidation resistance incomparison with pure zirconium diboride. Furthermore, these au-thors have detected the formation of a ZrB2-based solid solutionphase, due to tantalum entering the diboride lattice. Opila et al.26

studied the influence of TaSi2 additions to ZrB2–SiC compositionsand found a significant improvement in the oxidation resistance at16271C in air. Finally, it was reported that TaSi2 is beneficial as asintering additive (2 vol%) for a HfB2-based material,27 due toliquid phase formation.

In this contribution, the densification, microstructure andproperties of ZrB2 and HfB2 composites containing 15 vol%TaSi2 are presented and discussed. The volumetric fraction ofthe TaSi2 addition was chosen to allow for direct comparison ofthese composites to previously produced UHTC compositeswhere the matrix phases were the same but the secondary phasewas MoSi2.

20,21

II. Experimental Procedure

Commercial powders were used to prepare the ceramic compos-ites: hexagonal ZrB2 grade B (H.C. Starck, Karlsruhe,Germany), impurities (maximum content): C: 0.25 wt%, O: 2wt%, N: 0.25 wt%, Fe: 0.1 wt%, Hf: 0.2 wt%, particle sizerange 0.1–8.0 mm; hexagonal HfB2 (Cerac Incorporated, Mil-waukee, WI), �325 mesh, particle size range 0.5–5.0 mm, meanparticle size B1 mm (by scanning electron microscopy (SEM)analysis), impurities: Al (0.07%), Fe (0.01%), Zr (0.47%), andhexagonal TaSi2 (ABCR, GmbH & Co, Karlsruhe, Germany),�45 mm. The following compositions were prepared:

A. ZrB2115 vol%TaSi2, labeled ZBT.B. HfB2115 vol%TaSi2, labeled HBT.The powder mixtures were ball milled for 24 h in absolute

ethanol using zirconia milling media. Subsequently the mixtureswere dried in a rotary evaporator and sieved through a 60-meshscreen. Some preliminary tests were carried out to attempt topressurelessly sinter these materials but similar to results re-ported in the literature22 compositions with 10 or 15 vol% TaSi2contained significant amounts of porosity (B10%) as ascer-tained by SEM analysis. Hot pressing was, therefore, chosen todensify the TaSi2-contaning compositions. Hot pressing wasconducted in low vacuum (B100 Pa) using an induction-heatedgraphite die with a constant uniaxial pressure of 30 MPa, heat-ing rate 201C/min and free cooling. For each composition, themaximum sintering temperature was set on the basis of theshrinkage curve, i.e., 18501C/10 min and 19001C/15 min forZBT and HBT, respectively. The bulk densities were measuredby the Archimedes’ method. Crystalline phases were identifiedby X-ray diffraction (XRD) (Siemens D500, Karlsruhe,Germany). The microstructures were analyzed using (SEM,Cambridge S360, Cambridge, U.K.) and energy dispersive spec-troscopy (EDS, INCA Energy 300, Oxford instruments,Oxford, U.K.) on polished surfaces. Mean grain sizes, amountof porosity and of secondary phases were determined throughimage analysis on SEMmicrographs of polished surfaces using a

R. Cutler—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: [email protected] No. 24398. Received March 11, 2008; approved May 28, 2008.

Journal

J. Am. Ceram. Soc., 91 [10] 3285–3291 (2008)

DOI: 10.1111/j.1551-2916.2008.02593.x

r 2008 The American Ceramic Society

3285

commercial software program (Image Pro-plus 4.5.1, MediaCybernetics, Silver Springs MD).

