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Carbon nanotubes functionalization process for developing ceramic matrixnanocomposites

Jes�us Gonz�alez-Juli�an, Pilar Miranzo, M. Isabel Osendi and Manuel Belmonte*

Received 11th November 2010, Accepted 15th February 2011

DOI: 10.1039/c0jm03885g

The carbon nanotubes (CNTs) functionalization for developing ceramic matrix composites with an

optimum interface between the matrix and nanotubes is presented. The functionalization processes

were successfully performed by means of oxidation and boron nitride and silicon oxide coatings, which

were characterized, among others, by O2/N2 adsorption-desorption method, micro-Raman

spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. In all

cases, the functionalized CNTs exhibited an enhanced thermal stability compared to the original

nanotubes. Silicon nitride (Si3N4) nanocomposites having 5.3 vol% of multi-walled CNTs, both pristine

and functionalized, were developed. Dense nanocomposites having well dispersed nanotubes were

attained under all circumstances, although the nanocomposites containing the coated CNTs required

the use of a dispersant agent to prevent nanotubes agglomeration. Besides, slightly higher sintering

temperatures were needed to densify the composites containing the coated CNTs. A stronger

mechanical interlocking between the matrix and the nanotubes was achieved by the functionalization

processes, which led to some improvement of the mechanical properties of these nanocomposites,

compared to those containing pristine CNTs; and actually values that were close or even higher than

those measured for the blank Si3N4 were achieved.

1 Introduction

The extraordinary physical and mechanical properties of carbon

nanotubes (CNTs)1,2 have created a great expectation about their

capability for enhancing the properties of many structural

materials.3 In this sense, extensive research has been conducted in

polymer composites containing CNTs4 and to a lesser extent in

ceramic based ones.5,6 Focusing on the latter materials, the

development of ceramics/CNTs nanocomposites requires over-

coming three important challenges. First, a homogeneous

dispersion of the nanotubes within the ceramic matrix, avoiding

the common tendency of CNTs to form bundles due to Van der

Waals attraction forces, needs to be accomplished. Second, the

CNTs degradation during densification of the ceramic composite

due to the high temperatures (>1200 �C) normally required for

sintering ceramics must be prevented. Last but not least, an

optimum interface between the matrix and nanotubes is crucial

to achieve an effective load transfer and, as a result, the

enhancement of the mechanical response of the composite.

Among ceramic materials, silicon nitride (Si3N4) stands out

due to its excellent thermo-mechanical and tribological proper-

ties,7 which prompts its uses in many technological applications,

such as engine parts, ball bearings, or metal cutting and shaping

Institute of Ceramics and Glass (CSIC), Campus Cantoblanco, Kelsen 5,Madrid, 28049, Spain. E-mail: mbelmonte@icv.csic.es; Fax: +34-917355843; Tel: +34-917355863

This journal is ª The Royal Society of Chemistry 2011

tools.8 The addition of CNTs to Si3N4 pretends to enhance some

of their properties and, although scarce references can be found

in this sense, the change of Si3N4 from highly electric insulator to

semiconductor just adding very low CNTs concentrations was

reported,9,10 as well as outstanding tribological performances of

Si3N4/CNTs composites compared to the monolithic material.11

However, the mechanical properties (elastic modulus, hardness,

toughness, strength) remain as an unsettled matter, reporting

diverse mechanical responses, either negligible changes9,12,13 or

large increases14,15 as much as for other CNTs containing ceramic

based composites.16 Aspects as the type and purity of nanotubes,

processing conditions, Si3N4 microstructure (porosity, a/b ratio,

grain size and aspect ratio distributions) and, especially, the

matrix/nanotube interface have a great influence on the

mechanical properties.

In a previous work of the present authors,17 Si3N4/CNTs

nanocomposites were reported, which contained up to 8.6 vol%

of multi-walled carbon nanotubes (MWCNTs), showing a good

nanotubes dispersion and no evidence of MWCNTs degradation

after spark plasma sintering (SPS) at 1600 �C. Therefore, the

third important point enumerated above, that is the improve-

ment of the matrix/nanotube interface, becomes the goal of the

present work. To follow this, MWCNTs were effectively func-

tionalized through two different approaches: oxidation and

nanotube coating. The former was done by attaching oxygen

functional groups (O–H, C]O and COOH) to the outer wall of

the MWCNTs for creating a certain chemical surface roughness

J. Mater. Chem., 2011, 21, 6063–6071 | 6063

Table 1 Oxidation conditions, amount of oxygen on the nanotubessurface (DO2), and Raman intensity ratios between D and G bands (ID/IG) for each treatment. AR corresponds to the untreated as-receivedMWCNTs

