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Composites Science and Technology 68 (2008) 1710–1717

SCIENCE ANDTECHNOLOGY

Fabrication and mechanical properties of carbon short fiberreinforced barium aluminosilicate glass–ceramic matrix composites

Feng Ye *, Limeng Liu, Liangjun Huang

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

Received 11 September 2007; received in revised form 26 December 2007; accepted 6 February 2008Available online 13 February 2008

Abstract

Dense BaSi2Al2O8 (BAS) and Ba0.75Sr0.25Si2Al2O8 (BSAS) glass–ceramic matrix composites reinforced with carbon short fibers (Csf)were fabricated by hot pressing technique. The microstructure, mechanical properties and fracture behavior of the composites were inves-tigated by X-ray diffraction, scanning and transmission electron microscopies, and three-point bend tests. The carbon fibers had a goodchemical compatibility with the glass–ceramic matrices and can effectively reinforce the BAS (or BSAS) glass–ceramic because of asso-ciated toughening mechanisms such as crack deflection, fiber bridging and pullout effects. Doping of BAS with 25 mol% SrSi2Al2O8

(SAS) can accelerate the hexacelsian to celsian transformation and result in the formation of pure monoclinic celsian in Csf/BSAScomposites, which can avoid the undesirable reversible hexacelsian to orthorhombic transformation at �300 �C and reduce the thermalexpansion mismatch between the fiber and matrix.� 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Barium aluminosilicate; A. Carbon fibres; A. Short-fibre composites; B. Mechanical properties

1. Introduction

Advanced glass–ceramics, such as barium aluminosili-cate (BaSi2Al2O8 or BAS) typically exhibit high meltingpoints, low coefficient of thermal expansion, and goodoxidation resistance and dielectric properties [1]. For thesereasons, they have potential as matrix materials for ceramicmatrix composites with applications in electronic packag-ing, structural components and nuclear shielding [2–6].

Although celsian BAS is the stable phase below 1590 �Cand hexacelsian polymorph is stable above 1590 �C, hexa-celsian BAS can metastably exist below 1590 �C due tothe sluggishness of the hexacelsian to celsian transforma-tion [7]. Hexacelsian shows a large thermal expansioncoefficient of �8.0 � 10�6 C�1 and undergoes a rapid,reversible hexacelsian to orthorhombic transformation at�300 �C, accompanied by a large volume change of�3%. This structural transformation during thermal

0266-3538/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2008.02.004

* Corresponding author. Tel.: +86 45186413921; fax: +86 45186413922.E-mail address: yf306@hit.edu.cn (F. Ye).

cycling would result in microcracking of the BAS matrix.Therefore, it is needed to promote the kinetics of transfor-mation of hexacelsian into monoclinic celsian in order thatBAS glass–ceramic and its composites can be successfullyapplied.

On the other hand, like other glass–ceramics, the pureBAS ceramic matrix exhibits relatively low mechanicalproperties, and hence limits its use in many structuralapplications [8]. Over the past years, various kinds ofBAS based composites, including particulate-, whisker-,platelet- and fiber reinforced BAS composites have beenextensively investigated [5,6,9,10]. Continuous fiber rein-forced composites exhibit superior properties comparedwith the monolithic BAS matrix [5,6]. The high strengthand modulus of the fibers provide superior mechanicalproperties and can prevent catastrophic brittle failure incomposites. While improvements in mechanical propertieshave been demonstrated, ease of producibility and low costhave been sacrificed.

Short fiber reinforced composites have generated a greatdeal of attention due to their adaptability to conventional

Table 2Compositions of the investigated composites and hot-pressing conditions

Sample Csf content(vol%)

BAS content(vol%)

Hot-pressing conditions

Temperature (oC) Time (h)

B 0 0 1500 130B13 30 70 1300 130B14 30 70 1400 130B16 30 70 1600 120B 20 80 1550 130B 30 70 1550 140B 40 60 1550 130S 30 70 (25 mol% SAS) 1550 140S 40 60 (25 mol% SAS) 1550 1

F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717 1711

manufacturing techniques and low cost of fabrication [11].Another advantage of the use of short fibers instead of con-ventionally used long fibers is that the short fibers allow thedevelopment of an inner oxidation protection, i.e. the innershort fibers can be protected by the matrix [12]. However,fabrication and properties of short fibers-reinforced BAScomposites have not been reported.

