IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 19 (2008) 165701 (8pp) doi:10.1088/0957-4484/19/16/165701

Wetting of carbon nanotubes by aluminumoxideKantesh Balani and Arvind Agarwal

Department of Mechanical and Materials Engineering EC 3464, 10555 W Flagler Street,Florida International University, Miami, FL-33174, USA

E-mail: agarwala@fiu.edu

Received 15 January 2008, in final form 4 February 2008Published 20 March 2008Online at stacks.iop.org/Nano/19/165701

AbstractMost ceramic–carbon nanotube (CNT) composite processing utilizes solid state sintering, hencethe concept of wetting of CNTs by molten ceramic is absolutely new. In the present work on aplasma sprayed Al2O3–CNT nanocomposite, it is observed that molten Al2O3 spreadsuniformly on the CNT surface by forming a thin (∼20–25 nm) ceramic layer without anycracks. The wettability of the Al2O3–CNT system is associated with the surface tension andcapillary forces as captured from the evolution of microstructure. The dynamic equilibriumbetween melting and solidification of Al2O3 was deduced from the meniscus height, curvature,contact perimeter and projection area of solidified Al2O3 on the CNT surface. This interfacialphenomenon illuminates the mechanisms of microstructure evolution from Al2O3-coated CNTbridge structures to CNT mesh formation. Consequent ab initio modeling depicted distortediso-surface contours at the interface, suggesting partial bonding and good wettability of Al2O3

on the CNT surface.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Ceramic–CNT systems have been studied to obtain novelnanocomposites with improved fracture toughness [1–3] andelectrical conductivity [4]. The toughening behavior ofthe CNT-reinforced matrix is mainly linked to CNT crackbridging, CNT dispersion and crack deflection. The roleof the interface in toughening the nanocomposite via stresstransfer between in situ grown CNTs in an Al2O3 matrix hasbeen emphasized [5, 6]. The wetting behavior between theceramic and the CNTs could lead to the generation of newinterface(s) that may alter the load transfer and tougheningmechanism [3, 4, 7–10]. However, wettability studies onceramic–CNT composites are non-existent in the literatureowing to the limitations of solid state processing (namely hotpressing, sintering, and spark plasma sintering) utilized forthese studies [7, 11–15]. In this work, the wetting behaviorbetween aluminum oxide and multi-walled CNTs has beenstudied in plasma sprayed nanocomposite. Understandingof such a concept can extend the potential application ofoxide-coated CNTs for use as sensors, strong field emitters,electrically insulated CNTs in nanocircuits and highly toughceramic nanocomposites [1, 3, 8, 15].

It is extremely difficult to undertake liquid droplet anglestudies in a ceramic–CNT system because of the very highmelting points (e.g. 2333 K for Al2O3, and 3773 K forCNTs) [3, 8]. Since the surface is the sole contact with thesurrounding environment, fundamental concepts of capillarityand surface tension are the only direct measures in definingwettability [16]. Thereby, a theoretical model is constructedin correlating the wettability to the evolved microstructureof plasma sprayed Al2O3–CNT nanocomposite. The aim ofthis research is to understand the underlying wettability viarevealed microstructure and ab initio molecular simulation ofthe Al2O3–CNT interface.

2. Material and methods

As-received Al2O3 powder (particle size of 150 nm, andcrystallite size of ∼40 nm, obtained from Inframat®

Corporation, Farmington, CT) and 4 wt% multi-walled CNTs(obtained from NanoAmor, Houston, TX, with 95%+ purity,OD 40–70 nm, 0.5–2.0 μm in length) were commercially spraydried for improved CNT dispersion and enhanced powderflowability. Composite spray drying of Al2O3–4 wt% CNTresulted in spherical agglomerates of 15–60 μm (referred

0957-4484/08/165701+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

CNTs

Figure 1. SEM image of spray dried Al2O3–CNT agglomerate. Theinset shows the CNT dispersion in the powder agglomerate.

to as A4C-SD powder feedstock), figure 1, which are idealfor plasma spraying. The CNTs are uniformly dispersed,as seen in the inset of figure 1, in the powder agglomerate(at both the surface and core) [1]. Subsequently, A4C-SDpowder was plasma sprayed on AISI 1020 steel substrate withplasma parameters described in our earlier publication [1]. AnAccuraSprayTM (Tecnar Automation Ltee, QC, Canada) sensorwas used to measure the temperature and velocity of the in-flight particles during plasma spraying. Consequently, thedwell time and cooling rates are calculated from the time-of-flight and temperature of the in-flight particles. Microstructuralcharacterization of plasma sprayed A4C-SD coating wasperformed using field emission scanning electron microscopy(FESEM, JEOL JSM 6330F). SEM images were used formeasuring the meniscus height, curvature, contact perimeterand projection area of solidified Al2O3 and subsequentlycalculating the surface forces acting on the Al2O3–CNTsystem. The SIESTA 1.3 (Spanish initiative for electronicsimulations with thousands of atoms) simulation packagewas used for molecular modeling of Al2O3–CNT interfacialanalysis.

