A team of researchers at the U.S. Army Research Laboratory (ARL) has detailed industry progress toward developing an optimal sand-phobic thermal barrier coating for gas turbine engines in military and commercial fixed- and rotary-wing aircraft.
With some commercial and military aircraft often operating over sandy regions, existing engine inlet particle separators are not entirely efficient in filtering fine sand particles below 75 microns, affecting the durability of turbine blade thermal barrier coatings and overall performance of the aircraft engine.
The lab aims to make further improvements on the coatings” tolerance to these and similar foreign particulates that may be contained in the intake air. Such coatings are used in the high-temperature sections of the engine.
Over the years, through consistent efforts in advanced materials development, scientists and engineers have significantly improved the efficiency and power densities of gas turbine engines by increasing the turbine inlet temperature using high-temperature capable blade materials and coatings. Traditionally, foreign object damage (FOD) has been the primary concern in aviation and tank automotive gas turbine engines. Current state-of-the-art inlet particle separators equipped in gas turbine engines can remove most of the larger sized particles (greater than ~75 µm) from the inlet air. This, in combination with high-quality, erosion-resistant coatings on the compressor sections, has significantly reduced the risk from FOD in the cold sections of the engine. However, fine micron-sized particles (consisting of ash, soot, dust, and/or sand) can still pass through the engine and create problems in the hot-sections, where the operating temperatures of modern gas turbine engines typically surpass the melting point of many of these contaminants. The resulting impact, adhesion, melt infiltration and/or glassy solidification of these small particles can significantly damage the hot-section components within the gas turbine engine.
High pressure gas generator turbine blades typically experience inlet temperatures of 1300-1600 ºC, requiring thermal barrier coatings (TBCs) to be used as thermal insulators on the turbine blades to reduce blade cooling needs. Current state-of-the-art gas turbine blade technology has four layers in the high temperature turbine blade coating system that consists of different materials with specific properties and functions. These layers are (i) the Ni-based superalloy substrate, (ii) bond coat, (iii) thermally grown oxide (TGO) (Alumina, Al2O3), and (iv) the ceramic top-coat (YSZ, Yttria stabilized Zirconia). These discrete multilayer materials have shown vulnerability to intake flows that contain sand, dust, and/or fly ash, which adhere and react at high temperatures and cause premature component failure through both impingement and combined mechanical-thermal-chemical attack. Currently there are no valid fundamental physics based models to describe the complex phenomena of molten particulate impingement, adhesion and melt infiltration into the ceramic TBCs, followed by subsequent phase change to glassification and underlying chemical oxidation leading to failure/delamination of the coating layers. A collaborative research effort at the ARL, with the U.S. Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC), the U.S. Navy Naval Air Systems Command (NAVAIR), and the National Aeronautics and Space Admininstration (NASA), has been initiated to conduct research to understand the complex fine sand particulate interaction mechanisms with thermal barrier coatings and bulk material substrates.
Field-returned engine hardware from Army rotorcraft from Southwest Asia (SWA) shows a significant number of occurrences of sand-induced damage. Army rotorcraft engines are being pulled out of service at a substantial knockdown on the design life. Commercial jet engines flying over dust polluted urban areas in some parts of Far East Asia and South Asia also find sub-micron particles in a molten/semi-molten state after passing through the combustor. Novel TBCs which are also resistant to sand-induced surface degradation can greatly reduce vulnerability in hostile particle-laden environments and increase fuel efficiency of gas turbine engines for both the military rotorcraft and civil aircraft fleet. This paper outlines the continued research efforts at ARL to develop novel TBCs that are sand-phobic and have excellent resistance to impact wear and Calcium-Magnesium- Aluminum-Silicates (CMAS) adherence, as well as excellent thermal strain tolerance and very low thermal conductivity properties for improved performance of future gas turbine engines.
The article also discusss:
Particle Interactions Research Study.
Sand Particle Impact Simulation.
Sand Particle Melting Simulation.
Sand Particle Trajectory Simulations.
Experimental Efforts.
Scanning Acoustic Microscopy Analysis.