In present work, carbon short fiber reinforced BASglass–ceramic matrix composites were prepared by hotpressing. The role of doping of BAS with strontium alumi-nosilicate (SAS) on the BAS crystal phase formation wasinvestigated. The effects of sintering pressing parametersand fiber volume fraction on the mechanical performancewere also discussed.

2. Experimental

The materials used in this study were BAS glass–ceramiccomposites reinforced with different volume fractions ofshort carbon fibers (from 20 to 40 vol%). The BAS matrixpowders were synthesized through hydrolysis of alkoxides[13]. The mixed 0.75BaO � 0.25SrO � Al2O3 � 2SiO2 (BSAS)powders were also synthesized using the same process toimprove the hexacelsian-to-celsian phase transformationduring sintering. The compositions of BAS and BSASmatrix powders are shown in Table 2.

The carbon fibers used in this study (Jiling CarbonIndus., China) have a diameter of 6–8 lm, and its proper-ties are summarized in Table 1. The fibers were cut to alength of 3–5 mm before fabricating the composites. Theshort carbon fibers were firstly ultrasonically dispersed intoethanol. Then the matrix powders were added to the abovesolution. The mixed solutions were further mixed by ball-milling for 4 h. The slurries were dried and aggregates weredispersed by hand as required. The dried blends were thenhot-pressed in graphite dies at 1300–1600 �C for 1 h undera pressure of 35 MPa in a nitrogen atmosphere into disksof 50 mm diameter and 6 mm thickness. The densities ofthe samples were measured by Archimedes’ method in dis-tilled water at 20 �C.

The fracture toughness and flexural strength of the com-posites were measured in air at room temperature. All flex-ural bars were machined with the tensile surfaceperpendicular to the hot-pressing axis direction. Flexuralstrength measurement were performed on bar specimens(3 mm � 4 mm � 36 mm) using a three-point bend fixturewith a span of 30 mm. Fracture toughness measurementswere performed on single-edge-notch beam specimens(SENB) with a span of 16 mm, and a half -thickness notchwas made using a 0.33 mm thick diamond wafering blade.Six bars were tested for each composition.

Table 1Typical properties of carbon fiber

Diamater(lm)

Density(g cm�3)

Tensile strength,(MPa)

Tensile modulus(GPa)

6–8 P1.76 2930 200–220

The constitution phases of the composites were deter-mined by X-ray diffractometry (XRD). Fracture surfaceand crack propagation path produced by Vickers indenteron the composites were examined by scanning electronmicroscopy (SEM). The microstructures of the compositeswere characterized by transmission electron microscopy(TEM). Thin foil specimens taken normal to the hot-press-ing axis were prepared by dimpling and subsequent ion-beam thinning.

3. Results and discussion

3.1. Densification and phase characterization

The effect of sintering temperature and Csf content onthe densification of the composites are shown in Fig. 1. Itreveals that the relative density increases with increasingthe sintering temperature and decreases with increasingCsf content. The composites with different Csf contentcould be densified to over 95% of the theoretical densityafter sintering at 1500 �C for 1 h. The BSAS compositespossess a higher relative density than that of BAS com-posites with same fiber content, as shown in Fig. 2. It isdue to the lower eutectic temperature in BSAS systemduring sintering, which can more effectively promote thedensification.

The typical XRD patterns taken from the polished sur-faces of the hot-pressed composites are given in Fig. 3. Itreveals that the only crystalline phase present in BASmatrix composites is hexacelsian phase, and no celsianwas detected, as shown in Fig. 3a. Apparently, it is dueto the sluggish kinetics of hexacelsian to celsian phasetransformation [7]. On the contrary, for BSAS composites,celsian becomes the predominant crystalline phase, indicat-ing that the incorporation of 25 mol% SAS can effectivelypromote the hexacelsian- to celsian-BaAl2Si4O8 transfor-mation (as shown in Fig. 3b). In both the BAS and SASsystems, hexacelsian is always the first to form, but thekinetics of hexacelsian to celsian transformation in SASis very fast [14]. BAS and SAS can form solid solutionsin the entire composition range, and doping of BAS withSAS could assist the breaking of Ba–O or (Al,Si)–O bondsand hence promoting the polymorphic transformation of

1300 1400 1500 160090

92

94

96

98

100 aR

elat

ive

Den

sity

, %

Temperature, °C

30vol%Csf /BAS composites

0 10 20 30 4070

80

90

100

Csf /BAS composites

b

Rel

ativ

e D

ensi

ty, %

Csf content, vol%

HPed at 1550 °C for 1h

Fig. 1. Densification of the Csf/BAS composites as a function of the sintering temperature (a) and carbon fiber content (b).