Treatment

DO2

(wt%)RamanID/IGLabel Etching

Temperature/�C

Time/h

AR As-received — — 0.0 0.78OX1 Air 300 1 1.0 —OX2 NH4OH/H2O2 75 5 0.4 —OX3 HNO3 80 2 2.5 —OX4 HNO3 80 4 2.9 0.77OX5 HNO3 130 4 12.4 1.31OX6 HNO3/H2SO4 80 4 4.8 0.82OX7 HNO3/H2SO4 80 6 13.3 0.93

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that may enhance the mechanical interlocking with the matrix.

Among the widespread types of oxidation methods, wet chemical

methods18–20 and air furnace oxidation21 were selected. For the

nanotube coating approach, boron nitride (BN) and silicon oxide

(SiO2) were tried. It can be mentioned that BN coatings are

extensively used for carbon fibres composites to prevent the fibre

oxidation and to get a desirable anchorage between the rein-

forcement and the matrix as well.22 Based on this, the BN coating

of MWCNTs was pursued through a reaction between boric acid

and urea, and further nitriding at 1000 �C.23 For the SiO2

coating, the strategy was linked to sintering additives normally

required for Si3N4 densification. The additives generally are

a mixture of rare earth oxides and Al2O3 plus the native SiO2

always present in Si3N4 powders, which promote densification at

high temperatures through a liquid-phase sintering mechanism.7

In a similar way, the SiO2 coating in the MWCNTs could also

take part in the densification process by helping in the liquid

phase formation with the sintering additives used in the present

case (Y2O3 plus Al2O3). After cooling from the maximum sin-

tering temperature, the liquid turns to a glassy state surrounding

grains and also concentrating at triple points, probably affecting

the interface between CNTs and the Si3N4 matrix.

Here we present the results of the functionalization of as-

received commercial MWCNTs (MWCNTs-AR) by means of

different oxidation treatments, and BN and SiO2 coatings. To

establish the degree of success of the functionalization processes,

nanotubes were characterized by O2/N2 adsorption-desorption

curves, micro-Raman spectroscopy, Fourier transform infrared

spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS),

thermogravimetric analysis (TGA), field emission scanning

electron microscopy (FESEM) and transmission electron

microscopy (TEM). To explore the effect of the different func-

tionalizations in the matrix/nanotube interface, Si3N4/MWCNTs

nanocomposites containing 5.3 vol% of nanotubes were devel-

oped. The quality of the nanotube dispersion, the density of the

composites, and the possible nanotube degradation were quan-

tified and, furthermore, the matrix microstructure and the

mechanical properties of the nanocomposites were compara-

tively evaluated.

2 Results and discussion

2.1. Functionalization of MWCNTs

2.1.1. Oxidation treatments. The total amount of oxygen

(DO2) on the MWCNTs surface for the different oxidation

treatments, as determined by O2/N2 adsorption-desorption

method, is shown in Table 1.

The experiments carried out both in air (OX1) and using basic

etching (OX2) led to DO2 # 1.0 wt%, which seems rather low to

promote enough active anchoring centres between the matrix

and the nanotube. In order to increase DO2, acid treatments were

required and, in this sense, a HNO3 reflux at 80 �C for 2 h (OX3)

was selected as starting treatment.19 OX3 led to DO2 ¼ 2.5 wt%,

although an increase in the refluxing time up to 4 h (OX4) just

produced a slight increment of the nanotube oxidation (2.9 wt%).

However, when the oxidation reaction was carried out at 130 �C

(OX5), a considerably increase of the DO2 to values of 12.4 wt%

was estimated, which evidenced that the amount of oxygen

6064 | J. Mater. Chem., 2011, 21, 6063–6071

attached for this reaction mainly depended on the temperature,

the reaction time being far less important.

On the other hand, the effect of the strong acid mixture of

HNO3/H2SO4 was evaluated at 80 �C19 for two etching times

(4 and 6 h, OX6 and OX7 treatments, respectively), looking for

a higher chemical oxidation of nanotubes without the need to

increase the temperature. As a result, OX6 treatment augmented

DO2 to 4.8 wt%, which is a 60% raise than for the OX4 treatment

(DO2 ¼ 2.9 wt%) for the same etching conditions but using

HNO3 as unique reagent. When the reflux time of the acid

mixture was kept for longer times (6 h, OX7), the maximum

surface oxidation (13.3 wt%) of nanotubes was attained. There-

fore, the oxidation treatments under acid conditions were able to

modify the nanotubes surface with a wide range of oxygen-

containing groups, as will be addressed later.

