Fig nbsp xA SEM images showing the cross sectional
Microstructure and fracture resistance of graded/layered TBCs have been studied by various research groups , , , ,  and . These graded TBCs are found to possess a compositional change along the thickness direction (from heat-resistant ceramic layer to fracture-resistant alloy layer). The graded structure provides a gradual variation in the mechanical and physical properties of TBCs. For example, Young's modulus and coefficient of thermal expansion are found to be affected with layer composition  and . Further, owing to the gradual changes in the composition of graded coatings, the graded TBCs offer improved thermal shock resistance , fracture resistance  and  and Y-27632 strength among the coatings . In TBCs, it is expected that the joint of ceramic/alloy and elimination of sharp interface can lower the residual stress. This allows TBCs to resist the crack propagation and thus improves the reliability of TBCs. However, it has been reported in some of the studies that the cyclic oxidation performance of graded TBCs is not as good as expected , ,  and . This is due to the fact that the ceramic and metallic components in the graded regions are unstable during high temperature oxidation ,  and . Lee et al.  reported that the Mar-M-247 superalloy substrate spalled within the graded area after isothermal oxidation at 1000 °C for 100 h. For free standing air plasma sprayed (APS) graded TBCs, similar results have also been reported by other researchers  and  for YSZ + MCrAlY (M = Ni, Co and/or Ni + Co) systems at lower temperature (600 °C and 800 °C). One exception was that Mendelson et al.  produced a laser-glazed top-coat on the surface of YSZ + MCrAlY graded TBCs. They found better performance of cyclic oxidation in the laser modified graded TBCs as compared to the traditional TBCs. From the literature survey, it is clear that the TBCs produced by supersonic air plasma spraying (SAPS) process offer better combination of structural and physical properties , ,  and . The difference in microstructures of TBCs produced by APS and SAPS is ascribed to different designs of spraying guns. The SAPS gun with a Laval nozzle results in a high-energy plasma jet by improving the cooling system and pinching the arc. Besides, the powder was injected into the plasma jet to heat and accelerate the particles. Thus, the velocities of 2400 m/s and 500 m/s for the plasma jet and powder particles can be obtained through SAPS guns. In contrast, using APS guns, the velocities of the gas and particles are in the range of 300–800 m/s and 130–220 m/s, respectively .