carbon composites

Carbon.

1974, Vol.

12, pp. 233-241.

Pergamon

Press.

Printed

in Great

Britain

EFFECT OF GAS PHASE CONDITIONS RESULTANT MATRIX PYROCARBONS CARBON/CARBON COMPOSITES* Composite

M. L. LIEBERMAN and H. 0. PIERSON Materials Development Division 5313 Sandia Laboratories, Mexico 8’7115, U.S.A.

ON IN

Albuquerque,

New

(Received 20 August 1973) Abstract-The various microstructures obtained by the low temperature (< 1500°C) chemical vapor deposition (CVD) of carbon in carbon fiber substrates fall into three major types identified as rough laminar, smooth laminar, and isotropic. It is shown that the type of microstructure is a function of the temperature of deposition, the total gas pressure, and the carbon to hydrogen ratio of the source gas. These experimental results are found to be in good agreement with a previously reported carbon CVD model which relates the microstructure to a single parameter, the equilibrium gas phase C2H2/C6HR molar ratio. The densities and crystallographic parameters of the heat-treated composites are significantly affected by the type of microstructure. The rough laminar material has by far the most graphitic characteristics and is followed by the smooth laminar and isotropic materials, in that order.

1. INTRODUCTION

have been made in the development of carbon/carbon composites composed of fibrous substrates and chemically vapor deposited matrices. Previous studies have shown that these composites exhibit a wide range of properties which are affected by the structure and properties of the fibrous substrate[l-61 and the pyrolytic carbon (pyrocarbon) matrix[7-lo]. Three different optical microstructures of the matrix, similar to those observed on coated particles [ 1 l-181, have been repeatedly observed [5,7-10,19-251. Neither the phenomena which cause the various microstructures, nor the deposition conditions which yield them have been well determined. One effort to elucidate these parameters has been a proposed chemical vapor deposition Substantial

advances

“This work was supported Atomic Energy Commission.

by the United States 233

(CVD) model for carbon which relates the gas phase conditions to the resultant microstructures [26]. While the limited experimental data available support it, the model is unproven and, consequently, must be regarded as conjectural. The main purpose of the present investigation is to experimentally determine deposition parametermatrix microstructure correlations and to compare them with the predictions of the CVD model. An additional goal is to determine some of the microstructure-controlled properties of the composites. 2. EXPERIMENTAL

Substrates The two types of carbon fibers used in this study were made by the pyrolysis of viscose rayon and polyacrylonitrile (PAN) precursors[27]. The substrates were in the form of felt plates with bulk densities ranging from 0.1 to 0.3 g/cm’. 2.1

234

M. L. LIEBERMAN

PAN-based substrates were prepared by the following procedure[28]. A PAN tow of 10,000 fibers (average diameter of 12pm) was oxidized under tension via the method described in the literature[27]. It was then cut into 5cm lengths and felted in a modified laboratory carder and needle loom. Bulk density averaged 0.15g/cmS in the as-felted condition. Felts of different densities were obtained by pressing them at 200°C to a suitable thickness reduction. Details of the compression process have been given elsewhere [3,5]. The felt was then carbonized to 840°C; the resulting fiber diameter was approximately 8pm and bulk densities of the carbonized felts were 0.1, 0.2, and 0*3g/cm’. The rayon-based felt substrates were commercial carbon felts (Carbone Co.) which had an average fiber diameter of approximately 8pm and a bulk density of 0*095g/cmS. This felt has been widely used as a substrate in the fabrication of carbon/carbon composites [2]. 2.2 Matrix deposition The carbon felt substrates (typically 15 X 8 X 1.25 cm) were heated at the deposition temperature under flowing argon for one hour before the carbon-bearing gas was introduced. They were then infiltrated with pyrocarbon by the thermal gradient method at either local atmospheric pressure (625+ 5 Torr, in Albuquerque, New Mexico) or 12 +: 1 Torr. During each deposition experiment, four substrates (three PAN-based felts of different densities and one rayon-based felt) were simultaneously infiltrated. The substrates were mounted on a rectangular graphite mandrel by means of carbon yarn. Infiltration at local atmospheric pressure was performed in the apparatus shown schematically in Fig. 1. A modified version of this apparatus was used for infiltration at reduced pressure. Details of the CVD process at both reduced and atmospheric pressures have been given elsewhere [19,29]. Because of exhaust difficulties, it was not possible to maintain total gas flow at the same level in all

and H. 0.

