October 4-9, 2015
Thermal protection of metallic components by using ceramic coatings like thermal barrier coatings (TBCs) is widely used, in rocket engines, aircraft industry and gas turbines for power generation to reduce the substrate temperature up to 165°C. This technology diminishes the heat transfer in combustor, initial rotor blades and nozzles guide vanes, so that turbines have taken special advantage of these coatings to meet increasing demands for greater fuel efficiency, lower NOx emissions and higher power and thrust . Thermal barrier coatings, apart from thermal protection are also used to protect against abrasion, oxidation and corrosion .
The understanding of the mechanical behavior of TBCs in both as-sprayed and thermal loading conditions is of great importance for the evaluation of components life. However, it is challenging to determine the true mechanical behavior because of the substantial differences, between each layer (TC, BC, and substrate), since the difficulty to prepare the sample´s surface and its high contents of pores, cracks and defects.
This work reports on the mechanical characterization of APS 6-8 wt.% yttria stabilized zirconia (YSZ) as top layer bonded by an HVOF NiCoCrAlY layer to a superalloy substrate (Inconel 625) using the micro- and nano-indentation techniques on test specimens after being subjected to isothermal-oxidation at 1100°C and different time exposition (0, 200, 400, 600, 800, 1000, and 1700 hours).
Observations under optical and SEM microscopy were performed in order to study the variation of the mechanical properties such as hardness, young’s modulus, and fracture toughness within the ceramic top layer after isothermal oxidation.
The hardness and Young’s modulus of the ceramic layer showed an increase until 600h exposition which can be explained by sintering phenomenon. From 600h up to 1700h the Young’s showed a stabilization due to the equilibrium of sintering and phase transformation mechanisms, but hardness continued its increasing rate.
The fracture toughness of the ceramic coating showed a softening condition of about 23% during the whole range of time for isothermal oxidation as a consequence of the detriment of the ceramic layer.
Results show that sintering phenomena, TGO growth and phase transformation give a complex state of stresses of the coating system which will have great influence on its performance during regime operation. It is concluded damage of the coating is proportional to the extent of time exposition.
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