Chinese Journal of Aeronautics, (2015), 28(5): 1529–1538

Chinese Society of Aeronautics and Astronautics& Beihang University

Chinese Journal of Aeronautics

cja@buaa.edu.cnwww.sciencedirect.com

Imaging the interphase of carbon fiber composites

using transmission electron microscopy:

Preparations by focused ion beam, ion beam

etching, and ultramicrotomy

* Corresponding author. Tel.: +86 10 82339800.

E-mail address: leemy@buaa.edu.cn (M. Li).

Peer review under responsibility of Editorial Committee of CJA.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.cja.2015.05.0051000-9361 ª 2015 The Authors. Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Wu Qing, Li Min *, Gu Yizhuo, Wang Shaokai, Zhang Zuoguang

Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science andEngineering, Beihang University, Beijing 100191, China

Received 31 December 2014; revised 25 March 2015; accepted 9 April 2015

Available online 20 June 2015

KEYWORDS

Carbon fiber composite;

Chemical analysis;

Focused ion beams;

Interphase;

Ion beam etching;

Microstructure;

Ultramicrotomy

Abstract Three sample preparation techniques, focused ion beam (FIB), ion beam (IB) etching,

and ultramicrotomy (UM) were used in comparison to analyze the interphase of carbon fiber/epoxy

composites using transmission electron microscopy. An intact interphase with a relatively uniform

thickness was obtained by FIB, and detailed chemical analysis of the interphase was investigated by

electron energy loss spectroscopy. It shows that the interphase region is 200 nm wide with an

increasing oxygen-to-carbon ratio from 10% to 19% and an almost constant nitrogen-to-carbon

ratio of about 3%. However, gallium implantation of FIB tends to hinder fine structure analysis

of the interphase. For IB etching, the interphase region is observed with transition morphology

from amorphous resin to nano-crystalline carbon fiber, but the uneven sample thickness brings

difficulty for quantitative chemical analysis. Moreover, UM tends to cause damage and/or

deformation on the interphase. These results are meaningful for in-depth understanding on the

interphase characteristic of carbon fiber composites.ª 2015 The Authors. Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA. This is an

open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The use of carbon fiber composites has substantially increasedin the fields of aerospace, transportation, and sports goods due

to their excellent properties, such as high specific strength, highspecific modulus, and the ability to be tailored for specificapplications.1–3 At the same time, a great deal of scientific

efforts has been focused on the analysis of interfacial proper-ties and the approaches to improve, since it is well recognizedthat the interphase significantly impacts the final behavior of

1530 Q. Wu et al.

composites.4–6 At the core of these efforts lies in the need tounderstand the structure, mechanical, and physico-chemicalproperties of components, as well as their interactions at the

interphase region across multiple length scales starting fromnanoscale characterization. Based on this, nanomechanicaltechniques, such as dynamical modulus mapping,7 atomic

force microscopy,8,9 nanoindentation and nanoscratch,10,11

and fiber push-in and push-out tests,12–15 have been developedto reveal the thickness and mechanical properties of interphase

in specific composite systems. However, knowledge of theinterphase morphology as well as the structure and chemicalproperties between carbon fiber and polymer is still lacking atthe present time. Detailed studies of microstructure and

physico-chemical properties are favorable to understand theinterfacial functionary mechanism and the structure–propertyrelationship, which are critical to optimize composite behaviors.

Transmission electron microscopy (TEM) provides theunique combination of analytical techniques, for example,electron energy loss spectroscopy (EELS) and energy disper-

sive X-ray (EDX), which is suited and able to characterizethe structural and chemical information of a sample at thenanometer scale. However, such studies require the sample

to be transparent (approximately 100–150 nm) to the electronbeam and being much thinner is preferred for EELS analy-sis.16,17 Preparation of such a sample, especially for the inter-phase of heterogeneous materials, is both a science and a

challenge. Focused ion beam (FIB),18–20 ion beam (IB) etch-ing,21–23 and ultramicrotomy (UM)24–26 are three commonpreparation techniques. FIB uses a finely focused beam of ions

to bombard a target so that site-specific milling or cutting canbe performed.17 IB etching is a sputtering process in whichenergetic neutral atoms or ions from a cathode impinge on a

sample wafer, at an angle.16 UM produces an ultrathin sectionwith the thickness down to approximately 30 nm by creating amicro crack that progressively propagates into a sample.27

