0 Elsevier, Paris Nicalon ceramic fibres including Hi-Nicalon Ann. Chim. Sci. Mat, 2000, 25, pp. 523-528

RECENT ADVANCES IN NICALON CERAMIC FIBRES

INCLUDING HI-NICALON TYPE S

Hiroshi ICXIKAWA

NipponCarbon Co., Ltd., 2-6-1, Hatch&xi, Chuo-ku, TOKYO, ~!4-oo32, hpan

Abstract - Polymer-derived Nicalon Sic fiber produced commercially has been widely applied in high temperature Ceramic Matrix Composites (CMCs). Recently, Sic fiber with a low oxygen content (Hi-Nicalon) was developed and produced commercially using an electron beam curing. This fiber has a higher elastic modulus and creep resistance, and much higher thermal stability up to 1600” C than that of Nicalon fiber. High temperature mechanical properties of CMCs with Hi-Nicalon have much improved. However, creep and oxidation resistance of Hi-Nicalon CMCs are not satisfactory for high temperature structural materials, because Hi-Nicalon mainly consists of SIC micro-crystals and excess carbon. Therefore, we have developed an SIC fiber (Hi-Nicalon type S) with stoechiometric Sic composition and high crystallinity. Hi-Nicalon S fiber has high elastic modulus (420 Gpa), good oxidation resistance at 14OO”C, and excellent creep resistance. In this paper, the recent development of these oxygen free Sic fibres including physical and mechanical properties, thermal stability and an environmental resistance are reported.

@urn6 - R&en&s dkouvertes sur les fibres Sic Nicalon, en particulier la fibre Hi-Nicalon type S. Les fibres SIC Nicalon prod&es ?I partir de polym&es sont largement utilis&s pour fabriquer des materiaux c&amiques composites (CMCs). Recemment, la fibre Sic a t&s faible teneur en oxygene (fibre Hi-Nicalon), produite par traitement par faisceau Clectronique, a Ctt developpc5e a l’echelle commerciale. Cette fibre pos&de un module d’elasticite, une resistance au fluage et une stabilite thermique jusqu’a 16OO”C, beaucoup plus importants que la fibre Nicalon. Les proprietes thermomt!caniques des CMCs produits a partir de la fibrc Hi-Nicalon ont eti significativement ameliorees. Cependant, la resistance au fluage et a l’oxydation restent en de@ des esperances pour des applications thermostructurales. Ceci provient du fait que la fibre Sic est principalement constituee de microcristaux de carburc de silicium et dun excbs de carbone. Pour cette t&son, nous avons d&eloppC la fibre Sic de type Hi-Nicalon type S, dot&z dune composition Sic stoechiometrique et d’une meilleure cristallinite. La fibre Hi-Nicalon type S pos&de un module d’elasticite longitudinal de 420 GPa, une bonne resistance a l’oxydation a 1400°C et une excellente tisistance au fluage. Dans cet article, les n5centes avancees dans le developpement de cette fibre, ainsi que ses proprietes physiques et mecaniques et sa stabilite thermique sont presentees.

Renrints : Hiroshi ICHIKAWA, Nippon Carbon Co., Ltd., 2-6-1, Hatchobori, Chuo-ku, Tokyo, 104-0032, Japan

524 H. lchikawa

1. INTRODUCTION

In recent years, the demand for high performance ceramic matrix composites (CMCs) in high temperature applications has been contmuously increasing. CMCs are required with heat resistance above 15OO’C as structural materials for space planes and high-temperature gas turbine applications. CMCs are highly dependent on the properties of the reinforcement. A reinforcing fiber should have high environmental stability and sufficient mechanical properties even at high temperature. SIC fibres that have high tensile strength, high elastic modulus and good thermal stability are one of the best candidates for reinforcement. Especially polymer derived SIC fibres, which have the advantages of being flexible and having a fine diameter form over those from CVD or sintering processes [ 11. The Si-C-O fiber Nicalon synthesized from polycarbosilane (PCS) was produced mdustrially and applied widely to heat resistant materials and reinforcements for polymer, metal and ceramic matrix composites [2, 31. Nicalon fibres perform well as reinforcement for CMCs [4]. However, the conventional Nicalon has a temperature limit of 1200 “C in heat resistance [5].

