Materials Letters 63 (2009) 2035–2037

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Materials Letters

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Pressureless sintering ZrB2–SiC ceramics at low temperatures

Miao Zhu, Yiguang Wang ⁎National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an, China 710072

⁎ Corresponding author. Tel.: +86 29 88494914; fax:E-mail address: wangyiguang@nwpu.edu.cn (Y. Wan

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.06.041

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 April 2009Accepted 18 June 2009Available online 25 June 2009

Keywords:CeramicsSintering

ZrB2–SiC ceramics are prepared by pressureless sintering using ZrB2 powders and liquid polycarbosilane(LPCS) precursors. The LPCS can effectively reduce the sintering temperature. The phases of the sinteredceramics are characterized by X-ray diffraction, and their morphologies are observed by scanning electronmicroscopy. From these results, it is learned that LPCS can provide free carbon and silicon at hightemperatures. Therefore, the oxides on the ZrB2 surface can be removed by free carbon, and the densificationprocess can be promoted by silicon. These coupled effects make it possible to pressureless sinter the ZB2–SiCceramics at low temperatures.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

ZrB2 is one of the materials known as ultrahigh-temperatureceramics. It has a series of excellent properties, such as high meltingpoint, hardness, and chemical stability [1–4]. By adding SiC to ZrB2, theresultant ZrB2–SiC ceramics have better strength and oxidationresistance than ZrB2 alone [1,2,5–8]. Although ZrB2-based ceramicsexhibit those attractive properties, sintering these ceramics is difficultdue to their low diffusivities and surface impurities of the startingpowders [2,3,9]. In most cases, dense ZrB2–SiC ceramics are obtainedby hot pressing, which is limited to simple geometric shapes.Fabrication of complex components requires expensive and time-consuming diamond machining. However, in contrast to hot pressing,pressureless sintering would be good for fabrication of near-netshapes and reduce processing costs [2,3,8–10].

Many efforts have been made to pressureless sinter ZrB2 ceramics[3,8–18]. The sintering temperatures of ZrB2 ceramics decrease tobelow 2150 °C by adding additives such as MoSi2, B4C, C, ZrSi2, WC orMo [2–4,8–16]. The densification process is enhanced by eitherformation of liquid phase [4,11,16] or removal of oxide impuritiesfrom the surface of ZrB2 with these additives [6,9,10,13,14]. Recently,several studies have been carried out to sinter ZrB2-based ceramics byusing polymeric precursors to get dense ceramics at low temperatures[6,7]. It is found that the polymeric precursor can greatly enhance thedensification process. However, the mechanism for such a process isstill unclear. According to the results of composition analysis [6,7], it isbelieved that the residual carbon produced during the pyrolysis ofpolymeric precursors was the reason. It was thought that the residualcarbon could remove the oxides presented on the ZrB2 particlesurfaces through solid-state reactions [6,7]. However, this explanation

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is not persuadable because the carbon additive alone cannot have sucha great effect on reducing the densification temperatures.

The purpose of this paper is to reveal the real reasons for polymerprecursors to promote the densification process of ZrB2 ceramics. Inthis study, ZrB2 with liquid polycarbosilane (LPCS) were sintered atdifferent temperatures. The microstructure and composition of thesintered ceramics were analyzed. Finally, the function of polymericprecursors in sintering process was discussed based on these results.

2. Experimental procedure

The ZrB2 powder (0.5mm, BeijingMountain Technical DevelopmentCenter, Beijing, China) and LPCS (Laboratory of Advanced Materials, XiaMenUniversity, Xiamen, China)were used as the startingmaterials. TheLPCS was the precursor of SiC ceramics, and its pyrolysis process wasdescribed elsewhere [19]. The LPCS was diluted by acetone and thenmixedwith ZrB2 powder uniformly. Theweight ratio of ZrB2 to LPCSwasabout 4:1. Afterwards, the ZrB2–LPCS slurry was gradually dried bycontinuous stirring. The dried powder was then pressed into cylindricalpellets with a pressure of 10 MPa, followed by cold isostatic pressing at200 MPa for 60 s. Prior to sintering, the green compacts were heated to900 °C in flowing argon to convert the LPCS into ceramics. The sampleswere then sintered at 1600 °C, 1800 °C, 1900 °C and 2000 °C for 2 h,respectively. At temperatures below 1800 °C, a mild vacuum wasadopted. Above 1800 °C, the furnace was backfilled with flowing argon.

The bulk densities of specimens weremeasured by the Archimedesmethod. The measured bulk density was divided by the theoreticaldensity to obtain the relative density. The theoretical density wasestimated using the mixture rule. The morphologies of the sinteredceramics were observed by scanning electron microscopy (SEM, JEOL-6700F, Tokyo, Japan). X-ray diffraction (XRD) was used for the phaseanalysis, which was carried out by using a Rigaku D/max-2400diffractometer (Tokyo, Japan) with copper Ka radiation. Data were

Fig. 1. Relative density of the ZrB2–SiC ceramics as a function of sintering temperature.

Fig. 3. XRD patterns of the sintered ZrB2–SiC ceramics pyrolyzed at 900 °C (a) and thensintered at 1600 °C (b), 1800 °C (c), 1900 °C (d), and 2000 °C (e) respectively.

2036 M. Zhu, Y. Wang / Materials Letters 63 (2009) 2035–2037

digitally recorded in a continuous scan in the range of angles (2q)from 15° to 75°.

3. Results and discussion

Fig. 1 shows the relative density of the sintered specimens as afunction of temperature. It can be seen that the relative densityenhances with the increase in sintering temperature. At 1900 °C, therelative density reaches its maximum value. Further increasing thesintering temperature to 2000 °C, instead, the ceramic density willdecrease.

