Date of Award

3-23-2017

Document Type

Thesis

Degree Name

Master of Science

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 demanding aerospace applications such as aircraft engine hot-section components. In these applications the composites will be subjected to cyclic and sustained loadings at elevated temperature in aggressive combustion environments. Current aircraft engines employ Nickel-based superalloys in applications such turbine blades, where the metallic alloys must perform at or near their operating temperature limits in highly corrosive environments. The SiC/SiC composites, which offer low density, high strength and fracture toughness at elevated temperatures could potentially replace Nickel-based superalloy in aircraft engine applications. However, before the SiC/SiC composites can be safely used in advanced aerospace applications their durability at elevated temperatures in service harsh environments must be assured. Therefore a thorough understanding of mechanical performance of SiC/SiC composites and their constituents in service environments is critical to design and life prediction of these materials. When composite is subjected to mechanical loading in combustion environment, surface matrix cracks form. Then steam (one of the main component of the service environment) enters the composite through matrix crack and reacts with the SiC matrix to leach Si and become saturated with Si(OH)4. The silicic acid-saturated steam travels into the composite interior and attacks the oxidation prone reinforcing SiC fibers. Hence thorough understanding of performance and durability of advanced SiC fibers at elevated temperatures in silicic acid-saturated steam is of paramount importance. This effort investigates creep of Hi-Nicalon™ S SiC fibers at 900°C in air and in Si(OH)4 saturated steam. The fiber tows consisting of approximately 500 filaments with an average diameter of 12 µm were subjected to creep tests at 900°C using a unique testing facility developed at AFIT, Creep stresses ranged from 3.5 to 1180 MPa in air and from 3.5 to 800 MPa in Si(OH)4 saturated steam. Primary and secondary creep regimes were observed in all tests. Creep run-out defined as 100 h at creep stress was achieved at 736 MPa in air, but only at 3.5 MPa in Si(OH)4 saturated steam. Creep rates in Si(OH)4 saturated steam were approximately an order of magnitude higher than those in air. Post-test microstructural examination revealed passive oxidation of fibers tested in air or in steam, and showed no evidence of active oxidation.

AFIT Designator

AFIT-ENY-MS-17-M-277

DTIC Accession Number

Pending

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