Effects of pressure carbonization in the CC composite process

Materials Science and Engineering, A 143 (1991 ) 223-229

223

Effects of pressure carbonization in the C-C composite process T. Hosomura and H. Okamoto Aerospace Division, Nissan Motor Co., Ltd., 3-5-I Momoi, Suginami-ku, Tokyo 167(Japan)

Abstract In the resin-charring process of C-C composites, pressure carbonization of the matrix carbon is known to have some advantages. The carbon in the decomposition gas generated through the heating treatment of the matrix resin is caught in the carbonized matrices under the high ambient pressure condition. Owing to this phenomenon, a higher carbon yield can be attained, and, furthermore, controlling the structure of matrices becomes possible through pressure carbonization. This paper describes the results of experiments on the pressure carbonization of C-C composites and of conventional materials for matrices, such as phenol resin, furfuryl alcohol, coal tar pitch and petroleum pitch. The results can be summarized as follows: (1) phenol resin and furfuryl alcohol are not suitable for pressure carbonization, (2) the carbon yield of pitch increases markedly, especially when the size of the pitch molecule is small, (3) the pores in the carbonized matrices become smaller in size and more spherical in shape as the pressure rises and (4) such higher densification and smaller spherical pores result in a higher strength of the composites.

1. Introduction T h e fracture behaviour of C - C composites is that of a fragile material. This means that the fracture of the composite is usually initiated by the fracture of the matrix. T h e strength o c of a fibrereinforced composite such as C - C composites may be expressed by the following relation: O c ----- V f ~ m E f

(for

E m <~ Ef)

(1)

where Vr is the volume fraction of reinforcing fibres, ef and e m are the elongations of the fillers and the matrix and Ef is the Young's modulus of the fibres. W h e n Vr and Ef are given, the strength of the composite can be improved by increasing the elongation em of the carbonized matrix. Such an improvement in e m can be accomplished by arranging the combination of matrix and by tuning the temperature a n d / o r the pressure con-

dition during carbonization. However, in the case of C - C composites, it is necessary to densify the material sufficiently in order to increase %. T h e density of C - C composites manufactured through a resin-charring process under normal pressure is usually less than 1.7 g cm 3 even after several densification processes. T h e C - C composites with a density of less than 1.6 g cm 3 will be fragile owing to the collapse of fibres against some compressing a n d / o r bending loads. T h e following sections describe an experimental study on the effects of pressure carbonization on the properties of both resins and C - C composites.

2. Experiments and analyses 2.1. Materials

Phenol resin (resol type), furfuryl alcohol and three kinds (Table 1) of pitch were selected as

TABLE 1 Matrix precursor Resin

Type

Fixed C (%)

Quinoline (%)

Toluene (%)

Softening point (°f)

Pitch A Pitch B Pitch C

Coal tar Petroleum Petroleum

52 54 82

20 19.4 37

0.3 1.2 0

83 131 266

0921-5093/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

224 ~ ~

N2 (10-200 MPa)

~x~N~.~xN~N

Pressure

Vessel

~111 ~'' ~

Z

::::e;al

Barrier

J~

ll(l

polished and are observed through an orthogonal Nicol prism by a reflect polarization microscope at a magnification of 7.5-400. Each matrix precursor, weighing 3 mg on the platinum pan, is heated to 800 °C at a heating rate of 10 °C min-l under a helium gas pressure of 0.1, 2.5 or 5 MPa, and then the weight loss of the specimens is measured using high pressure thermogravimetry equipment. The results are given later in Fig. 4. The graphite crystallinity is measured with X-ray diffraction equipment. The degree of graphitization is estimated by obtaining the size of crystallite based upon A0 at 20 (26.0 °) of the 002 plane and the results will be given in Table 3. The porosity and the bulk density are measured by the toluene dipping method, and the results are shown later in Fig. 5. The flexural strength is measured by the threepoint bending test. A C - C composite specimen of size 50 mm ( L ) x 10 mm ( W ) x 1.4 mm (T) is tested under the testing condition L / D = 32 and the loading rate is 1 mm min- i.

Cooteiner

II ,,

t__.J

Relief Valve

Fig. 1. Pressure-carbonizingfurnace. 100

89.8

/5/..:-"

•~.

¢otn

so

.g_+ O" 45.1

20

T 8°'

+.°

................. 55

I $1

O Pitch A (IPitch B °Pitch C APhenol AIkFuran f

1

Atmosphere

I

10 30 Pressure

I 100

I 200

(MPa)

Fig. 2. Carbon yield variation with the pressure. matrix precursors. To manufacture the C - C composites, TSN (PETOCA Co., Ltd.) petroleum pitch fibres heat treated at a temperature of 850 °C are used. 2.2. Process

The pressure-carbonizing furnace used in this study is shown in Fig. 1 schematically. Sample materials are sealed with N2 gas in a stainless steel container, pressurized and heated along the preprogrammed process patterns. After the maximum pressure and the maximum temperature (650 °C) is attained, the temperature and the pressure are kept constant for an hour. The pressure is kept constant at the specified value by releasing gas when the pressure exceeds the preprogrammed pressure during the heat treatment. 2.3. Characterization