Since sample preparation techniques are very material-dependent, the selection of a suitable preparation techniqueis of great significance for TEM analysis. For carbon fiber rein-forced resin composites, the challenge arises because of the

huge mismatch in properties, such as modulus and hardness,between carbon fiber and resin. This brings great difficultyfor preparing a thin TEM sample with an intact interphase.

Based on this, herein, FIB, IB etching, and UM techniqueswere respectively used to prepare a TEM sample of a carbon

Fig. 1 Cure cycle and metallographic imag

fiber/epoxy composite. Our specific goals were to (1) identifythe capabilities of these preparation methods for TEM analysisof the interphase in the carbon fiber/epoxy composite, (2)

reveal the suitability and strength of each method for investi-gating which characteristic of the interphase, and (3) under-stand the microstructure, chemical components, chemical

bonding states, and thickness of the interphase.

2. Experimental

2.1. Materials

Unidirectional T300-3K-40B carbon fiber (7 lm in diameter,Toray Inc.)/epoxy (5228, Beijing Institute of AeronauticalMaterials) prepreg was impregnated by the hot-melt method

and cured in autoclave. The cure cycle and metallographicimage of the prepared composite are shown in Fig. 1. The com-posite has a uniform distribution of carbon fibers and a lowvoid content. The tensile modulus of the fibers is 230 GPa

(from TORAYCATM carbon fibers data sheet), and that forthe epoxy matrix is determined to be about 3.5 GPa accordingto GB/T 2567–2008.

2.2. Preparation methods

FIB experiment was performed in an AURIGA 40 (Carl Zeiss,

Germany) Dual Beam FIB-scanning electron microscope(SEM) system with a Ga+ ion source at 30 kV. The prepara-tion process is illustrated in Fig. 2. Firstly, the cross-section

of the composite was identified and targeted as the region ofinterest (yellow rectangular box in Fig. 2(a)). Secondly, a1 lm platinum (Pt) protective layer was deposited on the sur-face of the target milling area (see Fig. 2(b)), and then coarse

and medium milling was performed (see Fig. 2(c)). 20 nAbeam current was used for coarse milling until the samplewas left with a 2 lm thickness, and then 4 and 1 nA beam cur-

rent was for medium milling until a thickness of �1 lm wasreached. Water gas was used for fast material sputtering.Thirdly, the section was detached from the surrounding mate-

rial and transferred to a TEM half-grid for fine thinning (seeFig. 2(d)). Fourthly, fine thinning was carried out only atthe desired areas (two braces in Fig. 2(e)) until they becametransparent at 3 kV (SEM mode), using 600 pA and 240 pA

e of the prepared T300/epoxy composite.

Fig. 2 Series of SEM images visualizing the sample preparation process of longitudinal direction of T300/epoxy composite.

Imaging the interphase of carbon fiber composites using transmission electron microscopy 1531

beam current. The un-thinned part serves as a support frameto avoid bending and shrinkage deformations of the sample.Finally, the amorphous layer on the sample’s surface was

removed with a low-energy Ga beam (5 kV, 20 pA).For IB etching, a Minitom precision cut-off machine

(Struers Inc. Copenhagen/Denmark) was used to cut a 1 mmthickness sample from the composite block. Then, the sample

was carefully grinded and polished to 10–15 lm thickness by acombination of an automatic polish-grinding machine (StruersInc.) and manual operations. Intermittent IB etching, i.e.,

15 min milling followed by 10 min rest, was carried out in anRES 101 (BAL-TEC, GER) Ion Mill using a rotating stageand two ion guns with argon to prevent the temperature

increase of the sample and its surroundings. The voltage was6 kV and the ion current reached 2 mA. According to the thin-ning degree, the milling angle was lowered from 15� to 5� as

milling progressed.For UM, a Leica EM TXP fitted with a diamond knife was

used to trim down the cutting face of the composite block to ataper, which would provide a suitable cutting area to the knife.

