MICROSTRUCTURAL STABILITY AND CREEP-BEHAVIOR OF SI-C-O (NICALON) FIBERS IN CARBON-MONOXIDE AND ARGON ENVIRONMENTS

Constant-load creep-rupture tests were performed on single Si-C-O fibe rs (Nicalon). Test environments included pure carbon monoxide (CO), pu re argon gas, and a mixture of CO and argon gas with a CO partial pres sure of 40 kPa, Fibers were tested at temperatures of 1200 degrees-140 0 degrees C and at nominal applied stresses of 0.15-0.7 GPa, The as-re ceived and crept specimens were characterized by means of scanning ele ctron microscopy, transmission electron microscopy, X-ray photoemissio n spectroscopy, electron-probe microanalysis, Auger electron spectrosc opy, and thermogravimetric analysis, In pure argon, the microstructure of the Nicalon fiber was unstable, which was attributed to the decomp osition of the silicon oxycarbide phase, which resulted in CO and sili con monoxide gas evolution and silicon carbide grain growth, Fiber shr inkage was observed at temperatures < 1300 degrees C at low applied st resses. At high stresses, fibers exhibited only primary creep, In the CO/argon-gas environment, very limited grain growth and a smooth carbo n coating were observed at the fiber surface at temperatures < 1350 de grees C, At all applied stresses, fibers exhibited steady-state creep whose rates, strains, and times to failure were higher than those obse rved in argon, The apparent activation energy for creep of Nicalon fib ers in the CO/argon-gas environment was 435 kJ/mol, At temperatures > 1350 degrees C in the CO/argon-gas environment, however, the fiber beh aved as in pure argon, Tests in pure CO only resulted in lower strains to failure and thicker carbon layers on the fiber surface, A rheologi cal model based on the viscous how of a concentrated suspension mas; p roposed to describe the fiber deformation, The continuously decreasing creep rate in argon was suggested to be related to the continuous inc rease of the total solid volume fraction, which affects the fiber visc osity, On the other hand, the steady-state creep of Nicalon with a sta ble microstructure in the CO/argon-gas environment was characterized b y a Newtonian-type viscous flow, which supports the predictions of the model.