90

92

94

96

98 HP1550 °C, 1h

30S 40S 40B 30B

Rel

ativ

e D

ensi

ty, %

Fig. 2. Comparison of the relative densities of Csf/BAS and Csf/BSAScomposites.

0 200 400 600 800 10000.0

0.1

0.2

0.3

0.4 30B 30S

Lin

ear

ther

mal

exp

ansi

on

, %

Temperature, °C

Fig. 4. Line thermal expansion curves of the composites with 30 vol% Csf.

1712 F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717

BAS [15]. Therefore, it is an effective and suitable methodto accelerate the hexacelsian-to-celsian transformation bydoping BAS with certain amount of SAS. This result hadalso been reported by Bansal [16]. The realization of com-plete hexacelsian-to-celsian transformation in BAS matrixis expected to further improve the mechanical propertiesof BAS matrix composites.

The thermal expansion coefficient of the compositestrongly depends on the polymorphic form of BAS matrix.Fig. 4 shows the thermal expansion curves of the investi-gated composites as a function of temperature. It clearly

10 20 30 40 50

a

40B

30B

20B

H - Hexacelsian

HHH

HH

HHH

H

HH

H

Inte

nsi

ty

2 Theta,°

Fig. 3. XRD spectra of Csf/BAS (a) and

reveals that the BSAS composite possesses a lower thermalexpansion which is relatively linear over the temperaturerange tested. However, the BAS composite exhibits anabrupt large volume change in temperature range of290–320 �C due to the hexacelsian to orthorhombic trans-formation. The average liner thermal expansion coefficientsin the range 25–1000 �C were calculated to be 1.63 �10�6 �C�1 and 3.5 � 10�6 �C�1 for the two composites,respectively. Thermal expansion coefficients of pure BSAScelsian and BAS hexacelsian have been reported to be5.28 � 10�6 �C�1 [17] and 8.0 � 10�6 � C�1 [18], respec-tively. This implies that the incorporation of carbon short

10 20 30 40 50

b All the peaks from celsian

40S

30S

2 Theta,°

Inte

nsi

ty

Csf/BSAS composites after sintering.

F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717 1713

fibers has a pronounced effect on their thermal expansioncoefficients. The formation of monoclinic celsian in BSAScomposite can effectively reduce the average thermalexpansion coefficient and avoid the undesirable reversiblehexacelsian to orthorhombic transformation at �300 �C,which will benefit the mechanical properties, especiallythe high temperature performance.

3.2. Microstructural characterization

Figs. 5 and 6 demonstrate the typical microstructures ofinvestigated composite after sintering. The fibers are homo-geneously dispersed in continuous BAS matrix with alength of a few 100 lm, which is much shorter than the ori-ginal length. The length distribution curves of the shortfibers in composites are shown in Fig. 7. It clearly reveals

Fig. 5. SEM micrographs of Csf/BAS composites: (a) 20 vo

Fig. 6. SEM micrographs of Csf/BSAS composites

40 80 120 160 200 240 2800

5

10

15

20

25

30 a 20B 30B 40B

Freq

uenc

y, %

Length of carbon fiber, μm

Fig. 7. The length distribution of carbon fibers in the composites af

that the length of fiber decreases with the Csf contentincreasing. The high Csf content may enhance the possibil-ity of impingement among fibers and hence resulting infibers fracturing during hot pressing.

As shown in Fig. 8a and b, it was found that somecracks appeared in the BAS matrix. It undoubtedlyresulted from the thermal mismatch between the fiberand BAS matrix. In the investigated composites, the radialthermal expansion coefficient of carbon fiber between roomand 900 �C is about 8–12 � 10�6 �C�1, which is slightlyhigher than that of the BAS matrix (�8 � 10�6 �C�1). Itwill result in a trend for fibers debonding during cooling.Therefore, in the radial direction, the thermal expansionmismatch between fiber and matrix is acceptable. However,in the axial direction, the thermal expansion coefficient ofthe carbon fiber (�1.2–0 � 10�6 �C�1) is much smaller than

l% Csf/BAS; (b) 30 vol% Csf/BAS; (c) 40 vol% Csf/BAS.

