Effect of bond coat surface roughness on the thermal cyclic behavior of thermal barrier coatings

In this study, conventional two-layered TBCs with different bond coat surface roughnesses were produced by EB-PVD on Ni-based superalloy. The thickness of the bond coat and top coat was approximately 60 μm and 120 μm, respectively. Before ceramic deposition, the bond coat surface was blasted by different sizes of Al2O3 grit to form different surface roughnesses, denoted by the values of Ra, of 0.82 μm, 1.57 μm and 3.76 μm, respectively. Thermal cyclic testing was carried out by exposing to air at 1323 K for 0.5 h, and then cooling to room temperature within 5 min by forced air cooling. Rockwell surface indentation and Micro-Knoop cross-section indentation tests were employed to evaluate the adhesion between the bond coat and top coat. The roughness of the bond coat significantly affects the lifetime of TBCs in the presented roughness range. With increasing roughness Ra of the bond coat, the weight gain increased evidently during thermal cycling and the lifetime decreased. The lifetime was 730 hs (1460 cyclic times) for the TBCs with a surface roughness Ra of about 0.82 μm, however, only 530 hs was counted for the TBCs with the surface roughness Ra of 3.76 μm. Rockwell surface indentation and Micro-Knoop indentation tests also showed that the adhesion state between the bond coat and the top coat was affected by the bond coat surface roughness.