Enhancement of Hot Corrosion Resistance of Thermal Barrier Coatings through Modified Configuration of Bondcoat

Abstract

The Trend of applying coatings is widely increasing in industrial and aero turbines. Thermal barrier coatings (TBCs) are widely used to resist high temperatures and to protect the base metal from exposure to high temperatures. TBCs are becoming increasingly essential to protect the hot section components of gas turbine engines against oxidation and hot corrosion. Invariably, it is the corrosion of the coating that determines the need for refurbishment and the life of a component, and not the loss of high-temperature mechanical strength. In present work, a novel technique has been introduced to enhance the life of TBCs by applying thin layer of TiN (by physical vapor deposition method) on bondcoat for improving the oxidation and hot corrosion resistance of bondcoat. Standard TBC samples (Yttria stabilized zirconia thermal barrier coating (TBC) along with CoNiCrAlY bondcoat) were compared with TiN modified bondcoat TBC samples. Both TBC systems were exposed to high temperature under the presence of corrosive salts i.e. a mixture of V2O5 and Na2SO4 (50:50) for 50 hours. The characterizations of the coatings included X-ray diffraction analysis, scanning electron microscopy and optical microscope. It was observed that TiN modified samples showed better results in terms of oxidation resistance and delamination. The formation of Cr2Tin-2O2n-1 phases at the interface of topcoat-bondcoat, in TiN modified samples, were found responsible to enhance the thermal and oxidation properties of the TBC. The durability of coatings is evaluated by thermal cycling. After 225 cycles, the standard TBC samples spalled 30% of the topcoat, whereas, the TiN-modified samples spalled only 5%. Based on above results it could be concluded that TiN-modified coating interface was better thermal shock resistant and enhanced the life of coatings against hot corrosion. The effect of bondcoat thickness on the hot corrosion resistance was studied. Results indicated that TBCs with thick bondcoat exhibited superior hot corrosion resistance to the TBCs with conventional bondcoat. The reaction products were microscopically investigated and a detailed elemental diffusion of different alloying elements was investigated.