The sample after trimming has a smooth and flat surface, aswell as sharp edges. This is significant to obtain high-qualityultrathin sections. Ultrathin sectioning was conducted on a

Leica EMUC7/FC7 ultramicrotome with an ultra 35� diamondknife. The sectioning speed was 25 mm/s and the prepared sec-tions were picked up with a C-flat grid from the water trough.

2.3. Characterization

TEM imaging was performed using a JEM-2100F (JEOL)

field-emission electron microscope operating at an acceleratingvoltage of 200 kV.

EELS test was carried out on a Tecnai G2 F20 U-TWIN(FEI Inc.) field-emission electron microscope operating at

200 kV in the scanning transmission electron microscope(STEM) mode, providing an energy resolution of 0.6–0.8 eV.EELS is based on the acquisition of a spectrum of inelastically

scattered electrons. Digital images and energy loss spectra werecaptured using a Gatan imaging filter (GIF Tridiem) with adispersion of 0.2 eV/pixel and an energy shift of 270 eV.Spectral background was removed by fitting the pre-edge

background with a power law function. The beam convergenceangle a was 0.58 mrad. The distance from the projector cross-over to the recording plane was 438.5 mm, and that from the

crossover to the actual entrance aperture (2 mm in diameter)was 748.74 mm. Obtaining spectra in the STEM mode usinga camera length of 500 mm provided a collection semi-angle

(b) of 2.34 mrad.28 With the need of chemical analysis, multi-ple scattering was removed off from EELS spectra by theFourier-ratio deconvolution method. The sample thickness

can be measured from the low energy electron loss spectrum,since the amount of all inelastic scattering increases with thesample thickness.28,29 The thickness (t, nm) normalized tothe average mean free path (k, nm) is measured using the

log-ration method:

t

k¼ ln

ItI0

� �ð1Þ

where It is the total intensity in the spectrum and I0 is theintensity under the zero-loss peak. From the quantitative view,

k needs to be determined according to the following equations:

k ¼ 106FE0

EmInð2bE0=EmÞð2Þ

where E0 is the voltage, kV, F is a relativistic correction factorrelated with E0, and Em is the average energy loss, kV, whichcan be calculated by the average atomic number Z of amaterial.

1532 Q. Wu et al.

Em ¼ 7:6Z0:36 ð3Þ

F ¼ 1þ E0=1022

1þ ðE0=511Þ2ð4Þ

Since carbon is the main element in both fiber and resin, hereassume Z � 6 for all the three phases (i.e., fiber, interphase,and resin) of the T300/epoxy composite. Accordingly, k is cal-

culated to be approximately 169 nm. Thus, the thickness of theFIB-produced sample was in the range of 95–118 nm (0.56k–0.70k) with no obvious variation. For the IB etching-

produced sample, gradual increasing of the thickness from30 to 186 nm (0.18k–1.10k) was observed from the resin tothe fiber.23 The thicknesses of carbon fiber and epoxy in the

UM-produced sample were respectively about 50 and 12 nm(0.30k and 0.07k). It’s worth noting that k will decrease withincreasing content of nitrogen or oxygen, which have higheratomic numbers than carbon. Thus the true thicknesses for

the interphase and the resin might be 1%–3% lower.EDX tests were conducted on an analysis system (type

TEAMTM, from EDAX Inc.) with an energy resolution of

133 eV. The beam spot size of the EDX-line scan was 0.5 nm.

