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Reactive Hot Pressing of ZrB2SiC Composites

Guo-Jun Zhang, Zhen-Yan Deng, Naoki Kondo, Jian-Feng Yang, and Tatsuki Ohji*

National Industrial Research Institute of Nagoya, Nagoya, Aichi 463-8687, Japan

A ZrB2SiC composite was prepared from a mixture ofzirconium, silicon, and B4C via reactive hot pressing. Thethree-point bending strength was 506 43 MPa, and thefracture toughness was 4.0 MPam1/2. The microstructure ofthe composite was observed via scanning electron microscopy;the in-situ-formed ZrB

2and SiC were found in agglomerates

with a size that was in the particle-size ranges of the zirconiumand silicon starting powders, respectively. A model of themicrostructure formation mechanism of the composite wasproposed, to explain the features of the phase distributions. Itis considered that, in the reactive hot-pressing process, the Band C atoms in B4C will diffuse into the Zr and Si sites andform ZrB2 and SiC in situ, respectively. Because the diffusion

of Zr and Si atoms is slow, the microstructure (phase distri-butions) of the obtained composite shows the features of thezirconium and silicon starting powders.

I. Introduction

ZIRCONIUM DIBORIDE (ZrB2) is a hexagonal crystal of the AlB2type and is similar to titanium diboride (TiB2). ZrB2 has a highmelting point (3040C), high hardness (22 GPa), high electricalconductivity (9.2 106 cm), and good corrosion resistance.ZrB2 is wetted but not attacked by molten metals; thus, it has beenused as molten-metal crucibles, HallHeroult cell cathodes, andthermowell tubes for steel refining.1 The flexural strength andfracture toughness of ZrB2 ceramics are 200375 MPa and 45

MPam1/2

, respectively. The density of ZrB2 is 6.09 g/cm3

, and thethermal expansion coefficient is 5.5 106 K1.1

ZrB2 powder can be manufactured via the reaction of elementalzirconium and boron or via the reaction of ZrO2 with carbon andB4C or with carbon and B2O3.

1,2 ZrB2 ceramics usually areproduced via hot pressing or presureless sintering from ZrB2powder. However, the mechanical properties and oxidation resis-tance of monolithic ZrB2 ceramics are not satisfactory for appli-cations such as thermowell tubes. Thus, some components areadded to form ZrB2-based ceramic composites. For example, theaddition of SiC will improve the oxidation resistance,3,4 theaddition of BN will improve the thermal shock property, and theaddition of B4C will increase the hardness and toughness.

4

ZrB2ZrC and LaB6ZrB2 eutectic composites with high mechan-ical properties were prepared via directional solidification.5,6

Johnson et al.7 developed a method to prepare ZrB2ZrCzirco-nium composites with homogeneously distributed platelet-shapedZrB2.

TiB2SiC composites have been prepared from a mixture ofTiH2, silicon, and B4C,

8,9 and this in-situ reaction has been used to

produce TiB2Ti(C,N)SiC ternary composites10 and in-situ-

formed TiB2-toughened SiC.11 In the present study, the following

reaction will be used to prepare ZrB2SiC composite:

2Zr Si B4C 3 2ZrB2 SiC (1)

The volume contents of the composite formed from the above-mentioned reaction are 74.85 vol% ZrB2 and 25.15 vol% SiC. Thetheoretical density of the composite, according to the rule ofmixtures, is 5.37 g/cm3. This processing can avoid the effect of theoxidation of fine ZrB2 powder on the sintering in the conventionalprocessing. In this communication, the manufacturing process,mechanical properties, and microstructure of the composite will be

reported preliminarily.

