The determination of the delamination resistance in thermal barrier coating system by four-point bending tests

The delamination resistance in a plasma-sprayed thermal barrier coating (TBC) system was evaluated by means of a modified four-point bending test. In this work, in order to study the effect of the interface roughness between the bond coating (BC) and the top coating (TC), two kinds of the powder with the different particle size were used for spraying the BC material. In addition, the influence of the isothermal aging on the delamination resistance was also investigated. The experimental results indicate that the effect of the TC/BC interface roughness wasnʹt significant. The energy release rate Gc, which was estimated by the modified four-point bending tests, increased with increasing aging time at 1000 °C, on the other hand, the scatter of it decreased with increasing aging time. The crack mainly propagated in the top coating for as-sprayed condition and at the top-coating/bond-coating interface after thermal aging for 2000 h. The energy release rate Gc was correlated with the fracture strength of the weakest parts of the TBC specimen. The energy release rate Gc increased with thermal aging until a critical time due to the sintering of the top-coating expired. It can be expected that Gc will decrease with the thermal aging after the critical aging time because the top-coating/bond-coating interfacial strength decreases by the TGO growth. The critical aging time in this work is approximately 2000 h.