contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites

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nature materials | VOL 3 | AUGUST 2004 | www.nature.com/naturematerials 539

There has been growing interest in using high-aspect-ratio carbonnanotubes to toughen ceramic composites1. This originates fromthe notion that carbon nanotubes are exceptionally stiff and

strong2–8,and that by combining carbon nanotubes with brittle ceramicsone can impart some of the attractive mechanical properties of thecarbon nanotubes to the resulting composites. In one study, 10 vol.%multiwall carbon nanotubes (MWNTs) were incorporated in SiC, andonly a marginal increase in bend strength and toughness over SiCceramics without the MWNTs was found9. This could be due to the poor crystallinity of the MWNTs synthesized using vapour methods.Others have used an in situ vapour method to grow single-wall carbonnanotubes (SWNTs) or double-wall carbon nanotubes (DWNTs)within Al2O3 ceramics, but no improvements have been found inmechanical properties in the resulting composites over a referencecomposite10. In another study11, high-crystallinity MWNTs wereincorporated (arc method) in Al2O3 using hot-pressing, and a 25%increase in the toughness was observed relative to Al2O3 containing noMWNTs,as measured using the Vickers-indentation method.

Thin (20–90 µm thickness) amorphous alumina compositesreinforced with unidirectional, continuous MWNTs, have been testedusing nanoindentation12. In some composites, no cracking wasobserved, but collapse of the MWNTs under shear was seen. In othercomposites, crack deflection, crack bridging, and MWNTs pull-outwas observed, but toughness values were not ascribed to thosecomposites due to the complex nature of the indentation-damage andresidual stresses12.

Zhan et al.1 have used spark-plasma sintering (SPS) to densify arandom mixture of high-quality SWNTs (HiPco method13) in Al2O3

nanopowders, and they measured the toughness of the resultingcomposites using the Vickers-indentation method. The highestindentation-toughness values they report for Al2O3/10 vol.% SWNTcomposite specimens with densities 95% and 100% (theoretical limits)are 8.1 MPa m0.5 and 9.7 MPa m0.5,respectively.This,compared with theindentation toughness of fully dense pure Al2O3 of 3.3 MPa m0.5, is asubstantial increase1.

There has been growing interest in incorporating single-wall

carbon nanotubes (SWNTs) as toughening agents in brittle

ceramics. Here we have prepared dense Al2O3/SWNT

composites using the spark-plasma sintering (SPS) method.

Vickers (sharp) and Hertzian (blunt) indentation tests reveal

that these composites are highly contact-damage resistant,

as shown by the lack of crack formation. However, direct

toughness measurements, using the single-edge V-notch

beam method, show that these composites are as brittle as

dense Al2O3 (having a toughness of 3.22 MPa m0.5). This type

of unusual mechanical behaviour was also observed in SPS-

processed, dense Al2O3/graphite composites. We argue that

the highly shear-deformable SWNTs or graphite

heterogeneities in the composites help redistribute the

stress field under indentation, imparting the composites with

contact-damage resistance. These composites may find

use in engineering and biomedical applications where

contact loading is important.

Contact-damage-resistant ceramic/single-wall carbon nanotubes andceramic/graphite compositesXIAOTONG WANG1, NITIN P. PADTURE1* AND HIDEHIKO TANAKA2

1Department of Metallurgy and Materials Engineering, Institute of Materials Science,University of Connecticut,Storrs,Connecticut 06269-3136,USA2Advanced Materials Laboratory,National Institute for Materials Science,Tsukuba,Ibaraki 305-0044,Japan*e-mail: [email protected]

Published online:18 July 2004; doi:10.1038/nmat1161

nmat1161-print 7/8/04 3:33 PM Page 539

© 2004 Nature Publishing Group

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540 nature materials | VOL 3 | AUGUST 2004 | www.nature.com/naturematerials

Although the report by Zhan et al.1 is intriguing, there are severalissues that need to be resolved. In their study, they used the Vickers-indentation method for measuring the toughness, which is an indirecttoughness-testing technique: the validity of the toughness resultsdepends critically on the elastic/anelastic contact-mechanical responseof the material being tested14. Also, they did not present a tougheningmechanism responsible for the observed improvement in toughness.Although they have alluded to effects such as stronger interfacialbonding between SWNTs and Al2O3 and crack deflection in theircomposites1, those mechanisms are at odds with each other14–16.

