High-strength alkali-resistant sintered SiC fibre stable to 2,200 °C

Abstract

The high-temperature stability of SiC-based ceramics has led to their use in high-temperature structural materials and composites 1-3 . In particular, silicon carbide fibres are used in tough fibre-reinforced composites. Here we describe a type of silicon carbide fibre obtained by sintering an amorphous Si-Al-C-O fibre precursor at 1,800 ЊC. The fibres, which have a very small aluminium content, have a high tensile strength and modulus, and show no degradation in strength or change in composition on heating to 1,900 ЊC in an inert atmosphere and 1,000 ЊC in air-a performance markedly superior to that of existing commercial SiC-based fibres such as Hi-Nicalon. Moreover, our fibres show better high-temperature creep resistance than commercial counterparts. We also find that the mechanical properties of the fibres are retained on heating in air after exposure to a salt solution, whereas both a representative commercial SiC fibre and a SiCbased fibre containing a small amount of boron were severely degraded under these conditions 4 . This suggests that our material is well suited to use in environments exposed to salts-for example, in structures in a marine setting or in the presence of combustion gases containing alkali elements.Si-Al-C-O fibre was synthesized using polyaluminocarbosilane prepared by the reaction of polycarbosilane (-SiH(CH 3 )-CH 2 -) with aluminium(III)acetylacetonate. The reaction of the last two compounds proceeded at 300 ЊC in a nitrogen atmosphere by the condensation reaction of Si-H bonds in polycarbosilane and the ligands of aluminium(III) acetylacetonate accompanied by the evolution of acetylacetone 5 ; the molecular weight then increased by the cross-linking reaction with the formation of a Si-Al-Si bond.Polyaluminocarbosilane was melt-spun at 220 ЊC, and then the spun fibre was cured in air at 160 ЊC. The cured fibre was continuously fired in inert gas up to 1,300 ЊC to obtain an amorphous Si-Al-C-O fibre. This fibre contained a non-stoichiometric excess of carbon and oxygen of ϳ12 wt%. The Si-Al-C-O fibre was converted into a sintered SiC fibre by decomposition accompanied by the release of CO gas at temperatures from 1,500 to 1,700 ЊC and sintering at temperatures over 1,800 ЊC. In this sintering process, aluminium plays a very important role as a sintering aid. However, to obtain a very strong sintered SiC fibre, the content of aluminium in the fibre has to be controlled under 1 wt %. As can be seen from Fig. 1, such a sintered SiC fibre (with Ͻ1 wt% Al) showed a smooth surface (Fig. 1a) and a densified structure. Moreover, in this case, the sintered SiC fibre showed transcrystalline fracture behaviour (Fig. 1b). On the other hand, a sintered fibre with a large amount of aluminium showed intercrystalline fracture behaviour (Fig. 1d). These phenomena are presumed to be related to the upper concentration limit of solid-soluble aluminium in the SiC crystal 6 . From the transmission electron microscopy (TEM) image (Fig. 1c) of the low-Al sintered SiC fibre, no obvious second phase is obser...

Effect of Heat Treatment on Microstructure and Mechanical Properties of Stoichiometric SiC/SiC Composites

“…Actually, Hi-Nicalon TM Type-S and Tyranno TM -SA fibers, recently developed highly crystalline and near-stoichiometric SiC fibers, exhibited excellent thermo-mechanical properties. 3,4) In particular, the latter one is known to have superior oxidation resistance up to 2273 K in Ar and 1773 K in air. From these facts, this approach should be beneficial to SiC matrix.…”

“…In recent years, various approaches such as improvements of constituents, process optimization, thermal and environmental barrier coatings (TBC/EBC) have been pointed out to address this issue. [3][4][5][6] From the viewpoint of development of composite constituents, tailoring the high-crystallinity and near-stoichiometry composition is essential for the improvement of oxidation resistance. Actually, Hi-Nicalon TM Type-S and Tyranno TM -SA fibers, recently developed highly crystalline and near-stoichiometric SiC fibers, exhibited excellent thermo-mechanical properties.…”

Influence of Heat Treatment on Phases, Microstructures and Mechanical Properties of Laminated Fabric (SiC and Aluminosilicate)/Mullite Matrix Composites

“…Charac teristics of the starting bers and mullite ller in the prepa ration of green composites are listed in Table 1. The bSiC bers with 0.5 mass÷ O (Ube Industries Ltd., Yamaguchi) were produced from amorphous SiAlCO bers by the release of CO gas at temperatures at 15001700 C and by the sintering at temperatures over 1800 C. 15) After these treatments, the sintered SiC bers have a stoichiometric SiC composition and a wellcrystallized microstructure. According to the manufacturer, the SiC bers show no degradation in the strength on heating to 1900 C in an inert atmosphere.…”

ポリビニルシラザン由来の非晶質 Si-C-N モノリスの作製と圧縮変形

“…3,4 Many kinds of ceramic fibers have been developed using polymer precursor method, since continuous SiC fibers were obtained by this method in the 1970s. [5][6][7] Bulk amorphous ceramics such as Si-C-N and Si-B-C-N have been fabricated by pyrolyzing the isostatically pressed and warm pressed preceramic powders.…”