Recently, oxygen-free Si-C fibres (Hi-Nicalon) was developed. The tibres were produced from polycarbosilane (PCS) by electron beam curing and pyrolysis. The Hi-Nicalon fiber has a high elastic modulus and creep resistance and much improved thermal stability over the Nicalon fiber [6,7]. Therefore, Hi-Nicalon was successfully introduced in a wide variety of fabrication processes and promising mechanical properties of derived CMCs were achieved. However, I%-NicaIon fibres mainly consist of SIC micro crystals and amorphous carbon. These micro-crystals would cause creep deformation mom easily at high temperature, and excess carbon can degrade oxidation resistance.

We have synthesized the oxygen-free Si-C fibres with various C/Si atomic ratios by electron beam curing and pyrolysis from PCS fibres. Based on the study of thermomechanical properties with various ClSi fibres, the fiber with the C&i atomic ratio of 1.05 was selected as a new grade of Hi-Nicalon. This fiber is pyrolysed at high temperature under special conditions, and Sic crystals grew without decreasing the fiber strength. The stoechiometric, highly crystalline Sic fiber (Hi-Nicalon type S) was developed [8].

In this study, recent results of these fiber’s physical and mechanical properties, thermal stability and environmental resistance are reported.

2. PREPARATION OF Sic FlBRES

Figure 1 shows the fabrication process of Sic based fibres. A preceramic polymer, polycarbosilane (PCS), was synthesised from dimethyl dichlorosilane as a starting material. The Sic fibres were prepared by the melt spinning of polycarbosilane, followed by curing and pyroIysis [2,3].

While conventional Nicalon fiber was prepared by oxidation curing of polycarbosilane fiber, Hi-Nicalon fiber was made by radiation curing using an electron beam irradiation from 10 to 15 MGy in a helium gas flow. The SIC fibres with various ClSi ratios are prepared from electron beam cured PCS libres by pyrolysis in a hydrogen gas flow.

t Green Fiber

I Electron Beam Irradiation Oxidation

t t Cured Fiber

I I

(Hi-NICALON) (NICALON)

Figure 1 : Fabrication process of the Sic based fibres (Nicalon and Hi-Nicalon)

Uicalon ceramic fibres including Hi-Nicalon 525

3..CHARACTEFUSTICS OF Si-C FIBRES

3.1. Physical and mechanical nronerties

Based on the study of the various USi fibres, we have selected the fiber with C/Si atomic ratio of 1.05 as new grade of Hi-Nicalon fiber. The fiber was pyrolysed at high temperature under special conditions, and the crystallite size of the fiber grew without decreasing the strength. Then, we developed the stoechiometric Sic fiber (Hi-Nicalon type S) as a new grade of Hi-Nicalon.

TabIe Z shows the typical properties of Hi-Nicalon type S compared to other PCS-derived SIC fibres. The properties of type S are preliminary ones.

Table I. Typical properties of PCS-derived Sic based fibres

The fiber diameter and Tex of type S are rather smaller than the other two types of fibres. Tensile strength is 2.6 GPa, which is sufficiently high for the reinforcement of CMCs. Type S shows the highest modulus of 420 GPa and the highest density, 3.1 gZcm3. The specific resistivity of type S is much smaller, which can be attributed to the existence of a carbon layer at the fiber surface.

The Transmission Electron Microscopy (TEM) micrographs of Sic-based fibres arc shown in Figure 2. The grain size of three types of the fibres were obviously different from each other, and increased in the order Nicalon, Hi-Nicalon and Hi-Nicalon type S fiber. Each fiber’s grain size is approximately 5,10, and 20 nm, respectively.

Nlcalon SIC-O fiber

Hi-Nicalon SK fiber

HI-Nlcalon types Stolchiomatrlc SIC fiber

100 nm

Figure 2. TEM micrographs of the Sic based fibres

526 H. lchikawa

3.2. Thermal stability.

Figure 3 shows the tensile strength of the fibres after thermal exposure tests. Nicalon fiber exhibited low strength after exposure at 1400°C and no strength after 1500°C exposure. On the other hand, Hi-Nicalon and type S fibres retained good strength even after exposure at 1600°C. Hi-Nicalon type S fibms showed the highest strength, 1.8 GPa, after 10 hours exposure in argon gas at 1600” C.

Further tests at higher temperature were performed on Hi-Nicalon and type S. After exposure for 4

one hour at 2000°C in argon gas, Hi-Nicalon fiber maintained the same appearance and flexibility as that a before exposure and had moderate strength of 1.3 GPa. Hi-Nicalon type S after one hour exposure in an argon gas at 1800” C was quite stable chemically, since no 2 structural decomposition occurred and it exhibited a good strength of 1.9 GPa. It is evident that Sic-based fibres with reduced oxygen content exhibits superior 1 thermal stability.