PIERSON t EXHAUST

ARGON ISLIGHT POSITIVE PRESSURE) C

STEEL SHELL

Fig. 1. Schematic diagram of CVD system. experiments. The total carbon to hydrogen (C/H) ratio in the gas phase was varied by varying the flow rates of the methane and hydrogen. Three deposition temperatures were investigated, namely, 1250”, 1325” and 1400°C. The temperature of the susceptor was maintained constant to ZL10” and continuously recorded via a platinum-platinum 10 per cent rhodium thermocouple. Because of the nature of the thermal gradient infiltration process, however, the temperature within the felt varied with time and location. This was shown by monitoring the temperature during several deposition experiments (Fig. 2). The temperature near the middle of the felt rapidly reaches the level of the susceptor temperature, after which time it remains uniform. The outside felt temperature, however, remains consistently below the susceptor temperature although it gradually increases throughout the deposition. 2.3 Heat-treatment The infiltrated plates were heat-treated in an induction furnace to 3000 + 30°C. Time to reach maximum temperature was approximately 5.5 hr and the residence time at maximum temperature was 2 hr. Cooling to room temperature took approximately 16 hr with an initial cooling of 600°C for the first hour and 500°C for the second hour.

EFFECT OF GAS PHASE CONDITIONS

OV

0

4

I

1

1

1

8

12

16

20

I

24

HOURS DEPOSITION TIME

+-THERMOCOUPLE

B

THERMOCOUPLE A

DEPOSITION CONDITIONS: l@X CHq. WMIN 630 TORR O.Ig/cd RAYON SUBSTRATE

FLOW

Fig. 2. Temperature profiles in felt substrates during CVD. 3. CHARACTERIZATION OF HEATTREATED COMPOSITES Optical characterization of the matrix microstuctures was performed along cross sections of heat-treated plates which had been embedded in a plastic and successively polished with silicon carbide grits and 6p.m and 1 Frn diamond pastes. As in earlier work [5,7-10,19-251, the authors observed the three predominant microstructures shown in Fig. 3 which are termed smooth laminar (SL), rough laminar (RL), and isotropic (I), because of their appearances under polarized light. Rough laminar deposits are characterized by high optical activity and numerous irregular extinction crosses. Smooth laminar deposits are characterized by less optical activity than that exhibited by RL deposits, and large, well defined extinction crosses. Heat-treatment tends to cause circumferential cracks in the smooth laminar

235

material, which suggests that it is relatively rigid. Isotropic deposits are characterized by little, if any, optical activity, a fine grain structure, and an absence of cracks after heattreatment. Identification of microstructures is not always an easy matter because complex arrangements sometimes occur, i.e., more than one microstructure can exist in a small area of the felt or even around a single fiber. Samples of the heat-treated materials were polished with fine sand paper and examined visually to identify zones of the different micrestructures. The RL zone has a shiny, silvery appearance, is relatively soft, and leaves a tracing similar to that of a soft lead pencil. In contrast, the SL zone has a dull black appearance, is harder than the RL zone, and leaves a tracing similar to that of a hard lead pencil. The isotropic zone is more difficult to identify and is intermediate to the RL and SL zones in appearance. Figures 4 and 5 schematically show the areas in which the three zones were observed for the different deposition experiments. Heat-treated samples were visually classified as belonging to the different zones and a minimum of 12 from each zone were subjected to X-ray diffraction analyses performed with a General Electric XRD-6 diffractometer which employed Ni-filtered CuKa radiation. The interlayer spacing, dool, and the apparent crystallite size perpendicular to the layer planes, L,, were calculated from each (002) diffraction peak. These values primarily reflect the crystallographic parameters of the matrices because the fiber volumes are only 6-18 per cent. Results are summarized in Table 1 and are in agreement with available data[5] on RL and SL matrix microstructures, i.e., the RL material is the while the SL matrix is most graphitic, perhaps slightly more graphitic than the I matrix. Densities of the different microstructure types (without fibers) were determined by suspending ground samples in density gradient columns which contained solid density

236

M. L. LIEBERMAN

Fig. 3. Matrix microstructures.

