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Hi ‐Nicalon TM ‐ S SiC Fiber Oxidation and Scale Crystallization Kinetics
Author(s) -
Hay R. S.,
Fair G. E.,
Bouffioux R.,
Urban E.,
Morrow J.,
Hart A.,
Wilson M.
Publication year - 2011
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2011.04647.x
Subject(s) - crystallization , tridymite , materials science , cristobalite , kinetics , activation energy , amorphous solid , nucleation , chemical engineering , crystallography , crystal growth , mineralogy , composite material , chemistry , quartz , physics , organic chemistry , quantum mechanics , engineering
The oxidation and scale crystallization kinetics of Hi‐Nicalon TM ‐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 SiO 2 –SiC interphase. Crystalline scales were thinner than amorphous scales, except where growth cracks allowed much more rapid oxidation.