Residual stresses and structure of EB-PVD thermal barrier coatings exposed at high temperature

The substrate material was nickel based superalloy (In738LC), CoNiCrAlY was pressureless plasma-sprayed on the substrate as the bond coating. As the top coating, zirconia with 4 mol% yittria was made by a electron beam-physical vapor deposition (EB-PVD) with a rotation. The thickness of the bond coating was about 0.18 mm, and the thickness of the top coating was about 0.12 mm. The rotation speeds of specimens in the EB-PVD process were 5 rpm, 10 rpm and 20 rpm. The specimens were exposed at 1 273 K for 200 h. The in-plane residual stress was measured by a conventional X-ray method, and the out-of-plane residual strain was measured by a strain scanning method with hard synchrotron X-rays. For the specimens with 5 rpm and 10 rpm, the in-plane compressive residual stresses were released by the high temperature exposure. For the specimen with 20 rpm, the in-plane residual stress did not change by the high temperature exposure, because it was very small before the exposure. The in-plane residual stresses were large compression near the interface. For the specimens with 5 rpm and 10 rpm, the out-of plane stresses were small from the coating surface but became a compression near the interface. Both in-plane and out-of-plane residual stresses of the specimen with 20 rpm were very small. In the effect of decreasing the residual stress, the rotation speed of 20 rpm was the excellent condition. According to the observation with a scanning electron microscope, the feather-like columnar structure decreased after the high temperature exposure. This volume decrease caused the release of the in-plane residual stresses.