(a) smooth

and H. 0. PIERSON

laminar,

(b) rough

laminar,

(c) isotropic.

237

EFFECT OF GAS PHASE CONDITIONS

SUBSTRATE

/ MICROSTRUCTURE

PDZZTI RL I

Table

1. X-ray diffraction data for heat treated carbon/carbon composites

St

UIuIml

PAN

PAN RAYON 212 )

116

3.25 3.25

Ave. Range Ave. Range Ave. Range

Rough Laminar Smooth Laminar Isotropic

PAN

PAN PAN

-/

Apparent crystallit: Size, L, (A)

Interlayer spacmg, do,,,(A)

Microstructure

3.37 3.37 3.41 3.40-3.44 3.43 3.41-3.44

Ave. 385 Range 310-460 Ave. 125 Range 95-165 Ave. 90 Range 70-110

PAN RAYON 118

208

214 j

1112

3.0

2.0

6.0

8.0

PAN

0.1

PAN

0.2

PAN

0.3

in agreement

RAYON

0.1

eral hundredths

PAN

RAYON NOTE:

T = IdW-C

ples of each microstructure

/ 0.1

j

the densities the

diffraction

results,

/ 1

mmmmmrm

graphitization

0.1

~

m

L, and density

Fig. 4. Infiltration conditions and resultant micrestructures. In the cross sectional sketches, the bottom side is towards the susceptor.

were

F

T

SUBSTRATE 3 g/cm TYPE

matrix

tropic The

to smooth

Fig. 5. Infiltration conditions and microstructures. In the cross sectional sketches, the bottom side is towards the susceptor.

from

carbon

spectively, where the uncertainty represents the observed spread in the column for a given sample. Density values of two different sam-

phase

laminar. minimized

microstructure. CARBON

obtained with

CVD

different acetylenic

under those

mode1[26]. it is sufficient

microstructure

to different

Analytically,

as iso-

CVD

and predictions

that

gaseous

from

can now be made of the mat-

of this paper

the

both bulk

to rough

OF THE MODEL

purpose attributed

varies

laminar

conditions

the

by Re-

increase

is apparently

microstructures

deposition

densities

laminar

Model summary

rix

samples

2, show that for compo-

porosity

A comparison NOTE: GAS FLOWS ARE CH4 3.25 LIMIN. “2 3.25 LlMlN

of

while

pycnometer.

microstructure

open

extent

and porosity

PAN substrates,

4. APPLICATION

standards. Approximate values for the heattreated RL, SL and I pyrocarbons are 2.12 * O-01, 1.95 kO.05, and l-66? 0.02 g/cm’, re-

helium

displacement

with the rough

4.1

density

on composite

sults, given in Table

the

as the

of

with

increase.

of a Beckman

and helium

sev-

d002 decreases

displacement

sites with identical T

i.e.,

increases,

determined

means

to be

to within

(RL > SL > I) is consistent

0.2

Helium

other

of a g/cm” unit. The order

j 03

P i 6,O TORR

were found

with each

relative and

this is treated

CnHn/C,jH,i molar

For

the

to note types

are

quantities

of

aromatic in terms ratio,

various predicted

species. of the gas

hereafter

des-

ignated by the symbol I?. For given values of temperature, pressure, and C/H ratio, the equilibrium

value of R can be computed

[30].

238

M. L. LIEBERMAN

Table

2. Density

and H. 0.

PIERSON

and porosity of heat-treated composites*

Composite bulk density,” pn

carbon/carbon

Theoreticai Helium density of displacement Open density,” pNe composite,b pT porosity’

Zone

(g/cm”)

(gicm3)

RL SL I

I.86 -t 0.06 l-58 ” 0.05 1.45%0.04

2.01 * 0.02 1*79co*12 l-64 2 O-02

(g/cm”)

(%)

2.07 - 0.06 l-92 2 0.06 I-67 + 0.05

7 14 12

*Minimum of three samples per test; PAN-based O-2 g/cm” carbon felt used as substrate.