6

Figure3. Tensile strength of the Sic-based fibres after 10 hours exposure in an argon gas

3.3. Crew resistance

Figure. 4 shows the creep properties of some representative PCS-denved SIC fibres as compared to those of various ceramic fibres reported by DiCarlo [7.8]. Hi-Nicalon and type S fibres show much better creep resistance than conventional Nicalon fibres. Excellent creep resistance was obtained in the case of the annealed type S fiber, which is a stoechiometric composition with 3.5 nm crystallite size. TEM observation shows that this annealed fiber has a Sic grain size of approximately 200 nm, which is about 10 times larger than that of the as-fabricated fiber.

Temperature, “C

.6

m .4

‘0. %

0.3 .3

V Dow Coming .3 0 Carborundum 2 0 PRO-166 .l

.l 0 / ,

4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0x10-4

Reciprocal temperature, K-l

Fire 4. One-hour stress relaxation ratio of the PCS-derived SIC fibres and other polycrystalline Sic and Alumina fibres [lo]

Nicalon ceramic fibres including Hi-Nicalon 527

4. SUMMARY

Table II represents the properties of PCS-derived fibres; Nicalon, Hi-Nicalon and Hi-Nicalon type S. Oxygen-free Si-C fibres (Hi-Nicalon) were developed. The fibres were produced from polycarbosilane (PCS) by electron beam curing and pyrolysis. Hi-Nicalon has a high tensile strength of 2.8 GPa and a high elastic modulus of 270 GPa. The fiber retained a high strength and modulus after thermal exposure at 1600” C for 10 hours in argon atmosphere.

Table II Comparison of properties on PCS-derived Sic fibres

Tensile strength 8 3.0 GPa

Elastic modulus 0 200 GPs

Thermal stabllity 01473K

Oxidation resistance A

cmap rasietarm A

1.39 0.5 wt%

0 2.9 GPa

0 270 GPa

02O73K

*

0

Furthermore, the stoichiomeuic crystalline SiC fiber (Hi-Nicalon Type-S) was developed. The fiber has been prepared from e-beam cured PCS fibtes using special process conditions. The Hi-Nicalon S fiber has an excellent thermal stability up to 1600°C, as does the Hi-Nicalon fiber. The fiber also has higher elastic modulus (420 Gpa), excellent creep resistance and better oxidation resistance than other Si-C tibres.

Hi-Nicalon and Hi-Nicalon Type S fibms should be the best candidates for the reinforcement of ceramic matrix composites. Hi-Nicalon fibres were produced by Nippon Carbon Co., Ltd. commercially, and Hi-Nicalon Type S fibres also were provided to customers.

5. REFERENCES

[l] J. R. Strife, 5. .I. Btennan and K. M. Pmwo, Ceram. Eng. Sci. Proc., 11 (7-8) (1990) 871.

[2] S. Yajima, J. Hayashi, M. Omori and K. Okamura, Nature, 261(1976)683.

[3] T. Ishikawa, Composites Science and Technology, 51 (1994) 135.

[4] K.M. Prewo, J. J. Brennan and G K. Layden, Am. Ceram. Sot. Bull., 65 (2) (1986) 305.

[5] T. Ishikawa and H. Ichikawai, Proc. Electrochem. Sot. 88-5; Symp. II. Temp. Mater. Chem.,

(1987) 205.

528 H. lchikawa

[6] K. Okamura, M. Sato, T. Seguchi and S. Kawanishi, Proc. 1” Jap. Int. SAMPE Symp, (1989)

929.

[7] M. Takeda, Y. Imai, H. Ichikawa, T. Ishikawa, N. Kasai, T. Seguchi and K. Okamura, Ceram.

Eng. Sci. Proc., 14 (9-10) (1993) 540.

[S] M. Takeda, Y. Imai, H. Ichikawa and T. Ishikawa, Ceram. Eng. Sci. Pro&l5 (4) (1994) 133.

[9] G N. Morsher, J. A. DiCarlo and T. Wagner, Ceram. Eng. Sci. Proc., 12 (7-8) (1991) 1032.

[lo] J. A. DiCarlo, Composites SciexeandT~~,51(19!%)213