Effects of Al2O3 diffusion barrier layer (including Y-containing small oxide precipitates) and nanostructured YSZ top coat on the oxidation behavior of HVOF NiCoCrAlTaY/APS YSZ coatings at 1100 °C

•Nanostructured YSZ top coat slightly reduced double layered TGO growth rate and weight gain of TBCs.•Y was slightly incorporated in the Al2O3 (TGO) layer of HVOF NiCoCrAlTaY coating as Y containing small oxide precipitates.•The segregation of Ta-enriched oxides (TaOx) at the middle of TGO layer led to the formation of some cavities.•TaOx formation in TBCs including MCrAlTaY bond coats can be prevented by presence of Al2O3 film including fine Al-Y oxides.