Unsteady Thermal Stresses of Thermal Barrier Coatings under Cyclic Heating

The thermal stresses of the thermal barrier coatings (TBCs) under cyclic heating were measured using hard syn-chrotron X-rays of 70 keV at the beam line BL02B1 in SPring-8. We were able to quickly measure the unsteady stresses using the CdTe pixel detector which is an area detector. The two kinds of the TBC specimens were prepared. One was made by atmosphere plasma spraying (APS) and the other was made by suspension plasma spraying (SPS). The steady thermal stresses in both the APS and SPS specimens showed the same value during the heating and cooling processes. Therefore, the steady thermal stresses were caused by the miss match of the thermal expansions. The unsteady thermal stresses were larger than the steady stresses in both the APS and the SPS specimens. The unsteady thermal stresses also include the stress caused by the temperature gradient. According to the behavior of the unsteady thermal stresses measured, the thermal stress of the APS top coat did not exceed 100 MPa due to non-elastic deformation an sliding effect of lamella structure. For the SPS specimen, it was found that the tensile and compressive stresses were relaxed due to the columnar structure.