Three generations of small diameter ceramic fibres based on polycrystalline silicon carbide have been developed over a period of thirty years. This has been possible due to studies into the relationships between the microstructures and properties of the fibres. A variety of techniques have been employed by research teams on three continents. The fibres are made by the conversion of polymer precursors to ceramic fibres and all three generations are presently produced commercially. The nature of the precursor and the techniques used for cross-linking have been varied in order to optimise both properties and cost of manufacture. It has been possible to improve the characteristics of the fibres as the processes involved in the cross-linking of the precursor fibres have been better understood and the mechanisms governing both room temperature and high temperature behaviour determined. The result is that, although first generation fibres were limited by a low Young's modulus at room temperature and by creep and instability of the structure at temperatures far lower than those limiting the behaviour of bulk silicon carbide, the third generation fibres shows many of the characteristics of stoichiometric silicon carbide. This remarkable improvement in characteristics has been due to a thorough understanding of the materials science governing the behaviour of these fibres which are reinforcements for ceramic matrix composite materials.

A review of the development of three generations of small diameter silicon carbide fibres

Polysiloxanes [RSiO1.5]n with R=CH3 (PMS) and C6H5 (PPS), respectively, were transformed to Si–O–C ceramics of variable composition and structure upon pyrolysis in inert atmosphere at 800–1500°C. The electrical conductivities of the Si–O–C ceramics in air were measured at room temperature by using a shielded two point configuration. In situ measurements of the dc-conductivity during the pyrolytic conversion from the polymer to the ceramic phase were carried out up to 1500°C with four point contacted carbon electrodes in inert atmosphere. During polymer-ceramic conversion excess carbon precipitates above 400°C (PPS)–700°C (PMS). At temperatures above 800°C (PPS) and 1400°C (PMS) coagulation and growth of the carbon clusters results in a percolation network formation. While below the percolation threshold electrical conductivity can be described according to Motts mechanism by variable-range-hopping of localized charge carriers, regular electron band conduction due to the instrinsic conductivity of turbostratic carbon (8×10−4 (Ωcm)−1) predominates above. Thus, the in situ measurement of non-linear electrical property changes can be used as a microprobe of high sensivity to detect microstructural transformations during the pyrolysis of preceramic polymers.

Electrical conductivity measurements as a microprobe for structure transitions in polysiloxane derived Si–O–C ceramics

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

Extensive investigations of polymer-based ceramic materials taken within the Si-C-N-O-(H) system have been performed and previously published as Parts I – IV. This paper reports the main results, facts, trends and conclusions which can be drawn from them, regarding the physical and chemical behavior of these materials submitted to an increasing thermal treatment. Both similarities and discrepancies are described and explained, whatever the chemical composition of the ceramic, (i.e. containing Si-C or Si-C-N, Si-C-O, Si-C-N-O). Emphasis is made on the mechanisms, and the role of various parameters, either intrinsic (e.g. the role of compositional C, O, or N) or extrinsic (e.g. influence of the atmosphere), is examined. The overall results indicate, among others, that excess carbon (relative to the SiC stoichiometry) is always beneficial both to the structural stability of the ceramic and the mechanical properties (when measurable), while heteroatoms (O, N) are always finally detrimental. They thus sustain the current efforts to develop ceramic fibers from chemical systems as simple as possible, using oxygen-free curing processes. More generally, they provide guidelines for understanding, and somewhat predicting, the thermochemical behavior of any related materials.

Thermal behavior of (organosilicon) polymer-derived ceramics. V: Main facts and trends

Electrical, dielectric and microwave-absorption properties of polymer derived SiC ceramics in X band

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.

F Laanani

Papers

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