2.1. Ab initio molecular modeling of the aluminumoxide–CNT interface

SIESTA 1.3 modeling scales was operated on the Al2O3–CNT interface, created with a 1 × 1 × 1 crystal lattice ofα-Al2O3 interfacing 2 × 2 × 2 crystal layers of graphite.A plane wave basis set was used for the interfacial system,limiting in the z-direction and generating periodicity in the x–ydirection with a cell size of 4.928 × 4.928 × 26.4114 A

3and

α = 90◦, β = 90◦, and γ = 120◦. The standard Kohn–Sham self-consistent density functional was utilized with anLCAO (linear combination of atomic orbitals) basis set inlocal density approximations. The spin polarized Ceperly–Adler scheme (Perdew and Zunger) was used for definingthe Al, O and C exchange correlation functionals. ImprovedTroullier–Martins pseudopotential generation was employed todescribe nonlocal, and norm conserving interaction between

core and valence electrons. First-principle pseudopotentialswere generated from spin polarized non-relativistic groundstate components of Kleinman and Bylander projectors. Al wasdefined with ground state 3s23p13d0 with cutoffs 1.86, 2.25and 3.07 Bohr respectively [17], O as 2s22p43d04f0 groundstate with cutoffs 1.15, 1.15, 1.15, and 1.15 Bohr respectively,and C as 2s22p2 ground state with cutoffs 1.50 and 1.54 Bohrrespectively [17, 18].

The localized spin density (LSD) Hamiltonian wascalculated by matrix diagonalization to generate a self-consistent Kohn–Sham solution. The conjugate gradient (CG)method was used for coordinate optimization with limitingforce of 0.05 eV A

−1or 50 iterations, whichever came first.

Maximum displacement during the CG optimization run waslimited to 0.2 Bohr.

3. Results and discussion

3.1. Background of CNT toughening in plasma sprayedAl2O3–CNT nanocomposite

Composite spray drying of Al2O3–4 wt% CNT (A4C-SD)resulted in uniform dispersion of CNTs in the plasma sprayednanocomposite coating [1]. The role of CNT dispersion inenhancing the fracture toughness is described in our earlierwork [1], depicting an increase of fracture toughness up to43% (to 4.60 ± 0.27 MPa m1/2 in A4C-SD coating) whencompared to that of plasma sprayed Al2O3 without CNTreinforcement (3.22 ± 0.22 MPa m1/2). Enhancement of thefracture toughness was attributed to CNT chain-anchoring,CNT bridge formation, CNT meshing, and crack deflection [1],which largely depend on the improved wetting of the CNTsby Al2O3 [1]. Hence elucidation of the CNT wettingcharacteristics in strengthening the Al2O3–CNT ceramicsholds the key to enunciating the enhanced fracture tougheningof the nanocomposite.

3.2. Wettability and modeling of Al2O3 on the CNT surface

Figure 2 shows a CNT bridge that forms an anchor betweentwo plasma sprayed splats. Since the diameter of the as-received CNTs was 40–70 nm, thickening of the diameter to∼120 nm indicates the formation of an Al2O3 layer (∼20–25 nm thick) on the CNT surface. The neck formation at twosplats and uniform thickness indicate excellent Al2O3 wettingon the CNT surface. No cracking is observed in the thin (∼20–25 nm) layer of Al2O3 coating on the CNT surface. It canbe inferred that the nanolayer of Al2O3 exhibits ductility at asmall length scale. This reinforcement by individually coatedCNTs (by tough Al2O3 layer) thereby scales to enhancedtoughening of the ceramic nanocomposite because of enhancedCNT wetting to provide anchoring sites [19].

