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 with in the temperature range 1200-1400 degrees C. The strength reduction at ambient for temperatures less than or equal to 1600 degrees 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 degrees C. The thermal dependance of the creep strain rate strongly increases at temperatures greater than or equal to 1300 degrees C. This feature might be partly explained by the structural evolution of the fibre occurring above this temperature. Heat treated fibres (1400-1600 degrees C) exhibit a much better creep strength, probably due to their improved structural organization.