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Materials Science and Engineering A 528 (2011) 6079–6082

Contents lists available at ScienceDirect

Materials Science and Engineering A

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apid communication

park Plasma Sintering of ZrB2–SiC–ZrC ultra-high temperature ceramics at800 ◦C

lexandra Snydera,∗, Dat Quachb, Joanna R. Grozab, Timothy Fisherc, Stephen Hodsonc, Lia A. Stanciua

School of Materials Engineering, Purdue University, 701 West Stadium Ave., West Lafayette, IN 47906, United StatesDepartment of Chemical Engineering and Materials Science, University of California, Davis, One Shields Avenue, Davis, CA 95616, United StatesSchool of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47907-2088, United States

r t i c l e i n f o

rticle history:eceived 11 October 2010eceived in revised form 25 March 2011ccepted 6 April 2011

a b s t r a c t

The compositional effects in ZrB2–SiC–ZrC ultra high temperature composites with four different compo-sitions were investigated via Spark Plasma Sintering (SPS) at a maximum temperature of 1800 ◦C. Density,Rockwell hardness, and thermal conductivity were measured, along with structural X-ray diffraction(XRD) and microstructural characterization. The relative amounts of SiC and ZrC had an influence on the

vailable online 15 April 2011

eywords:interingeramicsanoindentation

composites’ density, mechanical and thermal properties.© 2011 Elsevier B.V. All rights reserved.

canning electron microscopy (SEM)

Zirconium diboride (ZrB2) and hafnium diboride (HfB2) areltra-high temperature ceramics (UHTCs) displaying severalnique properties, such as very high melting points (above 3000 ◦C),xtreme hardness, high electrical and thermal conductivity andow volatility. Previous research has indicated that because sili-on carbide (SiC) additions to ZrB2 and HfB2 can increase oxidationesistance, these composites are promising candidates for ultraigh temperature applications [1]. SiC can also function as a grainrowth inhibitor, increasing composite strength. Furthermore, theddition of ZrC is expected to enable the formation of a ternaryomposite with an increased resistance to ablation at high temper-tures. High melting points and oxidation resistance make theseomposites interesting candidates for thermal protection mate-ials for a wide range of applications, from the steel industry toeronautics.

Different methods have been employed for the processing ofHTCs with improved performance. One approach involves the usef sintering aids together with conventional sintering techniquesuch as hot pressing (HP) [2–4]. However, sintering aids often resultn different grain boundary phases, which in turn degrade the highemperature properties of ZrB2 or HfB2 based materials [1,5]. Here,

e investigate an alternative route for improving the properties of

rB2 ceramics, namely the addition of secondary and ternary phasesuch as SiC and ZrC, which restrain grain growth and, as opposed to

∗ Corresponding author. Tel.: +1 5705909699; fax: +1 765 4941204.E-mail address: snyder23@purdue.edu (A. Snyder).

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.04.026

sintering aids, improve the oxidation behavior at high temperature.The rapid heating and short holding times at maximum tempera-ture employed in Spark Plasma Sintering (SPS) have been shownto contribute to restricted grain growth and therefore, superiormechanical properties. In addition, SPS has the advantage of elim-inating the need for sintering aids, leading to improved materialproperties at high temperatures. SPS is a consolidation techniquethat uses a combination of pulsed electrical field application withhigh heating rates to densify, while preserving an ultra-fine grainsize in a large variety of powder materials that have proven to bedifficult to sinter by other methods [6–16].

In this communication, we present results related to our effortsto understand the composition–microstructure–properties rela-tionships in ZrB2–SiC–ZrC UHTCs sintered at 1800 ◦C via SPS. Thestarting powders were ZrB2 (325 mesh), SiC (<1500 grit), and ZrC(100 mesh) obtained from Sigma–Aldrich. The samples were mixedfor ball milling in polyethylene bottles with 200 mL of hexane. Thepowders were milled using yttria-stabilized zirconia media at aweight ratio of ten to one at 25 rpm, 160 rpm, and 180 rpm, andfor durations ranging from 8 to 16 h. Five different compositionswere prepared and will be referred to hereafter in terms of weightpercent of each compound as shown in Table 1. After milling, thesamples were immediately dried in a rotary evaporator at a tem-perature of 70 ◦C, a vacuum pressure of 200 mmHg (∼27 kPa), and

a speed of 150 rpm. After drying, the samples were brushed fromthe walls of the flask and collected in glass vials.

The powder composition and particle size were examined usingX-ray diffraction (XRD) and Transmission Electron Microscopy

6080 A. Snyder et al. / Materials Science and Engineering A 528 (2011) 6079–6082

Table 1Compositions and density values for SPS samples.