Vickers microhardness (HV1.0) was measured with a load of9.81 N, using a Zwick 3212 tester (ULM, Germany). Young’smodulus (E) was measured by the resonance frequency method on28 mm� 8 mm� 0.8 mm specimens using an HP gain-phase an-alyzer (Tokyo, Japan). Fracture toughness (KIc) was evaluated us-ing chevron–notched beams (CNB) in flexure. The test bars,25 mm� 2 mm� 2.5 mm (length by width by thickness, respec-tively), were notched with a 0.1 mm-thick diamond saw; the chev-ron-notch tip depth and average side length were about 0.12 and0.80 of the bar thickness, respectively. The specimens were frac-tured using a semiarticulated silicon carbide four-point fixture witha lower span of 20 mm and an upper span of 10 mm using a screw-driven load frame (Instronmod. 6025, HighWycombe, U.K.). Thespecimens, three for each composite, were loaded with a crossheadspeed of 0.05 mm/min. The ‘‘slice model’’ equation of Munz etal.28 was used to calculate KIc. On the same machine and withthe same fixture, the flexural strength (s) was measured at roomtemperature, 12001 and 15001C in air on chamfered bars 25mm� 2.5 mm� 2 mm (length�width�thickness, respectively),using a crosshead speed of 0.5 mm/min. For the high-temper-ature tests, a soaking time of 18 min was set to reach thermalequilibrium. Five specimens were tested at room temperature,three at 12001 and 15001C.

III. Results

(1) Densification

ZBT started shrinking at 15801C (Fig. 1) and required a max-imum temperature of 18501C to achieve a nearly full density

with a dwell time of 10 min. Its final bulk density was 6.6 g/cm3

and a mass loss of about 15% was measured. For HBT, themixture started shrinking at 17301C and the densification wascompleted at 19001C, with a holding time of 15 min (Fig. 1).The final bulk density was 10.7 g/cm3 with a mass loss of 8%.During all of the sintering cycles, a significant variation of thepressure inside the furnace vacuum chamber was observed. AtT412001C, the pressure increased from the initial value of60 Pa up to a maximum of 130 Pa, which occurred in the tem-perature range 13001–14501C. In the temperature range 14501–16501C, the pressure decreased reaching the initial value around16801–17001C. An example of pressure variation is shown in theinset in Fig. 1. The reason for this pressure variation was therelease of gaseous species, as explained in Section IV.

(2) XRD Analysis

A. ZBT: Hexagonal ZrB2, hexagonal TaSi2 and traces of tetrag-onal ZrO2 were detected along with a series of reflections thatwere attributed to a solid solution formed by Ta dissolution intothe ZrB2 lattice (Fig. 2(a)). Compared with pure ZrB2, thesepeaks were shifted towards higher angles, which indicates acontraction of the diboride unit cell. The unit cell parameters ofthis newly formed phase were a5 3.152 A and c5 3.485 A, i.e.,shorter than those of pure ZrB2 (a5 3.169A, c5 3.530 A).The shift was more pronounced at higher diffraction angles,as shown in the diffraction pattern of Fig. 2(b). On the basisof the Vegard’s rule, for the system ZrB2–TaB2 and hypothesiz-ing that only Ta can enter the ZrB2 structure, it can be estimatedthat the amount of Ta incorporated into ZBT was about18–20 at.%, giving (Zr0.8Ta0.2)B2 as the composition of the solidsolution.

B. HBT: The XRD pattern indicated (Fig. 3) that hexagonalHfB2, hexagonal TaSi2 and monoclinic HfO2 were the crystal-line phases. The diffraction spectrum is reported in Figs. 3(a)and (b). At high diffraction angles, splitting of the main reflec-tions of HfB2 was observed. Different from ZBT, the secondarypeaks for HBT were visible only at high diffraction angles(2y4601). These additional reflections were identified as a solidsolution formed by the incorporation of Ta into the HfB2 lattice.The unit cell parameters of this newly formed phase werea5 3.131 A and c5 3.440 A, i.e., shorter than those of pureHfB2 (a5 3.140 A, c5 3.470 A). According to the Vegard’s ruleapplied to HBT, the amount of Ta incorporated into HBT wasabout 18–20 at.%, giving (Hf0.8Ta0.2)B2 as the composition ofthe solid solution.

For both the composites, the nonuniform Ta distribution wasdue to the fact that the processing time was not sufficient to al-low for homogenization under the conditions used for densifi-cation. This feature is in agreement with the findings of Talmyet al.25 for ZrB2–Ta5Si3 composites.

