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.

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The stability of lanthanum orthophosphate (LaPO4) on SiC was investigated using a LaPO4‐coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO4 is reduced in the presence of SiC and reacts to form La2O3 or La2Si2O7 and SiO2 as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO2 film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO4 coating. Continuous LaPO4 coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO4 coating becomes morphologically unstable due to free‐energy minimization as the grain size reaches the coating thickness, which allows the SiO2 oxidation product to penetrate the coating.

Woven cloths of Nextelt 610 and 720 fibers were coated with monazite by precipitation. The cloths were first saturated with concentrated precursor solutions, and then submerged in warm water to initiate precipitation onto the fiber surfaces. Coatings were characterized by scanning electron microscopy, and transmission electron microscopy; thermogravimetric analysis was performed on LaPO 4 owders precipitated in solution under the same conditions as the coatings were deposited. Coating thickness distributions were measured and analyzed. Coated fiber strength was measured following heat treatment for 2 h at 12001C. Processing conditions which retain a substantial fraction of the uncoated fiber strength are identified, and are discussed in the context of current understanding of strength degradation in coated fibers. Strength retention of coated Nextelt 610 fibers following heat treatment was broadly insensitive to precursor solution chemistry and was more strongly affected by intercoat firings which govern the final coating microstructure. For fixed processing conditions, more strength degradation was observed in Nextelt 720 due to higher residual stresses in the fiber.

Monazite coatings were deposited on woven cloths and tows of Nextelt 610 fibers by heterogeneous nucleation and growth using solution precursors. Initial experiments revealed two coating regimes in which monazite was either precipitated both in solution and onto the fiber surfaces or only onto the fiber surfaces depending on the precursor solution concentration and fiber surface area. In both cases, regions of tightly packed fibers within cloth were uncoated. Image analysis of coated fiber cross sections revealed a strong correlation between fiber separation and coating thickness, suggesting that the coating of tightly packed fibers was limited by transport of the reactants in solution to these areas. By adopting a coating procedure in which the tightly packed regions are saturated with reactants before precipitation, more uniform coatings of monazite were obtained throughout the cloth; however, the strength of as-coated and heat-treated fibers was degraded and remains problematic.

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