TENSILE CREEP-BEHAVIOR OF A SILICON CARBIDE-BASED FIBER WITH A LOW-OXYGEN CONTENT

The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (< 0.5 wt%) have been examined up to 1600 degrees C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of beta-SIC crystallitesiaveraging 5-10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 degrees C in air is related to fibre surface degradation involving free carbon. Crystallization of SIC and carbon fu rther develops in both fibres subject to either creep or heat treatment at similar to 1300 degrees C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 degrees C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 degrees C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2-3, both in air and argon, and in the range 200-300 kJ mol(-1) in argon and 340-420 kJ mol(-1) in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.