To investigate further the unusual mechanical behaviour of suchcomposites, we have prepared our own high-density Al2O3/10 vol.%SWNT composites using SPS.With the aid of Raman spectroscopy—adefinitive tool for characterizing SWNTs17—we show that SWNTs arepresent in Al2O3/SWNT composites SPS-processed at temperatures ashigh as 1,550 °C in vacuum. Vickers-indentation tests on thesecomposites show little evidence of cracking, calling into question the validity of the use of the Vickers-indentation test for measuring thetoughness of these materials.To determine if SWNTs play a unique rolein determining the unusual mechanical behaviour of these composites,we have performed a critical experiment where, instead of SWNTs, wehave incorporated graphite in the composites. In SPS-processed, denseAl2O3/10 vol.% graphite composites we observe a similar lack ofcracking under Vickers indentation.This indicates that SWNTs may not

play a unique role in determining the unusual contact-mechanicalresponse of these composites.

We have also performed direct toughness measurements (single-edge V-notched beam or SEVNB) on both the composites and the dense Al2O3; the average toughness values of the dense Al2O3, theAl2O3/SWNT composite, and the Al2O3/graphite composite werefound to be 3.22 MPa m0.5, 3.32 MPa m0.5 and 3.51 MPa m0.5,respectively. Although the indentation results show that both theAl2O3/SWNT and the Al2O3/graphite composites are resistant tocontact or indentation damage, the toughness results show that thesecomposites are not tough. Based on some additional results fromHertzian indentation tests, we argue that the contact-damageresistance in these composites is related to the presence of shear-deformable SWNTs or graphite in the composites.

Figure 1 is a bright-field transmission electron microscope (TEM)image of the Al2O3/SWNT powder blend showing the well-dispersednature of the SWNTs and the well-mixed nature of the powder blend.Note that the Al2O3 powder is a mixture ofα-Al2O3 (larger particles) andγ-Al2O3 (smaller particles)1.

The densities of the four materials processed in this study aresummarized in Table 1. The theoretical density values for thecomposites were obtained from the literature, which are based on rule-of-mixtures calculations assuming the following density values1: Al2O3

3.97 g cm–3,graphite 2.25 g cm–3,and SWNTs 1.80 g cm–3.Figure 2a,b shows Raman spectra for the Al2O3/SWNT and the

Al2O3/graphite composites, respectively, along with Raman spectra forthe SWNT and graphite starting materials. In Fig. 2a the characteristicSWNT peak near 1,590 cm–1 and the shoulder near 1,570 cm–1 (ref. 17)are clearly observed in both the SWNTs and the composite, providing

300 nm

SWNTs

α−Al203

γ−Al203

Figure 1 Bright-field TEM image of the Al2O3/SWNT powder blend.The α-Al2O3

(larger particles), the γ-Al2O3 (smaller particles),and the SWNTs are evident.

1,200 1,300 1,400 1,500 1,600 1,700

SWNTsAl203/SWNT composite

Raman shift (cm–1)

Inte

nsity

1,200 1,300 1,400 1,500 1,600 1,700

Raman shift (cm–1)

Inte

nsity

GraphiteAl203/graphite composite

a

b

Figure 2 Raman spectra from starting carbon materials and composites.Comparisons of the Raman spectra from a,SWNTs and the Al2O3/SWNT composite and b,graphite powder and the Al2O3/graphite composite,show that the starting carbonmaterials survive the SPS processing.

Table 1 Densities of the materials prepared in this study.

Material Measured density Theoretical density % Density (g cm–3) (g cm–3)

Dense Al2O3 3.898 3.97 98.1

Porous Al2O3 3.596 90.6

Al2O3/10 vol.% SWNT 3.568 3.75 (ref. 1) 95.1

Al2O3/10 vol.% graphite 3.713 3.80 (ref. 1) 97.7



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definitive evidence for the presence of SWNTs in the Al2O3/SWNTcomposite. The peak and the shoulder in the spectrum for thecomposite are shifted to higher Raman wavenumbers, possibly due toresidual stresses imposed by the constraining ceramic matrix, quitesimilar to what is observed in polymer/SWNT composites18. In thecomposite, a broad peak appears near 1,350 cm–1, indicative offormation of some disordered graphite during sintering. In theAl2O3/graphite composite and the graphite powder (Fig. 2b) the characteristic disordered-graphite peaks near 1,580 cm–1 and1,350cm–1 (ref.17) are the only peaks observed,confirming the presenceof disordered graphite in that composite.

