Masters Theses

Date of Award

12-1993

Degree Type

Thesis

Degree Name

Master of Science

Major

Electrical Engineering

Major Professor

D.B. Koch

Committee Members

P.B. Crilly, M.A. Abidi

Abstract

Previous studies of ceramic composites have revealed the fiber-matrix interface to greatly influence the mechanical properties of the composite. A weak interfacial bond is desired to allow for fiber pull-out, resulting in the toughening of the composite. Nicalon&red;-fiber/SiC-matrix composites have been fabricated with a carbon layer between fiber and matrix in order to provide the necessary weakening. Carbon, however, is susceptible to attack by oxygen, rendering it unsuitable as an interfacial material at high temperatures. Metal oxides were deposited on Nicalon fibers by sol-gel methods in an effort to find oxidation resistant coatings which may provide the necessary weakening of the interfacial bond. Alumina, zirconia and titania were deposited on layers of Nicalon® cloth. A control sample with a pyrolytic carbon layer was fabricated as well. The cloth was placed in a holder and the composite densified by forced flow chemical vapor infiltration. The resulting composites were cut into bars and tested under four-point loading, both as-fabricated and after exposure to air at 1273 K for 1000 hours. Alumina and titania reacted with the fibers during processing to form mullite and TiC, respectively, as well as small amounts of graphite. These composite still displayed some weakening of the interfacial bond. Zirconia was stable, but bonded tightly to the matrix and fibers, resulting in weak and brittle behavior. The control sample displayed abundant fiber pull-out in the as-fabricated condition. Upon exposure to air at high temperature, the mullite layer was stable and the composite suffered no loss in strength or fracture toughness. The TiC layer and the control sample suffered some degradation and the composite lost significant strength and fracture toughness. SEM and TEM analysis revealed a strong correlation between debonding and interfacial smoothness.

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