II. Experimental Procedure

The starting powders were zirconium, silicon, and B4C; thecharacteristics of the powders are listed in Table I. The stoichio-metric powders, according to reaction (1), were mixed in propanolwith ZrO2(Y2O3) balls for 24 h in a plastic bottle and then dried.The composite was produced via reactive hot pressing in a graphitedie with a BN coating at a temperature of 1900C under a pressureof 30 MPa for 60 min in an argon atmosphere. The dried mixedpowder was placed into a cold graphite die and loaded to 3 MPabefore the temperature was increased. Reaction (1) is exothermicwith a large enthalpy of reaction; therefore, a slow heating rate

(10C/min) was adopted to prevent the reaction from becomingself-sustaining. To prevent the molten silicon from moving out ofthe die at high temperatures, the application of pressure was begunat 1550C and gradually increased to 30 MPa before the temper-ature reached a value of 1900C.

The obtained product, 25 mm in diameter and 5 mm thick, wascut into specimens and then ground with diamond wheel (800 gritsize) along the longitudinal direction of the specimens. Phasecomposition was determined via X-ray diffractometry (XRD)using CuK radiation. The density was tested using the water-displacement method. The three-point bending strength was mea-sured on bars with dimensions of 2.5 mm 3 mm 20 mm (fouredges were beveled using 1500 grit SiC abrasive paper); the spanwas 16 mm, and the crosshead speed was 0.5 mm/min. The

strength data were an average of five measurements. The fracturetoughness was measured via the indentation method, using a loadof 5 kg, according to Japanese Industrial Standard JIS R 1607.12

The calculation formula is

KIC 0.026E1/ 2P1/ 2a

C3/ 2 (2)

where P is the indentation load, a the half length of the indent, Cthe half length of the crack, and E the elastic modulus of thecomposite. E is calculated from the elastic moduli of the compo-nents (EZrB2 450 GPa and ESiC 414 GPa), according the ruleof mixtures. The hardness data and toughness data were an averageof ten measurements. Scanning electron microscopy (SEM) wasperformed to observe the microstructure of the composite.

R. W. Ricecontributing editor

Manuscript No. 189035. Received October 6, 1999; approved April 24, 2000.This work has been conducted as special research (KYOU-SOU TOKKEN)

supported by AIST, MITI, Japan.*Member, American Ceramic Society.

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III. Results and Discussion

According to the XRD pattern, only ZrB2 and SiC phases arepresent in the obtained composite, which shows that the reactionoccurred during the reactive hot-pressing process and is in accor-dance with reaction (1). This observation means that the ZrB2SiCcomposite can be produced through this in-situ reaction and thevolume contents of ZrB2 and SiC also can be adjusted by addingZrB2 or SiC powder to the mixed reactant powders.

The characteristics of the synthesized composite are listed inTable II. The relative density of the composite is considerablyhigh, and it is believed that the relative density can be improvedthrough optimization of the processing, such as via adjustment ofthe hot-pressing temperature and hold time.

The bending strength of the composite is 506 43 MPa, whichis 2 times higher than that of conventional monolithic ZrB2ceramics with a grain size of several tens of micrometers.1,4 Thedifference in grain size would be one of the main reasons for theimproved strength of the present composite. In addition, for aparticulate composite, particle bridging would work when apropagating crack meets the secondary-phase particles and im-proves the strength of the composite.13 In the present case, the SiCand ZrB2 can be treated as the secondary-phase particles and thematrix, respectively. In addition, for in-situ composites, the inter-faces between the component phases generally are cleaner andstronger than those of the conventionally processed compos-ites.11,14,15 Such phenomena also should exist in the in-situZrB2SiC composite and lead to a considerably high bendingstrength. The fracture toughness of the composite is somewhat lessthan that of monolithic ZrB2 ceramics. This phenomenon is

suggested to be related to the thermal expansion coefficients ofZrB2 and SiC (ZrB2 5.5 10

6 K1 and SiC 4.0 106

K1, respectively). According to the calculation method that wasproposed by Taya et al.,16 the calculated average residual stressesfor the ZrB2 matrix and SiC particles in the composite are 152MPa (tensile) and 452 MPa (compressive), respectively. (In thecalculation, the parameters are set as follows: Poissons ratios ofZrB2 0.25 for ZrB2 and SiC 0.14 for SiC, and the temperaturerange over which stresses are not relieved by plastic deformationis T 1200 K.) That is, the existence of the tensile stress in theZrB2 matrix will reduce the toughening effect by the particlebridging of the SiC particles. An approximate calculation, accord-ing to the Taya group, shows that the reduction in the fracturetoughness that is caused by the existence of the tensile stress in theZrB2 matrix is 0.45 MPam

1/2.

