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Composites Science and Technology 68 (2008) 1321–1327

SCIENCE ANDTECHNOLOGY

Hybrid nanocomposites: A new route towards tougheralumina ceramics

Kaleem Ahmad *, Wei Pan

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua

University, Beijing 100084, People’s Republic of China

Received 25 July 2007; received in revised form 2 November 2007; accepted 7 December 2007Available online 23 December 2007

Abstract

This paper describes a new class of alumina composites based on a novel hybrid microstructure design, in which multiwalled carbonnanotubes (MWNTs) and SiC nanoparticles are combined to give a new type system of ceramic nanocomposites. This design providesmutually redundant mechanisms to enhance the strength and toughness of the alumina matrix without deteriorating its intrinsic prop-erties such as hardness. The hybrid alumina composites with different MWNT contents (0, 5, 7, and 10 vol%) and 1 vol% of SiC werefabricated by spark plasma sintering. The resulting significant improvement in mechanical properties, specially the fracture toughness, issubstantiated by direct evidence for toughening mechanisms by MWNTs, observed for the first time in highly disordered MWNT/alu-mina composites.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Hybrid nanocomposites; A. Carbon nanotubes; B. Mechanical Properties; B. Fracture toughness; A. Ceramics

1. Introduction

In the last several years, different concepts for micro-structure design have been proposed to overcome theinherent brittleness of ceramics [1–3] but the predictedimprovements have not been achieved. Niihara first pro-posed a new design concept of ceramics and reported sig-nificant enhancement in the mechanical properties of SiCreinforced alumina composites [1]. The subsequent studieselsewhere showed similar, but lower increases in the bend-ing strength accompanied by small, if any, increases in thefracture toughness [4–7]. The improvements were attrib-uted to strengthening the grain boundaries by incorpora-tion of SiC fine particles [8–10]. The unique mechanicalproperties of carbon nanotubes (CNTs) [11,12] make themthe prime candidate as a high performance nanoscale rein-forcement in alumina for applications in various fields [13–

0266-3538/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2007.12.011

* Corresponding author. Tel.: +86 10 62795496; fax: +86 10 62771160.E-mail addresses: amd04@mails.tsinghua.edu.cn (K. Ahmad), panw@

mail.tsinghua.edu.cn (W. Pan).

15]. Several attempts were made to improve mechanicalproperties of alumina, specially the fracture toughnessusing carbon nanotubes [16–18]. However, the results weregenerally disappointing for multiwalled carbon nanotube(MWNT)/alumina composites [17,19–22], while a fewconflicting reports were published [18,23] for single wallcarbon nanotube (SWNT)/alumina composites. In thepresent microstructure design, the presence of CNTs atthe grain boundaries makes them mechanically weak[23,24], presumably due to agglomeration [16,23] as wellas existence of thermal residual stresses [25,26], whichresults in fracture along the grain boundaries [20,23,24,27]. For minimizing agglomeration different methodshave been used but the results are still not promising[17,19,20]. Recently, it has been reported that the uniquegrain boundary structures in CNT/alumina compositeshave a strong influence on the mechanical properties [24].However, at present no attempts have been made tostrengthen the grain boundaries by tailoring the design ofthe microstructure. In this work, we report a new class ofceramic nanocomposites, based on a hybrid microstructure

Fig. 1. Schematic of hybrid microstructure design of alumina reinforcedby MWNTs and SiC nanoparticles.

1322 K. Ahmad, W. Pan / Composites Science and Technology 68 (2008) 1321–1327

design of alumina reinforced by SiC nanoparticles andMWNTs. Unlike the previous microstructure designs, thehybrid microstructure design provides a mutually redun-dant toughening mechanism. First, by strengthening thegrain boundaries and toughening the matrix by nanosizeSiC particles and second, by fiber toughening mechanismsthrough MWNTs. In addition, the incorporation of SiCnanoparticles in alumina results in removal of residualstresses at the grain boundaries [9,10,28], as residual stres-ses are undesirable to optimize the cracking behavior inMWNT/alumina composites [25,26]. The best resultreported so far for MWNT/alumina composites is 80%improvement in the fracture toughness at the cost of 4%decrease in bending strength [16]. Whereas for SWNT/alu-mina composites, a threefold improvement in the fracturetoughness at the expense of 20% decrease in hardness hasbeen reported [27]. However, in each case, the enhancedtoughening was not supported by key evidence of potentialtoughening mechanisms. Several researchers have disputedthe threefold improvement as an overestimation [18,23,29].This necessitates that any significant enhancement must besupported by direct evidence of key toughening mecha-nisms [23,29,30]. In contrast to these previous works, inthe present study for the first time, the observed significantenhancement in the fracture toughness has been substanti-ated by direct evidence of fiber toughening mechanisms inhybrid alumina composites through a nanoscale dispersionof highly disordered MWNTs. The nanocomposites haveshown an improvement not only in the fracture toughness(�117%) but also in the bending strength (�44%), while thehardness remains almost unchanged.