-0.5

0.5

1.5

2.5

3.5

4.5

5.5

0

t (min)

Shr

inka

ge (

mm

)

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

T (

˚C)

ZrB2

HfB2

Ton:1580˚C

Ton:1730˚C

30252015105

Fig. 1. Shrinkage behavior and temperature profile of the composites.Inset: pressure variation inside the hot-pressing chamber as a function ofthe temperature.

90

ZrB2Zr-B-Ta ss

2-Theta°

20

ZrB2TaSi2Zr-B-Ta ssm-ZrO2

Inte

nsity

(a.

u)

2-Theta°

7060504030 150140130120110100

(a) (b)

Fig. 2. X-ray diffraction spectra of the ZrB2-15 vol% TaSi2 composite. (a) 2y: 201–701, (b) 2y: 881–1581. Reflections from CuKa2 radiation wereremoved.

3286 Journal of the American Ceramic Society—Sciti et al. Vol. 91, No. 10

(3) Microstructural Features

A. ZBT: Some examples of polished surfaces of ZBT are shownin Figs. 4(a)–(c). Little or no porosity (o1%) was observed, in-dicating that the relative density was higher than 99%. Theglobular ZrB2 grains visible in Fig. 4(a) had an average size ofabout 2 mm. These grains were surrounded by a Zr–Ta–B solidsolution, which appeared lighter in color (Fig. 4(b)). Accordingto quantitative EDS analysis, the composition of this solid so-lution was (Zr0.8Ta0.2)B2 in agreement with the compositionpredicted by the Vegard’s law. Pure TaSi2 was not clearly iden-tified in the composite, but a TaxSiy phase containing a certainamount of Zr was observed (Fig. 4(d)). This phase had a lightercolor than ZrB2 and (Zr,Ta)B2. By image analysis, the contentof the TaxSiy phase was estimated to be 3–4 vol%. ResidualZrO2 particles were also found along with silica-based phasescontaining various impurities. These oxide phases were attrib-uted to the oxygen impurities in the starting powders and/oroxygen contamination during powder processing. Spurious car-bide phases, such as Zr—Ta–C, SiC, or Si–C–O, were also de-tected in limited amounts (Fig.4(c)). Carbon impurities were

certainly present in the starting ZrB2 powder but some carbonenrichment could have also resulted from the polyethylene bot-tles used for the milling procedure. Moreover, in the graphite-rich sintering environment, CO generation could have inducedeither the carburization of metals or the carbothermal reductionof oxide species, as observed for other composites of the sameclass of materials.29,30

B. HBT: A fine microstructure with little porosity (o1%) wasobserved in HBT in Fig. 5(a). The mean HfB2 grain size wasaround 1 mm, which was similar to the starting particle size ofthe HfB2 powder, indicating that no significant grain coarseningoccurred during sintering. The lightest colored phase was iden-tified as HfO2 in agreement with the findings of the X-raydiffraction pattern. Significant fractions of HfO2 were foundin other composites produced from the same starting HfB2 pow-der as the present work,19,20 suggesting a large oxygen contam-ination. TaSi2 had a very irregular shape (Fig. 5(b)) and the waythis phase filled the spaces between the HfB2 grains indicatedpossible high-temperature ductile behavior. Other spuriousphases were mixed carbides, based on (Hf, Ta)C and SiC, whose

20

HfB2TaSi2

Hf-B-Ta ssHfO2

Inte

nsity

(a.

u.)

2-Theta˚94

HfB2

Hf-B-Ta ss

2-Theta˚10610410210098967060504030

(a) (b)

Fig. 3. X-ray diffraction spectra of the HfB2-15 vol% TaSi2 composite. (a) 2y: 201–701, (b) 2y: 941–1061. Reflections from CuKa2 radiation wereremoved.