A system of Al2O3 wetting on the CNT surface is definedthrough figure 3. In considering the theoretical modeling, itis assumed that molten Al2O3 spreads onto the CNT surfaceby capillary action, where it freezes instantaneously due to therapid cooling rates (∼4.6 × 106 K s−1) associated with theplasma spraying [1]. The spreading of molten Al2O3 and itsdynamic freezing depend on the surface tension. The resulting

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Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

Al2O3 coated CNT Bridge

Figure 2. CNT bridge structure showing the increase in CNTdiameter because of uniform Al2O3 coating on the surface. Neckformation at the two ends indicates wetting between the CNT andAl2O3.

microstructure (figure 2) is taken as the representative modelfor surface forces towards defining wettability. Since constantequilibrium contact angles are not experimentally observedowing to changing ‘true’ contact angles, it is a valid assumptionthat dynamic freezing is a representative model [20]. Directsurface forces acting on the molten Al2O3 surface can bedefined by surface tension and capillarity to describe theevolution of microstructural features. The capillary force fc

is defined as follows [21]:

fc = γ (cos θ1 + cos θ2)�/hm (1)

where γ is the surface energy (taken as ∼1.59 J m−2 forthe stable Al-terminated Al2O3 surface), � is the surfaceprojection area and hm is the meniscus height as shownin figure 3. Higher meniscus infers reduced capillarity,but increased surface tension leads to enhanced wetting ofCNTs by Al2O3. The surface tension force TS is defined asfollows [22]:

TS = lγ cos α (2)

where l is the perimeter contact, and γ cos α is the verticalcomponent of the surface tension (figure 3). Surface tensionis caused by the difference in the magnitude of surfaceforces where a difference in the forces of adhesion andcohesion results in wetting/dewetting of liquid droplets ontothe substrate.

Owing to the rapid kinetics involved in the plasmaspraying (the particle residence time in plasma is ∼4.1 ×10−4 s), it becomes extremely difficult to track and timeresolve the sequential progress of CNT wetting by Al2O3.Moreover, non-uniform heat is experienced by the powderagglomerates due to the Gaussian distribution of the plumeintensity. Variation in the heat experienced and trajectoryadopted by powder particles would also lead to variationin the wetting characteristics and microstructural evolution.Hence, it can be safely concluded that all the stages ofwetting can be captured from the microstructural images ofthe plasma sprayed Al2O3–CNT nanocomposite. Therefore,based on the microstructural snapshots and surface forcesexperienced by the in-flight powder particles, a sequentialwetting mechanism is proposed for the Al2O3–CNT system;see figure 4. Though the detailed explanation follows, theepitome of the wetting starts with flowing of Al2O3 particles(attached to the CNT surface) upon its heating and meltingduring plasma spraying. Consequently, molten Al2O3 coats theCNT surface and brings other coated CNTs closer because ofsurface tension. The interplay of capillarity and surface tensionresults in the evolution of differential microstructure (figure 5).Theoretical calculations performed on the system are presentedin table 1. The CNT diameter is assumed to be 70 nm andthe approximate angle of contact is calculated from the SEMmicrograph (figure 2). Meniscus height, perimeter contact andsurface projection area are the dominating factors which definethe capillary force and surface tension occurring at the Al2O3-coated CNT interface. Several values of meniscus height areassumed to theoretically calculate the perimeter contact andthe surface projection area. It must be mentioned that when themeniscus height is 35 nm (for a 70 nm diameter CNT), thereis no surface to support the top meniscus vertically. Hence,when α = 0, the increase of θ1 reduces the capillary force.The presence of a similar horizontal force (defined by θ2,

Figure 3. Theoretical representation of CNT wetting by a molten Al2O3 layer.

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Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

CNT

Al2O3

i) Al2O3 particles on CNT surface ii) Upon heating molten Al2O3

flows and coats the CNT

iv) Capillarity draws the coated-CNTs closer.

iii) Molten Al2O3 holds the coated-CNTs because of surface tension.

v) Many coated-CNTs come in contact and are drawn closer.

vi) Dynamic freezing of partially melted Al2O3

entraps CNTs to form trimodal microstructure. vii) Further seeping down of molten Al2O3 by

capillarity leaves behind CNTs mesh.

CNT mesh

Figure 4. Proposed snapshots of sequential CNT wetting by Al2O3.