2. Experimental

MWNTs were dispersed carefully to ensure maximumhomogeneity of the composites using the process describedby Zhan et al. [27,31]. In brief, the starting powder alumina(99.9% in purity, Chong Qing Tuo Yuan, China) was ultra-sonically mixed with different (0, 5, 7, and 10) volume (vol)percentages of MWNT (Green Chemical Reaction Engi-neering and Technology, China) along with 1 vol% ofnanosize SiC (99.9% in purity, Shijia Zhuang High-techCeramics Material, China) particles in ethanol. For furthermixing ball milling was performed for 24 h. After drying,powder mixtures were spark plasma sintered (Dr. Sinter1050, Sumitomo Coal Mining Co., Japan) at 1550 �C invacuum under a pressure of 50 MPa in a 20 mm innerdiameter cylindrical graphite mold. X-ray diffraction anal-ysis showed absence of any new phase. The relative densi-ties of sintered samples were measured by the Archimedes’smethod. The fracture toughness was measured by directtoughness measurement technique i.e. single edge notchbeam method on rectangular bars with size of about3 mm � 2 mm � 15 mm containing a pre-notch of length61.5 mm and width �0.25 mm at cross head speed of0.05 mm/min. The bending strength was measured by threepoint bending test on rectangular bars with size of about

2 mm � 3 mm � 12 mm at cross head speed of 0.5 mm/min. These tests were performed at room temperatureusing a universal testing machine (Shimadzu Servo PulserEHF-EG50KNT-10L, Japan). Four bars were tested foreach material and the mean value and standard deviationswere obtained. The hardness was measured by Vickersindentation method with a load of 5 kg for 15 s. Crackson polished surfaces were induced by Vickers indent.

3. Results and discussion

In hybrid design, the microstructure of the nanocompos-ite is formulated by dispersing second phase nanosize SiCparticles within the matrix grains and on the grain bound-aries present as intra/intergranular reinforcements whileCNTs are present at grain boundaries as intergranular rein-forcement as shown schematically in Fig. 1. The thermalexpansion mismatch between alumina and SiC nanoparti-cles produces an improvement in the fracture toughness,bending strength, and hardness in addition to improvementby MWNT. The nanosize SiC particles relieve the residualstresses in the matrix and along the grain boundaries bygenerating the dislocations around the particles [8,10]. Thisreduces the defects on the grain boundaries and strengthens

K. Ahmad, W. Pan / Composites Science and Technology 68 (2008) 1321–1327 1323

the grain boundaries by impeding the intergranular frac-ture mode of alumina. SiC has lower thermal expansioncoefficient than alumina so during cooling an embeddedSiC nanoparticle is subjected to radial compression, whilethe immediate surrounding matrix is in radial compressionalong with a positive hoop tension (Fig. 1). The elasticenergy associated with each SiC particle is insufficient toinitiate and propagate microcracks per se. However, anyexisting crack either resulting from processing flaws orotherwise presents that can propagate towards the SiC par-ticle due to internal tangential tension would be blunted bythe particle. While, the ropes like intertwining networks ofMWNT at the grain boundaries bridge the crack propaga-tion at the intergranular positions through fiber tougheningmechanisms. As a result, the networks restrict the fractureof grains and improve the toughness of the composites(Fig. 1). This new hybrid class of ceramic nanocompositeprovides mutually redundant mechanisms to improve thestrength and toughness of alumina matrix.

Four composites were fabricated by spark plasma sin-tering (SPS) at 1550 �C using 0, 5, 7 and 10 vol% ofMWNT, while SiC was kept at 1 vol%. It is worth mention-ing that CNTs have been proven stable under SPS at 1700–2000 �C [32,33], so any damage or change in the structureof CNTs is not expected to occur at 1550 �C, as reportedby several studies [22,23,30]. 1 vol% of SiC is sufficient tochange the fracture mode of alumina from intergranularto transgranular [34,35], as one of the main toughening

Fig. 2. (a) MWNTs (5 vol%) are uniformly distributed in hybrid alumina com10 vol% of MWNT with 1 vol% of SiC reinforced alumina composites. (d) HRinset shows interface between the particle and the matrix.

mechanisms in SiC/alumina composites is to change thefracture mode, which implies a reinforcement of grainboundaries [36,37]. The second advantage of SiC is thatradial cracks were observed by Vickers indentation. Thecracks in samples not containing SiC could not be observeddue to accommodation of shear deformation of CNTsunder the indenter [23,25,29,30]. These cracks helped touncover the actual mechanism for toughening by providingthe direct evidence through MWNTs in the nanocompos-ites. The third benefit is that the hardness is almost unaf-fected. Further improvements may result with high vol%of SiC, however the presence of SiC particles lowers thesintering rate [37,38]. Therefore, higher sintering tempera-tures are required to obtain the fully dense materials andthat may have adverse effects on carbon nanotubes.

