Properties and residual stress distribution of plasma sprayed magnesia stabilized zirconia thermal barrier coatings

Abstract

Thermal barrier coatings (TBCs) are employed to protect hot section components in industrial and aerospace gas turbine engines. Conventional TBCs frequently fail due to high residual stresses generated within the coating systems owing to the difference between coefficient of thermal expansion (CTE) of the substrate & deposited layers. Functionally graded-thermal barrier coatings (FG-TBCs) with gradual variation in composition and microstructure have been proposed to minimize the problem. In the present study, three different TBC systems, one duplex and two FG-TBCs, having three and five layers, respectively, were prepared by atmospheric plasma spray (APS) process on Nimonic 90 substrates using Ni-5Al as bond coat (BC) and magnesia stabilized zirconia as top coat (TC) materials. Five layers comprising the three TBC systems were also deposited separately as individual coatings to study their characteristic properties. The coatings were characterized by a scanning electron microscope. Microhardness and CTE of the individual coatings were also measured. Finite element analysis (FEA) using ANSYS 14.5 was performed to estimate the thermal residual stresses generated within the as-sprayed coating systems. The microstructure, microhardness and CTE in the five-layer FG-TBC changed gradually as compared with duplex and three-layer FG-TBC. The lowest radial, axial and shear stresses were generated in the five-layer FG-TBC compared with those of the duplex and three-layer FG-TBC. Increasing the number of layers with a gradient in composition from BC to TC having similar thickness of coatings increased the bond strength of the FG-TBCs. The bond strength of five-layer FG-TBC was found to be almost 1.5 times as high as that of the duplex TBC.