HEAT-RESISTANT INORGANIC FIBERS

Toshihiro Ishikawa1, Hiroshi Oda2

1 Tokyo University of Science, Yamaguchi, 1-1-1 Daigaku-Dori, Sanyo-Onoda, Yamaguchi 756-0884, Japan

2 UBE industries, Ltd., 1978-5 Kogushi, Ube, Yamaguchi, 755-8633, Japan

ABSTRACT

Up to now, many researchers have developed various inorganic fibers (For example: Glass fiber, Carbon fiber, Metal oxide fibers, Boron-based fiber, Single crystalline fibers, Eutectic oxide fiber, SiCCVD-based fiber, and Polymer-derived SiC-based fibers) for developing composite materials with lightweight and high fracture toughness. Of these, carbon fiber has established a very big market after being adopted in airplane applications. Besides the mechanical properties have been dramatically improved. And presently, SiC-polycrystalline fibers have been also addressed in the field of airplane engines for the severer applications at very high temperatures. Ube Industries, Ltd. (UBE) is one of suppliers of SiC-polycrystalline fibers. UBE's SiC-polycrystalline fiber (Tyranno SA) shows very high heat-resistance up to 2000°C and relatively high mechanical properties. And so, some programs on the CMC technology using Tyranno SA have been performed in the field of airplane engines. In these programs, the higher mechanical properties are expected for expanding the application parts. The historical tide on the technical issue of Tyranno SA is very similar to that of carbon fiber. And also, the control factors on the mechanical strength of this fiber are very similar. Now, the research on the mechanical properties of the SiC-polycrystalline fiber has been actively performed. In this paper, the aforementioned historical viewpoint of inorganic fibers and the important factors for controlling the mechanicalproperties of the SiC-polycrystalline fiber was reported.

INTRODUCTION

The first continuous inorganic fiber was developed about 70 years ago. The main objective was to obtain composite materials with light weight and high fracture toughness. Up to now, lots of inorganic fibers have been developed. The time series regarding the development of inorganic fibers are shown in Fig.1.

Fig.1 The time series regarding the development of inorganic fibers

Of these, glass fiber and carbon fiber are well known all over the world. In addition, aluminasilica fiber, single crystalline oxide fiber and silicon carbide fiber, which show excellent oxidation resistance at high temperatures even in air, have been developed and commercialized except for the single crystalline oxide fiber. Of these, glass fiber and carbon fiber made a lot of progress in a field of the reinforcement of plastics. Especially, carbon fiber has been already established a very big market. Oxide fibers such as alumina-silica fiber had been used for insulating materials of space shuttle as a form of fabrics, and so forth. However, up to now those oxide fibers have not been able to make a very big market such as the carbon fiber whereas the researches on composite materials using oxide fibers have been performed in many research laboratories [1]. On the other hand, silicon carbide fibers have achieved great progress in the specific characteristics [2-4]. As the result of development of the aforementioned inorganic fibers, lots of advanced researches concerning ceramic composite materials making the best use of the excellent physical properties (for example: mechanical strength, heat-resistance and oxidation-resistance, and so forth) have been performed actively. By the way, as mentioned before, carbon fiber has been already established a very big market because of the excellent mechanical properties along with very low density. However, in air, it is relatively difficult for the carbon fiber to show the high-temperature properties because of its low oxidation-resistance. On the other hand, silicon carbide can show balanced heat-resistance both in air and inert gas atmosphere. Accordingly, the development of SiC-based fiber had been attracted. The first developed SiC-based fibers have been produced since the mid-1960s by chemical vapor deposition onto tungsten or carbon filament core. However, as these types of SiC-based fiber had a large diameter, their applications were limited by their difficulties for use. After that, a SiC-based fiber with small diameters of about 10 micron meters were synthesized from organo-silicon polymer. This type of fiber was classified into a polymer-derived SiC fiber. The first polymer-derived SiC fiber was developed from polycarbosilane by Professor Yajima in the middle 1970s [5]. After that, many types of polymer-derived SiC-based fibers have been developed and commercialized [6,7]. These SiC-based fibers can show good mechanical strength and oxidation-resistance up to very high temperatures over 1000°C. So, research and development on composite materials using the SiC-based fibers have been widely performed [8-10]. In particular, stoichiometric SiC-polycrystalline fibers (Tyranno SA, and Hi-Nicalon Type S) show an excellent heat-resistance up to 2000°C [3,4]. Accordingly, representative airplane engine manufacturers have actively evaluated these fibers. However, to extend the application field, increase in the mechanical strength of these fibers is eagerly required. By the way, the production processes of the polymer-derived SiC fibers are very similar to that of carbon fiber [11]. That is to say, regarding the improvements of the mechanical strength of these fibers, the history of the development of the strongest carbon fiber would be very suggestive for the study on the increase in the mechanical strength of the SiC-based fibers. The tensile strength of firstly commercialized carbon fiber (T-300 produced by Toray Industries, Inc.) was only about 3 GPa. However, presently, the highest strength (about 7 GPa) has been successfully achieved by the same company [12,13]. Until the success, lots of defects contained in the inside and outside of each filament of the carbon fiber have been remarkably reduced. On the other hand, present tensile strengths of all commercial SiC fibers have been still around 3 GPa. The present stoichiometric SiC-polycrystalline fiber (Tyranno SA) also shows the almost similar strength. Tyranno SA is synthesized by further heat-treatment (~2000°C) of an amorphous Si-Al-C-O fiber, which is synthesized from polyaluminocarbosilane [3]. During the aforementioned further heat-treatment, the degradation reaction of the amorphous Si-Al-C-O fiber and the sintering of the degraded fiber proceed as well, accompanied by the release of CO gas and compositional changes, to obtain the dense structure. Since these changes proceed in each filament, a strict control should be needed to minimize residual defects on the surface and in the inside of each filament. Considering the aforementioned present strength (around 3 GPa) of the stoichiometric SiC-polycrystalline fiber (Tyranno SA), this fiber must contain some residual defects. So, the remarkable increase in the strength would be expected by an effective decrease in the residual defects. In this paper, the historical points of inorganic fibers and important things for development of heat-resistant inorganic fibers are described. Of these, the development story of the polymer-derived SiC fibers and the progress of highest heat-resistant SiC-polycrystalline fiber will be addressed.

HISTRICAL POINTS OF INORGANIC FIBERS

As mentioned before, since the first inorganic fiber (Glass fiber) was developed about 70 years ago, lots of inorganic fibers have been developed. The good information regarding the basic materials constructing the aforementioned inorganic fibers was published by W.C.Miller [14]. It is summarized in Table 1.

Table 1. The basic information of developed inorganic fibers