Abstract
In order to clarify the thermal and / or mechanical failure behavior of the plasma sprayed thermal barrier coating (TBC) system in connection with their coating characteristics such as the coating microstructures and interfacial natures depending on the coating process condition, two kinds of the failure analytical tests were conducted for TBC systems processed under different conditions. One was the high-temperature oxidation test, which was conducted at 1100℃ under both the isothermal and thermal cycle conditions. The other was the in-situ observation of mechanical failure behavior, which was conducted under the static loadings at ambient temperature; as the most fundamental aspect, by means of an optical microscopy. It was found that the thermal and mechanical failure behavior of TBC system depends strongly on the top-coat (TC) / bond-coat (BC) interfacial condition, the sorts of BC spraying process, the reheat-treatment (RHT) after spraying and so on. For the TBC system with vacuum plasma sprayed (VPS) BC as well as for that with air plasma sprayed (APS) BC, in particular, the RHT at an appropriate temperature in Ar atmosphere was found to be effective for improving the oxidation property. For the TBC system with APS-BC, however, it was impossible to prevent the crack growth into the BC interior under the tensile loading in spite of conducting the RHT, since the microdefects such as oxides and micropores within the APS-BC tend to provide an easy crack propagation path. In addition, it was clarified that the smoothening process on the BC surface is able to prevent perfectly the occurrence of the wart-like oxide (WO) during oxidation, but at the same time increases also the risk of the TC spalling under the mechanical loading. Furthermore, the effect of the coating process condition on the TC spalling resistance was discussed.
