Materials Science and Engineering -Mechanical properties and microstructure of Ermel

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Materials Science and Engineering A 387–389 (2004) 845–851

Mechanical properties and microstructure of carbon fibre reinforcedcarbon materials produced by chemical vapour infiltration

R. Ermel, T. Beck ∗, O. Vöhringer

Institute of Materials Science and Engineering I, University o f Karlsruhe (TH), Kaiserstr. 12, D-76131 Karlsruhe, Denmark

Received 25 August 2003; received in revised form 25 November 2003

Abstract

Correlations between mechanical properties, fibre architecture, matrix texture and fracture behaviour of carbon fibre reinforced carbon

CFC-materials produced by chemical vapour infiltration are investigated using results from tensile and four-point-bending tests, polarized

light microscopy and scanning electron microscopy. Mechanical testing was performed at temperatures from 20 up to 1600 ◦C. Samples

were tested in the as-received state and after heat treatment at T = 2200 ◦C. The heat treatment leads to an increasing Young’s modulus

and brittleness, which corresponds with an increasingly distinct layer structure of high textured matrix carbon. A rise of test temperature up

to 1400 ◦C also leads to a larger Young’s modulus and increasing brittleness. However, at T = 1600 ◦C the material shows a pronounced

pseudoductile behaviour. Scanning electron micrographs show in this case a lot of concentric cracksbetween the sublayers of the high textured

matrix. The fracture behaviour is explained by a multiple delamination between these layers.

© 2004 Elsevier B.V. All rights reserved.

Keywords: Carbon/carbon-composites; Fracture behaviour; High-temperature properties; Tensile test; Four-point-bending test; Microstructure

1. Introduction

High strength at temperatures up to 1800 ◦C and a good

chemical resistance in neutral and reducing environments

are outstanding properties of carbon fibre reinforced carbon

(CFC). The capability to pseudoductile fracture behaviour in

the case of an appropriate fibre architecture and matrix tex-

ture predestinates CFC as a material for components, which

undergo extremely high thermal and mechanical loadings.

Pseudoductile behaviour of such a composite consisting of

two brittle components occurs if an appropriate strength

of the fibre–matrix interface results from the manufactur-

ing process. Then, matrix initialized cracks deflect at the

fibre–matrix interfaces and a locally limited debonding be-

tween fibres and matrix takes place and the fibres will bridge

the crack. Additionally, fibre fracture during further load-

ing can cause so-called fibre-pullout, consuming additional

energy for crack propagation [1–3].

The fracture toughness of CFC can be further increased

if the crack deflection occurs not only at fibre–matrix inter-

∗ Corresponding author. Tel.: +49-721-608-4159;

fax: +49-721-608-7451.

E-mail address: [email protected] (T. Beck).

face but also in the matrix itself [4]. This behaviour requires

a matrix consisting completely, or at least partially, of high

textured pyrolytic carbon, which is deposited in concentric

layers around the fibre. Tests on chemical vapour infiltrated

(CVI) fibre-felts show, that a material with a larger volume

fraction of high textured matrix shows a more pronounced

pseudoductility than composites with medium or low tex-

tured matrices [5].

The CVI process can be described as follows: methane

diffuses into the fibre-preform, where it cracks into hydrogen

and elementary carbon and latter is deposited on the surface

of the fibres until the fibre structure is completely densified.

Because of an exact understanding of the CVI process [6,7],

it is possible to produce a composite of high density within a

time of a few days. The parameters of the infiltration process

must be optimized to get a complete densification combined

with the desired texture of the carbon matrix [8].

This work presents the results of four-point-bending and

tensile tests at temperatures from 20 up to 1600 ◦C on two

different CVI-CFC-materials. Some specimens were heat

treated at T = 2200 ◦C after manufacturing which results in

a weak partial graphitization of the high textured pyrolytic

matrix carbon. Fractographic investigations with SEM and

a microstructure analysis with polarized light microscopy

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2003.12.075



846 R. Ermel et al. / Materials Science and Engineering A 387–389 (2004) 845–851

Fig. 1. Microstructure of CVI-CFC.

(PLM) are used for the interpretation of the results of me-

chanical testing.

2. Experimental details

2.1. Investigation of the microstructure

The investigation of the microstructure is carried out by

polarized light microscopy (PLM) on specimens cut per-

pendicular to the fibre direction. The measurement of the

Fig. 2. Material 1: UD- and fibre-felt-layers (a) and material 2: 2D-0/0/90/90 (b).

extinction angle of polarized light characterizes the texture

of pyrolytic carbon, so that a separation in isotropic (ISO),

low (LT), medium (MT) und high textured (HT) carbon is

possible [9].

