Effect of temperature and ceramic bonding on BC oxidation behavior in plasma-sprayed thermal barrier coatings

This study was conducted to investigate formation and growth of thermally grown oxide (TGO) and mixed oxide clusters of Chromia, Spinel, and Nickel oxide (CSN) under the influence of temperature and the absence of top coat (TC). For this purpose, 21 plasma-sprayed thermal barrier coating (TBC) samples underwent isothermal loading at various temperatures with different time exposures and the effect of temperature and time exposure on the formation of TGO and CSN oxides were explored. It was concluded that the minimum temperature for formation of TGO and CSNs were 900 and 1040 °C, respectively, and the minimum time exposure for CSNs to start the formation process was 12 h. Furthermore, an isothermal loading at 1070 °C was applied on 5 samples without TC to study the formation trend of TGO growth and found that the growth rate of TGO is lower when TC exists in the samples. In order to explain the behavior of TGO growth in this state, a finite element model was executed and the applied stresses to TGO from bond coat (BC) were calculated. Results show that the order of stresses in the state with TC is much lower than the state without TC, which consequently leads to TGO separation in the latter state and, as a result, a weaker adhesion to the BC and lower TGO growth rate.