Fig. 4. Polished surfaces of the ZBT. (a) Overall view, (b) enlarged view showing pure diboride grains (1) surrounded by the solid solution (2).(c) Carbide grain. (d) EDS spectra related to the phases labeled in (b) and (c). Legend: (1) ZrB2, (2) (Zr,Ta)B2 (3) TaSi2, (4) (Zr,Ta)C. Beam acceleratingvoltage: 15 keV.

October 2008 Sintering and Mechanical Properties of ZrB2-TaSi2 and HfB2-TaSi2 Ceramic Composites 3287

morphologies were consistent with a liquid-phase sinteringmechanism. The origin of these carbides may be ascribed tothe interaction of the sintering environment with the startingpowders, as previously mentioned for ZBT. Analyzing the mi-crostructure in BSE imaging, many HfB2 grains exhibited acore-shell structure (Fig. 6). By EDS analysis, the outer shell wasestimated to be a solid solution with composition (Hf0.8 Ta0.2)B2

in agreement with the composition predicted by the Vegard’slaw. The microstructural and XRD analyses showed that the

formation of a solid solution was more pronounced in ZBT thanin HBT.

(4) Mechanical Properties

The mechanical properties are reported in Table I, along withsome data for two other composites with the same matrixphases, i.e., HfB2 and ZrB2, but with MoSi2 as the secondaryphase.20,21 Generally speaking, HBT had higher hardness, stiff-ness, and fracture toughness, but lower room-temperature flex-ural strength than ZBT. Both at 12001 and 15001C, the strengthof HBT (700 and 600 MPa, respectively) was higher than ZBT(600 and 370 MPa, respectively). With respect to the MoSi2-containing composites, it can be seen that TaSi2 additions in-creased hardness and toughness but had almost no effects onYoung’s modulus. The flexural strength of ZBT (840733 MPa)was significantly higher than the same matrix with MoSi2(704798 MPa). On the other hand, the strength of HBT(698758) and of the HfB2–MoSi2 (7427142) were not differ-ent from a statistical point of view, but in the latter case thevalues were affected by a very large data dispersion.

IV. Discussion

During the densification of ZBT and HBT, several phenomenaoccurred, the most important being the mass loss and partialsubstitution of Ta for Zr or Hf, implying that Ta has at leastlimited diffusivity in both borides. The mass loss can be attrib-uted to the interaction of TaSi2 either with B2O3, which is pres-ent as surface oxide on the boride particles, or with COgenerated inside the graphite-rich environment of the furnace.The pressure in the hot press vacuum chamber increased above13001C due to the formation of volatile species and reached itsmaximum (130 Pa) around 14001C. Using a commercial soft-ware package (HSC Chemistry for Windows 5, OutokumpuResearch Oy, Pori, Finland), some potential reactions betweenTaSi2 and CO, TaSi2, and B2O3 were analyzed in the range12001–19001C under a pressure of 100 Pa (i.e., approximately

Fig. 5. Polished surfaces of the HBT. (a) Overall view, (b) and (c) details of the microstructure, (d) EDS spectra related to the phases labeled in (b) and(c). Legend: (1) TaSi2, (2) Ta–Hf–C, (3) SiC-based phase. Spectra 1 and 2 collected at 15 keV, spectrum 3 collected at 6 keV to limit beam lateralspreading.

Fig. 6. Polished surface of HBT. Enlarged view showing pure diboridegrains (1) surrounded by the solid (Hf,Ta)B2 solution (2).

3288 Journal of the American Ceramic Society—Sciti et al. Vol. 91, No. 10

the vacuum level inside the hot-press chamber), considering purematerials (i.e., no solid solutions), with the activity of each phaseequal to one. For simplicity, the molar ratio between the phaseswas taken equal to one. The thermodynamic results are sum-marized in Figs. 7(a) and (b) and give the following indications:

(1) CO promotes TaSi2 decomposition with the formation ofSiO gas at T � 13001C and Si(g) at T � 14501C. ForT � 12001C, the formation of TaC and SiC is expected. In therange 14501–18501C, the formation of liquid Si is predicted. Thechange in the Gibbs free energy varies from �0.9 � 103 kJ at12001C to �1.3 � 103 kJ at 19001C.

