Fabrication, phase transformation studies and characterization of SiC-AlN-Al sub 2 OC ceramics

Abstract

SiC and AlN are two of the important high temperature structural ceramics. AlN and the 2H polytype of SiC are isostructural. Prior work has shown that they form an extension solid solution at temperatures {ge} 2000{degrees}C. At lower temperatures, the solid solution can undergo phase separation. Additionally, Al{sub 2}OC is also isostructural and can form extensive solid solutions with SiC and AlN. The formation of solid solutions in such refractory materials as well as the tendency to undergo diffusional phase transformations suggests that a potential exists to improve properties through alloying. The principal objective of the proposed work is to examine phase relations, phase transformations, the resulting precipitate morphologies and their influence on mechanical properties of SiC-AlN-Al{sub 2}OC ceramics. Formation of modulated structures have been documented in SiC-AlN ceramics in our work. It has been shown that modulations occur along directions other than the (0001) direction and this results in the formation of a tweed type of a microstructure. In the AlN-Al{sub 2}OC system, the occurrence of cellular precipitates as well as coherent, disc-shaped precipitates has been observed. During the past year, work has progressed in the following areas: (1) Phase separation in SiC-AlN system: The effect of coherency strain energy on the precipitate morphology. (2) High temperature creep of SiC-AlN ceramics containing modulated structures and SiC-Al{sub 2}OC ceramics. (3) Fabrication and characterization of damage-resistant SiC-AlN ceramics. Three manuscripts have been submitted for publication.