Figure 3 shows a bright-field TEM image of the Al2O3/SWNTcomposite. The α-Al2O3 grains can be easily identified, and they werefound to be 1–2 µm in size.Note that the small amount of the γ-Al2O3 inthe starting powder transforms to α-Al2O3 during sintering. We alsofound carbonaceous matter at the grain boundaries, quite similar towhat was reported by Zhan et al.1, however, we could not confirm that to be SWNTs in the limited TEM study.

Figure 4 shows a scanning electron microscope (SEM) image of afracture surface of the Al2O3/SWNT composite. The intergranularnature of the Al2O3 fracture (light regions) and what appear to beSWNTs (dark regions) at the grain boundaries are clearly evident.This is quite similar to what has been observed by Zhan et al.1.

Figure 5a shows representative SEM images of a Vickers indentationsite in the dense Al2O3. Classical radial cracking is clearly evident in themicrographs. Because these cracks are well-defined, we can readilycalculate the Vickers-indentation toughness of pure Al2O3 to be3.01 MPa m0.5 (average of five indentations) using the Anstis et al.19

equation. Figure 5b shows similar radial cracking in the porous Al2O3

subjected to Vickers indentation. Figure 5c shows representative SEMmicrographs of a Vickers indentation site in the Al2O3/SWNTcomposite. Although minor subsidiary cracking can be observedaround the indentation site, there is a complete absence of classicalradial cracks. Consequently, toughness measurement using the Vickersindentation method, in conjunction with the Anstis et al.19 equation, isnot valid in this case.A similar Vickers indentation response is observedin the Al2O3/graphite composite (Fig. 5d).

The hardness values, H, of the dense Al2O3, the porous Al2O3, theAl2O3/SWNT composite,and the Al2O3/graphite composite were foundto be 19 GPa,16 GPa,9 GPa and 14 GPa,respectively.

Figure 6 shows SEVNB toughness results. The average toughness(KIC) of the dense Al2O3 (3.22 MPa m0.5) is a typical value for fine-grained Al2O3, consistent with measurements by others using varioustechniques,and our own Vickers indentation toughness measurementsreported above. The average KIC values of the Al2O3/SWNT compositeand the Al2O3/graphite composite were found to be 3.32 MPa m0.5 and3.51 MPa m0.5, respectively. These data clearly show that, withinexperimental scatter,the toughness of the three materials studied here isthe same. Considering the fine scale of the Al2O3 grains (1 to 2 µm), theSWNTs (∼1 nm diameter, 1 to 10 µm long), and the graphite particles (1 to 2 µm diameter), it is not surprising that the Al2O3/SWNT and the Al2O3/graphite composites are as brittle as the dense Al2O3.Assuming SWNTs or graphite particles are active in deflecting andbridging the cracks behind the crack tip in the composites, the fine scaleof those reinforcing elements will result in only small crack-pathperturbations and small toughening-zone sizes relative to the crack size.This will render crack deflection and crack-bridging mechanismsineffective in these composites (see refs 14–16). It has been known forquite some time that one needs to increase the scale of the reinforcementto make ceramics tough; high-toughness ceramic-matrix compositesuse fibre reinforcements that are hundreds of micrometres in length15.

The Vickers indentation results clearly show that the Al2O3/SWNTand the Al2O3/graphite composites are highly resistant to contactdamage. It cannot be argued that the Vickers indentation conditions inthe composites are subthreshold; that is, the indentation load, PV, is nothigh enough to cause cracking. This is because the Vickers-indentationparameter14 (PVH3/KIC

4) for the Al2O3/SWNT composite and theAl2O3/graphite composite is 1.45 ×105 and 4.43 ×105,respectively,whichis an order of magnitude higher than the ‘universal’threshold parameterof ∼1.5 × 104 (ref. 14). In other words, Vickers-indentation threshold-loads of 2.5 N for the Al2O3/SWNT composite, and 0.8 N for theAl2O3/graphite composite,are significantly lower than the load of 24.5Nused here.