: (a) 30 vol% Csf/BSAS; (b) 40 vol% Csf/BSAS.

20 40 60 80 100 120 140 160 1800

5

10

15

20

25

30

35

40b

Freq

uenc

y,%

Length of carbon fiber, μm

30S 40S

ter sintering (a) Csf/BAS composites; (b) Csf/BSAS composites.

Fig. 8. SEM micrographs of the investigated composites showing the microcracks in the BAS matrix induced by thermal expansion mismatch between thefiber and matrix (indicated by white arrows): (a) and (b) Csf/BAS composites, (c) Csf/BSAS composites.

1714 F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717

that of the matrix. A significant tensile stress will be builtup in the matrix during cooling from the sintering temper-ature. The magnitude of this stress can be approximatelycalculated from the following formula [19]:

Fig. 9. TEM micrographs of the investigated composites: (a) Csf/BAS compcomposite; (d) [�312] diffraction pattern of the celsian BAS.

ra ¼ ðam � afÞDTEfV f=V f ½ðEf=EmÞ � 1� þ 1 ð1Þwhere DT is the temperature change during cooling, Vf isthe volume fraction of fiber, am and af are the thermalexpansion coefficients and Em and Ef are the elastic moduli

osite; (b) ½3�10� diffraction pattern of the hexacelsian BAS; (c) Csf/BSAS

F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717 1715

of the matrix and fiber, respectively. In the Csf/BAS system,the magnitude of the thermal mismatch tensile stress can becalculated by substituting values of am = 8 � 10�6 �C�1,af.a = 0, Ef = 200 GPa, Em = 70 GPa, DT = 1000 �C intothe above equation. The calculated tensile stress in BASmatrix is about 233 MPa for 20 vol% Csf/BAS compositeand 316 MPa for the composite containing 30 vol% fibers.It is greater than the strength of the monoclinic BASmatrix (�70 MPa), which results in transverse microcracksin the matrix. For the Csf/BSAS composites, the formationof BSAS celsian phase decreases the thermal expansioncoefficient of the matrix, and hence lessens the thermal mis-match between the carbon fiber and the matrix. But somemicrocracks in the matrix could also be observed, as shownin Fig. 8c. Therefore, it is necessary to find a more effectiveway to eliminate this interfacial thermal mismatch.

The typical TEM micrographs showing the fiber/matrixinterface region of the composites are presented in Fig. 9. Itcould be seen that the C short fibers have a good bond withthe BAS or BSAS matrix grains without obvious interfacialreaction or amorphous layer, indicating no chemical reac-tion between the fiber and the matrix during compositeprocessing. Microdiffraction analyses from the BAS andBSAS matrix in the composites confirm that they are crys-tallized hexacelsian BAS and celsian BSAS (Fig. 9c and d),which is consistent with the XRD results (Fig. 3).

0

50

100

150

200

250a

30B 40S 40B 30S

Fle

xura

l str

eng

th, M

Pa

Fig. 11. The effect of incorporation of SAS on the mechanical propert

0 10 20 30 40

50

100

150

200a

Flex

ural

str

engt

h, M

Pa

Csf content, vol%

Fig. 10. Mechanical properties of the Csf/BAS composites as a functio

3.3. Mechanical properties

The mechanical properties of the Csf/BAS compositesare shown in Fig. 10. Both the flexural strength and thefracture toughness increase with the fiber content from 0to 30 vol% Csf. The flexural strength and fracture tough-ness of the 30 vol% Csf/BAS composites reach 201 MPaand 3.39 MPa m1/2, increasing 127% and 92% comparedto the pure BAS matrix, respectively. The increase instrength of the composites is due to the good Csf/BASinterfacial bonding, which can effectively realize the loadto transfer from BAS matrix to fiber. The tougheningmechanisms of the composites are crack deflection andfiber pullout, which can be seen clearly from the observa-tions of fracture surfaces crack paths, as shown in Figs.13 and 14, respectively. Further increasing the volume frac-tion of fibers to 40%, the strength and toughness decrease.It is because that the high fiber content induces a largeresidual thermal mismatch stress in the matrix, whichmay result in more transverse cracks perpendicular to thecarbon fiber in the matrix, and hence decreasing themechanical properties. As shown in Fig. 11, for the com-posites with the same volume fraction of fibers, themechanical properties of the BSAS composites are higherthan that of BAS composites. The flexural strength andfracture toughness of the 30 vol% Csf/BSAS composites