0

50

100

150

200

250

300

0 20 40 60 80Meniscus Height (nm)

Su

rfac

e T

ensi

on

Fo

rce

(nN

)

8

9

10

11

12

13

Cap

illar

y F

orc

e (n

N)

Surface Tension Capillary Force

Al2O3

Coating on CNT CNTEntrapment in

Al2O3

Stage 3

Stage 1

Stage 2

CNT Mesh Formation

Surface forces on CNT-Al2O3 interface

Figure 5. Surface forces on a 70 nm diameter CNT with aluminumoxide meniscus.

with reduced vertical force) in such a situation delivers theminimum value of capillary force as we approach ∼35 nm(figure 5). Though the capillary forces appear much smaller,capillarity is also assisted by processes like gravity, impactspreading during deposition, and the presence of surfaceroughness [16].

Figure 5 describes various possible interactions inassimilating interfacial capillarity and surface tension with

respect to meniscus height of Al2O3 over CNTs in differentialmicrostructures. The meniscus height will vary dependingon the time and temperature experienced by the powderagglomerates during plasma spraying. As the meniscus heightincreases, the surface tension of molten Al2O3 is sufficient tocounter the capillary force [23]. Similarly, a low meniscusheight suggests ease of holding down molten Al2O3 rather thanallowing it to rise as a coating over the CNT. It must be clarifiedthat seeping down of Al2O3 can occur even at a later stagewhen the mass of molten Al2O3 dragged over the CNT cannotbe supported by the surface tension. The process is dividedinto three stages to elicit evolution of the microstructure arisingfrom the wetting of CNTs by Al2O3.

(a) Surface tension dominated region (stage 1).(b) Intermixed mode (stage 2).(c) Capillarity dominated region (stage 3).

Stage 1 is characterized by the occurrence of high surfacetension forces (∼236.2 nN), wherein molten Al2O3 freezesonto the CNT to form a coating; see figure 2. The processhas just enough time to allow quick flow of molten Al2O3

over CNTs via capillary action, and cause consequent freezingof Al2O3 as a coating. Though CNTs assist seeping downof molten Al2O3 by capillary flow, the surface tension forcedominates in the reduction of surface energy by holding moltenAl2O3 as a thin nanolayer over the CNT surface. Increase

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Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

Table 1. Theoretical calculations of capillary force and surface tension on the Al2O3–CNT interface (CNT diameter assumed to be 70 nm).

Meniscusheight(hm) (nm) θ1 (deg) θ2 (deg) α (deg)

Perimetercontact,(l) (nm)

Surfaceprojectionarea (�) (nm2)

Surface energy,(γ = 1.59 J m−2)

Capillaryforce( fc) (nN)

Surfacetension force(Ts) (nN)

10 16.39 13.34 64.29 30 40 1.59 12.61 40.425 15.36 18.85 25.72 75 90 1.59 11.32 116.330 24.20 17.86 0.00 105 110 1.59 9.83 167.050 14.04 15.64 −38.57 150 160 1.59 10.09 225.170a 14.87 16.32 −90.00 210 195 1.59 8.78 236.2

a Represents observed value from the micrograph, figure 1.

CNTs Entrapped in FM Region

CNT Fused With Surface

Figure 6. Intermixed mode showing CNTs entrapped in Al2O3

(stage 2).

in the CNT diameter indicates enhanced surface wetting bymolten Al2O3 and consequent coating of Al2O3 on the CNTsurface. This bridge-like structure improves the fracturetoughness of the nanocomposite by serving as anchors to theadjoining splats. The intermixed mode (stage 2) is describedby the entrapment of CNTs along the partially melted andsolidified Al2O3 regions, where wetting by Al2O3 is justoccurring, and there is just sufficient time to entrap a fewCNTs before they can merge as a mesh. On the one handwhere the surface tension holds the molten Al2O3 coating layeron the CNT surface, capillary forces balance the seeping ofmolten Al2O3. Consequent dynamic freezing captures thefully melted and resolidified Al2O3 region, partially meltedand resolidified Al2O3 region and entrapped CNTs, generatinga trimodal microstructure. Both the capillarity and surfacetension of Al2O3 play a key role in generating such a trimodalmicrostructure, figure 6, which is beneficial in enhancingthe fracture toughness of the nanocomposite. Tougheningenhancement occurs because of (i) second phase strengtheningby Al2O3 nanoparticles and CNTs, (ii) structural strengtheningby fully molten Al2O3 regions, (iii) energy absorption atpartially molten regions, and (iv) ductility enhancement bygrain sliding of nanoparticles.