Fig. 1 shows an example of a carbon fibre surrounded by

matrix-layers with different textures.

2.2. Mechanical testing

Mechanical testing of the CFC-samples is performed on

an electromechanical testing machine. Because of the sus-



R. Ermel et al. / Materials Science and Engineering A 387–38 9 (2004) 845–851 847

Fig. 3. Stress–strain curve from tensile test at T = 20 ◦C, surface and longitudinal strain-distribution of three states of this tensile test (a). Scanning

electron micrograph of the corresponding fracture surface (b).

ceptibility of CFC against oxidation the tests at high temper-

atures are carried out in a vacuum chamber at an air-pressure

of p < 10−8 bar. With inductive heating of the specimens test

temperatures up to 1800 ◦C can be reached. The crosshead

speed for four-point-bending tests is 2.0 mm/min and for

the tensile tests 0.5 mm/min. The integral strain is mea-

sured with a capacitive extensometer. Additionally an op-

tical extensometer based on grey-scale-correlation-analysis

allows for an integral as well as locally resolved determi-

nation of the strains even at the highest test temperatures

[10].

2.3. Specimen geometry

The bending samples have a size of L × B × H = 52mm

× 6 mm × 1.6 mm and the distances of the inner and the

outer support are L0 = 20mm and La = 45 mm, respectively.

Assuming a linear elastic behaviour the fictitious bending

strength σ R∗ is calculated according to:

σ ∗R =

3F m

La−Li

2

BH 2(1)




Table 1

Investigated materials

Designation Preform Preform-

manufacturer

Fibre

volume ratio

Fibre type

Material 1 UD/felt layer Dunlop About 20% HT

Material 2 2D-0/0/90/90 Sintec 22.5% HT

using F m as maximum load. The elastically calculated sur-

face strain results from the deflection between the inner

supports. The gauge length of the tensile specimens is L0

= 15 mm with a cross-section of B × H = 2 mm × 3mm.

3. Materials

The investigated CFC-materials consist of infiltrated

preforms with a different fibre architecture (Fig. 2). Ma-

terial 1 is made of two different layers, a unidirectional

(UD) C-fibre and a fibre-felt layer. Material 2 consists of a two-dimensional preform with a sequence of the layers

in 0◦ /0◦ /90◦ /90◦ direction. Within each layer, 80% of the

fibres are orientated in the main direction.

The specimens are taken from the infiltrated preform

in such a way that the directions of the load-axis and fi-

Fig. 4. Comparison between σ –εt curves of heat treated and as-received

specimens from tensile test (a) and four-point-bending test (b).

bres are parallel (Fig. 2). The sequence of the layers of

the specimens from material 1 are chosen in order to get

UD-layers at both surfaces of the samples. Table 1 sum-

marizes fibre architecture, fibre volume ratio and fibre

type.

The parameters of the CVI-process are the same for both

materials. Due to that, the matrix in both materials mostlyconsists of high textured pyrolytic carbon with a thin layer

of low textured pyrolytic carbon around the fibres. Material

1 is characterized by a density of ρ = 1.82 g/cm3 and an

open porosity of P = 7%, the respective values of material

2 are ρ = 1.76 g/cm3 and P = 10%.

Fig. 5. SEM-pictures of fracture surfaces of material 1 in the as-received

(a) and the heat treated state (b) after four-point-bending at T = 20 ◦C.




4. Results and discussion

4.1. Material 1

Under tensile loading material 1 in the as-received state

shows a marked pseudoductile behaviour with pronounced

fibre-pullout as presented in a typical stress–strain curve and

Fig. 6. Representative stress–strain curves for tensile and

four-point-bending tests (a) and scanning electron micrographs of the

fracture surface of four-point-bending (b) and tensile samples (c).

the corresponding fracture surface for a test temperature of

20 ◦C in Fig. 3. The fracture surface of four-point-bending

specimens shows less pronounced fibre-pullout compared

with tensile specimens.

Additionally, in Fig. 3a the surface and the local

strain-distribution in longitudinal direction is represented

in three states of the tensile test. Distinct concentrationsof strain can be found at a relatively low total strain of εt

= 0.45% (phase 2) before a crack on the surface can be

noticed.

4.2. Influence of heat treatment

An influence of a heat treatment at T = 2200 ◦C for

2 h can be seen in tensile as well as in four-point-bending

tests (Fig. 4). In both cases the Young’s modulus in-

creases slightly while the plastic strain to failure decreases.

