Thermal shock characteristics of functionally graded thermal barrier coatings

The thermal shock resistance of thermal barrier coating depends strongly on the shear stress generated by the thermal expansion mismatch between the ceramic and bond coat layer. Applying a functionally graded structure composing of NiCoCrAlY and YSZ along the coating can mitigate this effect. The paper studied the improvement of thermal shock properties with different number of intermediate layers (2-4) added over the temperature cooling range 900-30°C. Acoustic emission (AE) technique was utilised to determine the moment of occurrence of damage within the coatings, and thus help to identify the corresponding failure mechanisms. Cross section analysis of the coatings after thermal shock tests revealed that the coatings generally failed by two mechanisms: edge delamination and segmentation of zirconia topcoat. Failures in the coatings with 2 and 3 intermediate layers (total of 3 and 4 layers respectively in the overall coating) were dominated by edge delamination while the coating with 4 intermediate layers exhibited only segmentation of the top zirconia layer. This points to the fact that interfacial stresses were not critically affecting the integrity of the 5-layer coating (4 intermediate layers plus the ceramic top layer). The cumulative and rate energy results showed that the energy released by the coatings during the thermal shock tests were in the order of 3-layer coating > 4-layer coating > 5-layer coating. The 5-layer coating had demonstrated the best thermal shock resistance among the four coatings.