Oxidation Behavior of LTA/YSZ Intermixed Layer Coating

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).