Thermal Behavior of Y-PSZ TBCs in situ Remelted During Their Deposition

Abstract

Abstract In previous works, laser irradiation was associated to thermal spraying in the so-called MELTPRO process to improve the performance and the durability of TBCs. This study aims at presenting results concerning the thermal behavior of TBC's manufactured according to this process. Two types of severized thermal cycling were implemented: (i) “oxidizing” test: isothermal shocks were performed at different temperatures, ranging from 900°C to 1050°C down to 0°C; (ii) “quite non-oxidizing” test: thermal shocks were implemented from a temperature of 1100°C down to 50°C. Moreover, thermal annealing at 1100°C were performed to compare sintering phenomenon. TBC microstructure and its evolution during heat treating were characterized using image analysis, Knoop micro-indentation and XRD analysis. MELTPRO process was shown to increase twofold the lifetime of TBCs during isothermal shock tests. This is attributed to the fact that the columnar structure and the pore-crack architecture of remelted coatings improve the compliance property and decrease the permeability of TBCs. XRD analyses show that, in the Y-PSZ TBCs, the main phase is the metastable tetragonal (t’) phase both for as-sprayed and MELTPRO processed coatings. Moreover, remelted TBCs show a higher phase stability than as-sprayed TBCs during thermal shock tests: it seems that the remelted coatings have higher phase stability thanks to their pore architecture, which lead to a better compliance in relation to the thermo mechanical stresses, and so to a decrease in the stress variations undergone by the structure during the thermal cycles. During thermal annealing, it seems that MELTPRO processed coatings are less affected by sintering than as-sprayed coatings. Sintering phenomenon primarily concerns inter-lamellar cracks of non remelted areas. Besides, whatever the coating manufacturing process, the main remaining phase is the metastable tetragonal phase whatever the heat treating duration.