The proposed hybrid microstructure design (Fig. 1) canbe inferred from the high-resolution scanning electronmicroscopy (SEM) characterization of hybrid alumina com-posites (Fig. 2). The MWNTs (5 and 7 vol%) are uniformlydistributed in alumina matrix (Fig. 2a and b). The stronglyentangled ropes of MWNT with the alumina matrix, alsoreported by others [24,27], appear to envelop the aluminagrains like a net (Fig. 2b and c) as shown schematically inFig. 1. The fracture mode is mixture of intergranular andtransgranular (Fig. 2a) due to addition of SiC, while a purelyintergranular fracture mode was observed in CNT/aluminacomposites without SiC [23,27]. Fig. 2d shows that there is agood adhesion between alumina and SiC nanoparticles that

posite. (b, c) Networks of MWNT around the alumina grains in 7 andTEM picture shows typical microstructure of SiC/alumina composite and

Fig. 3. Relative density (RD) and mechanical properties of all samples.

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may leads to change in fracture path from intergranular totransgranular [37,39] and strengthened the grain bound-aries. Fig. 2a suggests that the microstructure can be bestdescribed by the hybrid microstructure design perceived ide-ally in Fig. 1. CNTs are present at the intergranular posi-tions and transgranular fracture mode indicates that theSiC particles are present at intra/intergranular positions asreported by several studies [2,8,37]. From these images(Fig. 2) we can conclude that the hybrid microstructureshown ideally in Fig. 1 has been achieved.

The relative densities of 0, 5, 7, and 10 vol% of MWNTalong with 1 vol% of SiC of each alumina compositereached at 99.5%, 98.2%, 97.2%, and 95.1%, respectively(Fig. 3). SiC inhibits densification of alumina [38]. Theincrease of MWNT contents may have negative effects onthe densification as observed in case of SiO2 [40]. Addition-ally, some clustering at 10 vol% of MWNT also affecteddensification adversely. The 1 vol% SiC/alumina compositewithout carbon nanotubes shows good improvement in thebending strength along with moderate increase in the frac-ture toughness over monolithic alumina. In pure alumina,grain boundaries contain tensile residual stresses resultingfrom anisotropic thermal expansion. Therefore, large crackalong the grain boundaries created by residual stressesdominates the strength and consequently, alumina exhibitsintergranular fracture mode. In case of SiC/alumina com-posites, the thermal expansion mismatch between aluminaand SiC nanoparticles play an important role in strength-ening and toughening the matrix. The addition of nanosizeSiC particles yields dislocations around the particles whichrelieves residual stresses in the matrix and along the grainboundaries and thus reduces the flaw size along the grainboundaries [10]. The increase in bending strength by addi-tion of 1 vol% of SiC may be attributed to decrease in crit-ical flaw size and reduction of tensile residual stresses in thematrix as reported earlier [2,9,10]. The intragranular frac-ture mode indicates the grain boundary strengthening bynanosize SiC particles and improves the bendingstrength[9,36]. The toughness enhancement in 1 vol%SiC/alumina composites may be ascribed to change in frac-ture mode from intergranular to transgranular, and crackdeflection due to internal stress around the SiC particles [2].

The addition of 5 vol% of MWNT has significantlyincreased the fracture toughness of hybrid alumina com-posite, while the bending strength has decreased slightly.When we increase the vol% of MWNT from 5 to 7, thefracture toughness also increases while other propertiesremain almost same (Fig. 3). The increase in the fracturetoughness with the increase of CNT vol% as reported ear-lier [19,27] indicates that fiber toughening by MWNTs isdominant. The better dispersion of MWNTs resulted inimproved density and mechanical properties of 5 and7 vol% of MWNT reinforced hybrid alumina composites(Fig. 3). However, for 10 vol% MWNT reinforced hybridalumina composite, the improvements are not very signifi-cant due to agglomeration of MWNTs. As the vol%increases to 10, the probability of agglomeration also

increases that result in low densification and loose bonding.Consequently, when the stress transfers to CNT agglomer-ates, they are easy to separate from the matrix due to poorload carrying capability that deteriorates the mechanicalproperties. The increase in agglomeration with the increaseof CNT vol% was also reported by others [19,27,40]. Addi-tionally, a decrease in bending strength was reported due toagglomeration for 12 vol% MWNT/alumina composites[16]. Zhan et al. showed a sharp decrease in hardness withthe increase of fracture toughness [27], while some otherauthors have reported an increase in hardness, but in their

