synthesis of silicon nitride-barium aluminosilicate self-reinforced ceramic composite by a two-step...

COMPOSITES

Composites Science and Technology 65 (2005) 2233–2239

SCIENCE ANDTECHNOLOGY

www.elsevier.com/locate/compscitech

Synthesis of silicon nitride-barium aluminosilicateself-reinforced ceramic composite by a two-step pressureless sintering

Feng Ye a,*, Limeng Liu a, Jingxian Zhang b, Mikio Iwasa b, Cai-Li Su b

a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR Chinab National Institute of Advanced Industrial Science and Technology, AIST Kansai, P.O. Box 433, Ikeda, Osaka 563-8577, Japan

Received 11 July 2004; received in revised form 5 April 2005; accepted 26 April 2005Available online 17 June 2005

Abstract

Dense 40wt%BAS/Si3N4 self-reinforced composite was synthesized by a two-step pressureless sintering process. The resultsshowed that the BAS glass–ceramic not only served as an effective liquid phase sintering aid for attaining full densification and com-pleting the a- to b-Si3N4 phase transformation, but also remained as a structural matrix. A bimodal microstructure could beobtained via this two-step process without the addition of special b-Si3N4 seeds. It is due that the first step could suppress the a-to b-Si3N4 phase transformation while allowing densification, and then at the higher temperature second step, the continued trans-formation could be used to facilitate the growth of the b-nuclei formed in the first step. The obtained composite exhibits excellentmechanical properties compared to unreinforced BAS matrix. The flexural strength and fracture toughness could reach 565 MPaand 7.4 MPa m1/2, respectively. The crack deflection, grain bridging and pullout are considered as major toughening mechanismsin this composite.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Barium aluminosilicate; Si3N4; Composites; Mechanical properties

1. Introduction

Silicon nitride ceramics are one class of the mostpromising materials for structural applications due totheir excellent high-temperature strength, good resis-tance to oxidation, high creep resistance, low coefficientof thermal expansion, good resistance to thermal shockand chemical attack [1,2]. However, their wide applica-tions are inhibited by the high manufacturing costs, be-cause it is difficult to sinter pure silicon nitride ceramicsbecause of the highly covalent bond character [3]. Gen-erally, gas pressure sintering (GPS) or hot pressing (HP)is essential to help densify silicon nitride ceramics andpromote the a- to b-Si3N4 phase transformation, and

0266-3538/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2005.04.015

* Corresponding author. Tel.: +86 45186413921; fax: +8645186413922.

E-mail address: yf306@hit.edu.cn (F. Ye).

hence resulting in the formidable price of the silicon ni-tride components. To lower the process cost and fabri-cate complex-shaped components, pressurelesssintering has become an attracted area of study in theSi3N4 research community [2,4,5].

Pressureless densification typically requires a highersintering-additive content (>15 vol%) compared withthat for pressure-assisted densification (about 5–10vol%). In most cases, the additives in Si3N4 end up asresidual grain-boundary glass phase, which softens anddegrades the properties at high temperature [1,6,7].Therefore, a good additive system should form a liquidphase at a low temperature (a low liquid eutectic tem-perature), which subsequently is crystallized completelyinto a compound with a high melting point. Obviously,the BaO–Al2O3–SiO2 system fulfills these requirements.This three-compound oxide system has a lowest liquideutectic around 1175 �C and readily crystallizes from

Table 1Composition of 40wt%BAS/Si3N4 composite (wt%)

Si3N4 Al2O3 SiO2 BaCO3

57.78 10.61 11.10 20.51

(10˚C/min)

1800˚C /2h1700˚C /40min

0.1MPa

Tem

per

atu

re,˚

C

Nit

rog

en p

ress

ure

,MP

a

2234 F. Ye et al. / Composites Science and Technology 65 (2005) 2233–2239

melt during cooling to a highly refractory compound,Ba2Al2Si2O8 (BAS), which has one of the most meltingpoint, 1760 �C, among the glass–ceramic materials [8].

Barium aluminosilicate (BAS) is an attracting consid-erable interest for diverse applications, such as struc-tural components, electronic packaging and matricesfor ceramic-matrix composites. However, pure BASglass–ceramic matrix exhibits relatively low mechanicalproperties, which limit their use in many structuralapplications. Methods have been explored to improveits mechanical properties by incorporation of some rein-forcement phases, such as whiskers [9], platelets [10] andcontinuous fibers [11]. While improvements in mechani-cal properties have been demonstrated, ease of produc-ibility and low cost have been sacrificed.

