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5190 |
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Effect of bondcoat and substrate chemistry on oxide growth and lifetime in suspension plasma sprayed thermal barrier coatings |
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Mohit Gupta* / University West, Sweden Nicolaie Markocsan / University West, Sweden Xin-Hai Li/ Siemens Turbomachinery, Sweden Björn Kjellman/ GKN Aerospace, Sweden |
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A Thermal Barrier Coating (TBC) system is designed to protect gas turbines from high temperatures and harsh environments. Development of TBCs allowing higher combustion temperatures is of high interest since it results in higher fuel efficiency and lower emissions. It is well known that nano-structured TBCs produced by Suspension Plasma Spraying (SPS) have lower thermal conductivity as compared to conventional systems due to their very fine porous microstructure. However they have not yet been commercialised due to low reliability and life expectancy of the coatings. Lifetime of a TBC system is highly dependent on bondcoat and substrate chemistry as it influences the interdiffusion characteristics and growth rate of Thermally Grown Oxide (TGO) layer. To enhance the lifetime of SPS TBCs, fundamental understanding of relationships between bondcoat-substrate chemistry, TGO growth rate, and lifetime is essential. The objective of this work was to study the effect of TGO growth rate on lifetime in SPS TBC systems by changing bondcoat and substrate materials. Experimental compositions of NiCoCrAlY bondcoat powders with different aluminium activity were investigated. The aluminium activity of these powders was estimated by a separate diffusion modelling study. A commercially available NiCoCrAlY was also investigated for comparison. High velocity air fuel (HVAF) spraying was used for bondcoat deposition while axial-SPS was used for yttria stabilised zirconia topcoat deposition. Same topcoat spray parameters were used for all bondcoat samples. TGO layer formation was examined by isothermal heat treatment of the samples for different time durations in a furnace. Lifetime of all TBC systems was examined by thermal cyclic fatigue and thermal shock testing. The growth of TGO layer in different TBC systems with time and the failure mechanism in each case have been discussed in this article. |