Processed specimens are embedded in epoxy resin. Then the surfaces of specimens are

3. Results and discussion 3.1. Matrices 3.1.1. Carbon yield

The carbon yields of matrices after pressure carbonization are shown in Fig. 2. It should be noted that the data for 200 MPa are less reliable than those for the other pressures since in the 200 MPa case a comparatively small specimen is used and more gas must be released. (1) Distinct improvement in the carbon yield due to the pressure carbonization is not seen for phenol resin and furfuryl alcohol. The mass fractions of carbon in phenol and furfuryl alcohol calculated from their molecular formula are 68% and 72-76% respectively. The carbon yields obtained are nearly 55% irrespective of the p r e s sure. This means that the carbon monoxide and carbon dioxide in the decomposition gas cannot be trapped under the testing condition of 200 MPa and 650 °C. If the oxygen atoms in the phenol resin and furfuryl alcohol are released from the matrix in the form of carbon monoxide, the resultant carbon yields will be 48% and 53-60% respectively. When the oxygen is released in the form of carbon dioxide, the corresponding carbon yields become 58% and 62-63% respectively. It is therefore highly possible that, in the case of phenol resin, carbon

225 TABLE 2 Pitch properties o

"" °

857

P

C

Resin

8O

i¢

70.3

v

65.6 59.0..- . . . .

so

-O

.--O"/.~,9. 51.6 . - -

c

'

p 4P

B A

Pitch A Pitch B Pitch C

~

tO

40

~/

2o

I

Aromatization Molecular ratio weight

1.(163 1.305 0.618

726 782 931

C yield At 0.1 MPa

At 10 MPa

45.2 54.4 84.5

85.9 86.4 89.8

,.~.2

1500 Atmosphere

I

2.5

I

5.0

Pressure

(MPa)

Fig. 3. Carbon yield variation with the pressure (high pressure thermogravimetryanalysis). //'- ,N,//~//'~

Pitch

A

1o00

uJ~ca500 0,., I

Atmosphere

I

1

lo 30 loo Pressure (MPa)

200

Fig. 5. Poresize variation with the pressure (matrices). I 10000

20000 500 Molecular weight

100

Fig. 4. Molecular weight distribution of pitches.

dioxide is the major exhaustion molecule while, in the case of furfuryl alcohol, both carbon monoxide and carbon dioxide take this role. (2) The improvement in the carbon yield is evident in the case of pitch matrices. Pressure carbonization results in a remarkable improvement in the carbon yield of pitches which would not have a high carbon yield under the normal pressure condition. This improvement in carbon yield due to the pressure carbonization becomes saturated at the pressure of 10 MPa (Figs. 2 and 3). The final carbon yields of three kinds of pitch vary between 86% and 90%. This shows that the carbon contents of hydrocarbon gases generated through the heating treatment are trapped and decomposed into carbon and hydrogen effectively under higher pressures. The carbon fraction in each pitch (pitches A, B and C) is about 93% and the oxygen fraction is negligibly small. There is therefore a very small possibility of carbon exhaustion accompanied by oxygen. Since

the carbon yields at high pressures almost reach 90%, it is thought that the degradation of the carbon yields due to molecules other than oxygen can be suppressed by pressure carbonization. (3) From the point of view of the carbon yield alone, there exists a pitch (pitch C) for which the yield ratio is high even without pressure carbonization. The peculiar property of pitch C is its low aromatization ratio and its high molecular weight (Fig. 4). This pitch has a higher content of insoluble quinoline (Tables 1 and 2).

3.1.2. Pores of matrices Figures 5 and 6 indicate the relationship between the carbonizing pressure imposed on pitches and the typical pore size in the carbonized matrices, as well as the shapes of the pore. The pore size is evaluated directly from the pictures. The pore size and its variation decrease in inverse proportion to the pressure. The shape of the pore becomes nearly spherical under high pressure conditions. 3.1.3. Microstructure of matrices The microstructures of matrices carbonized under the pressure at 0.1, 10, 30 or 200 MPa

226

0.1 MPa

30Mpa

200Mpa Fig. 6. Pore size variation with the pressure (pitch A, polarization microscope): (a) 0.1 MPa; (b) 30 MPa; (c) 200 MPa.

observed with the use of a polarization microscope are shown in Figs. 7 and 8. (1) The microstructures of the phenol resin and the furfuryl alcohol do not grow under high pressures and their optical properties are isotropic even after pressure carbonization. (2) The coal tar pitch A has a tendency to segregate with a fine mosaic texture while the bulk shows coarse fibrous textures (Fig. 7). (3) In the microstructures of pitches, an orthogonal texture growth is observed as the pressure increases (Fig. 8). (4) The crystallite sizes of matrices, which are carbonized under normal and higher pressures and are graphitized at 2300 °C, are measured. There are no noticeable differences between the data for the various cases as shown in Table 3.

