Title

Creep of Sylramic-iBN Fiber Tows at Elevated Temperature in Air and in Silicic Acid-Saturated Steam

Date of Award

6-18-2015

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD.

Abstract

Stressed oxidation experiments on Sylramic-iBN fiber tows were performed to evaluate the novel fiber's mechanical performance, creep behavior, and creep mechanisms. This research effort investigated creep response of Sylramic-iBN fiber tows at elevated temperatures in both air and in silicic acid-saturated steam environments. Creep experiments were conducted at creep stresses ranging from 127 to 762 MPa at 400 deg C and 500 deg C in order to examine the mechanical behavior of the Sylramic-iBN fiber tows at temperatures below and above the melting point of boria (450 deg C). Sylramic-iBN fibers are known to have excellent creep resistance, better than most other non-oxide fibers and significantly better than oxide fibers. These fibers have a near-stoichiometric SiC composition, so they are a strong candidate for use in advanced aerospace CMCs. This research effort is a pilot program on the study of the effects of steam on Sylramic-iBN fibers. Previous attempts to study creep of SiC fiber tows in steam at elevated temperatures at the Air Force Institute of Technology's (AFIT) Mechanics of Advanced Aerospace Materials Research Laboratory have yielded inconclusive results. Prior work at AFIT showed that the steam extracted silicon from the fiber tow and became saturated with silicic acid (Si[OH]4) . As the steam traveled along the SiC fiber, it also chemically altered the fiber surface producing inconsistent chemical compositions along the length of the fiber tow specimen. Along the fiber portion exposed to the unsaturated steam, the fiber experienced a chemical change leading to material loss, while the fiber portion exposed to the saturated steam experienced a silica scale growth along the fiber surface. A new test facility had to be designed to accurately evaluate the effects of steam on SiC fibers. The facility design incorporated an apparatus for saturating steam with silicic acid prior to entering the test chamber.

AFIT Designator

AFIT-ENY-MS-15-J-046

DTIC Accession Number

ADA625375

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