Enhanced reliability with bimodal microstructure and transformation-induced toughening in Al2O3-YSZ based thermal barrier coatings

Abstract

Thermal barrier coatings (TBCs) are thermally insulating coatings used in hot sections of gas turbines with temperature > 1400 °C. Present work pursues controlled bimodal microstructure consisting of micrometre-sized Al2O3 and spray-dried nanometre-sized Al2O3, reinforced with 20 wt% 3 mol% yttria stabilized zirconia (3YSZ) and 4 wt% carbon nanotube (CNT) as a thermal barrier coating (TBC) deposited onto Inconel 718 substrate with NiAl bond coat via atmospheric plasma spraying. Bimodal microstructure depicts fully melted and partially melted (~32–34 vol%) regions eliciting denser microstructure with less defects than conventional deposited Al2O3 TBC. Reinforcement with nanostructures 3YSZ, Al2O3 and CNT resulted a reduction in average stiffness of coatings (using nano indentation) from 139.25 GPa (μ-Al2O3) to 125.9 GPa (with 3YSZ) and 135.2 GPa (with 3YSZ+ CNT). However, the elastic recoverability of composite coatings is superior (50–55 %) compared to that of μ-Al2O3 coatings (47 %), indicating an enhanced crack propagation resistance. A pop-out behavior in nanoindentation is attributed to stress induced phase transformation of ZrO2 from tetragonal to monoclinic phase. Further, statistical analysis of nanohardness, and Young's modulus of composite coatings reveals bimodal Weibull moduli corroborating bimodality of microstructure. An increased reliability of bimodal coatings in conjunction with transformation toughening validates the superiority of Al2O3-based composite coating as a prospective TBC.