One important concern after the oxidation treatments of

MWCNTs was the preservation of their integrity during this

covalent surface modification. To check on this, the three char-

acteristic peaks of MWCNTs micro-Raman spectra were

analysed:24 (i) D-band at �1350 cm�1 that corresponds to defects

in MWCNTs, (ii) G-band at �1580 cm�1 that gives an idea of

their crystallinity, and (iii) G0-band at �2700 cm�1 associated to

second order Raman scattering that results in the creation of an

inelastic phonon. Raman spectra for the pristine (AR) and the

oxidized MWCNTs after OX6 treatment, this chosen as proto-

type of the highly oxidized nanotubes, are plotted in Fig. 1.

The intensity ratio between D and G bands (ID/IG) calculated

from the spectra was used as means to quantify the crystallinity

of the MWCNTs (Table 1). A raise in ID/IG is indicative of an

increase of number of defects in the nanotubes and, therefore, of

higher degradation. Only oxidized MWCNTs with a substantial

surface oxidation (DO2 $ 2.9 wt%) were investigated. Among

them, OX4 and OX6 did have similar Raman signal as the

pristine nanotubes. Moreover, FESEM micrographs on Fig. 2

did not show any appreciable changes between the original and

the OX6 nanotubes. However, for the treatments that gave

higher DO2 amounts (OX5 and OX7), ID/IG significantly

increased (Table 1), especially for OX5 with ID/IG ¼ 1.31 due to

the high temperature of the oxidizing reaction that considerably

damaged the nanotubes.

Based on the analysis of oxygen content and the Raman signal,

it can be assessed that OX6 oxidation was the most convenient

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 Micro-Raman spectra of the pristine (MWCNTs-AR) and

oxidized (OX6) nanotubes. The three main characteristic peaks (D, G

and G0) of the nanotubes are marked.

Fig. 2 FESEM micrographs for MWCNTs: (a) pristine and (b) after

OX6 oxidation treatment.

Fig. 3 FTIR spectra for (a) MWCNTs-AR and MWCNTs-OX and (b)

MWCNTs-SiO2.

Fig. 4 Scheme of the hydroxyl and carboxyl groups’ formation on the

nanotubes surface after OX6 oxidation treatment.

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functionalization treatment, as it gave enough oxygen coverage

of the MWCNTs while avoided their degradation. Therefore,

this treatment was the selected process for further experiments

and, from now on, they will be referred as MWCNTs-OX.

Going back to the issue of the kinds of oxidant functional

groups created after MWCNTs oxidation, FTIR spectra for

MWCNTs-AR and MWCNTs-OX were compared in Fig. 3a.

Both spectra showed absorptions peaks at 3448 cm�1 (O–H),

2925 and 2839 cm�1 (symmetric and asymmetric C–H), 1632

cm�1 (C]C), 1562 cm�1 (O–H from H2O) and 1125 cm�1 (C–O

from alcohol). The presence of the latter stretching peaks has

been linked to manufacturing and/or purification processes.19

Besides those peaks, MWCNTs-OX presented two peaks at 1717

and 1388 cm�1, characteristics of stretching vibration of the

carbonyl groups (C]O) and the bending deformation of O–H

element present in carboxylic acids (COOH), respectively. These

results confirmed the assumed covalent surface functionalization

This journal is ª The Royal Society of Chemistry 2011

of the nanotubes by the proposed oxidation treatments. A

schematic representation of the formation of the covalent

groups, hydroxyl and carboxyl, respectively, on the nanotube

surface after the treatments is represented in Fig. 4.

2.1.2. BN coatings. XPS was chosen to confirm the successful

BN coating on the MWCNTs through the chemical reaction

between boric acid and urea, and further thermal treatment

under nitrogen atmosphere. Fig. 5 shows the B1s, N1s, O1s and

C1s core-level de-convoluted spectra of the coated nanotubes.

