Development of Bond Layer and Coating Techniques in Thermal Barrier Coating

Abstract

Surface engineering plays an essential role in enhancing the performance of high pressure turbines, which work under the most severe conditions of temperature and mechanical loading. Recently used turbines for power generation and aero-jet aircraft engines applications experience incessant efforts towards increasing thermal efficiency. Thermal Barrier Coating (TBC) systems are economically demanded for efficient and environmental friendly gas turbine engines. Typically TBC comprises of Ni superalloy substrate coated with MCrAlY or Ni(Pt, Al) bond coat. Yttria-Stabilized Zirconia (YSZ) is deposited on bond coat as ceramic top coat layer. At elevated temperature, the bond coat generates a thermally grown oxide (TGO) layer. the grown oxide scales consist mainly of alumina (Al2O3), Cr2O3 (chromium oxides), NiO (nickel oxides), and (Ni, Co) (Cr, Al) 2O4 (spinel phases). The TGO effects durability and TBC failure by spallation of ceramic top coat. Recent study intended to extend the life of TBC that are exposed to cyclic oxidation conditions in gas turbine systems. This was accomplished by deposition of an intermediate thin protective α-Al2O3 layer on the raw surface of APS-CoNiCrAlY bond coat, which was deposited on superalloy substrate. The YSZ ceramic top coat was deposited on the top of Al2O3 layer by APS. The deposition of the alumina layer was done using slurry dip coating technique and sol-gel technique. α-Al2O3 has low oxygen diffusivity and provides excellent diffusion barrier. The new TBC system was compared with the standard TBC system concerning the behaviour under thermal cycling at 1150℃ in terms of microstructure and oxidation resistance. The samples were investigated using optical microscopy, Scanning Electron Microscopy (SEM) equipped by Energy Dispersive X-ray analysis (EDS), X-ray Diffraction (XRD) and Raman spectroscopy. The effect of the intermediate layer on the residual stresses was studied. The oxidation kinetics were evaluated by measuring the thickness of the TGO scale and cracks generated within ceramic top coat after thermal cycling using image analysis software. The Experimental evidence was gathered showing that the deposited alumina layer using both techniques slurry and sol gel resulted in an intermediate protective layer with a good adhesion to ceramic top coat and metallic Bond coat the presence of sol-gel overlay Al2O3 layer has the potential to reduce the crack length and TGO thickness by suppressing the formation of detrimental oxides like Cr2O3, CoO, NiO, and (Ni, Co) (Cr, Al)2O4. and formation of protective alumina compared to reference system. The Al2O3 layer acts as a barrier to diffusion of oxygen. This effect improves the oxidation resistance metallic CoNiCrAlY Bond and promotes the adhesion between bond and top coat, hence improving TBC durability. Keywords: Thermal barrier Coating; Al2O3 intermediate protective layer; Slurry Dip-Coating technique; Sol-Gel Technique; Thermal cycling behavior; α-Al2O3 Thermally grown oxide layer.