Comparison of Bend Stress Relaxation and Tensile Creep of CVD SiC Fibers

Abstract

Three different CVD SiC fibers were tested for bend stress relaxation (BSR) and tensile creep over a wide range of temperatures, times, and stresses. Primary creep was always observed, even for creep strains on the order of 2%. The BSR and tensile creep results were compared using simple linear viscoelastic principles. It was found that BSR results could predict the same time and temperature dependence as tensile creep; however, BSR‐predicted creep strains usually overestimated the magnitude of tensile creep strain. The time, temperature, and stress dependence were determined for all the fibers for the experimental conditions of this study. Some of the primary creep behavior can be explained by load‐sharing effects between the core and the CVD SiC substrate and some microstructural changes; however, the extent of primary creep cannot fully be accounted for from this work.

High temperature properties of SiC and diamond CVD-monofilaments

“…This behaviour might be assigned to concurrent SiC grain growth (the creep parameters indeed suggest a grain-boundary diffusion mechanism) and/or to load sharing due to the multilayered composite structure of the SiC-CVD filaments. 25 Steady-state creep is never observed for the free silicon rich filaments for the studied time range, whereas it appears only after a sufficient duration (t > 10 4 s) in the case of the Ultra SCS and SM2156 filaments. Raman analyses evidenced a significant SiC grain growth in the former, whereas crystallisation was strongly inhibited by the presence of the intergranular free carbon phase in the latter.…”

“…17). 25 The BSR resistance of the SM1156 filament is significantly lower than that of the SCS-6 (about 250 • C lower for a given m value), while they show similar thermal activation. 25 The Ultra SCS and SM2156 filaments show a BSR resistance improvement of about 150 • C with respect to the SCS-6 and a slightly lower thermal activation.…”

“…When the CVD-SiC filaments are tested in tension, using temperature and stress conditions similar to those examined here, the creep strain is usually less than 1%, a steady-state creep is never observed and the stress exponent n is very close to 1, a feature suggesting that the creep mechanism is controlled by diffusion. [25][26] The time and temperature dependence of the bending creep strain can therefore be described by the same above equation, provided that n ≈ 1. For all tested filaments, the bending creep curves recorded for various temperature and stress levels were fitted to simple power law: ε max (t) = Ct p .…”

“…The activation energies of the SCS-6, SM1156 and both the Ultra SCS and SM2156 filaments at high temperature, are all near the range of the silicon or carbon grain boundary self-diffusion (respectively, 612 and 554 kJ mol −1 ), 27-28 suggesting a common creep behaviour involving grain boundary sliding and controlled by grain boundary diffusion. [25][26] Their significantly different creep rates can be explained by the SiC grain size, as well as the nature of the grain boundary and particularly of the intergranular phase. The grain boundary sliding leads to intergranular normal stresses.…”

“…a function of the temperature-compensated time θ = t exp(−Q/RT) (where Q is the activation energy and R the universal gas constant R = 8.3144 J mol −1 K −1 ) and a power of the applied stress σ. The corresponding expression of the strain is: [25][26] …”