Sample name Composition (wt.%) Density (g/cm3) % Theoretical density

ZrB2 SiC ZrC

60-02-38 60 02 38 5.978 95.660-20-20 60 20 20 5.263 99.770-15-15 70 15 15 5.422 98.3

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initial powder acts as a grain growth inhibitor, with the sample con-taining 2% SiC having the highest grain size and the sample with 20%SiC showing the smallest grain size.

Table 2Hardness values for SPS samples.

Composition Rockwell A hardness

80-10-10 80 10 10 5.582 98.0100-0-0 100 0 0 5.59 90.5

TEM). Five samples prepared as described above were sub-equently processed using a Spark Plasma Sintering machineSumitomo, Japan). The SPS processing was performed to a max-mum temperature of 1800 ◦C and the samples were held at thisemperature for 4 min in each experiment. The following heatingegime was followed for all samples, with the total time for pro-essing completion from room temperature to 1800 ◦C adding upo 17 min:

room temperature to 600 ◦C in 3 min.600–650 ◦C: 1 min.650–1740 ◦C: 7 min.1740–1790 ◦C: 1 min.1790–1800 ◦C: 1 min.1800 ◦C: 4 min.

The temperature was measured on the die surface by a pyrom-ter. No significant temperature overshoot was observed. The loadas 18 kN (∼60 MPa on a 19 mm diameter die) and was applied

t the beginning of the experiments. The density of the samplesas measured by Archimedes’ method. TEM observations of the

nitial powders were performed with an FEI Tecnai 20 TEM withaB6 source operating at 200 kV. Scanning Electron MicroscopySEM) experiments were performed on the sintered samples, with aitachi S-4800 FEG SEM operating a 30 kV. The samples were sput-

er coated with a layer of conductive gold before observations. TheRD spectra were recorded with a Siemens D500 Powder Diffrac-

ometer (Cu K� radiation) in the range 2� = 20–80◦. The surface ofach sample was polished and the hardness was measured by thendentation method, on the Rockwell scale A, with a load of 60 kgor 10 s. Six measurements were taken for each sample, and thealues reported here are averages of these measurements.

A 3-point bending test was also performed to measure the flex-ral strength of the samples. Before the test, the diameter andhickness of all samples were measured using a digital caliper. The

easured thickness values of samples 60-02-38, 60-20-20, 70-15-5 and 80-10-10 were 1.60 mm, 1.70 mm, 1.75 mm and 1.83 mm,espectively. The diameter of all samples was 19.06 mm. The spanetween the two supports was set to be 14.60 mm. The test waserformed on a Sintech 30/D digital tensile test machine.

Thermal conductivity was measured for 100-0-0, as well as for0-10-10 and 75-15-15. A transient photoacoustic (PA) techniquehat has been described in detail in previous work [17,18] was usedo characterize the thermal properties of the material. For a multi-ayer structure, the PA technique can resolve bulk and componenthermal resistances as well as thermal diffusivities of each layer.sing the acoustic signal in conjunction with the model developed

n refs. [17] and [18], which is based on a set of one-dimensionaleat conduction equations, the thermal conductivity of the materialas determined using a least-squares fitting method.

Ball milling of the powders produces well-mixed samples

ith minimal loss during processing. It was found that varying

t different speeds did not have a significant effect on pow-er morphology or composition, but faster milling preventedowder agglomeration. Preventing agglomeration is of signifi-

Fig. 1. TEM image of 60-20-20 powder.

cant importance, as it is expected that only non-agglomeratedpowders can result in an enhanced densification of the ternarycomposites due to additions of secondary phases. Fig. 1 showsone representative TEM image of the initial powder sam-ples.

It was found in all cases that the average initial particle size ofthe powder varied between 0.25 and 0.5 �m. The powders werethen sintered by SPS and their densities, measured by Archimedes’method, can be seen in Table 1. The theoretical densities of thecomposites are lower than the 6.17 g/cm3 value for ZrB2 due to alower theoretical density of SiC (3.20 g/cm3). Sintering 100-0-0 at1800 ◦C via SPS resulted in only 90.5% of theoretical density. Thistemperature is lower than the temperature usually required for sin-tering pure ZrB2 (usually above 2000 ◦C). The density data suggestsa density enhancement with an increase of percentage of SiC in theternary system, with the highest percentage of theoretical densitybeing achieved for the composite with 20 wt% SiC (Table 1). There-fore, by carefully adjusting the SiC content of the composites, thereis a promise for fully dense compacts being obtained at tempera-tures lower than 2000 ◦C. The results are in good agreement withthe known fact that traces of oxides, such as B2O3 or ZrO2 on the sur-face of the powder particles result in an inhibition of densificationand enhancement of grain growth in non-oxide ceramics. The pres-ence of carbides in the samples is expected to lead to a reductionof oxide content and the formation of a secondary intergranularyphase, phenomena leading to an enhancement in densification andgrain growth inhibition [19–26].

