Preparation of Nanostructured Al2O3 powders for thermal-sprayed Nano Coatings

Turbine blades are generally protected by thermal barrier coatings (TBCs) against high temperature oxidation and corrosion. A novel method has been developed to prepare nanostructured Al2O3 powders for thermal-spraying (atmospheric plasma spray) with high flow-ability. In this method, nano Al2O3 powders are granulated and then heat treated at 2000C which become suitable to be used in thermal spraying equipments. The normal Al2O3 and granulated nano Al2O3 powders were sprayed by using an atmospheric plasma spray device onto the typical TBC, separately. Then, YSZ/ normal Al2O3 and YSZ/ nano Al2O3 coatings were oxidized at 1000°C for 24h and allowed to form the thermally grown oxide layer onto the bond coat (NiCrAlY layer). The flow-ability of the granulated nano Al2O3 powders was studied by using a Hall Flow-meter. The microstructural characterization showed that the granulated nano Al2O3 powders had very high flow-ability. The increased apparent density and flow-ability of the granulated nano Al2O3 powders had substantially reduced the micro-cracks and interconnected porosities in the coating in comparison with normal Al2O3 coating. The thickness of TGO layer in YSZ/ normal Al2O3 coating was higher in comparison with YSZ/ nano Al2O3 coating after oxidation. Thus by using nano Al2O3 as a third layer, the thickness of TGO layer and oxidized areas inside the bond coat decreased effectively which led to the less mechanical stresses and can cause the improvement of TBCs life.