There is a small amount of porosity in the composites (4.9% in theAl2O3/SWNT composite and 2.3% in the Al2O3/graphite composite).We argue that the porosity does not play a role in imparting contact-damage resistance to these composites. This assertion is based on thefact that the porous Al2O3,with much higher porosity (9.4% porosity),

500 nm

Carbonaceousmatter

Al203

Figure 3 Bright-field TEM image of the Al2O3/SWNT composite.Al2O3 grains andcarbonaceous matter at the grain boundaries are indicated.

500 nm

Al2O3

Figure 4 SEM micrograph showing a fracture surface of the Al2O3/SWNTcomposite. Intergranular nature of the Al2O3 fracture (light regions) and the presence of what appear to be SWNTs (dark regions) at some grain boundaries (arrows) and as agglomerates.



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shows classical radial cracking under Vickers indentation in Fig. 5b,just like the dense Al2O3 (1.9% porosity) in Fig. 5a.

The dichotomy between the toughness and the indentation responsecan be explained based on the following arguments. First, there is afundamental difference between contact loading (indentation) andbend test. In indentation, highly concentrated loads act on very smallareas,resulting in a state of intense confined-shear,whereas in a bend testthe crack tip is subject to mode I tension only13.Second,homogenous andheterogeneous ceramics respond to contact loading in fundamentallydifferent ways14,20. In homogenous ceramics, the intense confined-shearunder Vickers indentation produces anelastic deformation, and theelastic/anelastic mismatch results in the formation of radial cracks14.Highly heterogeneous ceramics also undergo anelastic deformationunder Vickers indentation. However, if those ceramics are sufficientlyweak in shear,considerable redistribution of stresses can take place underthe indenter, resulting in the suppression of long cracks. It is difficult todiscriminate between the shear-deformabilities of homogenous andheterogeneous ceramics using Vickers indentation because both deformanelastically under the intense confined-shear. This problem becomestractable in the Hertzian indentation (spherical indenter) test, whichprovides a more moderate state of confined-shear under the indenter20.

In the dense Al2O3 subjected to Hertzian indentation,the formationof Hertzian cracks, which is well documented in the literature21,22, isevident in the optical micrograph (Fig. 7a). The abrupt depressionobserved in the scanning optical-interference microscope (SOIM)topograph (Fig. 7a) is due to the Hertzian cracks,not anelasticity.Onceagain, the lack of anelasticity in fine-grained, dense Al2O3 subjected tosimilar Hertzian indentations is well documented21,22. In contrast, thereis an absence of cracks in the Al2O3/SWNT composite subjected toHertzian indentation (Fig. 7b,optical micrograph).However,a gradualdepression can be observed in the SOIM topograph (Fig. 7b),something akin to a ‘dent’. This type of depression representsanelasticity, which could not be delineated in the conventional opticalmicroscope,an instrument with lower depth-sensitivity than the SOIM.A similar response is observed in the Al2O3/graphite composite (Fig.7c),but the anelastic depression is shallower.

The type of mechanical behaviour observed in the Al2O3/SWNTand the Al2O3/graphite composites is reminiscent of commerciallyavailable mica-based glass-ceramics (for example, Macor; Corning,New York), which, although brittle (KIC = 1.5 MPa m0.5, ref. 23), arehighly contact-damage resistant under both Vickers and Hertzianindentation due to the presence of the shear-deformable mica in

a b

c d

20 µm20 µm

5 µm

5 µm

20 µm 20 µm

5 µm 5 µm

Figure 5 Low- and high-magnification SEM micrographs showing top views of Vickers indentation sites. a,b,Classical radial cracks can be seen in the dense Al2O3 (a) and in theporous Al2O3 (b).c,d,There is no evidence of classical radial cracks in the Al2O3/SWNTcomposite (c) or the Al2O3/graphite composite (d).



