Hi‐Nicalon^TM‐SSiC Fiber Oxidation and Scale Crystallization Kinetics

Abstract

The oxidation and scale crystallization kinetics of Hi‐NicalonTM‐S SiC fibers were measured after oxidation in dry air between 700° and 1400°C. Scale thickness, composition, and crystallization were characterized by TEM with EDS, supplemented by SEM and optical microscopy. TEM was used to distinguish oxidation kinetics of amorphous and crystalline scales. Oxidation initially produces an amorphous silica scale that incorporates some carbon. Growth kinetics of the amorphous scale was analyzed using the flat‐plate Deal‐Grove model. The activation energy for parabolic oxidation was 248 kJ/mol. The scales crystallized to tridymite and cristobalite, starting at 1000°C in under 100 h and 1300°C in under 1 h. Crystallization kinetics had activation energy of 514 kJ/mol with a time growth exponent of 1.5. Crystalline silica nucleated at the scale surface, with more rapid growth parallel to the surface. Crystalline scales cracked from thermal residual stress and phase transformations during cool‐down, and during oxidation from tensile hoop growth stress. High growth shear stress was inferred to cause intense dislocation plasticity near the crystalline SiO2–SiC interphase. Crystalline scales were thinner than amorphous scales, except where growth cracks allowed much more rapid oxidation.

Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

“…• C and 1 h at 1300 • C. Crystallization was always found to nucleate from the surface [24] with much more rapid growth parallel to the surface than through the thickness [153]. A strong correlation was seen between the crystallization of the scale and the onset of loss of strength.…”

“…This would suggest that the reinforcing fibers are exposed to silicic-acid saturated steam, rather than unsaturated Hi-Nicalon T M S fibers are known to have excellent creep resistance [19], better than most other non-oxide fibers and significantly better than oxide fibers. These fibers have a near-stoichiometric SiC composition (~1 at% oxygen and~2 at% carbon) [24], so they are a strong candidate for use in advanced CMCs. There is a good understanding of SiC oxidation kinetics, but not in a stressed state.…”

“…• C. The oxidation and scale crystallization kinetics of these fibers was also studied [24,153]. It was found that the amorphous silica scale, with some carbon content, forms initially, followed by crystallization into crystobalite and tridymite.…”

“…Several studies have investigated the thermal stability of SiC fibers in oxidizing environments and the effects of oxidation on SiC fibers and its relation to subcritical crack growth (delayed failure) [24,43,49,105,111,121,[123][124][125][126][142][143][144][145][146][147][148][149][150][151][152][153][154][155][156][157][158]. LaraCurzio modeled the effects of oxidation on the stress-rupture behavior of fiber bundles and presented results for Nicalon T M fibers.…”

“…Using data from previous efforts [24,142,153,164], compressive stresses of approximately 25 GPa (3.63 msi) were calculated. These stresses were caused by a volume expansion of 2.2 times during the formation of the silica, but were rapidly relaxed to negligible levels at temperatures above 1200…”