Comparison of microstructure and adhesion strength of YSZ and MSZ thermal barrier coatings in the presence and absence of Al2O3 interface layer

In the present study, the APS process was used to coat Al₂O₃, YSZ, and MSZ were applied on the surface of the IN738LC superalloy and NiCrAlY as the middle layer. The microstructure, porosity, and adhesion strength of these coatings were compared with the coating having the Al₂O₃ as the interlayer. Results showed that MSZ coating had less porosity compared to YSZ. Also, the presence of Al₂O₃ as an interface layer in the YSZ sample increased the porosity while in the MSZ sample the porosity decreased. With the presence of the Al₂O₃ layer, the size of the pores increased to the point that the largest porosity diameter in the Al₂O₃-MSZ sample was about 75 μm. Al₂O₃ showed highest adhesion strength (31 MPa) because of lower porosity. The high adhesion strength of Al₂O₃ can be attributed to diffusion and the formation of possible splats. The results showed that the Al₂O₃-MSZ sample had higher strength than Al₂O₃-MSZ with about 28 MPa. MgO seems to be a better and cheaper stabilizing additive than Y₂O₃.