Masters Theses

Date of Award

5-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

C. J. McHargue

Committee Members

J. E Spruiell, R. A. Buchanan

Abstract

The fiber-matrix interfacial forces in a composite strongly influence the strength and fracture behavior of the material. A weakly bonded interface leads to a low matrix fracture stress and low ultimate strength. As the composite is stressed, load is not transferred from the matrix to the fibers; thus, the properties of the reinforcement are not utilized efficiently. As cracks are generated, they progress as in a porous ceramic material, and upon complete matrix failure the fibers are pulled from the matrix with little resistance. An increase in interfacial friction yields a higher matrix cracking stress by hindering crack propagation and by utilizing the properties of the fibers through effective load transfer prior to matrix failure. Higher ultimate strength is also achieved because more energy is expended to overcome the interfacial forces to pull the fibers out of the matrix. An extremely strong bond does not allow for crack deflection or debonding at the fiber-matrix interface; therefore, a crack propagating in the matrix simply passes through the fibers undisturbed, resulting in brittle fracture. A median position between high matrix failure stress and ultimate strength must be chosen to obtain the gradual composite failure through effective fiber pullout.

Fiber-reinforced SiC composites fabricated by thermal-gradient forced-flow chemical-vapor infiltration (FCVI) have exhibited both composite (toughened) and brittle behavior during mechanical property evaluation. Detailed analysis of the fiber-matrix interface revealedthat a silica layer on the surface of Nicalon/SiC fibers tightly bonds the fiber to the matrix. The strongly bonded fiber and matrix, combined with the reduction in the strength of the fibers that occurs during processing, resulted in the observed brittle behavior.

The mechanical behavior of Nicalon/SiC composites has been improved by applying thin coatings to the fibers, prior to densification, to control the interfacial bond. Varying degrees of bonding have been achieved with different coating materials and film thicknesses. Fiber-matrix bond strengths have been quantitatively evaluated using an indentation method and a simple tensile test. The effects of bonding and friction on the mechanical behavior of this composite system have been investigated.

Fiber coatings not only weaken the fiber-matrix bond, increasing toughness, but can protect the fibers from chemical attack and hinder chemical interactions at the interface. Electron microscopy and surface analyses were utilized to characterize interface compositions and interactions. These results have been used to further characterize fiber-matrix interfaces and their influence on the mechanical behavior of these composite materials.

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