Microstructural development, phase stability, and toughening in mullite matrix composites
Lin, Chien-Cheng
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https://hdl.handle.net/2142/23799
Description
Title
Microstructural development, phase stability, and toughening in mullite matrix composites
Author(s)
Lin, Chien-Cheng
Issue Date
1991
Doctoral Committee Chair(s)
Zangvil, Avigdor
Department of Study
Engineering, Materials Science
Discipline
Engineering, Materials Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Mullite matrix composites with partially stabilized zirconia and/or SiC whiskers were fabricated by hot pressing of powders which had been prepared from Al, Si, Zr, and Mg (or Y) alkoxide precursors by a sol-gel process. A series of studies were conducted on microstructural evolution, oxidation behavior and toughening mechanisms, in order to extend the knowledge in these areas and provide a basis for the future development of these materials. X-ray diffraction, optical microscopy, high resolution scanning electron microscopy (SEM) and analytical transmission electron microscopy (TEM) were the major tools used in the present study. Following a detailed microstructural/microchemical characterization of the hot-pressed composites, their microstructural evolution was elucidated, by examining powder samples heat treated or hot pressed at various temperatures between 800 and 1400$\sp\circ$C. It was found that, during hot pressing, mullite developed from an intermediate Al-Si spinel phase, while zirconia could develop by (1) nucleation, growth and subsequent transformation of cubic ZrO$\sb2$ particles from the amorphous matrix; (2) dissolution and reprecipitation from mullite grains; (3) decomposition from zircon (ZrSiO$\sb4$), which had formed as an intermediate phase. The microstructural development in hot pressed composites was found to be significantly different from that in heat treated powders. The oxidation behavior of SiC whiskers in various matrices was investigated at temperatures ranging from 1000 to 1350$\sp\circ$C for up to 1000 hours. Oxidation products and morphologies were observed in both cross-sectional and surface-normal samples by TEM. Oxidation mechanisms were described in details, and two basic oxidation modes were defined based on relative oxygen diffusivities in the matrix and the oxide layer around SiC whiskers. They were verified, and a mixed oxidation mode was also identified in actual composites. It was shown that oxidation behaviors could be strongly influenced by the matrix composition and microstructure, as well as by the temperature. Finally, possible toughening mechanisms in mullite matrix composites were addressed, through direct observations on fracture surfaces by TEM and SEM. The question of additivity of such mechanisms was also discussed.
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