Oxidation Behavior of LTA/YSZ Intermixed Layer Coating

Oxidation Behavior of LTA/YSZ Intermixed Layer Coating

Pritee Purohit (Army Institute of Technology, India) and Shashikant T. Vagge (College of Engineering Pune, India)
DOI: 10.4018/978-1-5225-4194-3.ch005

Abstract

This chapter describes how for power generators like gas turbines and aero engines, the economic and environmental challenges are increasing day by day for producing electricity more efficiently. The efficiency of power generators can be increased by changing its operating conditions like inlet temperature and procedure. Currently, the inlet temperature to the industrial gas turbine is reaching up to 1400°C. Also, in aero engines, the ring temperature reaches around 1550°C. Therefore, the coatings used in aero engine applications undergo short duration thermal cycles at very high temperature. The mean metal temperatures reach around 950°C and can increase up to 1100°C. But in industrial gas turbines, it varies from 800 to 950°C. Operating temperature of industrial gas turbines slowly reaches to maximum and ideally remains constant for thousands of hours, unlike aero engines.
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Background

TBCs provide effective thermal resistance to the gas turbine engines components by maintaining higher operating and inlet temperatures and also optimum cooling requirements. Atmospheric plasma spray (APS), electron-beam physical vapor deposition (EBPVD), and solution precursor plasma spray process (SPPS) are used to apply metallic/ceramic spray on the superalloy substrates. In APS process compared to the other techniques the coating strain tolerance and TBC life is increased by the readily available cracks and splat morphology (Ghosh, 2015). The blades and vanes are air cooled. TBC is comprised of a ceramic topcoat which is made up of yttria stabilized zirconia and having lowest thermal conductivity, metallic bond coat applied on a superalloy (Seraffon, Simms, Sumner, & Nicholls, 2011). The TBC material composition should have the capability to sustain the temperature cycling, most extreme temperature and thermal and mechanical stresses. In commercial jet engines, TBC is expected to survive in thousands of take-offs and landings. In industrial gas-turbine engines, it should survive up to 30000 hours of continuous operation at high temperature up to 1000°C. TBC structure is much complex than other coatings due to its harsh and demanding operating conditions and multi-material nature.

TBC failure occurs due to many reasons. Few of them are, increase in its thermal conductivity with increase in temperature, stresses due thermal expansion coefficient mismatch of TBC materials, oxidation of metals due to availability of excess oxygen, variation in the material composition and hence continuously varying microstructure, properties and interfacial morphologies (Padture et al., 2002).

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