Development of Life Predictive Method For Thermal Barrier Coating Systems

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The objective of this research is to develop evaluation methods of charcterizing thermal barrier coating (TBC) systems and predictive methodology of their life with special reference to a leading edge of advanced gas turbine blades, The research results are summarized for the following individual task.Task (1) Preparation of TBC specimens : TBC specimens with a top coat of ZrO and a bond coat of NiCrAlY were prepared by air plasma splay and electron beam-physical vapor deposition techniques. The specimen geometry was a cylindrical tube for simulating the curvature of leading edges and the substrate was a Ni-based superal loy.Task (2) Evaluation of interface properties(2-1) Thermal shock/fatigue tests were conducted on the cylindrical specimens with the TBCs using a laser heating method. A air-forced cooling cycle was employed to simulate the thermal transient history during trips. An acoustic emission method was used to examine the fracture process in the laser heating experiment. It w … More as shown from the AE monitoring that the buckling of the top coating layer induced the final spallation of the TBC system. Furthermore, the effect of high-temperature oxidation on the spallation was evaluated in terms of the thermal gradient within the TBC and the number of cycles to failure.(2-2) Thermal stress analyses using a FEM method were performed with reference to the cylindrical specimen geometry. The interface crack was accounted for in the thermal stress analyses. The stress intensity factor was determined as a function of interfacial crack length which formed a foundation for the analysis of the spallation behavior of the TBCs.Task (3) The experimental data of interface crack growth obtained from the laser heating thermal shock/fatigue tests were used to produce the fracture mechanics based relationship of fatigue crack growth rate through the interfacial crack stress intensity factor as determined by the FEM, The relationship was then used to reproduce the fracture process in the laser heating experiment. It was demonstrated that the predicted critical interfacial crack length was close to that observed in the laser heating experiment, indicating the validity of the fracture mechanics based life predictive method. Less