“Tolerances reflect the range of measured values. “Given by prel,+ (1 - pr,,,/p,)pM, where pfel,= density of uninfiltrated feit, pf = density of carbon fibers, and pw = density of matrix. Values employed were ptelt= O-2 g/cm’, pr = 1.71 g/cm” for PAN, and PM= 2.12, 1-95 and l-66 g/cm”, for RL, SL, and I matrix microstructures, respectively. Uncertainties are based on 2 10% uncertainty in pIeIt. ‘Given by 100 (pHe- pBYpHe.

Figure

6 shows that, over the range of deposi-

tion parameters considered in this investigation, the value of R increases with increasing temperature, decreasing pressure, and decreasing C/H ratio. Under non-sooting conditions, the model predicts smooth laminar deposits for relatively low R values and isotropic deposits for relatively high values; empirically, it was found that rough laminar deposits form at intermediate R vaIues[30]. These results suggested the different microstructures occur when the values of R are approximately as follows: SL I 1; 1 s RL 5 10; 12 10. Because R varies over about eight orders of magnitude over the usual range of deposition conditions (950-15OO”C, 76-760 Torr, C/H = l/l-1/12), the earlier work indicated that the SL and I microstructures form under a wide variety of conditions, while the RL material apparently forms only under a narrow range of conditions. 4.2 ~~ll-~~n~dco~d~~~o~ During deposition via the thermal gradient process, the gas phase conditions are probably best defined in a low density sub-

10’

i

104

ld

R loi

101

1

10-l

10’ TEMPERANRE, "C

F’ig. 6. Dependence

of R on temperature, and C/H ratio.

pressure

239

EFFECT OF GAS PHASE CONDITIONS strate

immediately

For such made

a situation

which

phase,

adjacent

two assumptions

allow characterization

namely,

(1) the

ture,

identical

can be

that

tempera-

with

surface

the input these

is virtually

gas. Given

assumptions

information

to

identical

calculate

equilibrium

C,H,/C,H,

calculations

are reported

elsewhere

Table 3 presents fixed temperature

the calculated and pressure

ous experimental

C/H ratios.

initial

microstructures*

given

for comparison.

I with

decreasing

dient

C/H

ratio

(or

RL to

the

same

arrive

ter

than

change

that

observed

previously.

in the empirical

limiting

While value

a

of R

Table 3. Effect of C/H ratio on equilibrium ratio[30] and initial microC&/C& structure* C/H 114 I15 116 118 1112

CTH&H,i 20 30 ;: 150

Initial microstructure RL RL RL RL(rayon), I(PAN)t I

*Felt density = 0.1 g/cm”, 14OO”C, 625 Torr. tThis is the only case where different microstructures were observed for the two precursors. It is believed that near a phase boundary, lesser effects, such as a small difference in felt density, can cause the different microstructures.

‘The initial microstructures are those formed at the geometric center of the face of the felt adjacent to the mandrel.

tensive

variables

consid-

pressure,

re-

Again

CnH,/C,H,;

the with

are

value.

are ratio,

obtained

of how the in-

P, C/H) are

ratio

altered

The

SL<5(*3),

boundaries

is the follow-

5(t3)
1>70.

4. Effect of temperature on equilibCsH,/C6Hti ratio[30] and initial microstructure*

(T)“C

C,Hs/C,iH6

1250 1325 1400

2.7 8 40

*Felt 625 Torr.

Initial microstructure SL RL RL

density = 0.1 g/cm’,

C/H = l/6,

Table 5. Effect of pressure on equilibrium CnH,/C6H6ratio [30] and initial microstructuret (P) Torr 625 12

to

present

that the effect of the C,H,/C,H,;

on the phase

Table rium

3 with

under

microstructures

regardless

(T,

a

For the low density

microstructures

at a given

work suggests ratio ing:

to Table

and

to the

R values,

such

are consistent

initial

related

that

C/H ratio.

