This paper introduces the brief summary of the recent collaboration test results on thermal barrier coatings (TBCs) in the JSMS Subcommittee, which covers the measurements of elastic modulus, tensile strength, residual stress and thermal conductivity of the TBC specimens. Here, the round-robin TBC specimens consisting of 8% yttria stabilized zirconia, CoNiCrAlY alloy bond coat and Ni-base superalloy were prepared by plasma spraying method. The isothermal exposure and the thermal cycles were applied to the TBC specimens by several conditions at high temperatures, to measure both the residual stress and the remaining adhesion strength of the ceramic top, as well as to characterize the thermally grown oxide at the top coat/bond coat interface. The round-robin test results clearly demonstrated that the adhesion strength was significantly changed by the application of thermal *253 cycles and the isothermal exposure. Of particular important results was found in the remaining adhesion strength that were strongly dependent on the testing method to give the thermal cycles.

Low cycle fatigue tests at elevated temperature were conducted on a Ni-based directionally solidified superalloy subjected to transverse loading. To investigate the effect of the arrangement and the orientation of crystal grains on the crack initiation, the electron back-scatter diffraction (EBSD) method was applied on the surface of the tested specimens. In addition, finite element analysis that considered the plastic behavior of crystal grains and the crystal grain shape on the specimen surfaces was performed to evaluate the relationship between the crack initiation and the local stress that develops near the grain boundaries. The results are summarized below. As for the effect of crystallographic properties in the specimen surface, many cracks initiated near the grain boundaries that neighbored the grains whose secondary axis was inclined to the normal direction of specimen surface by more than 35 degrees because the longitudinal stress larger than the nominal stress developed near those boundaries. The crack location was not confirmed to correlate with the grain boundary misorientation between the neighboring grains on the surface. However, most cracks initiated near the grain boundaries that neighbored the grain whose secondary axis was more inclined than the grain located in the opposite surface. The results of FEM analysis revealed that high stress developed near those grain boundaries. In addition, the location where high resolved shear stress developed generally corresponded to the crack location and that it correlated with the number of crack initiation cycles.

Low cycle fatigue tests at high temperature were conducted on test specimens with small holes made of a Ni-based directionally solidified superalloy, which are intended as the cooling structures formed in the components in the fossil fuel power plant. The tests included those cases with and without a strain holding process, i.e., fatigue creep interaction (FCI) tests and low cycle fatigue (LCF) tests, respectively. The number of LCF crack initiation cycles of the one- and seven-hole specimens decreased compared to that of the smooth one. The number of FCI crack initiation cycles of a compressive hold case for the seven-hole specimen decreased compared to that of the LCF test, while that of a tensile hold case decreased further. The test results were evaluated based on the inelastic behavior around the center hole of the specimens, where the most serious inelastic strain occurred, using finite element analysis that takes into account the inelastic anisotropy of material properties. The number of crack initiation cycles of the LCF and the compressive FCI tests correlated with the maximum tensile stress around the hole, while that of all the tests correlated with the frequency-modified strain energy. We propose a method for evaluating cyclic inelastic behavior around a hole using cyclic Neuber’s rule for anisotropic materials to simply evaluate the failure life of actual components.

Low cycle fatigue tests at elevated temperature were conducted on test specimens with small holes made of a Ni-based directionally solidified superalloy, which are intended to be the cooling structures formed in components in fossil fuel power plants. The tests included cases with and without strain holding processes. The number of crack initiation cycles of the tests without hold processes for the one-and seven-hole specimens was about 1/30 that of the smooth one. The number of crack initiation cycles of the test with the compressive hold processes for the seven-hole specimen was smaller than that of the tests without hold processes while that of a tensile hold case was even smaller.The test results were evaluated based on the inelastic behavior around the center hole of the specimens, where the largest inelastic strain occurred, using finite element analysis that takes into account the inelastic anisotropy of material properties. The number of crack initiation cycles of the tests without hold processes and that with compressive hold processes correlated with the maximum tensile stress around the hole, while that of all the tests correlated with the frequency-modified strain energy by setting the appropriate material parameters based on the test results of smooth specimens.

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