0

1

2

3b

30B 40S40BFra

ctu

re t

ou

gh

nes

s, M

Pa

· m1/

2

30S

ies of the composites: (a) flexural strength, (b) fracture toughness.

0 10 20 30 40

Csf content, vol%

1.0

1.5

2.0

2.5

3.0

3.5 b

Frac

ture

tou

ghne

ss, M

Pa ·

m1/

2

n of C fiber content: (a) flexural strength, (b) fracture toughness.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

20

40

60

80

100

120

140

160

180

30vol%Csf/BAS

40vol%Csf/BAS

Lo

ad,

N

Displacement, mm

BAS

20vol%Csf/BAS

Fig. 12. Load–displacement curves for the investigated Csf/BAScomposites.

Fig. 14. SEM photograph of indentation crack propagation paths in30vol%Csf/BSAS composites, showing crack deflection and bridging bycarbon fibers.

1716 F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717

increase to 256 MPa and 3.45MPa m1/2. It is attributed tothe increase in relative density and the decrease in thermalexpansion mismatch stress between carbon (axial direction)and matrix. The increase in strength also reveals that theload can be still effectively transferred from BSAS matrixto carbon fiber, although its interfacial bonding strengthbetween fiber and matrix is weaker than that of BAS com-posites due to the lower thermal expansion coefficient ofcelsian.

The typical load–displacement curves for the BAS com-posites are given in Fig. 12. The monoclinic BAS fails in abrittle mode as expected. However, the composites rein-forced with Csf show a controlled fracture behavior. Itcan be seen that the composites extended elastically atthe beginning of the test. Beyond the elastic limit, theapplied load produced plastic deformation until the maxi-mum load was reached, and then the load dropped withincreasing displacement and forming a long tail due tothe fiber debonding and pullout.

The fracture behavior of the investigated composites canbe demonstrated clearly by SEM observing the fracturesurface and the indentation crack propagation path ofthe composites, as shown in Figs. 13 and 14. The crackalways propagates along the fiber/matrix interface(Fig. 14) and extensive fiber pullout is observed on the frac-ture surface (Fig. 13), indicating the strong interaction

Fig. 13. Fracture surfaces of the investigated composites showing extensive fi

between the propagation crack and the microstructureduring the fracturing, and resulting in tougheningbehavior.

4. Conclusions

(1) Dense BAS and BSAS glass–ceramic matrix compos-ites reinforced with carbon short fibers were fabri-cated by hot pressing. Carbon fibers have a goodchemical compatibility with the investigated glass–ceramic matrices and display good strengtheningand toughening effects.

(2) Doping of BAS with 25 mol% SAS can accelerate thehexacelsian to celsian transformation and result inthe formation of pure monoclinic celsian in Csf/BSAScomposites, which can avoid the undesirable revers-ible hexacelsian to orthorhombic transformation at�300 �C and improve the thermal expansion mis-match between the fiber and matrix;

(3) The mechanical properties of BAS glass–ceramicmatrix can be greatly improved by addition of carbonshort fibers. By incorporation of 30 vol% fibers, theflexural strength and fracture toughness of the com-posites reach 201 MPa and 3.39 MPa m1/2, increasing127% and 92% compared to the BAS matrix, respec-tively. On doping of BAS with 25 mol% SAS, themechanical properties of the composites can be fur-ther increased due to the formation of pure mono-clinic celsian in the composites.

ber pullout during fracturing. (a) 30vol%Csf/BAS; (b) 30vol%Csf/BSAS.

F. Ye et al. / Composites Science and Technology 68 (2008) 1710–1717 1717

Acknowledgement

This work was supported by Program for New CenturyExcellent Talents in University China (Grant No. NCET-04-0336) and Excellent Youth Foundation of HeilongjiangProvince of China.

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