Continuous SiC fiber, CVI SiC matrix composites for nuclear applications: Properties and irradiation effects

The fate and role of in situ formed carbon in polymer-derived ceramics

Non-oxide ceramic fibers are of considerable interest due to the ability to combine the high performance, especially high temperature thermal and creep resistance, with the structural advantages of fibers including their use as reinforcements for metal (MMCs) and ceramic matrix composites (CMCs). In this paper the development of CVD SiC fibers and three generations of polymer derived SiC fibers over the past 50 years are discussed, illustrating the effect of fiber precursor and processing on the microstructure and physical properties of the non-oxide ceramic fibers. Additionally recent advances in research and development related to fibers from SiC and SiCN systems are presented with discussion of the current focus on reducing the costs of the fiber processing, while increasing their thermostructural stability.

Ceramic Fibers Based on SiC and SiCN Systems: Current Research, Development, and Commercial Status†

A comparative study of the chemical composition and microstructure of Hi-Nicalon, Hi-Nicalon type S, Tyranno SA, Sylramic and Carborundum fibres has been conducted. This analysis has confirmed results already published but has also evidenced some original features. The Hi-Nicalon type S fibre has a near stoichiometric composition but it still contains some oxygen (≈1 at. %) and free carbon (≈2 at. %). The expected near stoichiometric composition of both the Tyranno SA and the Sylramic fibres is only effective near the edge region, while the core of the fibres contains some amount of free carbon (e.g., up to ≈14 at. % and ≈6 at. % respectively in large diameter fibres) as well as some residual oxygen (≈0.5 at. %). The composition of the Carborundum fibre is very close to stoichiometric SiC except rare and localised free carbon or B4C inclusions. The properties of the different fibres, some of them still beeing at a development stage, are discussed from a chemical and a phase composition point of view, on the basis of what is known about their respective preparation process.

Characterization of nearly stoichiometric SiC ceramic fibres

SiCf/SiC composites with BN interface were prepared through isothermal–isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (KIC) were studied for the composites. The tensile strength of the SiCf/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiCf/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

Microstructure and oxidative degradation behavior of silicon carbide fiber Hi-Nicalon type S

Thermal stability of the low-oxygen-content silicon carbide fiber, Hi-NicalonTM

Polycarbosilane-derived SiC fibers (CG Nicalon, Hi-Nicalon, and Hi-Nicalon type S) were exposed for 1–100 h at 1273–1673 K in air. Oxide layer growth and changes in tensile strength for these fibers were examined after exposure. The three types of SiC fibers decreased in strength as the oxide layer thickness increased. Fracture origins were located near the oxide layer–fiber interface. The Hi-Nicalon type S showed better oxidation resistance than the other polycarbosilane-derived SiC fibers after exposure in air at 1673 K for 10 h. This result was attributed to the nature of the silicon oxide layer on the surface of the SiC fibers.

Microstructure and Oxidation Behavior of Silicon Carbide Fibers Derived from Polycarbosilane

SiC-based fibers with various chemical compositions were synthesized using an irradiation-curing process. Polycarbosilane (PCS) fibers were cured by irradiation with an electron beam in a helium atmosphere. The cured PCS fibers were pyrolyzed at 1300°C under controlled hydrogen or argon atmospheres, and SiC fibers with C/Si of 0.84 to 1.56 were obtained. The fibers consisted of <1.0 wt% O, <0.2 wt% N, <0.1 wt% H, with the balance being Si and C. The mechanism of pyrolytic transformation of cured PCS to SiC-based ceramics was investigated using TG/DTA analysis. Greater mass losses were observed during pyrolysis in a hydrogen atmosphere than in argon. This result suggests that the hydrogen atmosphere suppresses H2 evolution and helps to remove excess carbon as CH4 during pyrolysis. The microstructure and mechanical properties of the resulting SiC-based fibers were found to be very dependent on their C/Si chemical compositions.

Jun-ichi Sakamoto

Papers

Microstructure and oxidative degradation behavior of silicon carbide fiber Hi-Nicalon type S

Thermal stability of the low-oxygen-content silicon carbide fiber, Hi-NicalonTM

Microstructure and Oxidation Behavior of Silicon Carbide Fibers Derived from Polycarbosilane

Effect of Hydrogen Atmosphere on Pyrolysis of Cured Polycarbosilane Fibers

Properties of polycarbosilane-derived silicon carbide fibers with various C/Si compositions