3.2. C-C composites The unidirectionally reinforced C - C composite used here is manufactured by employing petroleum pitch carbon fibre TSN (Petoca Co. Ltd.) and petroleum pitch B. As shown in Table

Fig. 7. Microstructure of pitch A after pressure carbonization at 30 MPa and 650 °C.

TABLE 3 Crystallite size of carbonized pitches Pitch

A A B B C C

Pressure (MPa) 0.1 10 0.1 10 0.1 10

Lattice spacing

Crystallite size

(A)

(A)

3.383 3.386 3.391 3.383 3.386 3.388

186 186 204 194 177 204

L = 0.92/(fl cos 0) where L is the graphite crystallite size, fl the half-width, 2 the wavelength of the Cu Ka line and 0 the angle of diffraction.

4, the bulk density, the porosity and the flexural strength of the C - C composites, manufactured through several impregnation, carbonization and graphitization processes, are measured. The graphitization is carried out at a temperature of 2300 °C.

227

Pitch

A

Pitch

Pitch

B

i

O. 1 MPa

10MPa

i

30MPa

i

200MPa

Fig. 8. Matrix microstructure variation with the pressure.

C

228 TABLE 4 Bulk density, porosity and flexural strength (C-C composites) Bulk density (g cm- 3) for the following number of cycles

Pressure (MPa) 0.1 10 200

1

2

3

4

7

1.08 1.24 1.34

1.22 1.34 1.57

1.44 1.49 1.98

-1.75 1.98

1.73 1.75 1.98

Porosity

(%)

Flexural strength (kgf mm- 2)

14 12 6

45 53 60

O Bulk density Z~Porosity m 2.0

--

m

A ~

.

~

15

_

,o .g

0.1 MPa porosity 14 % I

I

1

10

u

I 200

Pressure (MPa) Fig. 9. Variations in bulk density and porosity with the pressure.

3.2.1. Bulk density and porosity The correlations between the imposed pressure during the carbonization and the bulk density and the porosity of the C - C composites after graphitization are shown in Fig. 9 and Table 4. Figure 10 shows the polarization microscope picture of the three final products enclosed in a rectangle in Table 4. The bulk density shows a significant improvement as the carbonization pressure increases. It reaches 1.98 g cm-3 only after the third high densification treatment at a pressure of 200 MPa, while such a bulk density cannot be attained even after several treatments under the normal pressure condition. The pores become smaller, more spherical and more dispersed as the pl essure becomes higher, as can be seen from Fig. 10. 3.2.2. Flexural strength In Table 4 the results of the bending test, described in Section 2.3, of the final products manufactured by means of several cycles of impregnation, pressure carbonization and graphitization processes are summarized. The porosity

IOMPa porosity 12 %

200MPa porosity 6 % Fig, 10. Pore size variation with the pressure.

229

60

0

E

~ 5o

~

o

×

0 Pitch

,'i-

4ol

I

I

I

0.1

1

10

Pressure

B

I

I

J

200

(MPa)

Fig. 11. Flexural strength variation with the pressure.

O 60

0

~ v

"

O

O

50 O

~ 40, ,-r

I 10 Porosity

I

20

(%)

Fig. 12. Flexural strength variation with the porosity.

is diminished and the bulk density is increased by employing several pressure carbonization cycles. The highest flexural strength is obtained when the bulk density is the highest (Figs. 11 and 12).

68% and 71% respectively, the carbon yield obtained is 55% under either normal pressure or higher pressure. (2) Of the pitches, those with a large molecular weight or higher content of insoluble quinoline have a high carbon yield ratio even under low pressures. Those which show significant improvement in the carbon yield with pressure carbonization have a small molecular weight. The upper limit of the carbon yield is reached at a pressure of 10 MPa. (3) As the carbonization pressure increases, the fine mosaic texture changes into a coarse mosaic texture. (4) C - C composites manufactured by pressure carbonization have a higher strength because the porosity and the pore radius decrease (and thus the bulk density becomes large) on increase in the pressure. (5) The strength of C - C composites is proportional to the final bulk density. Therefore the case for a pressure of 200 MPa is most effective because it needs few cycles of densification processes to obtain a high bulk density. (6) Since, at high pressures, the pore radius r decreases inversely proportional to pl/3, the effect of the pressure above 200 MPa becomes rapidly saturated. Acknowledgments

Finally, in conducting the present experiments, the authors are very grateful to the authors of refs. 1-3.

4. Concluding remarks

Experiments on pressure carbonization in the resin-charring process of the C - C composites were conducted and the following results were obtained. (1) For phenol resin and furfuryl alcohol, for which the molecular carbon mass fractions are

References 1 M. lnagaki and M. Washiyama et al., Tanso, (141) (1990) 17-22. 2 M. Inagaki, K. Goto and M. Sakai, Tanso, (126) (1986) 111-113. 3 S. Kimura, and E. Yasuda et al., Tanso (125) (1986) 62-67.