Scripta Materialia 55 (2006) 911–914

www.actamat-journals.com

Preparation and mechanical properties of carbonnanotube reinforced barium aluminosilicate

glass–ceramic composites

Feng Ye,* Limeng Liu, Yujin Wang, Yu Zhou, Bo Peng and Qingchang Meng

Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology,

Harbin 150001, PR China

Received 16 April 2006; revised 9 July 2006; accepted 26 July 2006Available online 24 August 2006

Barium aluminosilicate (BAS) glass–ceramic composites reinforced with multi-wall carbon nanotubes (MWNTs) were fabricatedby hot-pressing. The results showed that BAS glass–ceramic served as an effective liquid phase sintering aid to promote the densi-fication of the composites. The incorporation of MWNTs could significantly improve the mechanical properties of the BAS matrix.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotube; Barium aluminosilicate; Composite; Mechanical properties; Toughening mechanisms

Ever since their discovery, carbon nanotubes(CNTs) have been considered the most promising rein-forcements for toughening ceramic matrices due to theirunique mechanical properties and very high aspectratios [1,2].

However, the CNT/ceramic composites developed upto now have shown much lower mechanical propertiesthan expected, and in some cases, even worse mechani-cal properties than those of the monolithic ceramicmatrices [3–7]. This is mainly due to the inhomogeneousdistribution of CNTs within the ceramic matrix and theweak interfacial bonding between CNTs and the matrix.

Several processes have been proposed to fabricateCNT/ceramic composites in order to improve the rein-forcement effectiveness of CNTs [7–10]. The most excit-ing progress is that reported by Zhan et al. [10]. Theysuccessfully achieved nearly fully dense Al2O3 nanocom-posites reinforced with 10% volume fraction single-wallCNTs (SWNTs) using the spark-plasma-sintering (SPS)process. The composites obtained had much enhancedfracture toughness. This was attributed to the high qual-ity components and rapid sintering technique used intheir study, which produced a reasonably homogeneousdispersion without damaging the nanotubes. But thetoughness in their study was measured using the Vickers

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indentation method, and also no direct evidence wasprovided to elucidate the reinforcement mechanismresponsible for the observed improvement in toughness.Wang et al. [11] recently reported that indentation meth-ods do not provide an accurate measure of the fracturetoughness in CNT-reinforced composites. So it is neces-sary to conduct macroscopic measurements of both thefracture toughness and fracture strength.

Although there have been considerable achievementsin terms of crystalline ceramic composites reinforcedwith CNTs, very few studies have been carried out forglass–ceramic matrices [12–14]. Ning et al. [13] foundthat incorporation of 5 vol.%CNTs could enhance thebending strength and toughness of the silicate glassmatrix by 88 and 146%, respectively. More recently,Boccaccini et al. [14] investigated the densificationbehavior and microstructural characterization of mul-ti-wall carbon nanotubes (MWNT)/borosilicate glasscomposites, but they did not report the mechanicalproperties of the composites obtained. The objective ofthis study was to improve the strength and fracturetoughness of a barium aluminosilicate (BAS) glass–cera-mic by reinforcing with MWNTs. Although MWNTsare not the best choice in terms of mechanical propertiesdue to the non-uniform axial deformations inside theMWNTs, they were selected for this study due to theirlower cost in relation to SWNTs. The mechanical prop-erties, microstructure and toughening mechanisms ofthe resulting composites were investigated.

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Table 1. The resultant properties of the sintered MWNT/BAScomposites

Materials Relativedensity(%)

Flexuralstrength(MPa)

Fracturetoughness(MPa m1/2)

BAS 100 84 ± 8 1.22 ± 0.055 vol.%MWNT/BAS 100 220 ± 10 2.31 ± 0.0810 vol.%MWNT/BAS 100 245 ± 11 2.97 ± 0.1015 vol.%MWNT/BAS 97 169 ± 16 2.12 ± 0.13