Recent research results have shown that rodlike b-Si3N4 could be grown in situ from a-Si3N4 in thepresence of liquid BAS by pressureless or hot-pressingsintering [12–15]. The resulted BAS/Si3N4 compositeexhibited an attractive combination of high strengthand fracture toughness. The room temperature flexuralstrength and fracture toughness of 30% BAS/Si3N4 com-posite could reach 962 MPa and 5.4 MPa m1/2, respec-tively [14]. Moreover, the composite could maintainthis high strength up to 1120 �C [14], indicating thatthe crystallized BAS matrix significantly benefits thehigh temperature strength. These excellent propertiesmake the BAS/Si3N4 ceramic-matrix composites partic-ularly interesting for both room and high temperatureapplications. However, the mechanical properties havenot reached their full potential, further toughnessimprovement is still necessary to achieve practical use.

Many efforts have beenmade in the past to improve thetoughness of Si3N4-based materials [16–19]. The researchresults revealed that obtaining a bimodal microstructure(i.e. abnormal grown Si3N4 grains surrounded by finema-trix grains) is an effective method to further improvestrength and fracture toughness [17,18,20,21]. Additionof b-Si3N4-based seeds has shown to be quite effectivefor producing the desired bimodal microstructure [22,23].

The main objective of this study is to obtain a bimo-dal microstructure by a two-step pressureless sinteringprocess without the addition of special b-Si3N4 seeds.This is achieved by promoting the densification whilesuppressing the a- to b-Si3N4 phase transformation inthe first step, followed by a second step at a higher tem-perature above the BAS melting point to complete thetransformation and grain growth to achieve a bimodalmicrostructure. The effect of microstructure on themechanical properties has also been discussed.

Time, h

Fig. 1. Schematic diagram of sintering patterns to fabricate40wt%BAS/Si3N4 composite by PLS.

2. Experimental

The material used in this study was 40wt%BAS/Si3N4

composite. High purity powders of BaCO3, Al2O3 and

SiO2 were used for forming BAS. Starting powders wereSi3N4 (E10 Grade, UBE Industries Ltd, Japan), Al2O3

(Grade A16SG, Alcoa), BaCO3 (99.9%Grade, RareMetallic Co., Ltd., Japan) and SiO2 (99.9%Grade, RareMetallic Co., Ltd., Japan). When calculating the overallcompositions, 2.38wt%SiO2 (according to the manufac-turer�s specifications) on the surface of Si3N4 powderwas taken into account, and the overall starting compo-sitions are listed in Table 1.

The starting powders were wet milled in ethanol for12 h with silicon nitride balls as the mixing media, andthen dried at 40 �C in a rotary evaporator and sieved.The mixed powders were uniaxial pressed in a steel moldof B55 mm at a pressure of 20 MPa, followed by coldisostatic pressing with 300 MPa. Pressureless sinteringwas carried out in a graphite-resistance furnace undera nitrogen atmosphere. The heating program is indi-cated in Fig. 1. A two-step sintering procedure was ap-plied, i.e. the samples were first heated to 1700 �C at arate of 10 �C/min and held there for 40 min and theywere then further heated to 1800 �C at a rate of 20 �C/min and held there for 2 h under 0.1 MPa of nitrogen.

The bulk densities of the sintered specimens weremeasured according to Archimedes� principle. Crystal-line phases of the produced composites were character-ized by X-ray diffraction (XRD). The relative amountof a- and b-Si3N4 phase was determined from XRD pat-tern by the method outlined by Messier et al [24].

The fracture toughness and the flexural strength ofthe composites were measured in air at room tempera-ture. Flexural strength measurements were performedon bar specimens (3 · 4 · 36 mm) using a three-pointbend fixture with a span of 30 mm. Fracture toughness

F. Ye et al. / Composites Science and Technology 65 (2005) 2233–2239 2235

measurements were performed on single-edge-notchbeam specimens (SENB 3 · 4 · 36 mm) with a span of20 mm, and a half-thickness notch was made using a0.33-mm thick diamond wafering blade. At least sixspecimens were tested for each test condition.