The fracture surfaces of the sintered samples are shown in Fig. 2.After sintering at 1600 °C, the samples have a linear shrinkage of about17%. However, there are still a lot of pores inside (Fig. 2a). As thesintering temperature increases to 1800 °C, the porosity decreases(Fig. 2b) and the shrinkage is nearly 20%. It is also found from thefracture surfaces that pores aremainly located at the grain boundaries.As far as we know, during the sintering process, some impurities likeB2O3 and the gaseous products of the reactions between oxides and

Fig. 2. Morphologies of the fracture surfaces of ZrB2–SiC ceramics sin

reductions can evaporate out, which will lead to the pores betweenthe grain boundaries. With the increase in the sintering temperature,the diffusion is enhanced and the grain coarsening is enlarged. Thedensity of sintered samples thus increases. However, if the graingrows too fast, the pores will be entrapped in the samples, leading tothe decrease in density. This may be the reason for the low relativedensity of the samples sintered at 2000 °C.

The XRD patterns of the specimens sintered at different tempera-tures are shown in Fig. 3. As can be seen, there are two phases in thesamples treated at 900 °C (Fig. 3a): ZrB2, the main phase; ZrO2, theoxide impurity on the surface of ZrB2. There are no visible signals fromLPCS because its pyrolyzed products are mainly amorphous phase[19]. Whereas, at 1600 °C, new phases of ZrC and SiC appear, while thephase of ZrO2 almost vanishes (Fig. 3b). It is believed that SiC comesfrom LPCS, which will crystallize at temperatures over 1400 °C [19].Meanwhile, free carbon will be formed during the crystallization

tered at 1600 °C (a), 1800 °C (b), 1900 °C (c), and 2000 °C (d).

Fig. 4. SEM and EDS analysis of substance deposited on graphite paper wrapped thespecimens during pressureless sintering.

2037M. Zhu, Y. Wang / Materials Letters 63 (2009) 2035–2037

process of LPCS. Therefore, the following reactions may happen toreduce ZrO2 and to form ZrC [12–14].

ZrO2 þ B2O3ðlÞ þ 5C ¼ ZrB2 þ 5COðgÞ ð1Þ

ZrO2 þ 3C ¼ ZrC þ 2COðgÞ ð2ÞAfter sintering at 1800 °C, the samples have only the phases of ZrB2

and ZrC (Fig. 3c). The ZrC contents are calculated to be about 3 wt.%.However, SiC phase is not detected in the samples. Previous studyindicated that the following reactionwill be favorable at temperaturesabove 1750 °C [20].

SiC þ ZrO2 ¼ ZrC þ SiOðgÞ þ COðgÞ ð3ÞReaction (3) contributes to the vanishing SiC at high temperatures.

However, due to the limitation of ZrO2 contamination, the SiC couldnot be totally consumed by the above reaction. As known, the ZrO2 isabout 9 wt.% in the starting ZrB2 powders, and the LPCS could provideabout 20 mol% SiC in the final composites according to the calculationbased on LPCS pyrolysis behaviors [19]. Even though all of the ZrO2

were consumed, there still was more than 10 mol% SiC. Therefore,there are other reasons to cause the absence of SiC in the specimens.

During sintering ZrB2–LPCS samples in vacuum, the vacuum of thefurnacewildly increase in the temperature range of 1440–1700 °C. It isbelieved that some substances evaporate out. Moreover, a layer ofloose faint yellow substances is found on the graphite papers thatwrapped the specimens during vacuum sintering. SEM result (Fig. 4)indicates that these substances are finely crystallized. According to theanalysis of EDS (Fig. 4), the compositions of the crystalline are siliconand carbonwith an atomic ratio close to 1:1, which demonstrates thatthe substance is SiC crystalline.

Subsequently, it is thought that silicon will evaporate out duringsintering other than the reactions (1)–(3) to causeweight loss and SiCvanishing. Silicon is believed to originate from the pyrolysis of LPCS. Itis generally thought that the structure of polymer precursors will berearranged during pyrolysis [21]. The silicon domain and carbondomain do exist during the rearranged process. At temperatureshigher than its melting point (1410 °C), the silicon will easilyevaporate out to deteriorate the vacuum.

The residual carbon from LPCS plays an important role to removethe oxide impurities on the ZrB2 surface according to reactions (1) and(2), which is favorable for the densification process [3,6,9,13,14].However, the ability of carbon to reduce the sintering temperature islimited [6,12–14]. The silicon is the key to lower sintering temperatureand promote the densification process. The LPCS can provide silicon,which is liquid at temperatures higher than 1450 °C. The liquid siliconcan promote the diffusion of materials. When silicon volatilizes, theformed vapor pressure leads to much shrinkage of the specimens,which is very favorable for sintering. As a comparison, ZrB2–SiC pelletis prepared without added LPCS. Even sintering at 1800 °C for 2 h, thepellet shows no obvious shrinkage. On the other hand, the volatiliza-tion of silicon results in a lot of pores, which are difficult to be filled bydiffusion. This is the reason why the mechanical properties ofpressureless sintered ZrB2–PCS ceramics are not good enough [6].Therefore, controlling the contents of LPCS is very important. It is thenext topic of our research.

4. Summary

ZrB2–SiC were pressureless sintered at low temperatures usingZrB2 and LPCS. LPCS can provide free carbon and silicon at hightemperatures. Free carbon can remove the oxides on the ZrB2 surface,and silicon can promote the densification process. These couplingeffects make it possible to pressureless sinter ZrB2-based ceramics atlow temperatures. This methodology can also be used to sinter otherultrahigh-temperature ceramics such as HfC, TaC, or HfB2.

Acknowledgement

This work is financially supported by the Chinese Natural ScienceFoundation (Grant # 90176023).

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