SiC fiber strength after low pO₂ oxidation

Abstract

Hi‐Nicalon™‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO2 of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO2, 1400°C with 100 ppm pO2 or higher, and 1300°C with 50 ppm pO2 or higher. When active oxidation did not occur, a glass SiO2 scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO2. Fiber strengths were lowest after heat treatment at 1300°C and a pO2 of 3.7 ppm, 1400°C and a pO2 of 20 ppm, and 1500°C and a pO2 of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.

In situ Y2Si2O7 coatings on Hi‐Nicalon‐S SiC fibers: Phase formation and fiber strength

“…36 However, if this layer is consumed in the reaction with YPO 4 , active oxidation of SiC fibers results in a pitted surface creating flaws that degrade fiber strength. 37 The correlation between fiber strength and the Y 2 Si 2 O 7 fraction in the coating for fibers with 40-100 nm SiO 2 layers (pre-oxidized for 0.5 and 3 hours) is consistent with degradation by active oxidation after consumption of the SiO 2 layer. Weaker correlation between Y 2 Si 2 O 7 fraction and fiber strength for fibers with thicker SiO 2 layers is consistent with incomplete reaction between SiO 2 and YPO 4 , where residual SiO 2 on fiber surfaces protects them from attack by active oxidation.…”

“…Passive pre‐oxidation can protect the fibers from subsequent active oxidation at low pO 2 . However, if this layer is consumed in the reaction with YPO 4 , active oxidation of SiC fibers results in a pitted surface creating flaws that degrade fiber strength . The correlation between fiber strength and the Y 2 Si 2 O 7 fraction in the coating for fibers with 40‐100 nm SiO 2 layers (pre‐oxidized for 0.5 and 3 hours) is consistent with degradation by active oxidation after consumption of the SiO 2 layer.…”

Static fatigue of Hi‐Nicalon™‐S fiber at elevated temperature in air, steam, and silicic acid‐saturated steam

“…The fiber tow has 500 filaments with an average filament diameter of 12 µm. Fiber properties are extensively reported elsewhere . Fiber tow was carefully examined during preparation of test specimens.…”

“…Hi‐Nicalon™‐S SiC fibers are used in many of these SiC‐SiC CMCs . Oxidation can severely degrade SiC fiber strength, and is therefore a vital concern for SiC‐SiC CMCs, particularly in combustion environments and in steam …”

“…[10][11][12][13][14][15][16][17] Hi-Nicalon™-S SiC fibers are used in many of these SiC-SiC CMCs. 5,6 Oxidation can severely degrade SiC fiber strength, [18][19][20][21] and is therefore a vital concern for SiC-SiC CMCs, particularly in combustion environments and in steam. 19,21,22 Oxidation of SiC in air, low pO 2 , CO, CO 2 , and steam has been studied.…”