After application of a carbon coating, the polishedand fracture were examined by scanning electron micro-scope (SEM). The microstructures of the compositeswere also characterized by transmission electron micros-copy (TEM). Thin foil specimens were prepared by dim-pling and subsequent ion-beam thinning.

3. Results and discussion

3.1. Densification and phase characterization

Fully densified compacts were obtained after the two-step pressureless sintering. A typical X-ray diffractionpattern for 40wt%BAS/Si3N4 composite is shown inFig. 2. Phases identified consist primarily of b-Si3N4

and hexacelsian BaAl2Si2O8. Only a trace of residuala-Si3N4 phase was detected by XRD, indicating thatthe a- to b-Si3N4 phase transformation is almost com-plete during sintering. No celsian was detected in thiscomposite. Apparently, it is due to the sluggish kineticsof hexacelsian to celsian phase transformation [25]. Thedensity and phase assemblages of the composite aftersintering at 1700 �C at 40 min were also measured, asshown in Table 2. Although the high relatively densityand nearly complete crystallization of BAS matrix couldbe obtained after the first-step sintering, only �16%

20 25 30 35 40 45 50

H

β: β -Si3N4Η:Η: hexacelsian

βHH ββββ

ββ

HH

β

β

Hβ

β

H

Inte

nsi

ty

2 Theta

Fig. 2. XRD spectra of 40wt%BAS/Si3N4 composite after the two-stepsintering.

Table 2Phase content and mechanical properties of the 40wt%BAS/Si3N4 composite

Sintering condition Relative density (%) Phase assembly

HP1700 �C, 40 min 97.2 Hexacelsian, b-Si3a-Si3N4 (84%)

HP1700 �C, 40 min/1800 �C, 2 h 99.9 b-Si3N4, hexacelsia

a-Si3N4 transformed, suggesting that the dissolution ofa-Si3N4 starting particles into the liquid and transportcapability of liquid phase are limited under this condi-tion. So it is necessary to further increase the sinteringtemperature and extend the sintering time at the sec-ond-step to realize the complete the a- to b-Si3N4 phasetransformation, which is very important to further im-prove the mechanical properties of BAS/Si3N4

composite.As stated above, the BAS glass–ceramic served not

only as an effective liquid phase sintering aid for attain-ing full densification and completing the a- to b-Si3N4

phase transformation, but also remained as a structuralmatrix.

3.2. Microstructural characterization

Fig. 3 demonstrates the typical microstructures ofinvestigated BAS/Si3N4 composite after the two-stepsintering. Specimens were over-etched by melt NaOHto reveal the b-Si3N4 grains distribution. A distinct bi-modal microstructure is obtained in which a smallamount of large Si3N4 grains are surrounded by fineSi3N4 grains and the elongated b-Si3N4 grains are ori-ented randomly in a continuous BAS matrix. MostSi3N4 grains have width less than 0.5 lm with a highaspect ratio of >20. The abnormal grown elongatedb-Si3N4 grains have diameters larger than 1.5 lm. Thisbimodal microstructure is expected to benefit to themechanical properties, because it will contribute tocrack-bridging toughening mechanism [15].

TEM image of 40wt%BAS/Si3N4 composite isshown in Fig. 4(a). It reveals that a nearly fully den-sified material with elongated b-Si3N4 grains orientedrandomly in the fine and nearly continuous matrixof BAS, and there is no evidence of microcracks atany location in the micrographs. No a-Si3N4 grainswere observed under TEM, which is consistent withthe XRD results. A crystallized BAS matrix phasewas observed between Si3N4 grains. This phase canbe distinguished from the b-Si3N4 phases by its wet-ting, non-facetted nature. Microdiffraction analysisindicated this phase to be hexacelsian BAS (Fig.4(b)), and energy dispersive spectral (EDS) analysisfrom the wetting intergranular phase also conformsthat the BAS composition is BaAl2Si2O8, as shownin Fig. 4(c). Although some amorphous phases were

Vickershardness (GPa)

Flexuralstrength (MPa)

Fracturetoughness (MPa m1/2)

N4 (16%), 12.2 ± 0.4 380 ± 15 4.0 ± 0.5

n, 11.2 ± 0.3 565 ± 18 7.4 ± 0.4

Fig. 3. Typical microstructure of 40wt%BAS/Si3N4 composite showing the in situ grown b-Si3N4 whiskers.

