Multifunctional coating interlayers for thermal-barrier systems
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Introduction Thermal-barrier systems are an excellent example of multilayered, multifunctional material assemblies that have enabled substantial improvements in the performance and efficiency of gas turbine engines. The combination of a metallic substrate, an intermetallic (or mixed metallic and intermetallic) interlayer, and a low conductivity ceramic topcoat permits the system to operate near, or even above, the melting point of the substrate in a highly oxidizing combustion environment.1–7 Intermetallic coatings have long been a central requirement for high temperature operation of propulsion and power generation systems. In early applications in aero- and land-based turbines (before ceramic thermal barriers were developed), environmental coatings typically served a single function. Aluminide-type coatings based on NiAl and NiCoCrAl-family coatings became the standard systems for oxidation protection, while diffusion chromides and overlay CoNiCrAl materials were used to protect against hot corrosion.1,6 As turbine operating temperatures have increased, fuels have become cleaner, substrate alloys have evolved to refractory-rich nickel-based single crystals, and environmental coatings have become (by necessity) multifunctional. In current turbine components (see Figure 1 in the introductory article for a depiction of a gas turbine), a single coating deposited on a turbine blade may be expected to:1–9
• Resist Type II hot corrosion in cooler sections (e.g., in blade shanks). • Resist Type I hot corrosion in intermediate-temperature regions (e.g., below the blade platform and in cooler sections of airfoils). • Resist high-temperature oxidation in the hottest sections (e.g., blade tips, platforms, and airfoils). • Maintain adhesion to the thermal-barrier coating (TBC). • Provide environmental protection in the event of TBC spallation. • Minimize interdiffusion and limit transformations at the interface with the nickel-based substrate. The challenges of developing intermetallic bond coatings that will reliably perform in present and future engineering systems are addressed in this article. A perspective on presently available bond coating systems and their functionality is given first. The suite of processing approaches for interlayer synthesis and challenges in controlling structure and composition are then addressed. Aspects of the bond coating that limit system performance are reviewed. New experimental and characterization tools and computational design approaches are highlighted, along with future prospects for discovery of new materials combinations and for prediction of their performance in complex thermo-chemical-mechanical environments.
T.M. Pollock, Materials Department, University of California, Santa Barbara; [email protected] D.M. Lipkin, GE Global Research, Fairfield, CT; [email protected] K.J. Hemker, Department of Mechanical Engineering, Whiting School of Engineering, The Johns Hopkins University; [email protected] DOI: 10.1557/mrs.2012.238
© 2012 Materials Research Society
MRS BULLETIN • VOLUME 3
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