Date of Award

3-24-2016

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD.

Abstract

Aircraft structural and engine components are subjected to elevated temperature and steam environments during operation. Turbine blades experience particularly harsh conditions that are approaching the operating temperature limits of current Nickel-based superalloys even with active cooling. Ceramic matrix composites (CMCs), which demonstrate high specific strength and specific stiffness and maintain these properties at elevated temperatures such that active cooling is not required, are prime candidates to replace superalloys as the materials for turbine blades. Ceramic matrix composites are composite materials that consist of a ceramic matrix with fiber reinforcement. This research investigated a silicon carbide (SiC) fiber, Hi-Nicalon™ S, which is a candidate for reinforcement of CMCs with SiC-based matrices. Hi-Nicalon™ S fiber tows, consisting of approximately 500 filaments, were creep tested at 700°C in air and in silicic acid-saturated steam. The steam was saturated with silicic acid in order to simulate the interaction that would occur in a SiC-SiC CMC as the steam environment enters through matrix cracks and leaches Si from the matrix while moving towards the fibers. Creep stresses ranged from 2.96 to 1256 MPa in air and from 2.96 to 798 MPa in silicic acid-saturated steam. Creep run-out was defined as 100 h at creep stress. Creep strains were measured and steady-state creep rates were determined in all tests. In air creep runout was achieved at stresses as high as 1023 MPa, while in saturated steam creep runout was achieved only at 750 MPa. Creep rates were approximately an order of magnitude greater in saturated steam than in air. Post-test fiber microstructure was examined using a scanning electron microscope (SEM). Microscopy revealed the presence of a passive oxidation layer on the fibers tested both in saturated steam and in air but no evidence of active oxidation.

AFIT Designator

AFIT-ENY-MS-16-M-234

DTIC Accession Number

Pending

Comments

AFIT-ENY-16-M-234 as AFIT designator on SF298.

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