Advanced Techniques for Evaluation of Thermal Shock Durability of Thermal Barrier Coating

Although increasing the thickness of thermal barrier coating (TBC) enhances its thermal protection capability, it generally decreases its thermal shock durability. This is caused by an increase of thermal stress due to the increase of the temperature differential between the TBC surface and the metal surface. In order to realize the optimum TBC thickness, the thermal shock durability of TBC should be evaluated precisely and quantitatively. This study focused on two advanced techniques to evaluate the durability of TBC that were developed through joint research with the National Institute for Materials Science (NIMS). The first technique utilizes a new acoustic emission (AE) measurement system called continuous wave memory (CWM), which can analyze AE events more precisely than conventional AE systems. The second technique is a noncontact strain measurement method using laser speckles, which can measure the strain on the TBC surface at high temperature. These techniques were combined with a laser thermal cycle test to establish the “𝜖H-N diagram”, which shows the relationships between the hot shock strain 𝜖H in the laser thermal cycle test and the cycle number N to failure for different TBC thicknesses. In these experiments, CWM was used to determine the TBC failure more precisely, while the laser speckle method was used to measure the thermal shock strain 𝜖H. The obtained 𝜖H-N diagram revealed the durability of TBC for various thicknesses and was transformed to an evaluation map to predict the TBC thickness limitation. For real parts, this evaluation map is useful to estimate if a thicker TBC will fail or not by comparing the maximum strain it is predicted to undergo by numerical analysis.