3. Results and discussion

3.1. FIB preparation for composite interphase

Fig. 3(a) presents the TEM image of the FIB-producedT300/epoxy, including the bright epoxy resin, the dark carbonfiber, and the in-between interphase region. The periodical

arrangement of lighter and darker double stripes along thefiber axis is seen at the top right corner, which is ascribed tothe fiber cortex. The epoxy resin is the amorphous structure.The interphase region is further magnified in Fig. 3(b); how-

ever, the detailed nanostructure cannot be analyzed. The mainreasons are associated with two aspects. Firstly, the sample isnot thin enough, and thus the electron beam transmitted

through the sample, which carries the interior structure infor-mation of a material, is less. Secondly, there are many blackspots in Fig. 3(b), particularly obvious in the region marked

by a yellow ellipse, which is the evidence of Ga+ ion implan-tation. The implantation amount of Ga is investigated by the

Fig. 3 TEM image of FIB-produced composite, show

EDX line-scan by performing through a structural repeatingunit, as shown in Fig. 4(a). The obtained Ga X-ray intensityvariation versus the position is illustrated in Fig. 4(b). Low

Ga intensities locate at about 0–0.3 lm and 0.9–1.16 lm, andthe high intensities are at 0.5–0.7 lm. Two gradual transitionregions at probably 0.3–0.5 lm and 0.7–0.9 lm are observed.

The Ga+ ion implantation capability is different in the threephases of the composite, and the bulk carbon fiber has a betterGa+ resistance due to its crystal structure than the resin matrix

and the interphase. Six spots (the red dashed circles inFig. Fig. 4(a)) were selected to calculate the atomic contentsof elements, particularly for Ga. As listed in Fig. 4(c), theGa contents are no more than 0.48, indicating that Ga+ ion

implantation is well controlled by Pt-strap prior to FIB-milling. Compared with the light elements contained in thecomposite, the relatively high atomic number 31 of Ga makes

it eye-catching in the image. Therefore, such a small amount ofGa would hinder fine structural analysis of the interphaseunder a high resolution.

The former result indicates uniformity of the FIB-producedcomposite, and thus detailed chemical analyses are feasible byEELS, performed at 12 probe-points along 550 nm crossing

the interphase region, as shown in Fig. 5(a). The C-K, N-K,and O-K edges, respectively starting at around 287, 402,540 eV, are clearly identified in the EELS spectra (Fig. 5(b)).A feature at approximately 330 eV or 350 eV is a multiple-

scattering resonance (MSR), which represents the energy ofthe standing wave set up when the excited wave is scatteredback from the parent atom’s second nearest neighbors.30,31

The MSR is related with the C–C bond length,30 and theshorter the bond length is, the lower energy is needed to scatterback. Fiber has a high degree of graphite crystallite, which has

minimum heteroatoms and aliphatic molecules (plenitude inthe interphase and the epoxy resin). Thus, probe-points 1–3are inferred at the fiber cortex. Other probe-points 4–12 are

at the interphase and the resin, while the exact belongings needfurther analysis.

The process of inner-shell ionization is one of the principalinelastic interactions and the inner-shell ionization edges of

elements can be used for chemical bonding analyses of a sam-ple, especially sensitive for light elements. Thus, chemicalbonding states were further analyzed from the energy-loss

ing three regions: fiber, interphase and epoxy resin.

Fig. 4 EDX line-scan results of the FIB-produced T300/epoxy from carbon fiber to epoxy resin to carbon fiber.

Imaging the interphase of carbon fiber composites using transmission electron microscopy 1533

near-edge structure (ELNES) of C K-, N K-, and O K-edges,as shown in Fig. 5(c) and (d). Since chemical bonding analy-sis for carbon fiber/epoxy composites is rarely reported, the

spectra discernible features are assigned based on relevant car-bon materials and organic polymers.32–38 In Fig. 5(c), the CK-ELNES shapes of probe-points 1–3 are similar to those of

graphitic carbon, while those of other probe-points 4–12 agreewith those of amorphous carbon.39 The sharp and clear fea-tures (peaks 1) of probe-points 1–3 at (287 ± 0.3) eV and