B1s spectrum (Fig. 5a) exhibited peaks at 191.1 and 192.8 eV,

which were associated to the binding energies of B–N and B–O

bonds, respectively.25 The N1s peak at 398.4 eV (Fig. 5b) that

corresponds to the binding energy of N–B bond also confirmed

the formation of BN.26 In addition, both the peak assigned to B–

O bonds (Fig. 5a) and the second peak at 400.0 eV observed in

the N1s spectrum (Fig. 5b), which matched to a O–NB bonding,

J. Mater. Chem., 2011, 21, 6063–6071 | 6065

Fig. 5 XPS spectra of (a) B1s, (b) N1s, (c) O1s and (d) C1s core levels of

the MWCNTs after BN coating process.

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corroborated that BN coating was anchored to oxygen groups.25

These anchoraging groups between CNTs and BN were almost

certainly produced during the first oxidizing stage of the coating

process. In fact, peaks with binding energies of C–O (532.1 eV)

and C]O (533.6 eV)27 were identified in the O1s spectrum

(Fig. 5c), which also appeared in the C1s plot (Fig. 5d) at 286.2

(C–O) and 288.7 eV (C]O).28 Finally, the characteristic binding

energies of MWCNTs,26,28 i.e. C–C and p–p* bonding at 284.7

and 291.3 eV, respectively, were also observed (Fig. 5d).

Fig. 6 TEM micrographs of BN coated MWCNTs at (a) low and (b)

high magnification.

6066 | J. Mater. Chem., 2011, 21, 6063–6071

Therefore, XPS analysis confirmed the development of the BN

phase on MWCNTs through oxygen anchorages.

The TEM observations of MWCNTs-BN (Fig. 6) showed

a continuous and rough film on the nanotubes surface of few

nanometres that can be assumed without doubt as the BN

coating.

2.1.3. SiO2 coatings. The two treatments, SiO2-1 and SiO2-2,

with different weight ratios of the pairs MWCNTs-AEAPS and

NH4OH : TEOS, led to continuous coatings on MWCNTs of

�20 nm (Fig. 7a) and �10 nm (Fig. 7b), respectively. Moreover,

the process conducted to the coating of most of the nanotubes, as

FESEM micrograph depicts (Fig. 7c). TGA of the coated

nanotubes allowed estimating the amount of SiO2 incorporated

in each treatment by subtracting to the total weight of the sample

the weight loss due to the MWCNTs thermal oxidation (Fig. 8a).

Then, the resulting value will correspond to the SiO2 residue,

which is not affected by the thermal process, giving 83 and 46

wt% SiO2 content in the SiO2-1 and SiO2-2 coatings, respectively.

As the aim of this functionalization process was to get an

uniform and continuous coating of the nanotubes but limiting at

the same time the SiO2 excess to preclude extensive formation of

liquid during the sintering of the ceramic composite, SiO2-2 was

selected as the most appropriate coating treatment. The

MWCNTs coated following this process will be identified in

further experiments as MWCNT-SiO2. The FTIR analysis

shown in Fig. 3b allowed the identification of the characteristic

stretches of the original nanotubes (MWCNTs-AR) and the

three new bands at 798, 1096 and 3214 cm�1 which were assigned

to Si–OH, Si–O–Si and Si–OH stretching, respectively, in perfect

agreement with the SiO2 coating over these MWCNTs.

Fig. 7 TEM micrograph of coated MWCNTs by SiO2-1 treatment (a),

and TEM (b) and FESEM (c) micrographs of MWCNTs coated by

SiO2-2 treatment.

This journal is ª The Royal Society of Chemistry 2011

Fig. 8 TGA spectra of (a) SiO2 coated MWCNTs and (b) pristine and

functionalized MWCNTs.

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2.1.4. Thermal stability of functionalised MWCNTs. TGA

analysis of MWCNTs-AR, MWCNTs-OX, MWCNTs-BN and

MWCNTs-SiO2 (Fig. 8b) showed differences in the thermal

stability of the nanotubes as a function of the functionalization

process. In this sense, the pristine MWCNTs-AR remained

undamaged up to �500 �C, temperature at which the thermal

oxidation of these nanotubes started. The oxidation was

completed at �630 �C, leaving a residue of about 5 wt% of the

initial mass of nanotubes, which comes from the metallic cata-

lysts of the nanotube synthesis. The thermal degradation of

MWCNTs-OX presented two stages: an initial weight loss below

500 �C of �10 wt%, due to the decarboxylation of the carboxylic

groups and the elimination of hydroxyl groups attached to the

MWCNTs walls,18 and a subsequent oxidation at �500 �C

similarly to the pristine MWCNTs but at slower rate. For these

treated nanotubes, the complete oxidation ended at 690 �C,

higher temperature that the 630 �C of the original nanotubes,

and, furthermore, no residue was detected because the residual

catalytic metallic particles were removed by the oxidation

treatment.