912 F. Ye et al. / Scripta Materialia 55 (2006) 911–914

The materials used in this study were BAS glass–ceramic composites reinforced with different volumefractions of MWNTs (from 5 to 15 vol.%). Startingmaterials were BAS glass–ceramic powder andMWNTs. The BAS glass–ceramic powder was synthe-sized through the hydrolysis of alkoxides [15]. TheMWNTs (supplied by Shenzhen NANO tech. PortCo., Ltd., China), fabricated by catalytic pyrolysis ofhydrocarbon, had dimensions of 60–100 nm in diameterand 5–15 lm in length. The MWNTs were first dispersedin ethanol using an ultrasonic bath for 2 h. BAS-ethanolslurry was added to the dispersed mixture and then thefinal mixtures were ball-milled in ethyl alcohol usingZrO2 balls for 12 h. The slurries were subsequently driedat 40 �C in a rotary evaporator. The dried powder mix-tures were hot-pressed in graphite dries at 1600 �C for1 h under a pressure of 20 MPa in a nitrogen atmo-sphere. BAS glass powder without any MWNT was alsohot-pressed at 1600 �C to investigate the effect ofMWNTs on the mechanical properties of BAS glass–ceramic. The densities of the samples were measuredusing Archimedes’ method in distilled water at 20 �C.

Fracture toughness and the flexural strength of thecomposites were measured in air at room temperature.All flexural bars were machined with the tensile surfaceperpendicular to the hot-pressing axis direction. Flex-ural strength measurements were performed on barspecimens (3 mm · 4 mm · 36 mm) using a three-pointbend fixture with a span of 30 mm. Fracture toughnessmeasurements were performed on single-edge-notchbeam specimens (SENB) with a span of 16 mm, and ahalf-thickness notch was made using a 0.1 mm thick dia-mond wafering blade. At least six specimens were testedfor each test condition.

Crystalline phases of the produced MWNT/BAScomposites were characterized by X-ray diffraction(XRD). Fracture surfaces and the crack propagationpaths (produced by Vickers indenter) were examinedby scanning electron microscopy (SEM). The micro-structures of the composites were characterized by trans-mission electron microscopy (TEM). Thin foil specimenstaken normal to the hot-pressing axis were prepared bydimpling and subsequent ion-beam thinning.

After sintering at 1600 �C for 1 h, near fully denseMWNT/BAS composites were achieved except for thecomposite with 15 vol.%CNTs, as shown in Table 1,

Figure 1. Transmission electron micrographs of 5 vol.%MWNT/BAS compboundaries (a), and directly bonded with BAS grains (b). The arrows indica

indicating that the BAS glass–ceramic served as an effec-tive liquid phase sintering aid to attain full densification.BAS has been proven to be an effective sintering aid tothe densification of ceramic composites [16,17]. In gen-eral, it is very difficult to fabricate dense ceramic com-posites with high CNT contents via a conventionalpowder process, because CNTs greatly inhibit the graingrowth of the matrix, which is detrimental to the mate-rial densification [8–10,13]. The dense compositesobtained were expected to possess superior mechanicalproperties.

The results of the XRD analyses indicated that noinformation on the CNTs was obtainable by this tech-nique, which is consistent with the literature [7,14] whichreported that XRD is not effective in revealing the pres-ence of CNTs. Only the hexacelsian BaAl2Si2O8 phasewas revealed in the sintered samples without any othercrystalline phases or non-crystalline phases, indicatingthe excellent crystallization capability of BAS glass.No monoclinic BAS was detected due to the sluggishtransformation of hexacelsian to celsian phase [18].

Typical TEM micrographs of the composites areshown in Figure 1. It can be seen that the CNTs havea good bond with the BAS matrix grains without anyobvio us interfacial reaction or amorphous layer, sug-gesting that the nanotubes were not damaged duringthe hot press-sintering. The MWNTs are distributedalong the BAS matrix grain boundaries (Fig. 1a), whichis quite similar to what was reported by Zhan et al. inSWNT/Al2O3 composites [10]. Intimate contact betweenCNTs and BAS grains was observed (Fig. 1b).

The three-point bending strength of the composites asa fraction of MWNT content is show in Figure 2a. Theflexural strength of the composites increases with theincrease in volume fraction of MWNTs from 5 to

osite showing that the CNTs were distributed along the BAS grainte the CNTs.

F. Ye et al. / Scripta Materialia 55 (2006) 911–914 913

10 vol.%. The addition of 10 vol.%MWNTs increasesthe BAS glass–ceramic matrix strength from 84 to

0 5 10 150

50

100

150

200

250

Fle

xure

str

eng

th, M

Pa

Volume fraction of CNTs, %

0 5 10 151.0

1.5

2.0

2.5

3.0

Fra

ctu

re t

ou

gh

nes

s, M

Pam

1/2

Volume fraction of CNTs, %

Figure 2. Variation of flexural strength (a) and fracture toughness (b)of MWNT/BAS composites as a function of MWNT content.

