Evaluation of Cyclic Hot Corrosion Resistance of Plasma-Sprayed Composite Coating in Na2SO4-60%V2O5 Molten Salt Environment

The hot corrosion resistance of air plasma-sprayed 8YSZ-Al2O3-multiwall carbon nanotubes (MWCNTs) has been analyzed and compared to that of 8YSZ coating. The feedstock constituents were blended to produce 8YSZ-20% alumina-0%MWCNT (type II), 8YSZ-19% alumina-1%MWCNT (type III), and 8YSZ-17% alumina-3%MWCNT (type IV) feedstock. After the cyclic hot corrosion (cHC) test in Na2SO4-60%V2O5 molten salt, the morphology of coatings was analyzed using EDS, SEM, and XRD while the coating cross-section properties were determined by a nanoindentation test and the infiltration resistance of the topcoats was estimated from the Weibull modulus (m) of the bond coat (BC) properties. The formation of YVO4 and phase transformation of zirconia from tetragonal to monoclinic were found as important degrading mechanisms of the coatings. The difference in coefficient of thermal expansion also proved to be detrimental to the performance of the coating. The zirconia monoclinic phase content for the 8YSZ(type I), type II, type III, and type IV coatings was around 86, 80, 92, and 89%, respectively, after the cyclic hot corrosion test. The Weibull analysis of the Young's modulus of the bond coat showed that the type IV coating exhibited the highest heterogeneity in Young's modulus. Finally, the addition of MWCNT was found to be detrimental in the cyclic hot corrosion test as it led to the cracking of the coatings.