The BN coating of the nanotubes increased their thermal

stability, preserving the nanotubes against the oxidation up to

temperatures around 600 �C, which means 20% higher thermal

stability than either MWCNTs-AR or MWCNTs-OX. The

thermal process left a residue of 10 wt% attributable to the BN

coating. Regarding the MWCNTs-SiO2, the thermal analysis

plot showed a weight loss up to 500 �C due to the decomposition

of the organic coupling agent (AEAPS) and a second loss at

temperatures above 500 �C, at which point the oxidation of the

nanotubes slowly went on ending at 720 �C. A residue of about

This journal is ª The Royal Society of Chemistry 2011

50 wt% of the initial mass that corresponded to the SiO2 coating

left after the thermal cycle.

2.2. Si3N4/MWCNTs nanocomposites

2.2.1. Microstructural characterization. Table 2 summarizes

the apparent density, r, and the relative density with respect to

the theoretical value, rth, calculated assuming the rule of

mixtures for the matrix and for the MWCNTs.

For the SPS set point temperature of 1575 �C, homogenous

and almost fully dense ($99.4% rth) Si3N4/MWCNTs-AR and

Si3N4/MWCNTs-OX nanocomposites were obtained (Fig. 9a

and 9b and Table 2). However, coated MWCNTs containing

composites exhibited lower densities (rth of 96.2% and 97.5% for

Si3N4/MWCNTs-BN and Si3N4/MWCNTs-SiO2, respectively),

as a consequence of the MWCNTs agglomeration forming large

bundles within the matrix as seen in Fig. 9c and 9d.

It appears that the stability of the original nanotubes disper-

sions was modified by the coatings; therefore, the addition of

a dispersant agent (sodium dodecyl benzene sulfate, SDBS) in

aqueous media was tried to disentangle these coated nanotubes.

Thus, aqueous coated MWCNTs suspensions with a SDBS/

MWCNTs concentration ratio of 0.25 were sonicated in an

ultrasonic bath for 3.5 h. Afterwards, the stable suspensions were

freeze-dried and redispersed in ethanol, following from this point

the same procedure employed for the Si3N4/MWCNTs materials.

Besides, the SPS end temperature was raised up to 1600 �C to

enhance the densification as the coatings formed a much stiffer

CNTs network precluding densification, which required an

additional input of thermal energy similarly to what happens

when sintering composites containing non-sinterable particles of

high aspect ratio.29 With this new processing route, coated

MWCNTs showed a better dispersion within the matrix (Fig. 9e

and 9f), and densities of 98.5 and 99.3 % of rth were achieved for

the BN and SiO2-coated nanotubes containing nanocomposites,

respectively (Table 2).

The ID/IG values assessed from the micro-Raman spectra of

the different nanocomposites (Table 2) confirmed the absence of

nanotubes degradation due to the sintering process.

Regarding the Si3N4 matrix, the a-phase content increased

from 40%, for the monolithic material, up to 65%, for Si3N4/

MWCNTs-BN nanocomposite (Table 2), this behaviour being

related to the lower final density of this nanocomposite. Besides,

the CNTs produced some matrix refinement, with the average

size of Si3N4 grains, d50, decreasing from �300 nm for the

monolithic to 220–250 nm for the nanocomposites. This result

supports that the grain growth stage of the liquid phase sintering

mechanism was somehow suppressed by the presence of nano-

tubes.17 The slight differences observed in d50 with the type of

functionalized MWCNTs were associated to distinct a-phase

contents, i.e. higher content will lead to lower d50.

2.2.2. Mechanical properties. The Elastic modulus (E),

Vickers hardness (HV) and fracture toughness (KIC) of the

nanocomposites as a function of the CNT functionalization

process were determined by Vickers indentation using a universal

testing machine provided with a hardness measurement unit

(Fig. 10).