Once the molten Al2O3 flows onto the CNT surface, thesurface tension of the liquid attracts the surrounding Al2O3-coated CNT and merges them together. After enough Al2O3-surfaced CNTs are accumulated together, it becomes difficultto hold molten aluminum oxide by surface tension alone.

CCNNTT MMeesshh

Figure 7. Capillarity dominated CNT mesh (stage 3).

Owing to the increased volume of surrounding liquid (moltenAl2O3), the capillarity reduces the surface tension by itsseeping out molten Al2O3 and leaving the CNT structure asa mesh; see figure 7 (stage 3). Oozing of molten Al2O3

from the CNT surface is assisted by factors such as impactflow, CNT capillarity, surface roughness, and gravity forces.The network of CNT mesh serves as an intertwined structuredfabric in enhancing the fracture toughness by bearing stressand providing two-directional sledging. This mesh has a verythin layer (a few nanometers) of molten Al2O3, which in a truesense is a reinforcement by flow of Al2O3 ceramic onto theintricate surface of the CNT. Thickening and smoothening ofCNTs is visible in figure 7.

Hence, excellent wettability of Al2O3 on CNT surfacesis observed via formation of Al2O3-coated CNTs and CNTmesh formation (figures 2 and 7). It is re-emphasizedthat no cracking was observed in the nanolayer of Al2O3

coated on CNTs. Rapid solidification kinetics, inherent toplasma spraying, might be reasoned to attribute the enhancedwettability observed in the current work. The nucleationfrequency (Iυ ) of Al2O3 on the CNT surface at a temperature(T ) can be expressed as follows [24]:

Iv = Kv exp

(−�G∗ f (θ)

kT

)(3)

where Kv is the kinetic parameter, θ is the wetting angle,�G∗ is the excess free energy of the critical nucleus, andk is the Boltzmann constant. The enhanced excess free

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Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

Aluminum Oxygen

Carbon

Plane A(Below Interface)

Plane B (Graphitic Interface)

Plane C (Al2O3 Interfacial Layer)

Plane D (Interface Pseudo Plane)

ab

d

c

Figure 8. Al2O3–CNT system defined in ab initio computational modeling. (a) Uninfluenced graphitic planes below the interface, (b) nearinterface graphitic planes with arrowheads indicating distortion of iso-surface energy contours, (c) Al2O3 interfacial layer showingAl-terminated atoms with arrow heads, and (d) distortion of iso-surface energy contours at the pseudo-interface indicating partial bonding.

energy for nucleation is balanced by the reduction in thewetting factor f (θ ) because of the rapid solidification kineticsin plasma spraying. The enhanced wettability (of Al2O3

on CNT surfaces) by the reduction of the wetting angle isreinforced by the reduction in the wetting factor [25] (givenby the expression { f (θ) = (2 + cos θ)(1 − cos θ)2/4}).Heterogeneous nucleation allows further wetting of the CNTsurface by rapid solidification of Al2O3 by reducing the excessenergy barrier of nucleation [16, 25].

3.3. Thermodynamics and molecular modeling of theAl2O3–CNT interface

Capillarity and surface tension may be influenced by theinterfacial interaction between Al2O3–CNT. Hence, it becomescritical to visualize the molecular phenomenon at such a level.The surface energy of O-terminated Al2O3 is in the range4.45–10.83 J m−2, whereas Al-terminated Al2O3 possesses asurface energy of 1.59 J m−2 [14]. Though the CNT surfaceis very stable with a surface energy of 0.2 J m−2, nanosurfacesoften are complicated and depict non-intuitive behavior. Al–graphite can lower the surface energy to the 0.02–0.4 J m−2

range, making a strong possibility of a stable system [26].Ooi’s molecular modeling of Al–graphite depicted no bondingat the interface [26]. In addition to the absence of oxygen inthe system, representation of a specific cross-sectional plane ofthe Al–C interface showed that overall bonding at the interfacewas absent [26].

Enhanced wetting raises a question of interface reactionbetween Al2O3 and C, with the possible reaction presentedin (4).

2Al2O3 + 6C → Al4C3 + 3CO2. (4)

The very low activity of Al4C3 (∼6.8889 × 10−19 at2200 K, near the melting point of Al2O3) was obtainedfrom FactSage thermochemistry software [27]. Moreover CO(during partial reduction) and CO2 product gases will tendto destabilize the interface. The free energy of formationof Al4C3 is 700.2 kJ and 558.4 kJ at 2200 K and 2500 Krespectively [27]. Consequently, there are no stable reactionproducts of the Al2O3 reaction with C, (4), with the activitiesof Al2O3 and C (graphitic) remaining unity. Moreover, theabsence of prism planes in CNTs further restricts the bondingof carbon with aluminum and oxygen. X-ray diffraction didnot reveal the formation of any new reaction product [1].