Figure 1 shows the microstructure of the composite. Theparticle size of SiC is generally 3 m, whereas that of ZrB2 islarger and in the range of 310 m. These pictures indicate that the

distributions of the in-situ-formed ZrB2 and SiC phases in thecomposite are not homogenous on a microscale level. That is, theZrB2 phase generally gathers together to form larger agglomerates,and in these agglomerates, the particle size of ZrB2 is large. TheSiC phase also gathers together to form agglomerates, but the sizesof these agglomerates are smaller than those of the ZrB2 phase.The sizes of the agglomerates of ZrB2 and SiC are in the particlesize ranges of the starting powders of zirconium and silicon,respectively, when the data from Table I are compared with Fig. 1.This observation suggests that such phase distributions are relatedto the particle size of the starting powders. Figure 2 shows theformation mechanism of this composite in the reactive hot-pressing process. Figure 2 shows that, in the reactive hot-pressingprocess, the B and C atoms in B4C will diffuse into the Zr and Sisites, respectively, and then form ZrB2 and SiC in situ. Because the

diffusion of Zr and Si atoms is slow, the formed agglomerates ofsynthesized ZrB2 and SiC possess the features of the starting

Table I. Characteristics of the Starting Powders

Starting powder Particle size (m) Chemical composition Manufacturer

Zirconium 43 98% pure; impurities include 0.02% Al, 0.1%Ca, 0.06% Fe, 1% Hf, 0.1% Mg, and 0.08% Ti

High Purity Chemicals, Saitama, Japan

Silicon 10 99.9% pure; impurities include 0.001% Ca,0.008% Cr, 0.03% Fe, and 0.004% Ni

High Purity Chemicals, Saitama, Japan

B4C 1 75% boron, 20%25% carbon Denki Kagaku Kogyo Co., Ltd., Tokyo, Japan

Table II. Characteristics of the Reaction-SynthesizedZrB

2SiC Composite

Property Value

Phase composition, via XRD ZrB2 and -SiCRelative density (percentage of

theoretical density)97.67%

Bending strength 506 43 MPaFracture toughness 4.0 MPam1/2

Vickers hardness 21.0 GPaGrain size SiC, 3 m; ZrB2, 310 m

Fig. 1. SEM micrographs of the ZrB2SiC composite ((a) polishedsurface and (b) fracture surface); the white phase is ZrB2, and the dark grayphase is SiC.

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powders. According to the above-described model, it is believedthat a ZrB2SiC composite with a fine and homogeneous micro-structure and better mechanical properties can be obtained usingstarting powders with a fine particle size. The results of thisproblem and the details of the phase-formation mechanism will bereported after future investigation.

IV. Conclusions

A ZrB2SiC composite was prepared via reactive hot pressing

of a mixture of zirconium, silicon, and B4C powders. The bendingstrength of the obtained composite was 506 43 MPa, and thefracture toughness measured by Vickers indentation technique was4.0 MPam1/2. The microstructure of the composite possesses thefeatures of the zirconium and silicon starting powders. A modelwas proposed to explain this characteristic of the microstructure. Inthe reactive hot-pressing process, B and C atoms will diffuse intothe Zr and Si sites, respectively, and then form ZrB2 and SiC insitu. The diffusion of Zr and Si atoms was slow; therefore, thein-situ-formed ZrB2 and SiC were present in agglomerates whose

sizes were in the particle-size ranges of the zirconium and siliconstarting powders.

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Fig. 2. Microstructure formation mechanism of the ZrB2SiC composite

in the reaction-synthesis process, depicting the transformation from (a) thepowder compact to (b) the final microstructure of the composite.

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