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Vacuum 102 (2014) 51e53

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Vacuum

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Influence of heat treatment temperature on microstructure andthermal expansion properties of 2D carbon/carbon composites

Ying-bo Fei, Jin-hua Lu*, He-jun Li, Ling-jun Guo, Zi-shan ChenState Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China

a r t i c l e i n f o

Article history:Received 1 September 2013Accepted 21 October 2013

Keywords:Carbon/carbon compositesHeat treatment temperatureThermal stressThermal expansion

* Corresponding author. Tel.: þ86 29 88492272.E-mail address: lujinhua@nwpu.edu.cn (J.-h. Lu).

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a b s t r a c t

The influence of heat treatment from 1900 to 2650 �C on microstructure and thermal expansion prop-erties of thermal gradient chemical vapor infiltration (TCVI)-infiltrated 2D carbon/carbon compositeswas investigated. The structure evolution was characterized by polarized light microscopy, X-raydiffractometer, Raman spectroscopy and thermal expansion behavior was studied by thermal dilatom-eter. The results revealed that with the increasing heat treatment temperature, successive microstruc-tural changes in carbon matrix and fibers occurred, and cracks and pores in the composites increased.These resulted in a 35.6% decrease of CTE in Z direction and a 13.9% decrease in XY direction in thecomposites heat-treated at 2650 �C compared with as-deposited composites. The CTE mainly dependedon the thermal expansion of matrix in Z direction while it relied on that of the fibers in XY direction.

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As new-type thermo-structural materials, carbon/carbon (C/C)composites play a significant role in the fields of aeronautics andastronautics, such as nose tip of reusable space vehicles, airplanebrake discs, ultra-lightweight mirrors and heat sinks [1,2]. Theseapplications need materials to have low coefficient of thermalexpansion (CTE), which depends on the components of C/C com-posites (i.e., carbon fibers, matrix and pores). When C/C compositesare used at high temperature, microstructure will be changed andthermal stress will be produced in the composites because ofdifferent CTEs of fibers and matrix [3e5], which might significantlyinfluence the thermal expansion properties.

The heat treatment of C/C composites is widely used in the in-dustry after the manufacturing process, which would finalize themicrostructure and change the internal stress of C/C composites [6].Zhao [7] and Li [8] et al. have reported that heat treatment couldimprove CTE of C/C composites, while the optimal heat treatmenttemperature on the CTE of composites is still lack of concreteresearch.

In this work, thermal expansion evolution of 2D C/C compositeswith the increasing heat treatment temperature is characterizedand correlatedwithmicrostructural changes to clarify the influenceof heat treatment temperature on the CTE of composites.

2D needled carbon felts with a density of 0.58 g/cm3 are fabri-cated by alternatively stacked non-woven layers and carbon fiberwebs by a needle-punching technique, and then deposited with

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Pyrocarbon (PyC) by thermal gradient chemical vapor infiltration(TCVI), using natural gas as carbon source to obtain C/C composites.The as-deposited C/C composites are marked as S1. Part of the as-deposited samples are then heat-treated at 1900 �C, 2100 �C,2500 �C and 2650 �C, marked as S2, S3, S4 and S5, respectively.

Texture of the samples is characterized by polarized light mi-croscopy (Leica DLMP, PLM). X-ray diffractometer (PAN alyticalX’pert PRO) using Cu Ka radiation and Raman spectroscopy(RENISHAW VIA) with a 514 nm argon ion laser is used for char-acterization of microstructure.

CTE of the samples is measured in the direction parallel (XYdirection) and perpendicular (Z direction) to the non-woven layerby a DIL402C Dilatometer (NETZSCH Germany). Measurement isconducted in argon atmosphere from RT to 1400 �C with a heatingrate of 5 �C/min. The sample dimensions are F6 mm � 20 mm. CTEis calculated from CTE ¼ (DL/L0)/DT, where DL/L0 is the frictionalchange in length of the samples due to temperature change DT.

Fig. 1 shows CTE curves of C/C composites with the increasingheat treatment temperature in Z (a) and XY (b) direction. CTE valuesinitially decrease to the minimum and then increase with the in-crease of temperature. It can be clearly seen that CTEs in both di-rections decrease distinctly with the increasing heat treatmenttemperature over the whole temperature range. However,compared with Z direction, CTE values in XY direction are smallerand have a lower increasing rate above 1000 �C.

The influence of increasing heat treatment temperature oncrystal structure is given in Table 1. Due to an increase of heattreatment temperature, the height of the layer stack height Lc

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Fig. 1. CTE curves of C/C composites with different HTT temperatures in Z (a) and XY (b) direction.

Y.-b. Fei et al. / Vacuum 102 (2014) 51e5352

increases from 6.57 to 16.74 for heat treatment at 2650 �C while theinterlayer spacing d002 slightly decreases. The diminishing FWHMvalues suggest a narrow 002 peak and amore preferred orientation.Correspondingly, the graphitization degree increases to 30.2% forthe sample treated at 2650 �C. This is also confirmed by thedecrease of d002 and the pronounced increase of Lc after heattreatment. Thus, a highly orientated structure is obtained in thecomposites with the increasing heat treatment temperature.

In addition, the results of Raman demonstrate the increase ofcrystal size La, which is consistent with XRD results. Raman spectra(Fig. 2) exhibit a first-order graphite peak at about 1580 cm�1 (Gband) and a disorder-induced peak at 1360 cm�1 (D band). Therelative intensity ratio of D and G bands (R ¼ ID/IG) decreasesgradually with the increasing heat treatment temperature, varyinginversely with the in-plane crystal size La. Meanwhile, Fig. 2 illus-trates that the Raman peaks slightly shift to lower wave-numberwith the increasing heat treatment temperature, which indicatesthe existence of residual tensile stress in PyC matrix.