Figure 3. SEM micrographs of the typical fracture surfaces of MWNT/BASBAS composite.

Figure 4. Indentation crack propagation of 10 vol.%MWNT/BAS composi(indicated by white arrows); (b) is an enlarged micrograph of the section ma

245 MPa. This notable increase in strength (192%) ishigher than that for the BAS composites reinforced withthe same content of SiC whiskers [19], SiC platelets [15]or short carbon fibers [21]. It indicates that the load canbe effectively transferred from the BAS matrix toMWNT due to the good MWNT–BAS interfacial bond-ing. However, the strengthening effect of MWNTsreduces with a further increase in the MWNT volumefraction to 15%: the strength decreases from the245 MPa recorded for the 10 vol.%MWNT/BAS com-posite to 169 MPa, still a much higher strength thanthe monolithic BAS matrix though. The decrease ismainly attributed to this composite’s lower relative den-sity due to the agglomeration of CNTs.

The fracture toughness of the composites shows asimilar trend as the flexural strength, as demonstratedin Figure 2b. Incorporation of 10 vol.%MWNTs in-creases the fracture toughness of BAS glass–ceramicfrom 1.22 MPa m1/2 to 2.97 MPa m1/2 (143% increase),indicating the good toughening effect of the MWNTs.The toughening mechanisms could be seen clearly fromthe observations of the fracture surfaces and the inden-tation crack propagation paths, as shown in Figures 3and 4, respectively.

The fracture surfaces of the composites obtained afterflexural strength tests are shown in Figure 3. TheMWNTs were homogeneously dispersed within theBAS matrix in both 5 vol.%MWNT/BAS (Fig. 3a) and10 vol.%MWNT/BAS (Fig. 3b) composites. There area large number of pullout CNTs and residual holes leftby CNTs, indicating the presence of an ideal CNT–BASinterfacial structure suitable for crack deflection and thepullout mechanism. The extensive crack deflection and

composites: (a) 5 vol.%MWNT/BAS composite, (b) 10 vol.%MWNT/

te showing the bridging effect of MWNTs during crack propagationrked by white square in (a).

914 F. Ye et al. / Scripta Materialia 55 (2006) 911–914

CNTs pullout undoubtedly resulted in the increase infracture toughness. Since the elastic modulus of theCNTs is much higher than that of the BAS matrix, themodulus-load-transfer also increases toughness bytransferring stresses at a crack tip to regions remotefrom the crack tip, hence decreasing the stress intensityat the crack tip.

Figure 4 shows typical SEM micrographs of the crackpropagation paths produced by Vickers indentation. Itcan be observed that a large number of CNTs in thewake of propagation crack bridge the two crack surfaces(indicated by white arrows in Fig. 4), which stronglysupport the crack bridging effect during crack pro-pagation.

As stated above, the dense BAS glass–ceramic matrixcomposites with MWNTs also have mechanical proper-ties as high as those composites reinforced withwhiskers, platelets or short fibers. However, the effec-tiveness of using MWNTs for composites applicationhave been in doubt because of the very weak van derWaals forces between the outer and inner shell of theMWNTs [19]. Therefore, the results of this studystrongly support the reinforcement effects of MWNTs.The homogeneous distribution of CNTs in the matrixand the suitable interfacial bonding between CNTsand the matrix may be the most important factors ineffectively achieving the strengthening and tougheningof CNT/ceramic composites.

Dense MWNT/BAS glass–ceramic matrix compos-ites were successfully fabricated by hot-pressing. TheMWNTs were homogeneously dispersed within theBAS matrix. The incorporation of MWNTs improvedboth the flexure strength and fracture toughness of theBAS glass–ceramic matrix. The flexural strength andfracture toughness of the composites containing10 vol.%MWNTs were increased by 192% and 143%,respectively, compared to the BAS matrix. The maintoughening mechanisms were MWNTs pullout andbridging.

Supported by National Natural Science Foundationof China (Grant No. 50372014) and Program for NewCentury Excellent Talents in University China (GrantNo. NCET-04-0336).

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