K. Ahmad, W. Pan / Composites Science and Technology 68 (2008) 1321–1327 1325

work there is no consistent relationship between hardnessand fracture toughness [19,41]. Zhan et al. reported 9.30GPa hardness value for 10 vol% SWNT/alumina compos-ite [27], while in our study, at the same densification level(relative density �95.2%), and MWNT vol%, the hardnessof the nanocomposite is �14.45 GPa. The decrease in hard-ness is very low due to addition of SiC.

The transmission electron microscopy in some studies[22,27], has already shown that the interface betweenCNT graphene wall and alumina appears to be sharp,which in turn suggests a good bonding between them[22,27]. The good bonding between MWNTs and aluminaas well as the presence of a network structure of MWNTsat intergranular positions as reported in several studies[24,31] may result in strengthening and toughening of thenanocomposites by MWNTs. There are possibly two fac-tors complementing each other in toughening and strength-ening the alumina matrix. Firstly, the low vol fraction ofSiC nanoparticles that is strengthening the grain bound-aries and impeding the intergranular fracture mode. Sec-ondly, the homogenous mesh formation of MWNT(Fig. 2b and c) ropes around the alumina grains, whichobstruct intragranular fracture mode. In addition, theSEM observation of crack bridging, crack deflection, andMWNT pull outs provide direct evidence of fiber toughen-ing by the MWNTs (Fig. 4). Fig. 4a and b shows toughen-ing of the matrix by crack bridging and crack deflection

Fig. 4. (a) Crack bridging and crack deflection by continuous mesh of MWNTsClose up of selected area in Fig. 4a. (c) Crack bridging by single MWNT inMWNT pull outs in 7 vol% of MWNT and 1 vol% of SiC reinforced alumina

mechanisms. The crack bridging by MWNTs restrain thecrack opening and reduces the driving force for crack prop-agation. This is evident as the crack opening size is dimin-ishing gradually by the mesh of MWNTs and crack path istortuous due to crack deflection by the MWNTs (Fig. 4a).The marked rectangular area (Fig. 4a) has been magnifiedin Fig. 4b and shows that MWNTs remain intact to bridgethe crack by providing sufficient toughening. The debond-ing occurs at the atomic scale and the work requires pullingMWNTs out against residual sliding friction at the inter-face imparts significant fracture toughness to the aluminamatrix. The crack bridging and crack deflection byMWNTs lead to improvements in extrinsic and intrinsictoughness of the alumina matrix, respectively [2]. The pullouts (Fig. 4d) show that MWNTs bear significant stressesby sharing the portion of the load. These pull outs are moreconspicuous due to load transfer of alumina matrix to theouter shell of MWNTs. Xia and colleagues also reportedtoughening mechanisms on specially designed highlyordered parallel array of MWNTs on amorphous nanopor-ous thin alumina matrix (20 and 90 lm thickness) [25].However, the overall composite toughness was difficult tomeasure due to complex nature of residual stresses andcrack bridging [23,25]. It is the first time to expose theunderlying toughening mechanism in highly disordered realsituation. Our study shows fracture toughness improve-ment �117% and increase in bending strength �44% for

in 5 vol% of MWNT and 1 vol% of SiC reinforced alumina composite. (b)5 vol% of MWNT and 1 vol% of SiC reinforced alumina composite. (d)composite.

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1 vol% of SiC and 7 vol% of MWNT reinforced aluminacomposite, while keeps hardness almost unchanged.

4. Conclusion

In summary, alumina composites reinforced by 0, 5, 7,and 10 vol% of MWNT with concurrent reinforcement of1 vol% of SiC nanoparticles were fabricated by sparkplasma sintering based on hybrid microstructure design.Significant improvements in mechanical properties havebeen attributed to the synergetic effects of MWNT andSiC in strengthening and toughening the alumina matrix.The standard toughening mechanisms in fiber reinforcedceramic matrix composite have been observed by MWNTsat the nanoscale. Low vol% of SiC and MWNT can beused successfully to improve the mechanical properties ofalumina specially, the fracture toughness without affectingother intrinsic properties. This is a preliminary study moredetailed work is required to evaluate quantitatively thestrength and toughness imparted by SiC particles andMWNTs in the hybrid nanocomposites.

Acknowledgements

We thank National Natural Science Foundation of Chi-na (Grant Nos. 50232020 and 50572042) and Higher Edu-cation Commission of Pakistan for financial support.

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