Microstructural evolution of the latest generation of small‐diameter SiC‐based fibres tested at high temperatures

Abstract

SummaryThe new generation of silicon-carbide-based fibres made from organosilicon precursors, cross-linked by electron irradiation, have been compared with the earlier fibres which have undergone cross-linking in air. The latest fibres, known as Tyranno Lox-E and Hi-Nicalon, possess a lower oxygen content (Ϸ5 wt% and Ϸ0 : 5 wt%) whereas the NLM202 fibres contain 12 wt% and the Tyranno Lox-M 13 wt% of oxygen. The Tyranno fibres have been produced with a precursor similar to that used to produce the Nicalon fibres, but modified by the addition of titanium. All fibres possess a structure composed of b-SiC grains, free carbon aggregates, with no crystallized titanium compounds in the Tyranno fibres and an oxygen-rich intergranular phase, except in the Hi-Nicalon fibre. The Hi-Nicalon fibre has the largest grain size and its free carbon content is higher than in the NLM202 fibres. For all the fibres, the b-SiC grains grow when the temperature increases, whilst the strengths and Young's moduli decrease. The NLM202 shows the least change in grain size and tensile properties. The Hi-Nicalon is stiffer and stronger than the others at high temperature. TEM results show that grain growth is isotropic, even during creep tests. The growth depends on the nature and amount of the intergranular phase. Mechanical changes as a function of temperature can be explained by external oxidation during tensile tests in air and internal oxidation facilitated by the nanoporosity, which is greater in the Tyranno than in the Nicalon fibres. The presence of the oxygen-rich phase in the three fibres containing the most oxygen decreases the creep resistance. Titanium does not improve the mechanical properties and the creep resistance beyond 1523 K and does not have any positive influence in limiting the SiC grain growth. Tyranno fibres are less well stabilized than the Nicalon fibres. The Hi-Nicalon fibres have been shown to possess consistently better mechanical properties at all temperatures, including creep resistance, than the other fibres studied. All the fibres are sensitive to external oxidation at high temperature.

X‐ray Diffraction Study of the Effect of High‐Temperature Heat Treatment on the Microstructural Stability of Third‐Generation SiC Fibers

“…These values are close to those obtained by XRD meaning that most grains are single CDD. The initial grain size of the HNS fibers appears much higher than the one of the previous generation of High Nicalon fibers, as observed by Hochet et al . and it is in agreement with Sauder and Lamon's observations .…”

Tyranno SA3 fiber–ZrB2 composites. Part I: Microstructure and densification

“…In order to weaken the interface between fiber and matrix, we explored the interactions between ZrB 2 and these more refractory Tyranno fibers. The main differences between Hi-Nicalon and Tyranno SA3 fiber are related to the processing: both derive from a polycarbosilane precursor, but, for the second type of fiber, Al is present too as sintering agent [10][11][12][13]21]. Subsequently the polymer is spun and subjected to reticulation by electron irradiation in the first case and in air in the second one.…”

“…Subsequently the polymer is spun and subjected to reticulation by electron irradiation in the first case and in air in the second one. Then the pyrolysis is conducted at 1500°C for Hi-Nicalon and at 1800°C for Tyranno [11,12]. These different processes generate major differences in the microstructural features of the fiber: in Hi-Nicalon, SiC mean grain size of the crystallites is around 5 nm and is much more reactive, owing to the still unstable system containing turbostratic carbon and amorphous Si-C-O.…”

“…They have smaller diameter, 7.5 lm compared to 15 lm of the Hi-Nicalon, and are reported to possess higher stability at high temperature and thus are expected to display better performances in extreme environment. This type of refractory fibers is therefore considered more attractive owing to the excellent mechanical properties coupled with resistance to neutron irradiation, improved thermal conductivity and thermal stability [12,13].…”

Ceramic Filament Fibers – A Review

“…Annealing of Hi‐Nicalon fibers at high temperatures (1300–1600 °C) in an H 2 O and O 2 containing atmosphere leads to the formation of a silica layer on the fiber surface with a structure of α‐cristobalite crystals dispersed in an amorphous SiO 2 phase. The thickness of the layer increases with the annealing temperature 126–146…”