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those materials20. Thus we argue that, in the case of the Al2O3/SWNTcomposite,the presence of the SWNTs at the Al2O3 grain boundaries (asshown by Zhan et al.1 and in Figs 3 and 4) is likely to make the compositehighly shear-deformable under indentation.Whereas, in the case of theAl2O3/graphite composite, the shear-weak graphite is likely to impartshear-deformability to that composite, but to a lesser extent comparedwith SWNTs in the Al2O3/SWNT composite. This is because, unlikeSWNTs in the Al2O3/SWNT composite, which are at the Al2O3 grainboundaries, the graphite is distributed as discrete particles within the composite.

The mechanical behaviour of these composites fits the generalpattern of enhanced contact-damage resistance observed inheterogeneous ceramics containing shear-weak second phases (discreteor intergranular)20,24. In the intense confined-shear field under theindenter, the contact damage in such ceramics is characterized bydistributed shear anelasticity in the form of microstructure-localizedshear-sliding (mode II) along numerous interfaces. This type ofdistributed damage causes significant redistribution of stresses underthe indenter, and prevents the formation of long cracks that wouldotherwise develop.

In closing, although the Al2O3/SWNT and the Al2O3/graphitecomposites we have prepared are not tough, their contact-damage-resistance properties are attractive for applications where contactloading is prevalent. This presents new opportunities because ceramicsand their composites are being used increasingly in contact-mechanicalapplications such as bearings, valves, nozzles, seals, wear parts, armour,and prostheses, to name a few.

METHODS

PROCESSINGWe used the same Al2O3 starting powder as Zhan et al.1 (Baikowski International, Charlotte, North

Carolina). Purified SWNT ropes produced using the HiPco process13 were obtained from a commercial

source (Carbon Nanotechnologies, Houston, Texas), and they are similar to the ones Zhan et al.1 used.

The disordered graphite was also obtained commercially (Aldrich Chemicals, Milwaukee, Wisconsin),

and had an average particle size of 1 to 2 µm. The as-received SWNTs or graphite were dispersed either in

methanol or dimethyl formamide with the aid of ultrasonic agitation. It was determined that the nature

of the dispersant did not make any difference in the final composites in terms of their densities and

microstructures. The Al2O3 powder was added to this solution and ultrasonically agitated, to result in

either 90 vol.% Al2O3 + 10 vol.% SWNT or 90 vol.% Al2O3 + 10 vol.% graphite in the solution. The resulting

slurries were ball-milled for 24 h (zirconia-balls media; Tosoh, Tokyo, Japan), and were subsequently

dried on a hot plate while being stirred. The Al2O3/SWNT powder blend was observed in the TEM

(EM420, Philips Electron Optics, The Netherlands) to investigate the dispersion of the SWNTs

and mixing.

The powder blends were placed in either 20 mm or 40 mm diameter graphite dies and they were

spark-plasma sintered in a SPS unit (Sumitomo Coal Mining Company, Tokyo, Japan). The SPS

conditions were as follows: pressure 40 MPa, peak temperature 1,450 to 1,550 °C, hold time at peak

temperature 3 to 10 min, pulse duration 12 ms, and pulse interval 2 ms. Note that the temperatures used

here are higher than those that Zhan et al.1 used.

Dense and porous Al2O3 reference specimens were prepared using the above Al2O3 starting powder,

which was pressed in a graphite die (60 mm diameter) at 50 MPa, followed by cold-isostatic pressing

(AIP, Columbus, Ohio) at 350 MPa. The pressed pellet was pressureless sintered at 1,600 °C for 30 min or

1,350 °C for 60 min in a furnace (Thermolyne, Dubuque, Iowa) in air, to result in a fine-grained (∼1 to

2 µm) dense or porous Al2O3, respectively.

The resulting discs (∼3.5 mm thickness) of the dense Al2O3, the porous Al2O3, the Al2O3/SWNT

composite, and the Al2O3/graphite composite were cleaned, and their densities were measured using the

Archimedes principle, with distilled water as the immersion medium.

CHARACTERIZATIONThe Al2O3/SWNT and the Al2O3/graphite composites, and the corresponding SWNTs and graphite

starting materials were characterized using Raman spectroscopy (Ramanscope 2000, Renishaw,

Gloucestershire, UK). The radiation source was a laser of 514.5 nm wavelength.

