Date of Award

3-2020

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD

Abstract

Advanced aerospace applications such as aircraft turbine engine components, hypersonic flight vehicles, and spacecraft reentry thermal protection systems require structural materials that have superior long-term mechanical properties under high temperature, high pressure, and varying environmental factors, such as moisture. Because of their low density, high strength and fracture toughness at high temperatures SiC fiber-reinforced SiC matrix composites are being evaluated for aircraft engine hot-section components. In these applications the composites will be subjected to various types of mechanical loadings at elevated temperatures in oxidizing environments. Because their constituents are intrinsically oxidation-prone, the most significant problem hindering SiC/SiC composites is oxidation embrittlement. Typically the embrittlement occurs once oxygen enters through the matrix cracks and reacts with the fibers and the fiber coatings. The degradation of fibers and fiber coatings is generally accelerated in the presence of moisture. Environmental Barrier Coatings (EBC) were developed specifically to address degradation of CMCs due to oxidation by protecting the composite surface from the oxidizing environment. Before ceramic matrix composites with EBCs can be used in aerospace applications, their structural integrity and long-term environmental durability must be assured. A thorough understanding of the mechanical behavior of the candidate CMC with EBC at relevant service temperatures is critical to design with and life prediction for these materials. Tension-tension fatigue performance of a SiC/SiC composite with an EBC was investigated at 1200°C in laboratory air and in steam. The composite has a melt-infiltrated (MI) matrix consolidated by combining CVI-SiC with SiC particulate slurry and molten Si infiltration and is reinforced with laminated woven SiC (Hi-Nicalon™) fibers. The EBC consists of a Si bond coat (targeted at 127 μm) and an Ytterbium disilicate (Yb2Si2O7) top coat (targeted at 254 μm). The EBC was applied via Air Plasma Spraying (APS). Basic tensile properties of the composite with the EBC were evaluated at 1200°C. Tension-tension fatigue was examined for maximum stresses ranging from 110 to 140 MPa in air and in steam. To assess the efficacy of the EBC, experimental results obtained for the coated composite are compared to the results obtained for a control composite without the EBC. The presence of the EBC had a moderately beneficial effect on the composite performance. Fatigue run-out defined as survival of 200,000 cycles was achieved at 120 MPa in air and in steam for the EBC containing composite, but only at 110 MPa for the uncoated CMS. The retained properties of all specimens that achieved fatigue run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. A sharp decrease in cyclic lifetimes with increasing maximum stress observed for both the coated CMC and the control CMC is attributed to significant processing defects present in both composites.

AFIT Designator

AFIT-ENY-MS-20-M-287

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