Fig. 4. (a) TEM micrograph of 40wt%BAS/Si3N4 composites after sintering by the two-step process, showing rod-like Si3N4 grains and BAS matrix;(b) ½�1 2 �1 0� diffraction pattern of the BAS; (c) EDS spectra of the BAS; (d) EDS spectra of the b-Si3N4 grains.

2236 F. Ye et al. / Composites Science and Technology 65 (2005) 2233–2239

expected in the vicinity of Si3N4 grains, none were de-tected by microdiffraction or diffusion TEM within thelimits of resolution for the techniques used. Althoughb-Si3N4 grain has an extensive solid solubility withAl2O3 [26], EDX analysis of the b-Si3N4 grains re-vealed no substitution of silicon or nitrogen in theSi3N4 structure by metal ions (Fig. 4(d)), which indi-cates that no Si–Al–O–N composition was formedduring processing.

A typical TEM micrograph of the composite afteronly the first-step sintering at 1700 �C for 40 min is

shown in Fig. 5, revealing mostly untransformed equi-axed a-Si3N4 grains, which correlated well with theXRD analysis.

In general, the addition of larger b-Si3N4 seeds isneeded to dominate the formation of larger elongatedgrains to obtain bimodal microstructure in Si3N4-basedceramics [22–24,27]. But, in this study, a bimodal micro-structure could be obtained via a two-step PLS processwithout the addition of special b-Si3N4 seeds, whichcan be explained in the following way. During the fist-step sintering, the densification rate is much high than

Fig. 5. TEM micrograph of 40wt%BAS/Si3N4 composites aftersintering at 1700 �C for 40 min, showing mostly untransformed a-Si3N4 grains.

F. Ye et al. / Composites Science and Technology 65 (2005) 2233–2239 2237

the a- to b-Si3N4 phase transformation rate. After sin-tering at 1700 �C at 40 min, almost full densification isachieved, while the fraction of the a- to b-Si3N4 phasetransformation is only �16%. Because activation energyfor crystallization of BAS glass is low, about 588 kJ/molfor bulk BAS glass [25], the crystallization of BAS ma-trix is nearly complete after sintering at 1700 �C accord-ing to the XRD result and hence resulting that theresidual liquid phase is insufficient to assist the a- tob-Si3N4 phase transformation. The subsequent sinteringat a higher temperature (1800 �C) over the melting pointof BAS (about 1760 �C) for extending time, could facil-itate the a- to b-Si3N4 phase transformation and thegrowth of the large b-nuclei formed in the first stepwhich serve as seeds for the grain growth of large b-Si3N4 grains. This two-step sintering process has alsobeen successfully applied by Mitomo [18] and Chen[27] in fabricating self-toughen Si3N4 and a-siliconceramics, respectively.

However, Yu et al. [14] did not obtained a bimodalmicrostructure in 30wt%BAS/Si3N4 composite by pres-sureless sintering at 1920 �C. Obviously, it is attributedto the high a- to b-Si3N4 phase transformation rate dur-ing sintering because of the used high sintering temper-ature. After only sintering at 1920 �C for 5 min, arelative density of 97% of theory density with �68% ofa-Si3N4 being transformed to b-Si3N4 was achieved[14]. Thus, in this case, densification, grain growth andthea- to b-Si3N4 phase transformation occur concur-rently, and hence resulting in a unimodal microstructureafter extending sintering. The use of b-seed crystals isrequisite to obtain bimodal microstructure under thissintering process [28].

3.3. Mechanical properties

The mechanical properties of the investigated40 vol% BAS/Si3N4 composites are shown in Table 2.It indicates that the mechanical properties of the inves-tigated composites at room temperature are significantlygreater than that of non-reinforced BAS. The pure BASmatrix exhibits a very low room temperature flexuralstrength (<100 MPa) [12]. However, the average roomtemperature flexure strength of 40wt%BAS/Si3N4 com-posites is 565 MPa, which increased by 565% comparedto the BAS matrix, indicating that in situ grown rod-likeb-Si3N4 whiskers provided a good strengthening effectby means of the load transfer effect from BAS matrixto Si3N4.