J. Mater. Chem., 2011, 21, 6063–6071 | 6067

Table 2 Microstructural features of the blank Si3N4 and the Si3N4/MWCNTs nanocomposites: apparent (r) and relative densities (rth), Ramanintensity ratios between D and G bands (ID/IG), a-phase content, average grain size (d50), and average aspect ratio (AR50)

Material r /g cm�3 rth (%) ID/IG a-phase (%) d50 /nm AR50

MWCNTs-AR — — 0.78 — — —Si3N4 3.23 99.9 – 40 297 1.8Si3N4/MWCNTs-AR 3.15 99.4 0.86 48 237 1.5Si3N4/MWCNTs-OX 3.16 99.5 0.87 56 220 1.5Si3N4/MWCNTs-BN 3.06 98.5 0.83 65 220 1.6Si3N4/MWCNTs-SiO2 3.17 99.3 0.75 44 251 1.7

Fig. 9 FESEM micrographs of the fracture views showing Si3N4/

MWCNTs nanocomposites obtained by means of the standard process-

ing method, as a function of the nanotubes type: (a) MWCNTs-AR, (b)

MWCNTs-OX, (c) MWCNTs-BN and (d) MWCNTs-SiO2. Nano-

composites developed using the improved processing route based on the

addition of a dispersant agent are shown in (e) Si3N4/MWCNTs-BN and

(f) Si3N4/MWCNTs-SiO2.

Fig. 10 Mechanical properties of the blank Si3N4 and the Si3N4/

MWCNTs nanocomposites for the different functionalization processes:

(a) Elastic modulus (E), (b) Vickers hardness (HV) and (c) fracture

toughness (KIC).

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TEM micrograph of Si3N4-MWCNTs-AR (Fig. 11) showed

the characteristic microstructure of these nanocomposites, with

MWCNTs located at grain boundaries and appearing twisted

and bent. This location, together with the high deformability of

CNT30 and the presumable low attachment between the original

CNTs to the matrix (Si3N4 plus glassy phase), suggests that

a poor load transfer between matrix and nanotubes exists.

Besides, the nanotubes seem to act as rollers that enhance the

grain boundary sliding when load is applied, increasing defor-

mation and, correspondingly, giving an effective lower elastic

modulus.

On the other hand, the nanocomposites containing function-

alized MWCNTs regained the matrix mechanical parameters

compared to those of the non-functionalized CNTs nano-

composites. In this way, E increased from 175 to 212 GPa when

the nanotubes were oxidized, and up to 225 GPa with the SiO2

coated CNTs, which is indicative of a higher load transfer

between the matrix and these nanotubes, probably due to the

favoured chemical locking. Nevertheless, these values are still

6068 | J. Mater. Chem., 2011, 21, 6063–6071

lower than that of the blank Si3N4 (297 GPa) because, as other

studies have previously reported,31,32 the stress transfer from the

outer to the inner walls in MWCNTs is commonly poor and

decreases with the number of internal walls, leading to a lower

effective elastic modulus in MWCNTs composites.

HV evolution followed a similar trend to E, i.e., all function-

alization methods increased the hardness values of the nano-

composites compared to 12.5 GPa of the Si3N4/MWCNTs-AR

composite (Fig. 10), achieving a maximum value of 18.1 GPa for

Si3N4/MWCNTs-OX, a value quite close to that of the mono-

lithic material (18.9 GPa).

This journal is ª The Royal Society of Chemistry 2011

Fig. 11 TEM micrograph of Si3N4/MWCNTs-AR nanocomposite.

MWCNTs are pointed out by arrows.

Fig. 12 FESEM micrographs showing details of the fracture surfaces of

(a) Si3N4/MWCNTs-OX, (b) Si3N4/MWCNTs-BN and (c) Si3N4/

MWCNTs-SiO2 nanocomposites. Toughening mechanisms such as

MWCNTs bridges and pull-out can be observed.

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As mentioned before, the KIC parameter for Si3N4 based

ceramics is very sensitive to the matrix grain size and aspect ratio

distributions and, therefore, it is difficult to determine the real

toughening effect of CNTs if dissimilar microstructures are

compared. For that reason, the decrease from 4.5 for monolithic

material to 2.9 MPa$m1/2 for the Si3N4/MWCNTs-AR nano-

composite was affected by both coarser and more elongated

grains of the blank Si3N4 (d50 ¼ 297 and AR50 ¼ 1.8, Table 2)

