MICROSTRUCTURAL EVOLUTION OF THE LATEST GENERATION OF SMALL-DIAMETER SIC-BASED FIBERS TESTED AT HIGH-TEMPERATURES

The new generation of silicon-carbide-based fibres made from organosil icon precursors, cross-linked by electron irradiation, have been compa red with the earlier fibres which have undergone cross-linking in air. The latest fibres, known as Tyranno Lox-E and Hi-Nicalon, possess a l ower oxygen content (approximate to 5wt% and approximate to 0 . 5wt%) 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 simi lar to that used to produce the Nicalon fibres, but modified by the ad dition of titanium. All fibres possess a structure composed of beta-Si C grains, free carbon aggregates, with no crystallized titanium compou nds in the Tyranno fibres and an oxygen-rich intergranular phase, exce pt 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 beta-SiC grains grow when the temperature inc reases, whilst the strengths and Young's moduli decrease. The NLM202 s hows the least change in grain size and tensile properties, The Hi-Nic alon 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 th ree fibres containing the most oxygen decreases the creep resistance. Titanium does not improve the mechanical properties and the creep resi stance beyond 1523 It and does not have any positive influence in limi ting the SiC grain growth. Tyranno fibres are less well stabilized tha n the Nicalon fibres. The Hi-Nicalon fibres have been shown to possess consistently better mechanical properties at all temperatures, includ ing creep resistance, than the other fibres studied. All the fibres ar e sensitive to external oxidation at high temperature.