The fracture toughness of investigated compositefabricated by the two-step PLS process is7.4 MPa m1/2, which is much higher than BAS matrixglass ceramic (<2 MPa m1/2) [12]. The high fracturetoughness is undoubtedly attributed to the high con-centration of rod-like b-Si3N4 grains. On the otherhand, the thermal expansion coefficient for hexagonalBAS (8 · 10�6/�C) significant exceeds that of b-Si3N4

(3.3 · 10�6/�C) [12], so it will cause residual tensilestress at the interface and compressive stress in Si3N4

grain after cooling to room temperature. This thermalmismatch stress most likely contributes to the promo-tion of crack deflection, the formation of bridginggrains and also enhancement of whisker pullout [26].The fracture toughness of investigated materials sinter-ing by the two-step process is also higher than all thereported results in BAS/Si3N4 composites [13–15].Obviously, it is attributed to the formation of a bimo-dal microstructure in this study.

The mechanical properties of the 40 vol% BAS/Si3N4

composites sintered only by the first step process (i.e. at1700 �C for 40 min) were also measured, as shown inTable 2. It revealed a much lower strength (380 MPa)and toughness (4.0 MPa m1/2) compared to that fabri-cated by two-step sintering, which is due to the incom-plete a- to b-Si3N4 phase transformation in the sampleafter only the first-step sintering.

The typical fracture surface of the compositeobtained after flexural strength test is shown inFig. 6. The fracture mode is intergranular, indicatingthe presence of weak grain boundary structure suitablefor crack deflection and pull-out during fracturing.The interaction between elongated grains and thepropagating cracks can be demonstrated more clearlyby observing the crack propagation paths induced bylocalized heating during TEM observation, as shownin Fig. 7. It more clearly reveals that the propagatingcrack always deflected along the elongated b-Si3N4

grains. Interfacial debonding, crack deflection andcrack bridging were commonly observed along thecrack paths.

Fig. 7. Crack propagation of the investigated BAS/Si3N4 composite,indicating that the cracks could be deflected and arrested by elongatedb-Si3N4 grains.

Fig. 6. Fracture surfaces of 40wt%BAS/Si3N4 composite after sinter-ing, showing crack deflection and elongated Si3N4 grains pulloutduring fracturing.

2238 F. Ye et al. / Composites Science and Technology 65 (2005) 2233–2239

4. Conclusions

1. Dense 40wt%BAS/Si3N4 self-reinforced compositewas synthesized by a two-step pressureless sintering.In this composites, BAS glass–ceramic not onlyserved as an effective liquid phase sintering aid forattaining full densification and completing the a- tob-Si3N4 phase transformation, but also remained asa structural matrix.

2. A bimodal microstructure could be obtained via theselected two-step process without the addition of spe-cial b-Si3N4 seeds. It is due that the first step couldsuppress the a- to b-Si3N4 phase transformationwhile promoting densification, and then at the higher

temperature second step, the continued Si3N4 phasetransformation could be used to facilitate the growthof the large b-nuclei formed in the first step.

3. The obtained 40wt%BAS/Si3N4 composite exhibitsexcellent mechanical properties compared to unrein-forced BAS due to the formation of a bimodal micro-structure. The flexural strength and fracturetoughness could reach 565 MPa and 7.4 MPa m1/2,respectively.

Acknowledgement

Supported by the National Natural Science Founda-tion of China, Grant No. 50372014.

References

[1] Lange FF. Silicon nitride polyphase system: fabrication, micro-structure, and properties. Int Metal Rev 1980;1:1–20.

[2] Hoffmann MJ. High-temperature properties of Si3N4 ceramics.MRS Bull 1995;10(2):28–32.

[3] Kijama K, Shirasaki S. Nitrogen self diffusion in silicon nitride. JChem Phys 1976;65:2668–71.

[4] Hayashi T, Munakata H, Suzuki H, Saito H. Pressurelesssintering of Si3N4 with Y2O3 and Al2O3. J Mater Sci1986;71(3):3501–8.

[5] Hirosaki N, Okada A, Matoba K. Sintering of Si3N4 with theaddition of rare-earth oxides. J Am Ceram Soc1998;71(3):C144–7.

[6] Knickerbocker SH, Zangvil A, Brown SD. High-temperaturemechanical properties and microstructures for hot-pressuredsilicon nitrides with amorphous and crystalline intergranularphases. J Am Ceram Soc 1985;68(4):C99–C101.

