Fine-diameter polycrystalline SiC fibers

Composites Science and Technology 51 (1994) 167-171

hi i I

FINE-DIAMETER POLYCRYSTALLINE SiC FIBERS J. Lipowitz, T. Barnard, D. Bujalski, J. Rabe, G. Zank Advanced Ceramics Program, Mail Stop 500, Dow Coming Corporation, Midland, Michigan 48686-0995, USA

A. Zangvil & Y. Xu Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA (Received 31 July 1992; accepted 15 November 1992) Carbon Company, Japan; Tyranno ceramic fiber, 3 a S i - C - O - T i composition from Ube Industries, Ltd, Japan; and H P Z ceramic fiber, 4 a S i - N - C - O composition from Dow Corning Corporation, USA. Typical properties for these fibers are shown in Table 1. The continuous phase in these fibers, which controls mechanical properties, is amorphous. 5 Nicalon and Tyranno fiber contain B-SiC crystallites dispersed in the amorphous phase. The elastic moduli are considerably lower than the full density polycrystalline values of 315 GPa for Si3N4 and 460 GPa for SiC. The fibers are thermochemically metastable and undergo gas loss (CO, some SiO, and N2, if present) and crystallization at -1300°C (Nicalon and Tyranno ceramic fiber) or ~1450°C (HPZ fiber) in inert atmospheres. These fibers are useful up to approximately 1200°C in oxidative atmospheres. A crystalline SiC fiber is desired for hightemperature CMC use because modulus, oxidation resistance and thermal stability would be improved compared to these primarily amorphous fibers. Furthermore, a SiC fiber should display desirable creep resistance and high thermal conductivity as is found for SiC in bulk form.

Abstract

Various organosilicon polymers have been converted to small-diameter, polycrystalline silicon carbide fibers by melt-spinning, cross-linking and pyrolyzing to high-temperature in argon. Several wt% boron was doped into the fibers before pyrolysis. Use of polycarbosilane precursor gave 8-10 ixm diameter fibers having up to 2.6 GPa tensile strength, 450 GPa elastic modulus, 3.1-3.2 g/cm 3 density. The microstructure consists of >95 wt% [J-SiC crystallites of 30-40 nm average crystallite size. Stoichiometric fibers or fibers having excess carbon content have been prepared. Fiber has been thermally aged under inert conditions at 1800°C for 12 h with minimal strength and microstructural change. Stoichiometric fiber maintains higher strength after oxidative aging at 1370°C. Current processing efforts are aimed at preparing the fiber in continuous tow form. Keywords: [J-SiC ceramic fiber, microstructure, characterization, tensile strength, elastic modulus, thermal stability INTRODUCTION

Small-diameter ceramic fibers, which are sufficiently flexible to permit weaving and knitting, provide many desirable properties for use in continuous-fiber ceramic-matrix composites (CMC) intended for high-temperature performance in oxidative and inert atmospheres. ~ Such fibers are also useful in polymerand metal-matrix composites. The fine-diameter, non-oxide ceramic fibers have generally been prepared by polymer precursor routes. Products which have reached commercial status include: Nicalon T M ceramic fiber, 2 a S i - C - O composition from Nippon

PROCESS

Polymer-derived ceramic fbers which contain sufficient excess carbon convert to polycrystalline SiC on heating at -1600°C or above in an inert atmosphere. Oxygen in these fibers is lost mainly as CO (some SiO may be lost) and nitrogen (if present) is lost as N2, shown schematically in eqns 1 and 2: SiCaO b ~

Composites Science and Technology 0266-3538/94/$07-00 © 1994 Elsevier Science Limited.

SiC + CO(+SiO)

SiCaObNc he"ff~atSiC + CO(+SiO) + N2 167

(1) (2)

.I. Lipowitz et al.

