Pierre Olry

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of β-SiC nanocrystals (≈5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.

Correlation between microstructure and mechanical behaviour at high temperatures of a SiC fibre with a low oxygen content (Hi-Nicalon)

An oxygen free Si–C fibre has been studied in terms of the chemical, structural and mechanical properties produced as a function of annealing treatments. In spite of its high thermal stability with regard to a Si–C–O fibre the Si–C fibre was subject to moderate SiC grain growth, organization of the free carbon phase and densification within the temperature range 1200–1400°C. The strength reduction at ambient for temperatures ≤1600°C could possibly be due to SiC coarsening or superficial degradation. Bend stress relaxation (BSR) and tensile creep tests show that the as-received fibre undergoes a viscous flow from 1000°C. The thermal dependance of the creep strain rate strongly increases at temperatures ≥1300°C. This feature might be partly explained by the structural evolution of the fibre occurring above this temperature. Heat treated fibres (1400–1600°C) exhibit a much better creep strength, probably due to their improved structural organization.

Tensile testing at high temperatures of ex-PCS Si-C-O and ex-PCSZ Si-C-N single filaments

A microtensile tester consisting mainly of an induction-heated furnace, a 0–2 N load cell, a 0.1/1 μm sensitivity straining device and hot grips has been designed and used to test ceramic single ceramic filaments at 25–1600°C under vacuum (0.1 Pa) or in controlled atmospheres. Both failure strength and Youngs modulus were measured with an isothermal gauge length of 30 mm. A system compliance correction was applied for each test temperature and material. The apparatus was used to characterize an ex-poly-carbosilane Si-C-O fibre (Nicalon NLM-202) and an ex-polycarbosilazane Si-C-N experimental single filament almost free of oxygen (γ-ray curing). Both materials exhibit a significant strength loss at 1200–1600°C when tested under vacuum, assigned to a decomposition process with an evolution of gaseous species (SiO/CO or N2) and the formation of a mechanically weak decomposition surface layer. Conversely, the Si-C-N filament undergoes no strength loss when tested in an atmosphere of nitrogen (P=100 kPa) at 1200°C, the decomposition being impeded by the external nitrogen pressure. In all cases, no significant decrease in Youngs modulus was observed.