The same effect of grain growth restraining promoted by thepresence of SiC and ZrC in the composites is confirmed once againby the results of the microstructural analysis. Fig. 2 shows the frac-ture surface SEM image of four SPS composites. The average grainsizes for the reported composites ranged between 4 and 5 �m. Theparticle size analysis showed that a higher percentage of SiC in the

60-20-20 92.24 ± 3.1160-02-38 88.32 ± 16.970-15-15 89.14 ± 6.9980-10-10 91.23 ± 3.87

A. Snyder et al. / Materials Science and Engineering A 528 (2011) 6079–6082 6081

Table 3Thermal conductivity values for SPS samples.

Composition Density(g/cm3)

Thermalconductivity(W/mK)

Lower bound(W/mK)

Upper bound(W/mK)

70-15-15 5.405 71 62 8280-10-10 5.625 71 61 84

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Table 4Flexural strength data for SPS samples.

Composition Flexural strength (MPa)

100-0-0 32.8060-02-38 98.6960-20-20 123.8870-15-15 165.83

100-0-0 5.59 50 47 56

Table 2 shows the values obtained from a Rockwell hardnessesting with 60 kg loading. The general trend observed is that hard-ess values tend to increase with increasing amounts of siliconarbide, which also corresponds to other findings in literature [27].he highest hardness value was obtained for 60-20-20, with theecond highest value for 80-10-10. Equal percentages of SiC andrC seem to lead to the highest hardness values, with the sampleaving 20% SiC displaying a higher hardness value than the sampleaving only 10% SiC.

Table 3 presents the values obtained for thermal conductivity forhree of the five compositions that were able to be tested. Zimmer-

ann et al. state that the thermal conductivity should increase withncreasing silicon carbide content [28]. This trend was observed inhe thermal conductivity values, which increased from 50 W/mKor 100-0-0 to above 70 W/mK for the samples containing equalmounts of SiC and ZrC. However, it is surprising that 80-20-20 dis-layed a thermal conductivity value that is indistinguishable fromhat of 70-15-15. Any small differences in the reported range can bettributed to slight differences in the porosities of the two samples,hich have very close densities, in terms of percentage of theo-

etical density. A slightly higher porosity could be responsible forlightly reducing the thermal conductivity of this composition, buthe most significant trend is an increase of 40% in thermal conduc-ivity for 70-15-15 and 80-10-10 as compared to that of 100-0-0.uo et al. observed a similar trend in thermal conductivity [29],howing that SiC is the most significant contributor to enhance-ent of thermal transport in these composites. The addition of a

iC phase, with a higher thermal conductivity than ZrB2, results indecrease of the heat flow resistance and therefore an increase

n thermal conduction. In contrast, ZrC has a lower thermal con-uctivity and results in an enhancement of the thermal transportesistance.

Fig. 2. SEM images of the fractured surfaces of the SPS samples

80-10-10 302.29

Flexural strength values for each sample were calculated usingthe following relation:

�f = 3PL

(2bd2)

In the equation, P refers to the maximum strength the speci-men experiences, and L is the support span (mm). Variables b andd represent the width of test sample (mm) and the thickness of thespecimen (mm). As shown in Table 4, the flexural strength values ofall samples were lower than values reported in literature [30]. Aninvestigation of results in other studies leads to the conclusion thatalthough the grain size in ZrB2 ceramics plays an important role inthe flexural strength, it is not of singular importance [31]. Instead,the strength limiting factor seems to be the maximum SiC grain sizeinstead of the average ZrB2 grain size. We therefore can ascribethe lower than expected flexural strength we obtained to someobserved agglomeration in the starting powders. However, the dataindicates a general increase trend in flexural strength with increasein the weight percentage SiC for equal ZrB2 weight percentage con-tent, suggesting that the presence of SiC as a secondary phase hasa positive effect on the mechanical properties of the composites.

In summary, SPS processing of five different ZrB2 based compo-sitions, with different amounts of SiC and ZrC added was performedat a relatively low temperature (1800 ◦C) in a total time of 17 min.The results indicated that SiC and ZrC additions resulted in anenhancement in densification. A higher percentage of SiC resultsin grain growth inhibition, enhanced thermal conductivity, andincreased Rockwell A hardness and strength. Further research is

necessary to determine the effect of additional sintering parame-ters, such as heating rate, on the composites properties.

. (A) 60-02-38; (B) 70-15-15; (C) 80-20-20; (D) 60-20-20.

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[29] S.Q. Guo, Y. Kagawa, T. Nishimura, D. Chung, J.M. Yang, J. Eur. Ceram. Soc. 28

082 A. Snyder et al. / Materials Science a

cknowledgement

The authors are grateful for the financial support provided byhe National Science Foundation through award CMMI# 0758584.

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