168

Table 1. Typical properties of commercial and developmental textile grade ceramic fibers (fiber tow)

Ceramic grade Nicalon fiber Tyranno ceramic fiber X9-6371 HPZ ceramic fiber

Filament diameter (#m)

Density (g/cm ~)

Tensile strength (GPa)

Elastic modulus (GPa)

Coefficient of thermal expansion (ppm/K)

10-20 8-10 10-12"

2-55 2-4 2.4

2-8 3.2 2"8

t80 180 180

4-0 3.1 3

" Equivalent circular diameter for this oval fiber. On loss of these gases (thermal decomposition), a coarse granular SiC containing large voids between the grains is formed and no tensile strength is retained. Figure 1 shows a Nicalon S i - C - O fiber which had contained 12 wt% oxygen before heating. A similar fiber derived from polycarbosilane, which was cured with NO2 and contained several wt% boron distributed throughout the fiber, lost CO above

~4

1400°C and crystallized to a stoichiometric, polycrystalline SiC fiber, as shown in Fig. 2. These fibers retained their tensile strength after conversion from a Si-C-O composition to a polycrystalline SiC composition. Polycrystalline SiC fibers have been prepared from polycarbosilane 6 and polysiloxane 7 polymer precursors. Polymers were utilized which contain sufficient carbon to react with the silicon after high temperature loss of oxygen (mainly as CO) and nitrogen (if present). Pyrolysis was generally performed in argon to a temperature above 1600°C. The best fiber properties have been obtained using the polycarbosilane precursor. Polycarbosilane was melt-spun, oxidatively cured with NO2, doped with BCI3 gas, and pyrolyzed to > 1600°C in argon. ~ Stoichiometric SiC fibers or fibers containing an excess of carbon ( > 1 0 wt%) can be prepared from polycarbosilane by control of the amount of oxygen introduced during the cure process. Polycarbosilane without oxygen incorporation pyrolyzes to SiC containing - 1 5 wt% excess carbon. PROPERTIES

Fig. 1. SEM image of ceramic grade Nicalon fiber (11 wt% oxygen) after heating above 1600°C in argon.

Fig. 2. Boron containing SiC fiber, derived from polycarbosilane, after heating above 1600°C in argon. Tensile strength was 2.6 GPa and elastic modulus 450 GPa.

AND

STRUCTURE

The microstructure of the SiC ceramic fiber formed from the various polymeric precursors is similar. X-ray diffraction shows a typical /3-SIC pattern having an average crystallite size of 30-40 nm by line broadening measurements of the (111) diffraction peak (Fig. 3). Up to several per cent of s-SiC structure was present, and in some cases graphitic-like carbon was detected by a weak (0002) diffraction peak. A SEM image of a high-strength, high-modulus SiC fiber prepared from polycarbosilane is shown in Fig. 2. Properties of SiC fibers prepared from polycarbosilane are listed in Table 2 and are as would be expected for a polycrystalline SiC fiber. The elastic modulus is approximately twice that obtained for the polymerderived fibers shown in Table 1. Elastic modulus is reduced markedly by the presence of excess carbon, from - 4 5 0 GPa for stoichiometric SiC to - 3 0 0 GPa for fiber containing - 1 0 wt% excess carbon. Density is > 97% theoretical for stoichiometric SiC. Fractog-

Fine-diameter polycrystalline SiC fibers

Carbon

10

20

169

O/

30

t

t

l

t

q

40

50

60

70

80

Two - Thets Degrees

(CuKot)

Fig. 3. X-ray diffraction pattern of polycarbosilane-derived SiC fiber. Table 2. Properties of SiC fiber prepared from polycarbosilane

- - Average tensile strength up to a 2.6 GPa at 25 mm gage length - - Elastic modulus up to 450 GPa - - Fiber diameter 8-9/x m - - Density to >3100 kg/m 3 (>97% theoretical) - - Critical stress concentration factor (K~¢) - 3 MPa m "2, based on strength/flaw size relationship --Coefficient of thermal expansion 5.1 ppm/K (300-1600 K) - - F i b e r length <-100 mm - - 87% strength retention after 1800°C/12 h aging in argon; no microstructural change after aging

raphy showed that the weaker fibers generally fracture in tension at internal flaws or at kink sites. Fiber retained 87% of its initial r o o m - t e m p e r a t u r e strength after exposure to argon at 1800°C for 12 h (Table 3). Strength retention after 1370°C exposure in air for 12 h depends on the excess carbon content. Fiber which contained excess carbon retained only 35% of initial strength. Final strength after exposure was 0.6 GPa. SEM examination showed a ring of voids formed at the silica-fiber interface after oxidation (Fig. 4). These - 1 / z m diameter voids at the b o u n d a r y of a - 1 / z m thick silica layer a p p e a r to be the critical flaws which reduced strength. The stoichiometric SiC fiber retained 66% of initial strength after the same air exposure; final strength was 1.6 GPa. SEM imaging

Fig. 4. Carbon-rich SiC fiber after exposure to air at 1370°C for 12 h.

