Date of Award

3-22-2018

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

Advanced SiC/SiC ceramic matrix composites (CMCs) are being considered for a variety of applications. Of interest is their ability to withstand exposure to high temperature in an aggressive oxidizing environment. The presence of steam creates such an environment. As steam traverses across a SiC/SiC composite and through cracks in the SiC matrix, it becomes saturated with silicic acid, Si(OH)4. It is essential to understand the long-term impacts and durability of SiC/SiC CMCs that have been exposed to such a demanding environment. The present research investigated creep of Hi-Nicalon™ S SiC fibers at 1000°C in air and in silicic acid saturated steam. The fiber tow consists of 500 fiber filaments with an average diameter of 12 μm. Creep tests were conducted in tension using a dead weight creep test setup. Creep stresses ranged from 3.5 to 800 MPa in air and from 3.5 to 500 MPa in silicic acid saturated steam. Creep run-out was defined as 100 hours under load. Creep run-out was achieved at stresses of 500 MPa in air and 300 MPa in saturated steam. Both primary and secondary creep regimes were observed in all tests with the transition to secondary creep occurring early in the lifetime of each test. Steady-state creep rates were found for each test. The steady-state creep rates in saturated steam were approximately an order of magnitude larger than those in air. Passive oxidation was found on the fibers tested in both environments using scanning electron microscopy (SEM). No evidence was found of active oxidation.

AFIT Designator

AFIT-ENY-MS-18-M-271

DTIC Accession Number

AD1056615

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