[7] Cheong DS, Sanders WA. High-temperature deformation andmicrostructure analysis for silicon nitride-scandium (III) oxide. JAm Ceram Soc 1992;75(12):3331–6.

[8] Drummond CH, Lee WE. Crystallization of a barium alumino-silicate glass. Ceram Eng Sci Proc 1989;10(9–10):1485–502.

[9] Ye F, Yang JM, Zhang LT, Zhou WC, Zhou Y, Lei TC.Fracture behavior of SiC-whisker-reinforced barium aluminosil-icate glass–ceramic matrix composites. J Am Ceram Soc2001;84(4):881–3.

[10] Gu JC, Ye F, Zhou Y, Lei TC. Microstructure and mechanicalproperties of SiC platelet reinforced BaOAl2O32SiO2 (BAS)composites. Ceram Int 2000;26(8):855–63.

[11] Bansal NP. Celsian formation in fiber-reinforced barium alumi-nosilicate glass–ceramic matrix composites. Mater Sci Eng A2003;342(1–2):23–7.

[12] Fang Y, Yu F, White KW. Microstructural modification toimprove mechanical properties of a 70% Si3N4–30% BAS self-reinforced ceramic composite. J Mater Sci 2000;37:4411–7.

[13] Yu F, Nagarajan N, Fang Y, White KW. Microstructural controlof 70% silicon nitride–30% barium aluminum silicate self-reinforced composite. J Am Ceram Soc 2001;84(1):13–22.

[14] Richardson KK, Freitag DW, Hunn DL. Barium aluminosiliconreinforced in situ with silicon nitride. J Am Ceram Soc1995;78(10):2662–8.

[15] Ye F, Chen S, Iwasa M. Synthesis and properties of bariumaluminosilicate glass–ceramic composites reinforced with in situgrown Si3N4 whiskers. Scripta Mater 2003;48:1433–8.

F. Ye et al. / Composites Science and Technology 65 (2005) 2233–2239 2239

[16] Beacher PF, Hwang SL, Hsueh CH. Using microstructure toattack the brittle nature of silicon nitride ceramics. MRS Bull1995;2:23–7.

[17] Kim HD, Han BD, Park DS, Lee BT, Becher PF. Novel two-stepsintering process to obtain a bimodal microstructure in siliconnitride. J Am Ceram Soc 2002;85(1):245–52.

[18] Mitomo M. In-situ microstructural control in engineering ceram-ics. Key Eng Mater 1999;161–163:53–8.

[19] Goto Y, Thomas G. Phase transformation and microstructuralchanges of Si3N4 during sintering. J Mater Sci 1995;30:2194–200.

[20] Hirao K, Nagaoka T, Brito ME, Kanzki S. Mechanical propertiesof silicon nitrides with tailored microstructure by seeding. JCeram Soc Jpn 1996;104(1):54–8.

[21] Beacher PF. Microstructural design of toughened ceramics. J AmCeram Soc 1991;74(2):255–69.

[22] Hirosaki N, Akimune Y, Mitomo M. Microstructure character-ization of gas-pressure-sintered beta-silicon nitride containing

large beta-silicon nitride seeds. J Am Ceram Soc1994;77(4):1093–7.

[23] Hirao K, Nagaoka T, Brito ME, Kanzaki S. Microstructurecontrol of silicon nitride by seeding with rodlike beta siliconnitride particles. J Am Ceram Soc 1994;77(7):1857–62.

[24] Messier DR, Riley FL, Brook RJ. The reconstructive a/b siliconnitride phase. J Mater Sci 1978;13:1199–205.

[25] Bansal NP, Hyatt MJ. Crystallization behavior and properties ofBaO Æ Al2O3 Æ 2SiO2. J Mater Res 1989;4:1257.

[26] Ziegler G, Heinrich J, Wotting G. Review: relationship betweenprocessing, microstructure and properties of dense and reaction-bonded silicon nitride. J Mater Sci 1987;22:3041–86.

[27] Chen IW, Rosenflanz A. A tough sialon ceramics based on a-Si3N4 with a whisker-like microstructure. Nature 1997;389:701–4.

[28] Fang Y, Yu F, White KW. Bimodal microstructure in siliconnitride-barium aluminum silicate ceramic-matrix composites bypressureless sintering. J Am Ceram Soc 2000;83(7):1828–30.