Gradient thermal cycling test was performed on atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) to examine the influence of the bond coat temperature on the thermal cyclic lifetime. The TBC system consisted of Inconel 738 substrate, cold-sprayed NiCoCrAlTaY bond coat (BC) and plasma-sprayed yttria-stabilized zirconia (YSZ) top coat. Every thermal cycling test included 70s heating, 50s holding and 120s cooling. The temperature of top YSZ surface was about 1150 °C, while the bond coat temperatures were 900°C, 950°C and 1000°C, respectively. Results indicated that the lifetime of APS TBCs is significantly affected by bond coat temperature. A model to predict the relative thermal cyclic lifetime of TBCs at different bond coat temperature was established by taking account of main factors affecting the lifetime of TBCs. It was found that the predicted results are consistent with the observed results. As a result, the change of lifetime of APS TBCs mainly depends on the growth rate of thermally grown oxide (TGO), elastic modulus of YSZ and thermal expansion mismatch difference between YSZ and substrate caused by different bond coat temperature.

This paper investigates the effect of the thickness of the thermally grown oxide (TGO) layer on the isothermal cyclic lifetime of plasma-sprayed yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The TBCs were prepared with different initial TGO thicknesses through pre-oxidation. The results showed that the lifetime of the TBCs decreased significantly as the TGO thickness increased. There was a critical TGO thickness of 5 μm, beyond which the TBCs failed rapidly. The failure mode of the TBCs also changed with the TGO thickness, with cracking primarily occurring along the YSZ lamellae interfaces for TGO thicknesses below the critical thickness, and through the TGO or the interface between the TGO and YSZ coating for TGO thicknesses above the critical thickness.