Fig. 5. Near-stoichiometric SiC fiber after exposure to air at 1370°C for 12 h.

showed a - 1 /zm thick silica layer with no voids present (Fig. 5). Fractography showed tensile failure at regions of thick, bulbous silica growth. Structure of the SiC fibers is described in Table 4. A thin ( - 5 0 nm) carbon-rich surface layer is seen on all fibers by scanning A u g e r depth profiling. Figure 6 shows an example. The carbon-rich layer is believed

Table 3. Thermal stability of SiC fiber prepared from polycarbosilane

Ageing conditions

Tensile strength (GPa) Initial

12 12 12 12

h/1600°C/argon" h/1800°C/argon" h/1370°C/aiff h/1370°C/air b

1-85 + 1-74 + 1.74 + 2.44 +

° SiC fiber with excess carbon. b Near-stoichiometric SiC fiber.

0.51 0.10 0.32 0.49

Aged 1.70 + 1.52 + 0-61 + 1.62 +

0-51 0-40 0.19 0.52

Weight change

% Strength retained

0 -0-54 ---

91 87 35 66

(%)

J. Lipowitz et al.

170 Table 4. Structure of SiC fiber prepared from polycarbosilane

P

- - Composition is controllable from near-stoichiometric /3-SIC to >10wt% excess carbon - - O x y g e n and nitrogen content <0.1 wt% - - Average fl-SiC crystallite size by X-ray line broadening is 30-40 nm --/3-SIC crystallite size by TEM ranges up to 0.5/~ m - - All fibers have carbon-rich outer layer, -50 nm thickness - - Carbon-rich fiber has three layers: (i) outer -50 nm is carbon-rich (ii) next 1-2 ~m is stoichiometric fl-SiC (-0.5/zm crystallite size) (iii) interior is carbon-rich /3-SIC (up to 0.1 p~m crystallite size) --/3-SIC crystallites contain numerous twinning and stacking faults - - Excess carbon between SiC grains is graphitic-like Fig. 8. TEM image showing stoichiometric region of a SiC fiber. 100 ~

~

J. Brennan U.T.R.C.

8o E O '< 60

C

o

,o o U

10

30

50

100

200

300 400 500

Distance from Flber Surface, nm

Fig.

6. Scanning

Auger depth profile stoichiometric SiC fiber.

of

a

near-

Fig. 7. SAD analysis of SiC fiber showing twinning and stacking faults (streaking) in/3-SIC,

to be formed by evaporation of silicon from the surface region during the pyrolysis process. By T E M techniques, the fiber contains /3-SIC crystallites with numerous stacking faults. Small area electron diffraction (SAD) analysis shows twinning and stacking faults as is often found in /3-SIC crystallites (Fig. 7). Beneath the carbon-rich surface, fiber containing excess carbon has a 1-2 ~ m layer of equiaxed /3-SIC crystallites of 0.1-0.5 /~m size (Fig. 8). The interior consists of - 0 . 1 ~ m and smaller /3-SIC crystallites dispersed in a graphitic-like phase (Fig. 9). High-resolution lattice imaging resolved the 0.336 nm spacing of the graphitic structure (Fig. 10). Stoichiometric SiC fiber contains 0.1-0.5 ~ m equiaxed /3-SIC crystallites.

Fig. 9. TEM image showing carbon-rich region of a SiC fiber.

Fine-diameter polycrystalline SiC fibers

171

REFERENCES

Fig. 10. High-resolution lattice image of the carbon-rich region of a SiC fiber showing the graphitic layers. The authors are currently working to continuous fiber in multifilament tow form.

obtain

ACKNOWLEDGEMENTS We gratefully acknowledge the support and encouragement of the NASA-Lewis Research Center. We thank John Brennan of the United Technology Research Center for providing scanning Auger depth profiles of SiC fibers.

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