2008 Volume 57 Issue 6 Pages 596-602
In this work, in order to study the effect of the interface roughness between the bond coating and the top coating, two kinds of the CoNiCrAlY powder with the different particle size were used for spraying the bond coating material. In addition, the bond coat was sprayed by either the low-pressure plasma spray (LPPS) method or the high velocity oxy-fuel (HVOF) one. The adhesion strength of the TBC top coat was evaluated as a function of the isothermal exposure time by means of the modified 4-point bending test. In addition, in-situ observations of the initiation and propagation of delamination cracks were carried out. The experimental results indicated that the delamination cracks initiated and propagated at the intersplat boundaries in the top-coating for the as-sprayed TBC specimens. After isothermal exposure at 1000°C, the microcracks were generated in the top-coating and the thermal grown oxides (TGO) grew at the top-coating/bond-coating interface. The delamination of the thermal exposed specimen occurred by the mixed fracture mode of the microcracks and top-coating/TGO or TGO/bond-coating interfaces. There was little difference of the adhesion strength by the bond-coating process in the as-sprayed conditions. On the other hand, the LPPSed bond coat specimen which has the rough interface exhibited lower adhesion strength compared with other specimens after thermal exposure due to the remarkable growth of the mixed oxide type of the TGO.