probe-points 10, 11 at 289.3 eV are respectively ascribed tothe C‚C p* resonance and the overlap of C‚O p* and C–H r* resonances. In comparison, peaks 1 of other probe-points (4–9 and 12) at (288 ± 0.4) eV become wider, which

could be affected by C‚C p* and C‚O p* resonancestogether. Peak 2 at (295.0 ± 0.5) eV is a C–C r* resonance.Based on the bond length correlation,26 peaks 3 at 299.2 eV

for probe-points 1–3 are C–N r* resonances. Sizing agentsof both the T300 and the matrix are both epoxy type,40 sothe bond length of C–O in epoxy is large due to the existence

of angle tension, which makes the electron cloud not well over-lapped. Thus, peaks 3 of probe-points 4–12, ascribed to thesuperposition of C–O r* and C–N r* resonances, are shifted

lower. By contrast with probe-points 4–9 and 12, peaks 3 ofprobe-points 10, 11 at 298.4 and 298.8 eV are slightly higher,which might be affected by higher contents of nitrogen. Theweak features of peaks 4 at (302 ± 1.3) eV are observed for

probe-points 4–12, which is associated with the r* resonanceof C‚O bonds. In Fig. 5(d), two features at 402 eV (peak 5)and 410 eV (peak 6) of the N–K edge are the 1s to p* and 1s

to r* anti-bonding orbitals. Peaks 7, 8, and 9 are respectivelyO‚C p*, O–C r*, and O‚C r* resonances, and only probe-points 10 and 11 observe peaks 9. From above analyses, the

low Ga intensity, the position of MSR, and the C K-ELNESshapes confirm that probe-points 1–3 are at the carbon fiber.In addition, the different features, positions, and assignments

of peak 1 and peak 3 from the C K-ELNES indicate that theinterphase region is located between probe-points 4 and 9,about 200 nm in thickness. Probe-points 10 and 11 are in the

epoxy resin.The element amounts, calculated by the Digital Micrograph

software, are dependent on beam energy, convergence semi-

angle, collection semi-angle, and the collected EELS spectra.The results are plotted in Fig. 6. Taking account of the semi-quantitative accuracy of this method, only the integer part of

percentage is deemed to be valid. There is about 98% carbonin the carbon fiber (probe-points 1–3), and a gradual decreaseof carbon content from 88% to 81% is observed in theinterphase region (probe-points 4–9). The resin (probe-points

10, 11) has the minimum carbon content of about 70%. Theratio of nitrogen to carbon (N/C) is basically the same of3% except higher at the epoxy resin (about 8%) due to the

existence of amine cure agent. No oxygen is detected in thecarbon fiber. The ratio of oxygen to carbon (O/C) shows anincreasing trend from probe-point 4 (10%) to 6 (19%), and

then basically is constant until a significant jump betweenprobe-points 9 and 10.

3.2. IB etching and UM preparations for interphase

The results of the IB etching-produced carbon fiber/epoxycomposite and the high resolution transmission electronmicroscopy (HRTEM) image of its interphase are shown in

Fig. 7. Fig. 7(a) shows the amorphous epoxy resin (light grey),the oriented graphite crystallites of fiber (dark grey), and theintact interphase region (grey) between them. Fig. 7(b) exhi-

bits a gradual variation from nanoscale crystalline to amor-phous structure as getting to the matrix. Nanocrystals thatconsist of a stack of aromatic layers, two or three nanometers

in length, are basic structural unit. Our previous work shows

Fig. 5 EELS spectra analysis of the interphase region in the energy loss range of C K-, N K-, and O K-edges.

Fig. 6 Variation of element contents with different EELS probe-

points.