(2) B2O3 promotes TaSi2 decomposition with the formationof SiO gas at T412001C, TaB2 at T � 12001C and Ta atT � 17001C. At T � 14001C, the emission of BO also occurs.

For this reaction the change in the Gibbs free energy variesfrom �1.9 � 103 kJ at 12001C to �2.6 � 103 kJ at 19001C.

The above thermodynamic predictions are in partial agree-ment with the SEM observations. TaSi2 decomposition andconsequent SiO volatilization was certainly confirmed byweight losses, the increase in pressure in the furnace cham-ber, XRD data and microstructural analyses. No Ta or TaB2

species were detected either by XRD or by SEM analy-sis. However, these species are expected to form the solidsolution with the borides. There was also evidence of carbideformation, which were mixed phases of Ta and Zr (or Hf). Inboth ZBT and HBT, Si-based phases with a very irregularmorphology suggesting liquid-phase behavior were often de-tected. Possible origin of these phases can be either carbo-thermal reduction of residual silica or carburization of liquidsilicon.

As far as densification is concerned, therefore, the two keymechanisms are the formation of solid solutions and Si- or SiO2-based liquid phases.27 Due to the significant TaSi2 decomposi-tion observed in ZBT, it is believed that in this compositethe densification was primarily governed by the solid so-lution formation. In the case of HfB2 both mechanisms wereequally active.

The presence of solid solutions with unknown propertiescomplicates the analysis of the mechanical properties. The fol-lowing discussion will therefore focus on the properties of well-characterized phases. HBT was harder than ZBT because theHfB2 matrix is harder than the ZrB2 matrix. To give somereference values, the Knoop hardness measured with a load of160 g is about 23.5 GPa for HfB2 and 20.5 GPa for ZrB2.

31 Thehardness of the composites was lower than the pure matrices asthe hardness of TaSi2 is about 15.6 GPa.23 Because the hardnessof MoSi2 is about 12 GPa,32 the TaSi2-reinforced compositeswere harder than the MoSi2-reinforced composites. The higherYoung’s modulus of HfB2 than ZrB2 made HBT stiffer thanZBT.33 Also in this case, the presence of TaSi2 lowered theYoung’ modulus of the composites relative to pure matrices.The elastic modulus of the composites was not sensitive to thechange of secondary phase, although MoSi2 has a higher stiff-ness (E5 425 GPa34) than TaSi2. One possible explanation forthis behavior is that the MoSi2-reinforced composites containeda not negligible content of silica (4%–5%) which decreased thestiffness.21 HBT was about 10% tougher than ZBT, whilst theroom-temperature strength of ZBT was about 20% higher thanHBT. The reasons for this crossover are beyond the scope of thisstudy and may be the focus of a future work. The flexuralstrength was the property most influenced by the type of thesecondary phase. Replacing MoSi2 with TaSi2, in fact, increasedthe strength of the ZrB2-based composite and reduced the datadispersion of the HfB2-based composite. Fractographic analysisof specimens fractured at room temperature showed that in theMoSi2-based composites, large MoSi2 agglomerates were pres-ent on fracture surfaces.18,20 No TaSi2 agglomerates were in-stead detected on the fractured surfaces of ZBT and HBT,implying that TaSi2 dispersed more uniformly in the matricesthan MoSi2. At elevated temperatures, the strength of HBT washigher than ZBT. A similar crossover was observed for the samematrices containing MoSi2, indicating that the overall strengthmay be dictated by the matrix. At 12001C the load-displacementcurves of ZBT and HBT were linear up to fracture. At 15001C a

Table I. The Mechanical Properties of the Tested Composites in Comparison with the Same Matrices Reinforced with MoSi2