The influence of the heat treatment is more pronounced

in tensile tests. An increase of strength due to the heat

treatment observed in tensile tests cannot be noticed in

four-point-bending tests. The reason might be a larger scat-

ter of the flexural strength, caused by a different fracture be-

haviour. This can be seen in one case in which delamination

of a large area between unidirectional and felt-layer super-

poses the influence of fibre–matrix delamination. The cor-

responding σ R–εR,t curve is marked by an arrow in Fig. 4b.

An investigation of the fracture surfaces by SEM shows

for the heat treated sample (Fig. 5b) a more distinct laminar

structure of the high textured pyrolytic carbon, compared

to the as-received state (Fig. 5a). In addition, the matrix is

characterized by many cracks parallel to the layers. However,

the extent of pullout of fibres and matrix layers seems tobe reduced by the heat treatment. PLM investigations did

not show any change of the matrix texture due to the heat

treatment.

4.3. Material 2

A comparison between the stress–strain behaviour of ma-

terial 2 in tensile- and four-point-bending tests at T = 20 ◦C

Fig. 7. Stress–strain curves for T = 20, 1100, 1400 and 1600 ◦C.




Fig. 8. SEM-pictures of fracture surfaces after four-point-bending tests at T = 1600 ◦C (a) and at T = 20 ◦C (b).

is presented in Fig. 6a. A larger plastic elongation is found

in the tensile test. This agrees with SEM examinations of

the fracture surface after tensile and bending tests which

showed a larger extent of fibre-pullout and delamination of matrix layers for tensile specimens (Fig. 6b and c).

4.4. Influence of test temperature

Four-point-bending tests were carried out at temperatures

up to 1600 ◦C. The resulting surface stress–strain curves are

shown in Fig. 7 for temperatures of 20, 1100, 1400 and

1600 ◦C. Young’s modulus increases slightly with tempera-

ture while the scatter of the strength values decreases with

T . Up to T = 1400 ◦C, all samples show a brittle fracture be-

haviour. However, at T = 1600 ◦C the plastic fracture strain

of 0.3% is much higher than at lower T . An explanation is

given by SEM-observation of the fracture surfaces at high

magnification at which a high textured matrix streaked with

a lot of concentric cracks can be detected (Fig. 8a). The sur-face is very rough while the surface of the samples tested at

temperatures up to 1400 ◦C is much smoother (Fig. 8b). Ac-

cordingly, delamination between matrix layers can explain

the pseudoplastic behaviour at T = 1600 ◦C. Fibre-pullout

seems to be of subordinate significance, because the fracture

surfaces show a similar structure at all test temperatures.

5. Summary

Two CVI-CFC materials with a different fibre architec-

ture but with mainly the same matrix texture were tested




in four-point-bending and tensile tests at temperatures up

to 1600 ◦C. For both materials tensile loading led to a

more pronounced pseudoductility and fibre-pullout than

four-point-bending tests.

The consequence of heat treatment of material 1 at T

= 2200 ◦C after infiltration is a slightly higher Young’s

modulus and a lower plastic fracture strain than inas-received state. Scanning electron micrographs showed

a more distinct layer structure of the high textured ma-

trix and a smaller fibre and matrix layer pullout in the

case of heat treated specimens. A significant change of

the texture caused by heat treatment was not detected by

PLM.

Increasing test temperatures up to 1400 ◦C lead to increas-

ing Young’s modulus and a decreasing plastic fracture strain.

However, an influence of T ≤ 1400 ◦C on the fracture sur-

face and matrix microstructure cannot be noticed. Tests at T

= 1600 ◦C resulted in a relatively large plastic fracture strain

of about 0.3%, due to an increasing number of concentric

cracks within the high textured matrix. So, this pseudoplas-tic behaviour is attributed to delamination between matrix

layers.

Acknowledgements

The present study was performed in the Collaborative Re-

search Center 551: “Carbon from the gas phase”. Financial

support by the German Research Foundation (DFG) is grate-

fully acknowledged. The authors thank Prof. K.J. Hüttinger

for the infiltrated samples.

References

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1993.

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[4] B. Reznik, D. Gerthsen, Carbon 41 (2003) 57.

[5] B. Reznik, M. Guellali, D. Gerthsen, R. Oberacker, M.J. Hoffmann,

Mater. Lett. 52 (2002) 14.

[6] K.J. Hüttinger, Chem. Vap. Deposit. 4 (1998) 151.

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[10] GOM mbH, Aramis – Verformungsmessung nach dem Rasterver-

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