The Al2O3–graphite system utilized for modeling theinterface is presented in figure 8. An Al-terminated α-Al2O3

crystal is utilized in modeling such an interface owing to thestability of the α-Al2O3 crystal. The iso-surface contours fromab initio computational modeling for an unaffected graphite(0001) layer are presented in figure 8(a). This shows theperiodic and regular energy contours between the carbonatoms. As the interface approaches, the interference fromthe aluminum oxide surface on the graphite layer is depictedby the distortion of the periodic energy contours, figure 8(b).The influence from the surface atoms of the aluminum oxidecrystal depicts pseudo-bonding between aluminum and carbonalong the interface. The O-terminated aluminum oxide surfacemight further destabilize the interface by the formation of COor CO2. The thermodynamics of the reaction as presented inequation (4) clearly obviates the necessity of considering thisproduct.

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Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

The Al-terminated aluminum oxide crystal shows threealuminum atoms at the interface, figure 8(c), which furtherconfirms the contribution of aluminum atoms in the distortionof iso-surface energy contours (shown in figure 8(d)). Pseudo-bonding over the aluminum atoms (in the Al2O3 crystal)implies strong bonding with carbon. Since high polarityindicates strong metallic bonding in aluminum [26], distortediso-surface contours confirm the pseudo-metallic bond at theAl2O3–CNT interface. On the Al-terminated surface, the weakbinding energy with silver (∼0.5 eV) and low activation barrier(∼0.25 eV) might allow rapid diffusion and bonding on thissurface [28]. Hence, the Al-terminated Al2O3 crystal presentsstability (when compared to that of an O-terminated Al2O3

crystal), and has been utilized in the current modeling work.Direct evidence from the molecular modeling result clearlyshows the possibility of enhanced interfacial bonding betweenAl and C. This makes sense because a crystal with high surfaceenergy will try to adhere to a new surface in order to minimizeits overall energy [29].

Some experimental data already demonstrates the stabilityof the Al–C interface (energy ∼0.02–0.4 J m−2) in comparisonto self-existing aluminum oxide or graphite crystals [26]. It isquite viable that partial bonding at the interface interconnectsthe CNTs and encourages strong wettability. Moreover, theunconnected region acts as an energy sink during impactto further enhance the interfacial strength. Though thesemolecular simulations consider the Al2O3–graphite interface,this model closely mimics the surface properties of the Al2O3–CNT interface. This combination of interfacial linking bridgesthe gap that had existed in describing the wetting of the CNTswith aluminum oxide.

4. Conclusions

The interface wettability of the Al2O3–CNT system wasexplained in terms of surface tension and capillarity. Thiscomplex interfacial phenomenon was theoretically modeledin terms of extracting the surface forces. The evolutionof microstructures such as the Al2O3-coated CNT bridgestructure and CNT meshing was attributed to the interplay ofsurface tension and capillarity dominated stages respectively.Mixed mode elicited entrapment of CNTs in the Al2O3 matrixto generate trimodal microstructure. The improved wettingcharacteristic has resulted in a microstructure that assists thetoughening mechanisms. Computational modeling of theinterface depicted partial bonding of Al-terminated Al2O3 withcarbon at the interfacial region, suggesting improved wettingand stability of the system.

Acknowledgments

Office of Naval Research (N00014-05-1-0398) funding isacknowledged to enable the performance of this work.KB acknowledges an FIU Dissertation Year Fellowship.The authors thank Professor George Dulikravich and MrRaymon Morales, MAIDROC (Multidisciplinary Analysis,Inverse Design, and Robust Optimization Control), FloridaInternational University (FIU), for providing disk space and

a platform for computational modeling. Mr S Bakshi, FIU, isacknowledged for his assistance with the plasma spraying andinteresting discussions on surface forces. A Garcia, J Junquera,P Ordejon, D Sanchez Portal and J M Soler are acknowledgedfor distributing the SIESTA 1.3 molecular modeling packageand user’s guide.

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Nanotechnology 19 (2008) 165701 K Balani and A Agarwal

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