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Fatigue Behavior of an Advanced Melt-Infiltrated SIC/SIC Composite with Environmental Barrier Coating at 1200°C in Air and in Steam

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.

Modern and future aircraft jet engines require increased thermal efficiency to extract the necessary energy during fuel consumption for high velocity flight. One way of improving engine efficiency is through the increase of the turbine’s temperature gradient or the difference between the hottest and coldest temperatures in the engine during operation.

The deficiencies of nickel-based superalloys or high-performance alloy materials in turbine engine technologies has grown apparent as modern turbine engine hot section operating temperatures exceed these material’s stable operating ranges. Any additional cooling to the current systems to prevent melting of the superalloys would detrimentally lower the thermal gradient of the engine. For this reason, novel high temperature materials which do not require extensive cooling are necessary for the improvement of turbine engine technologies.

Ceramic matrix composite (CMC) materials have been developed as successors to superalloys. These composites maintain their properties at high temperatures due to the nature of the constituent ceramic materials while also benefiting from a reinforcement phase, which increases toughness compared to a monolithic or bulk ceramic. The accompanying figure shows general regions of operating temperatures and specific strengths for comparison of superalloys, CMCs, and various other materials. The weak bonding between the matrix and reinforcement phases of CMCs prevents brittle, catastrophic failure exhibited in a bulk ceramic due to deceleration of crack propagation and a simulated ductile region caused by matrix failure prior to reinforcement failure.

The complexities of composite production compared to alloy or monolithic ceramic production lend these materials to extensive characteristic variation between processing techniques. Each variation of material constituent and processing technique must be characterized to adequately understand the composite. Additionally, due to oxidation-prone constituents that hinder the composite through oxidation embrittlement and surface recession, environmental barrier coatings have been developed to protect the composite surface.

This research has been limited to identification and characterization of a single CMC composed of silicon carbide matrix and Hi-NicalonTM silicon carbide reinforcement fibers processed through meltinfiltration (SiC/SiC – MI) with a boron nitride (BN) interphase for weak fiber-matrix bonding. Additionally, the specimens have been grit-blasted and coated with a silicon (Si) bond coat and an ytterbium disilicate (Yb2Si2O7) environmental barrier coating.

Ten EBC/Hi-N/MI-SiC specimens were subjected to cyclic fatigue testing at various maximum stress levels to determine fatigue life of the specimens in air and steam at 1200°C along with the retention of tensile properties if run-out (200,000 cycles) was achieved. This data was compared to prior research on a set of identical but uncoated CMC specimens.

A SiC/SiC composite was selected for research due to the thermal, mechanical, and chemical stability of silicon carbide. Thermally, as shown in the accompanying figure, this CMC maintains adequate strength at elevated temperatures which surpasses competing materials. Mechanically, the composite exhibits a nearly ductile region prior to failure despite a fully ceramic composition. This stems from the prevention of instantaneous catastrophic failure through crack prevention. Chemically, silicon carbide creates a natural protective oxidation layer at high temperatures but suffers from oxidation degradation at temperatures below 1000°C. To stymie this material degradation, an EBC was applied to all specimens composed of a 5 mil Si bond coat and a 10 mil Yb2Si2O7 topcoat. Application of the coating may have other benefits such as filling pores remaining from the melt-infiltration process. The EBC is assumed to maintain a uniform dry film thickness and infinitesimal load carry. Finally, SiC has a high strength to density ratio which is ideal for any aircraft application due to weight reduction without sacrificing strength.

This work was done by Thaddeus M. Williams for the Air Force Institute of Technology. For more information, download the Technical Support Package (free white paper) below. AFIT-0002

Topics:
Aerospace Aircraft Coatings & Adhesives Coatings, colorants and finishes Coatings, colorants, and finishes Composite materials Composites Energy Engine efficiency Engines Fatigue Fuel consumption Jet engines Materials Materials properties On-board energy sources Powertrains Propulsion Vehicles
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Fatigue Behavior of an Advanced Melt-Infiltrated SIC/SIC Composite with Environmental Barrier Coating at 1200°C in Air and in Steam

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    Aerospace & Defense Technology Magazine

    This article first appeared in the December, 2021 issue of Aerospace & Defense Technology Magazine (Vol. 6 No. 7).

    Read more articles from this issue here.

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    Overview

    The thesis titled "Fatigue Behavior of an Advanced Melt-Infiltrated SiC/SiC Composite with Environmental Barrier Coating at 1200°C in Air and in Steam" by Thaddeus M. Williams, presented to the Air Force Institute of Technology, investigates the mechanical properties and fatigue behavior of a specific composite material designed for high-temperature applications. The research is particularly relevant for aerospace engineering, where materials must withstand extreme conditions.

    The study focuses on a melt-infiltrated silicon carbide (SiC) composite that is coated with an environmental barrier coating (EBC). This coating is essential for protecting the composite from environmental degradation, particularly at elevated temperatures. The research aims to understand how this composite behaves under monotonic tension and tension-tension fatigue testing at 1200°C, both in air and steam environments.

    Chapter 4 of the thesis presents the results of high-temperature testing, including tensile properties and fatigue performance. The data collected from these tests provide insights into the material's durability and its ability to maintain structural integrity over extended cycles of loading. The study highlights the significance of environmental factors, such as the presence of steam, on the fatigue life of the composite.

    The methodology includes careful specimen preparation and testing procedures, ensuring accuracy and repeatability in the results. Specimens were subjected to various conditions to simulate real-world applications, and the fracture surfaces were analyzed using scanning electron microscopy (SEM) to understand failure mechanisms.

    The findings indicate that the environmental barrier coating plays a crucial role in enhancing the fatigue life of the SiC/SiC composite, particularly under high-temperature conditions. The research concludes that the composite exhibits promising fatigue behavior, making it a suitable candidate for applications in aerospace and other high-performance environments.

    Overall, this thesis contributes valuable knowledge to the field of material science and engineering, particularly in the development of advanced composites for high-temperature applications. It underscores the importance of understanding material behavior under extreme conditions to improve the reliability and performance of aerospace components.

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