Composition

Label

Density

HV E KIc sRT s12001C s15001CExperimental Relative

Vol% g/cm3 % GPa GPa MPa �m1/2 MPa MPa MPa

ZrB2-15 TaSi2 ZBT 6.6 99 17.870.5 444724 3.870.1 840733 598725 37475HfB2-15 TaSi2 HBT 10.7 99 21.970.5 52875 4.170.1 698758 703724 597746ZrB2-15 MoSi2

w — 6.0 99 14.970.5 45274 3.570.6 704798 — 333731HfB2-15 MoSi2

z — 10.2 99 20.670.4 53075 3.870.1 7427151 664728 548720

Mean71 standard deviation. wSciti et al.21; zSciti et al.20

1100–0.2

0.0

0.2

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1.2

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2.0 1 mol TaSi 2+ 1 mol B2O3

Ta2O5

BO

B2O3

SiO2

TaB2

Ta

SiO (g)

Rea

ctio

n pr

oduc

ts (

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)

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0.0

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SiC

Si (l)

Rea

ctio

n pr

oduc

ts (

mol

)

Temperature (˚C)

2000190018001700160015001400130012001100 200019001800170016001500140013001200

Fig. 7. Molar content of the products deriving from the reactions: (a) 1 mol TaSi211 mol CO(g), (b) 1 mol TaSi211 mol B2O3, as a function of thetemperature and at constant pressure of 100 Pa.

October 2008 Sintering and Mechanical Properties of ZrB2-TaSi2 and HfB2-TaSi2 Ceramic Composites 3289

slight departure from linearity was observed only for HBT. Thissuggests that softening of the secondary phase is not likely to bethe main reason for the high-temperature strength decrease.More likely, it was the combination of the high-temperaturefracture toughness and the oxidation attack that determined thehigh-temperature strength behavior. Both composites, in fact,underwent significant surface modification at the test tempera-tures (Fig. 8), due to oxidation. Glass formation can either bluntcracks leading to strength retention or introduce new defectscausing strength degradation. Work on quantifying oxidationresistance is in progress. The retained strength of ZBT at 15001Cwas about 45% of the room-temperature value, but for HBTthis ratio was 86%. Among borides, strength retention at ele-vated temperature has only been approached by a HfB2-20vol% SiC composite densified by spark plasma sintering.5,27

V. Conclusions

ZrB2 and HfB2 plus 15 vol% TaSi2 additions were densified to99% of relative density by hot pressing in the temperature range18501–19001C. For both compositions, gaseous phases were re-leased during sintering and mass losses were measured aftersintering. The final microstructures of both composites were al-most pore-free with small grain sizes (1–3 mm). The most im-portant microstructural features were the partial decompositionof TaSi2 and the formation of solid solutions in the (Zr,Ta)B2

and (Hf,Ta)B2 systems. By thermodynamic analysis, the decom-position of TaSi2 was attributed either to the interaction withCO species which form in the graphite-rich reducing sinteringenvironment, or to the interaction with B2O3 present as oxideimpurity on boride particle surfaces.

The HfB2-based composite was harder (22 GPa), stiffer (528GPa), and tougher (4.1 MPa m1/2) than the ZrB2-based com-posite. However, the latter had a higher room-temperaturestrength (840733). On the other hand, when tested at elevated

temperature the flexural strength of HBT was higher than ZBT.Comparing the same matrices with MoSi2 in place of TaSi2, itwas shown that TaSi2 increases the room-temperature proper-ties and the high-temperature strength. Remarkably, the re-tained strength of the HBT composite at 15001C (600 MPa)was about 86% of the room-temperature value.

Acknowledgments

The authors wish to thank Daniele Dalle Fabbriche for the hot pressing routes.One of the authors (L. S.) gratefully acknowledges the financial support of theRegional Project MATMEC.

The authors would like to thank the referees and the associated editor for theiruseful and constructive suggestions for this work.

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Fig. 8. (a), (b) Examples of fracture surface morphology (detail) and cross section after the 15001C test of a ZrB2-15 vol%TaSi2 specimen, (c) Fracturesurface after the 12001C test of a HfB2-15 vol% TaSi2 specimen, (d) HfB2-15 vol% TaSi2 specimen surface after the 15001C test showing a dense glassylayer containing HfO2 crystals.

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