Masters Theses

Date of Award

12-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Peter K. Liaw

Committee Members

C. R. Brooks, N. Yu, J. D. Landes

Abstract

Nicalon® plain-weave fiber fabric-reinforced silicon carbide (SiC) matrix composites with various fiber/matrix interface coating thicknesses have been successfully fabricated by novel forced chemical vapor infiltration (FCVI) methods. The influence of the interface coating thickness on the fracture behavior of continuous fiber-reinforced SiC composites has been investigated. Experimental methods including four-point flexure testing, double notch compression interlaminar shear testing, and chevron-notch-three- point bending toughness testing have been used to characterize the mechanical behavior of the composite. In addition, scanning electron microscopy (SEM) has been used to investigate the failure mechanisms of the various test specimens. Experimental results indicate that fiber coating thickness significantly alters the fracture behavior of SiC composites. The fracture strength, work of fracture, interlaminar shear strength, and toughness exhibit a maximum as the coating thickness increases. Based on the experimental results and the SEM fractography, a mechanistic understanding of the fracture behavior is provided. Furthermore, a theoretical model is formulated to quantify the influence of coating thickness on fracture behavior. The predicted fracture behavior was found to be in good agreement with the experimental result. Based on the experimental results, the SEM observations, and the model, the failure of these composites is controlled by the nature and extent of microcracking which is governed by the forces at the fiber/matrix interface. Since the fiber coating provides a means for regulating the interfacial forces, it controls the failure mechanisms, and therefore, the mechanical properties through moderation of the interfacial forces.

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