compared to the nanocomposite (d50 ¼ 237 nm and AR50 ¼ 1.5),

which would lead to higher KIC values for the monolithic

material, and the poor matrix-nanotube mechanical interlock in

Si3N4/MWCNTs-AR that would limit the development of

toughening mechanisms due to the nanotubes. However, once

more, the functionalized MWCNTs were not only able to reverse

that trend, augmenting KIC up to 3.5 and 4.0 MPa$m1/2 for Si3N4/

MWCNTs-OX and Si3N4/MWCNTs-BN, respectively, which

had the finest microstructure (d50 ¼ 220 nm), but also enhancing

the mechanical response of the monolithic material in � 7% (4.8

MPa m1/2), as it happens for Si3N4/MWCNTs-SiO2 with still

a 15% finer microstructure (d50 ¼ 251 nm) than Si3N4.Therefore,

this was plain evidence of the stronger mechanical interlocking

between functionalized MWCNTs and the Si3N4. Furthermore,

FESEM observations of the fracture surfaces of the different

materials (Fig. 12) corroborated the excellent anchorage of the

functionalized nanotubes to Si3N4 grains, as well as numerous

examples of MWCNTs bridging and pull-out mechanisms, both

typical of fibre reinforced ceramic composites. As a result,

functionalized CNTs enhance the fracture toughness of Si3N4

materials if similar microstructures are compared, although that

enhancement is not as remarkable as in oxide-based/CNTs

composites.6

Further improvements in the mechanical properties could be

attained considering new approaches, such as the functionali-

zation of single-walled CNTs or the use of multilayered coatings

to change the residual stress field in the composite.

3. Experimental

Functionalization treatments

Commercial MWCNTs (MWCNTs-AR, 30 nm diameter and

1–5 mm length, Nanolab Inc., USA) were used. Chemical

oxidation processes were performed in air, basic and acid media.

This journal is ª The Royal Society of Chemistry 2011

For the treatment in air, MWCNTs-AR were placed into

a furnace under air atmosphere at 300 �C for 1 h using heating

and cooling rates of 10 �C min�1. The basic treatment was done

mechanical stirring MWCNTs-AR in a mixture of ammonium

hydroxide (NH4OH) and hydrogen peroxide (H2O2) (volumetric

ratio 1 : 1); meanwhile the acid treatments were carried out in

nitric acid (HNO3) or in a mixture of HNO3 and sulfuric acid

(H2SO4) (volumetric ratio 3 : 1). After each chemical oxidation

procedure, the nanotubes were thoroughly washed with deion-

ized water until pH¼ 7 was obtained and, then, filtered and dried

at 120 �C during 12 h.

A previous oxidation step was required before the BN coating,

for which the OX6 route was chosen as the most effective

oxidation treatment. Next, the oxidized nanotubes were soaked

at 55–60 �C for 1 h in a solution of boric acid (H3BO3) and urea

(CO(NH2)2) (molar ratio 1 : 6). Then, MWCNTs were filtered,

air dried at 120 �C and heat treated at 1000 �C for 3 h under

flowing N2 atmosphere. The heating rate was 10 �C min�1.

SiO2 coating was also carried out in two steps. First,

MWCNTs-AR were dispersed in ethanol using an ultrasonic

bath during 10 min. Afterwards, an excess of 3-2-amino-

ethylaminopropyldimethoxymethysilane (AEAPS) was added

and the suspension was refluxed and magnetically stirred at

100 �C for 6 h. The nanotubes were filtered and repeatedly

J. Mater. Chem., 2011, 21, 6063–6071 | 6069

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washed with ethanol to remove the residual AEAPS and, then,

redispersed in ethanol for 30 min using an ultrasonic bath; later

on, a solution of ammonium hydroxide (NH4OH) and tetrae-

thylorthosilicate (TEOS) was added. Two weight ratios of

MWCNT : NH4OH : TEOS (1 : 180 : 60 and 1 : 45:15) were

used to develop two different SiO2 coatings labelled as SiO2-1

and SiO2-2, respectively. These suspensions were vigorously

stirred during 24 h at room temperature and, finally, nanotubes

were filtered and washed with ethanol to remove any SiO2 excess.

Preparation of nanocomposites

Si3N4/MWCNTs nanocomposites containing 5.3 vol% of nano-

tubes were prepared as described elsewhere.13 In summary, both

MWCNTs and the ceramic powders (Si3N4 plus 2 wt% of Al2O3

and 5 wt% of Y2O3) were separately dispersed in ethanol and,

then, mixed in an ultrasonic bath under continuous stirring. In

the particular case of nanocomposites containing coated

MWCNTs, the weight corresponding to the coatings was esti-

mated to recalculate the amount of nanotubes necessary to keep

a total MWCNTs content of 5.3 vol%. The densification process

was carried out in a graphite die using the spark plasma sintering

technique (SPS, Dr Sinter, SPS-510CE) at 1585–1600 �C for

5 min under a vacuum atmosphere of 6 Pa, applying a uniaxial

pressure of 50 MPa. A Si3N4 blank specimen was equally pro-

cessed for comparison.