1534 Q. Wu et al.

that the O/C is 15% for the resin and 4%–9% for the inter-phase region in the IB etching-produced sample,23 which areboth lower than the results in the FIB-produced sample. The

reason might be ascribed to the thin thicknesses of the resin

and the interface regions (lower than 50 nm) that are unableto sustain electron beam irradiation and are easier to be pene-

trated. Moreover, the sample thickness changes in the threephases, produced by the IB etching method itself. Thus, esti-mation of the interphase width based on a TEM image of

the IB etching sample is not accurate. Related methods anddetailed descriptions can be found in our previous work.23

Here, we will not elaborate.For UM, the sample cutting is conducted along two differ-

ent directions, normal and parallel to the fiber axis respectivelyon the composite, as shown in Fig. 8(a) and Fig. 9(a). Thecutting in ultramicrotome is always in the vertical direction.

When the fibers are lying horizontally, the prepared compositeultrathin section is given in Fig. 8(b). It evidently shows twodifferent structures, which are the stripe structure of the car-

bon fiber and the homogeneous structure of the epoxy. Oneof the interphases is reasonably preserved, while on the secondthere is a gap. During sectioning, the upper surface of the sec-

tion is under tension, and the lower surface is under compres-sion, similar to a curved cantilever beam in Fig. 8(c). In fact,the cutting force (F) on the section can be decomposed intotwo directions, i.e., F1 and F2 in Fig. 8(d). The tension at

Fig. 7 TEM image showing three regions: fiber, interphase, and epoxy resin of the IB etching-produced composite.

Fig. 8 Schematic of sectioning process when cutting direction is normal to fiber axis, TEM image of the as-prepared carbon fiber/epoxy

ultrathin section and force acted on the section.

Imaging the interphase of carbon fiber composites using transmission electron microscopy 1535

the upper cutting surface can be partly offset by shear stress F2,but at the lower original surface, high compression tends to

cause failure, particularly in the interphase region. For theinterphase at the upper cutting surface, a magnified TEMimage is shown in Fig. 8(e), in which the resin region is

smooth with an amorphous structure and the fiber regionreveals aligned fibril textures. However, whether the regionin-between the fiber and the resin is the original interphase

or not remains unclear, because the fiber is pressed againstthe epoxy resin by the cutting force. Note that there are manyfine lines normal to the fiber axis in Fig. 8(b), which should beattributed to the fine imperfections of the cutting edge.

In Fig. 9(a), the composite block was cut longitudinally tothe fiber axis. Fig. 9(b) shows that the carbon fiber is seriously

distorted and damaged. As the knife is acted on the section,pressure is applied on the fiber and the resin simultaneously.

In this case, the carbon fiber tended to be damaged due to brit-tleness, and shear stress (s) concentrated along the interphaseregion. A sketch of stress on the section is depicted in

Fig. 9(c). Therefore, the intact interphase structure cannotbe identified from the distorted and fractured carbon fiber.

3.3. Comparison of different preparation methods

Three preparation methods used in this paper are compared inanalyzing the interphase of the carbon fiber/epoxy composite,as summarized in Table 1. FIB is a suitable and convenient

preparation technique for chemical analysis of the interphase.

Fig. 9 Schematic of sectioning process when cutting direction is parallel to the fiber axis, the TEM image of the as-prepared carbon

fiber/epoxy section and actual applied force acted on the section.

1536 Q. Wu et al.

A relatively uniform thin section area and a small incidentangle of ion beam are the benefits for TEM and EELS analy-

sis. However, the Ga+ ion implantation limits high resolutionanalysis of fine structures and lattice imaging. IB etching is afeasible method for structure and bonding state analyses of

the interphase region, but an uneven thickness of the sectionbrings difficulty for quantitative chemical analyses.Moreover, in order to avoid sample damage and artifact effect

of IB etching, extra care and awareness are necessary for

Table 1 Comparison of three preparation techniques for interphas

No. Comparison aspects FIB IB

1 Capability for TEM analysis of

interphase

A* A*

2 Interphase integrity Intact Inta

3 Suitable for interphase:

(1) structure NA� A*

(2) chemical component A* BA

(3) chemical bonding A* A*

(4) width (thickness) A*, directly from chemical

analysis

A*,

4 Drawbacks Ga+ ion implantation

5 Preparation time & efficiency Fast (2 h) Tim

day

6 Preparation cost Expensive Hig

Note: A* indicates Applicable; NA� indicates Not Applicable; BA§ indic

operation, including the degree of grinding, the sample cool-ing, and the milling parameters. For the UM method, distor-

tion and fracture inevitably occur in fine structures,particularly in the interphase region and the carbon fiber ofthe composite. Quality of the knife edge is vital for acquiring

a high-quality ultrathin section; however, the diamond knifecan be easily damaged by the hard carbon fibers. Hence,UM is not suitable for investigating the interphase of carbon

fiber/epoxy composites in TEM.