Cross-sections of the dense Al2O3, the porous Al2O3, the Al2O3/SWNT composite, and the

Al2O3/graphite composite were polished to a 1 µm finish using routine metallographic procedures.

The polished sections were observed in an SEM equipped with a field-emission source (JSM6335F, JEOL,

Tokyo, Japan). The fracture surface of the Al2O3/SWNT composite was observed in the SEM.

The Al2O3/SWNT powder blend and the composite were characterized using the TEM. In the case

of the powder blend, TEM specimens were prepared by placing a small amount of the powder onto a

TEM grid. In the case of the composite, TEM specimens were prepared using routine methods applicable

to ceramic materials. This entailed successive steps of mechanical thinning, dimpling and ion-beam

milling (DuoMill, Gatan Corp., Pleasanton, California) to perforation. The resulting specimens were

observed in the TEM operated at 100 keV.

MECHANICAL TESTINGPolished specimens of the dense Al2O3, the porous Al2O3, the Al2O3/SWNT composite, and the

Al2O3/graphite composite were indented at several locations (five indentations per material) using a

Vickers diamond pyramid with a load, PV, of 24.5 N (or 2.5 kg) under ambient conditions. The indentation

sites were examined in both an optical microscope (Nikon, Tokyo, Japan) and the SEM. The diagonals of

each indentation impression (2aV) were measured using the optical microscope, and the hardness values

for the three materials were calculated, H = PV/2aV2 (ref. 14). In the case of the dense Al2O3, the radial-

crack diameters (tip-to-tip surface traces; 2c) were measured in the optical microscope, and the

indentation toughness values were calculated using the Anstis et al.19 method, KIC = 0.016(E/H)0.5PVc–1.5;

for the dense Al2O3, Young’s modulus (E) and hardness (H) values of 390 GPa and 19 GPa, respectively,

were used.

The polished specimens of the dense Al2O3, the Al2O3/SWNT composite, and the Al2O3/graphite

composite were subjected to Hertzian indentation using a tungsten carbide ball of radius 3.97 mm with

load of 2,000 N (204 kg) under ambient conditions. These experiments were performed using a universal

mechanical-testing machine (Instron, Canton, Massachusetts). The Hertzian indentation sites were

observed using both the optical microscope and an SOIM (Zygo, Middlefield, Connecticut).

Bar specimens (4 mm wide, 3 mm thick , 25 mm long) were machined out of the dense Al2O3, the

Al2O3/SWNT composite, and the Al2O3/graphite composite, and their toughness was measured using the

SEVNB standard method25,26. Each bar specimen was V-notched using standard procedure25,26, and it was

subsequently tested in a semi-articulating four-point bending fixture (10 mm inner span, 20 mm outer

span) using the universal mechanical-testing machine, under ambient conditions. The fracture load and

the geometrical dimensions (specimen, loading fixture) were used to calculate the toughness KIC

(refs 25,26). Four bar samples were tested for each material.

Received 18 March 2004; accepted 17 May 2004; published 18 July 2004.

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DenseAl203

Al203/SWNT

Al203/Graphite

5

4

3

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1

0

K IC (M

Pa m

0.5 )

Figure 6 SEVNB toughness results for the dense Al2O3, the Al2O3/SWNT composite,and the Al2O3/graphite composite. Each histogram is an average of four measurements,and error bars represent the data range.

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Research Foundation.

Correspondence and requests for materials should be addressed to N.P.P.

Competing financial interestsThe authors declare that they have no competing financial interests.

a

b

c

90 µm90 µm

+2.9

–4.4

µm

Figure 7 Top-view optical micrographs (left) and corresponding oblique-view SOIM topographs (right) of Hertzian indentation sites. a,Hertzian cracks can be seen in the denseAl2O3, in both the optical micrograph and the SOIM topograph.b,c,There is no evidence of Hertzian cracks in the Al2O3/SWNT composite (b) the Al2O3/graphite composite (c), in either theoptical micrographs or the SOIM topographs. In the SOIM topographs in (b) and (c) anelastic deformation is evident.The colour-coded depth scale and the horizontal scale bars shown in bapply to a and c as well.



contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites

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