山崎 泰広

Herein, fatigue and crack propagation tests were conducted using the small-punch-fatigue (SPF) test to verify its applicability to assess fatigue strength and fatigue crack propagation behavior. During the SPF test, a fatigue crack first initiated at the specimen center and then propagated in the radial direction. In most cases, three fatigue cracks propagated from the specimen center to the radial direction in point symmetry. X-ray computed tomography revealed that the crack initiated by the SPF test had a quarter ellipse shape. Elastic finite element (FE) analysis was conducted to estimate the stress state and stress intensity factor (SIF) of the specimen, and an approximate equation was obtained as a function of the applied load. The experimental results comprising fatigue lives and crack growth rates obtained via the SPF test were compared with the conventional uniaxial test data as functions of the stress amplitude and SIF determined via elastic FE analysis, respectively. This comparison showed that the SPF test can be applied to the fatigue test and fatigue crack propagation test.

The suspension plasma spray (SPS) method is expected to become a novel coating method because it can achieve various microstructures using a suspension with submicron spray particles. Thermal barrier coatings (TBCs) with a columnar structure, which might achieve high strain tolerance, can be obtained using the SPS technique. This study evaluated the internal stress distribution of the suspension-plasma-sprayed thermal barrier coating (SPS-TBC) with different columnar structures using hybrid measurement using high-energy synchrotron X-ray diffraction analysis and laboratory low-energy X-rays. The relationship between the microstructure and the internal stress distribution of the SPS-TBC was discussed on the basis of the experimental results. In addition, the in-plane internal stress was decreased by decreasing the column diameter. The thin columnar microstructure of the SPS-TBC has superior strain tolerance. The internal stresses in the SPS-TBC are periodic decrements caused by stress relaxation in porous layers in its column.

The suspension plasma spray (SPS) technique has recently attracted attention due to the variety of microstructures achievable using submicron spray particles mixed with a solvent to form a suspension. Thermal barrier coatings (TBCs) with a columnar microstructure can be obtained using SPS. Because of its unique columnar microstructure achieving high strain tolerance, a top coat sprayed using this technique can efficiently suppress thermal cycling damage in SPS–TBCs. However, there are few reports on the effect of microstructure on the damage behavior of SPS–TBCs with a columnar structure. In this study, the effect of the top coat microstructure on the thermal cycling fatigue of SPS–TBCs was investigated. The size of the complex oxide formed on the top coat–bond coat interface and the length of the cracks at the interface (formed as a result of thermal cycling) increase with the column diameter in the top coat of SPS–TBCs. The thermal spray conditions affect the thermal cycling fatigue life of SPS–TBCs. The SPS–TBC with fine columnar structure exhibits superior thermal cycling fatigue life.