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
This Brief includes a Technical Support Package (TSP).
Fatigue Behavior of an Advanced Melt-Infiltrated SIC/SIC Composite with Environmental Barrier Coating at 1200°C in Air and in Steam
(reference AFIT-0002) is currently available for download from the TSP library.
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