e analysis in carbon fiber/epoxy composites.

etching UM

NA�

ct Deformed or broken

–§ –

–

by tilting the holder19 –

(1) Uneven thickness variation

(2) Complex preparation process

(3) Low success rate of sample

preparation

Expensive diamond knife is easy

to damage

e-consuming (generally a few

s or weeks)

Simplification & Time efficiency

h Low

ates Barely Applicable.

Imaging the interphase of carbon fiber composites using transmission electron microscopy 1537

4. Conclusions

Three methods, namely FIB, IB etching, and UM, were con-ducted for TEM sample preparation to study the interphase

of a carbon fiber/epoxy composite. The section qualities wereanalyzed for different preparation techniques and the effectson structure and composition of the interphase were discussed.

(1) FIB can produce intact morphology of the interphasewith a relatively uniform thin area, allowing detailedanalyses of the chemical compositions and the bonding

states. According to the bonding states, a transitioninterphase area of 200 nm thick is estimated, throughwhich the O/C increases gradually from 10% to 19%

and the N/C is almost constant at 3%. Ga+ implanta-tion is observed in the composite, and its relatively highatomic number (compared with carbon and oxygen)

obscures further fine structure analysis of the interphase.(2) The IB etching technique shows that the interphase

region is a transition area from basically an amorphous

structure to a crystalline structure. However, the unevensection thickness, caused by the method itself and quitediffering sputter yields of fiber and resin, brings diffi-culty for fine chemical analysis within the interphase.

In order to avoid sample damage and artifact effect ofIB etching, utmost care and awareness are necessaryfor operation during the milling process.

(3) UM tends to cause mechanical damage and/or deforma-tion in the interphase region, which is not suitablefor interphase investigation of carbon fiber/epoxy

composites.

These results are meaningful for accurate and in-depthunderstanding on the interphase characteristic of carbonfiber/epoxy composites.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (Grant No. 51273007) and theProgram for New Century Excellent Talents in University ofChina (NCET). Mr. Shi Wei and Mr. Ma Benxiao are thanked

for FIB sample preparation. Thanks are also due to Ms. QiXiaoying for EELS testing.

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Wu Qing is a Ph.D. student in the School of Materials Science and

Engineering at Beihang University. Her main research interest is

interphase/interface of composites.

Li Min is an associate professor and Ph.D. advisor in the School of

Materials Science and Engineering at Beihang University. Her current

research interests are CNT composite materials, interphase/interface of

composites, evaluation of high performance carbon fiber, and theory

analysis of composite manufacturing processes.

Gu Yizhuo is an associate professor in the School of Materials Science

and Engineering at Beihang University. His area of research includes

quality control theory and technology of advanced composite manu-

facturing, high performance matrix resins, nano-composite materials,

and special functional composite materials.

Wang Shaokai received his Ph.D. degree in materials science from

Beihang University in 2011, and then began his postdoctoral research

at Florida State University. His main research interests are high

thermal conductive composite materials, CNT/fiber hybrid compos-

ites, and functional application of CNT film.

Zhang Zuoguang is a professor and Ph.D. advisor in the School of

Materials Science and Engineering at Beihang University. His area of

research includes molding process theory and manufacturing quality

control of advanced polymer matrix composites, hybrid composites,

high impact composites, microstructure and performance evaluation of

carbon fiber and its composites, lightweight sandwich structure com-

posites, and special functional composites.