Characterization

O2/N2 adsorption-desorption method (LECO TC-436) was used

to analyze the amount of oxygen on the nanotubes surface. FTIR

spectroscopic measurements (Bruker IFS60v) were done in KBr

pellets prepared with very small amount of the different nano-

tubes. MWCNTs-BN were characterized by XPS (VG ESCA-

LAB 200R) as well. The spectra were obtained with

a hemispherical electron analyzer employing a Mg Ka 120W

X-ray source. The thermal stability of the different MWCNTs

was followed from 25 to 1000 �C with a thermogravimetric

analyzer (STA 409, Netzsch, Selb) under air atmosphere and

with a heating rate of 10 �C min�1. Micro-Raman spectra were

taken with a Renishaw inVia equipment, using excitation energy

of 2.54 eV and a laser wavelength of 514 nm, to study the possible

degradation during both nanotube functionalization and

composite sintering. FESEM (S-4700, Hitachi) and TEM (125

kV, Hitachi H-7100 and 400 kV, Jeol JEM-4000 EX) were used

to observe the functionalized MWCNTs, their dispersions, as

well as the microstructure of the sintered nanocomposites.

Composites were viewed in both fractured and polished form,

in the last mode the specimens were plasma etched in a CF4/5

vol% O2 plasma at 100 W for 25 s to reveal grain boundaries.

Apparent density of the specimens was determined by the water

immersion method. Crystalline phases and a/b-Si3N4 trans-

formation degree were determined by XRD (Bruker D5000,

Siemens) procedures using a well-known expression.33 Average

Si3N4 grain diameter (d50) and apparent aspect ratio (AR50) were

estimated on FESEM micrographs of polished and etched

samples using image analysis techniques. At least 1000 features

were counted for each specimen.

6070 | J. Mater. Chem., 2011, 21, 6063–6071

Mechanical parameters such as elastic modulus (E), hardness

(HV) and fracture toughness (KIC) were measured using an

instrumented microindenter (Zwick/Roell, Zhu 2.5). Vickers

pyramid indenters were used at different loads depending on the

type of test, 98 N for E and HV and 196 N for E and KIC. At least

five well-defined indentations were performed for each load and

specimen. KIC was calculated using the expression by Miranzo

et al.34

4. Conclusions

MWCNTs were successfully functionalized by means of oxida-

tion and BN and SiO2 coatings. Strong acid conditions promoted

the effective surface oxidation of MWCNTs, without signs of

degradation, attaching hydroxyl and carboxyl groups to the

outer wall of the nanotubes. Chemical reaction between boric

acid and urea, and further thermal treatment under nitrogen

atmosphere produced a continuous BN coatings few nanometers

thick on MWCNTs. The two step process with AEAPS and

NH4OH : TEOS, respectively, led to the development of 10 nm

SiO2 coatings. All the functionalized CNTs exhibited an

enhanced thermal stability compared to pristine nanotubes,

MWCNTs-BN being the most thermally stable.

Dense Si3N4/MWCNTs nanocomposites with good nanotubes

dispersion within the matrix and without signs of degradation

were attained, although those containing coated CNTs required

both a dispersant agent to prevent the formation of nanotubes

agglomerates and slightly higher sintering temperatures. The

chemical surface roughness acquired by CNTs with the oxidation

treatment enhanced the mechanical interlocking between the

matrix and the nanotube, and similarly occurred with the BN

and SiO2 coatings on the nanotubes. Both treatments led to

improvements of the mechanical properties, compared to the

nanocomposites containing pristine CNTs, such as for the KIC

values that were higher than the toughness of the blank Si3N4.

The choice of the most effective functionalization process should

be based on the final application of the material considering key

factors such as CNTs thermal stability, simplicity and cost of the

composite manufacturing process or the mechanical response of

the material.

Acknowledgements

The authors wish to thank Prof. J. L. G. Fierro from Institute of

Catalysis and Petrochemistry for assistance with XPS tests. The

financial support of Spanish Ministry of Science and Innovation

(MICINN) through project MAT2009-09600 is recognized.

J. Gonz�alez-Juli�an acknowledges the financial support of the

JAE (CSIC) fellowship program.

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