It is evident that after heat treatment at different temperatures,the microstructure of C/C composites remarkably changes. The re-sults of CTE tests and structural studies reflect the close correlationbetween the microstructure and thermal expansion properties.

Heat treatment leads to successive changes in the matrix andfibers. In Z direction, CTE mainly depends on the c-axis (perpen-dicular to graphene layers) thermal expansion of PyC. It can beinferred from the results of XRD and Raman investigations that theincreasing heat treatment temperature causes crystal growth andpreferred orientation structure. This is due to the texture reorga-nization of PyC matrix that most of graphene layers in the matrixare oriented parallel to the fiber axes. The structural changes pro-mote stronger bonding force between graphene layers, whichlimits the expansion in the matrix and decrease the CTE. Thus, CTEin Z direction of the composites decreases gradually with theincreasing heat treatment temperature.

PLM micrographs of the samples (S1, S3 and S5) are shown inFig. 3. The extinction cross around carbon fiber indicates that PyCtexture is medium texture and has no obvious change by heattreatment. After heat treatment, interstices and homocentric

Table 1Influence of increasing heat treatment temperature on crystal structures.

Sample 2q(002)/� FWHM/� d002/nm Lc/nm Graphitization degree g/%

S1 25.84 1.241 0.3446 6.57 e

S2(1900 �C) 25.90 0.792 0.3439 10.29 3.48%S3(2100 �C) 25.96 0.647 0.3434 12.56 8.14%S4(2500 �C) 26.06 0.520 0.3418 15.69 25.6%S5(2650 �C) 26.10 0.487 0.3414 16.74 30.2%

annular cracks are observed in PyC of S3 and S5, the number as wellas the width of these cracks increases as the heat treatment tem-perature increase up to 2650 �C. This is consistent with the Ramanresults that tensile stresses in the matrix increase the crystallinelattice spacing and the open porosity [9].

The distinct decrease of CTE in Z direction with increasing heattreatment temperature could also be explained with the effect ofinterstices and cracks. With expansion of fibers and matrix, themismatch of CTEs between fibers and matrix can lead to consid-erable stresses. When these stresses reach a certain limit, fibersmay debond from matrix following the formation of interstices.Moreover, new stress generates because of the larger crystalinteraction, which derives from structure reorganization andcrystal growth. This stress results in the propagation of previouscracks and formation of new cracks in PyC matrix. Interstices andcracks provide enough space for partial expansion, the expansion offibers or matrix fills this space, so there is no noticeable change ofmacroscopic size of the composites.

Different from Z direction, CTE in XY direction is related to thethermal expansion of both matrix and in-plane fibers. For 2D C/Ccomposites, except for the fibers of web layers arranging disorderly,half of carbon fibers are parallel to X direction (test direction), theother half are in Y direction (perpendicular to test direction) and afew needled carbon fibers are in Z direction. Anisotropy of carbonfiber determines its axial CTE ((�0.72 to 0.93) � 10�6 K�1) is lowerthan its radial CTE (about 8.85�10�6 K�1) [10]. It could be assumedthat CTE may be controlled by axial expansion of fibers in X di-rection and radial expansion of fibers in Y direction.

With the increasing heat treatment temperature, the crystalorder of fibers also changes. The reorganization of graphenelayers in the fibers leads to different change of radial and axialCTE of fibers. As a result of the graphene layers re-lining to thefiber axis, the increase of the heat treatment temperature causes

Fig. 2. Raman spectra of the five samples S1, S2, S3, S4, S5.

Fig. 3. PLM micrographs of sample S1 (a), S3 (b) and S5 (c).

Y.-b. Fei et al. / Vacuum 102 (2014) 51e53 53

the increase of the radial CTE values whereas the axial CTE de-creases [11]. Though the radial CTE of fibers increases, the radialexpansion of fibers is strongly restricted by the expansion of PyCmatrix. It has been revealed previously that for highly anisotropypyrocarbon, the CTEs are 28 � 10�6 K�1 along c-axis, while forisotropic pyrocarbon the CTE is in the range of 4e6 � 10�6 K�1

[10]. Namely the c-axial CTE of PyC (Fig. 3) studied here is higherthan or equal to the radial CTE of fibers at least. With theincreasing heat treatment temperatures, the c-axial expansion ofPyC is much greater than radial expansion of fibers that inhibitsthe expansion of fiber in Z direction. Actually, the expansion iscontrolled by a combination of matrix and fibers in XY direction.The increase in radial CTE of fibers can be ignored and thedecreased axial CTE of fibers plays a leading role in thedecreased CTE of composites in XY direction.

Furthermore, the decreased CTE in XY direction can be attrib-uted to the formation of pores and cracks. The number of pores inthe matrix and among fiber bundles is gained with the increasingheat treatment temperature. Pores offer room for the expansionand then decrease CTE. The pores number of S1 is the least and S5 isthe most. Accordingly, CTE of S5 is the smallest and CTE of S1 is thelargest. The increasing cracks still act like pores that provide certainspace for expansion.

In summary, increasing heat treatment temperature can effec-tively decrease the CTE of C/C composites. The decreased CTE isclosely related to the microstructural changes caused by heattreatment. Higher heat treatment temperature contributes to a

better preferred orientation, more cracks and higher porositywithin the heat-treated C/C composites, which decrease CTE of thecomposites. In addition, with the increase of heat treatment tem-perature, the decrease in CTE in Z direction of composites mainlydepend on c-axial thermal expansion of PyC matrix, while thedecrease in CTE results from the axial CTE of fibers.

Acknowledgments

This work has been supported by Industry-University-ResearchCollaboration project of Ministry of Education of Guangdongprovince, National Natural Science Foundation of China (51002124)and (51202194).

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