Date of Award

6-14-2018

Document Type

Thesis

Degree Name

Master of Science in Aeronautical Engineering

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD.

Abstract

Innovations in SiC based ceramic matrix composites (CMCs) have yielded exceptional high-temperature properties and performance in aggressive oxidizing environments. These material characteristics provide potential avenues for future advancements in many applications where current metallic alloys perform near their operating temperature limits in a harsh environment. As steam (typically present in these environments) enters through cracks in the matrix of a SiC/SiC composite, it leaches Si and becomes saturated with silicic acid, Si(OH)4, prior to attacking the reinforcing SiC fibers. Therefore, it is paramount that a thorough understanding of the performance and durability of SiC fibers be obtained under these conditions. This research effort investigated the creep behavior of Hi-Nicalon™ S SiC fibers at 1100°C in air and in silicic acid-saturated steam. The fiber tows that were tested consist of approximately 500 fiber filaments with an average diameter of 12 μm. Creep tests were performed with creep stresses ranging from 3.5 to 650 MPa in both air and in silicic acid-saturated steam. Regions of primary and secondary creep were observed in all tests, with the transition from primary to secondary creep occurring within the first hour of each test. Creep run-out was defined as 100 hours at creep stress and was achieved at 600 MPa in air, but only at 3.5 MPa in saturated steam. The creep rates achieved in saturated steam were approximately an order of magnitude higher than those achieved in air. Post-test analysis revealed passive oxidation of the fibers tested in air and in saturated steam. Crystallized scale growth was found on fibers tested in saturated steam.

AFIT Designator

AFIT-ENY-MS-18-J-072

DTIC Accession Number

AD1056664

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