Abstract
YVO4crystals and monoclinic ZrO2are known as hot corrosion products that can considerably reduce the lifetime of thermal barrier coatings during service. The hot corrosion resistance of two types of air plasma sprayed thermal barrier coating systems was investigated: an Inconel 738/NiCrAlY/YSZ (yttria-stabilized zirconia) and an Inconel 738/NiCrAlY/YSZ/nano-Al2O3as an outer layer. Hot corrosion test was accomplished on the outer surface of coatings in molten salts (45% Na2SO4+ 55% V2O5) at 1000°C for 52 hour. It was found that nanostructured alumina as outer layer of YSZ/nano-Al2O3coating had significantly reduced the infiltration of molten salts into the YSZ layer and resulted in lower reaction of fused corrosive salts with YSZ, as the hot corrosion products had been substantially decreased in YSZ/nano-Al2O3coating in comparison with normal YSZ coating after hot corrosion process.
Highlights
Thermal barrier coatings (TBCs) are extensively used to protect turbine blades against high temperature oxidation and corrosion
The TBC systems usually consist of an MCrAlY bond coat (M = Ni and/or Co) as an oxidation-resistant layer, yttria-stabilized zirconia (YSZ) as a thermally insulating ceramic top coat, and a substrate (Ni-based superalloy) [1,2,3,4,5]
It can be observed that (see Figures 2(b) and 2(c)) there are a high number of nanoAl2O3 grains in a granulated particle which can be considered as a plasma sprayable powder in air plasma spray (APS) method
Summary
Thermal barrier coatings (TBCs) are extensively used to protect turbine blades against high temperature oxidation and corrosion. Low-quality fuels usually contain impurities such as Na and V which can form Na2SO4 and V2O5 corrosive salts on the coating of turbine blades [7, 8]. The penetrated salts can react with yttria (the stabilizer component of YSZ) and depletion of the stabilizer and phase transformation of tetragonal zirconia to monoclinic zirconia can occur in a very rapid and effective manner during cooling [7, 8]. This phase transformation is accompanied by 3–5% rapid volume expansion, leading to cracking and spallation of TBCs [9]
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