R values.

felts the observed

transition

grea-

than

microstructures

apparently i.e.,

increasing

is somewhat

temperature rather

observed

for given

however,

tempera-

the mandrel

2 indicate

that the variables

are

spectively,

C,Hs/CeH,; ratio) is consistent with predictions of the model; the value of R at which the occurs,

in Fig.

may exist. 4 and 5 are similar

the calculated

observed

from

shown

eration

[30].

and the vari-

trend

only be 100°C below

the exception

of these

in O-1 g/cm3 felts are The

deposition

(gas phase equilibrium

need

condition Tables

R values for The

logarithmically

actual

temperature to reduce the value of R from 70 to 10. Measurements of the temperature gra-

appropriate

Details

varies The

remembered

to the de-

additional

the

ratios.

parameter

temperature.

10 to 70 may

be

ture)

pressure,

sufficient

this

from

it must

temperato that of

the deposition yield

appreciable,

temperature

temperature)

to the mandrel

and (2) the C/H ratio adjacent

position

seem

of the gas

deposition

ture (i.e., gas phase equilibrium is virtually

for the RL microstructure

to the mandrel.

C,H&H, 40 4 X lo4 (at 15 Torr)

Initial microstructure RI. I

tFelt density = 0.1 g/cm’, C/H = l/6, 1400°C.

240

M. L. LIEBERMAN

4.3 Complex conditions More than one matrix microstructure is commonly obtained in an infiltrated substrate. This is presumably a result of variations in R caused by local variations in temperature and/or C/H ratio. While no attempt has yet been made to determine local values of R within a substrate, one can surmise trends in this parameter and, consequently, infer trends in microstructure. In this manner qualitative predictions based on the carbon CVD model can be compared with microstructure trends obtained under complex conditions, i.e., conditions under which temperature and/or C/H ratio are (is) not well defined. structure transitions are For example, often obtained through the thickness of a felt (Figs. 4 and 5). Since the thermal gradient process results in a lower deposition temperature near the outer portions of the substrate than near the inner ones, the value of R should also be reduced near the outside. From the carbon CVD model one would infer structure transitions from the inside to the outside of the felt to occur in the order I + RL+ SL. In almost all cases* in which structure transitions occur through the substrate thickness, the observed trends are in qualitative agreement with the predicted order. The effect of felt density, or fiber volume, on microstructure can also be examined. An increase in the fiber volume has the effect of increasing the effective residence time of the gas in the hot zone. The gas phase reactions occurring there have the net effect of producing more gaseous molecules than originally enter the hot zone, i.e., 2 CH,+ C,H,+ H,+ C,H, + 2H, + C2H2 + 3H, --, 2C + 4H,. Thus, a net flow of gas occurs from the inside to the outside of the felt. The chemical reactions also indicate that a hydrogen concentration “The only exceptions occur in run No. 223 and the complexity of deposits which occur here causes some uncertainty in the identification of the micrestructures.

and H. 0.

PIERSON

gradient exists through the felt thickness. As the fiber volume increases, gaseous diffusion occurs at a reduced rate. This implies that as the fiber volume increases, the hydrogen concentration near the inside of the felt increases (the local C/H ratio decreases) which results in an increased equilibrium C2H,/C6H6 ratio. From the carbon CVD model, therefore, one predicts microstructure changes in the order SL + RL + I with increasing fiber volume. In all of the observed cases, the effect of increasing the fiber volume under otherwise identical conditions has been to change the initial microstructure in this manner. 5. SUMMARY Carbon felt/pyrocarbon matrix composites have been prepared which exhibit smooth laminar, rough laminar, and isotropic matrix Densities and crystallogmicrostructures. raphic properties of the heat-treated composites are significantly affected by matrix type, with the rough laminar material possessing the most graphitic properties. The conditions under which the different types form are in agreement with predictions of a previously reported carbon CVD model. The present work suggests that the value of the C2HK6H6 ratio in the gas phase affects the microstructure boundaries approximately as follows: SL < 5; 5 < RL < 70; I > 70. For cases in which the local intensive deposition parameters are not well defined, the trends in microstructure are in qualitative agreement with the predictions of the model. Acknowledgements-The authors wish to thank S. F. Duliere and B. L. Butler for the X-ray diffraction and density measurements, respectively, and J. F. Smatana for technical assistance throughout the study.

1. Stover E. 69-27, Vol. 2. Stoller H. Granoff B.

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EFFECT

.9 Pierson

4.

5. 6.

7.

H.

9.

10.

11.

12. 13. 14.

15

OF GAS PHASE

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CONDITIONS

241

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