A kinetic metallization technique, which is one of the cold spraying systems, has been studied as a new coating system for metallic bond coats of thermal barrier coatings for components used in hot section of advanced gas turbines. In this study, in-situ residual stresses in atmospheric plasma sprayed yttria-stabilized zirconia (YSZ) top coating with two different bond coat spraying systems, deposited by a low pressure plasma spraying and a cold spraying, were evaluated and compared by thermal cycle tests. From the results of 1st thermal cycle, in the case of the plasma sprayed bond coat, a tensile residual stress was observed at the elevated temperature up to 400°C. Relaxation of the residual stress was started beyond 400°C. On the other hand, the gradual increase of tensile residual stress was observed up to 1000 °C in the case of cold sprayed bond coat. In addition, transition behaviors of residual stress between plasma sprayed and cold sprayed coatings were varied in 3-thermal cycles.

In the thermal spraying technique, the process includes some problems such as insufficient cohesive strength between particles in the coating. It is considered that the laser treatment of thermal spray coating will be effective method to improve the coating properties. In this study, an yttrium aluminum garnet (YAG) laser treatment was carried out on the sprayed coating in order to examine the effect of laser beam on the coating properties and post laser treatment method and hybrid spraying method were carried out. Irradiation laser power conditions were 1.5kW, 2.0kW and 2.5kW, and defocus distance conditions were ±0mm, +30mm, +60mm, +90mm, changing the energy density and the traverse speed conditions were 100mm/s, 200mm/s, 300mm/s. In post treatment method the affected region in the Cr 3 C 2 -Ni-Cr coating is smaller with increasing traverse speed. Laser energy density affected the microstructure. In the case of defocus distance ±0mm and traverse speed is 100mmm/s there are three different regions, melted region, heat affected region and no affected region in the coating. The micro Vickers hardness of melted region of coatings was lower and heat affected region of coatings was higher than no laser treatment coating. It was considered that with post laser treatment method it was very difficult to treat the coating uniformly. In the blast erosion test, the wear-out rate of hybrid spray coating was lower than that of the HVOF spray coating. It is considered that the improvement of micro Vickers hardness of Cr 3 C 2 -Ni-Cr hybrid sprayed coating resulted from the precipitation of fine chrome carbide in the matrix and their strengthening affect cohesive strength between lamella in the coating. In WC-Co hybrid sprayed coating, the improvement of micro Vickers hardness resulted from the porosity decrease. The coating structure was different even in the identical processing condition by the place. The micro Vickers hardness of melted area of coatings was lower and heat affected area of coatings was higher than no laser treatment coating.

An investigation was carried out into the effect of the interface roughness between the metallic bond coat and the ceramic topcoat on internal stresses in a thermal sprayed ceramic thermal barrier coating (TBC). To evaluate the effect of the interface roughness on the residual stress in the top coat, the specimens with two kinds of bondcoat roughness (rough type and smooth type) were prepared. The in-plane stresses of the specimens were measured with laboratory X-rays. The in-plane stresses for the both of the rough and smooth specimens were about 60MPa and independent of the roughness of the bond coat. Using high energy X-ray, the stress of the rough specimen was compressive and the stress of smooth specimen was tensile. This tendency is different from the result measured by laboratory X-ray. This difference in the stress value is coursed by the out-of-plane stress. Theses stresses in the topcoat were estimated by the hybrid method, that is to estimate out-of-plane stress using laboratory X-rays and high-energy synchrotron X-rays. As a result, the larger the roughness of the bond coat became the larger out-of-plane-stress become.

Multi-layered thermal barrier coatings (TBC) having different functions were proposed for the hot section components of land-based gas turbines. This paper describes the multi-layered TBC with an oxidation resistant layer. A conventional duplex TBC and a triplex TBC, in which an aluminized layer was added to the conventional duplex TBC to increase oxidation resistance, was prepared. It was confirmed by a burner rig test that the triplex TBC with the aluminized layer is resistant to oxidation and shows high durability in a thermal cycle test, compared with the conventional duplex TBC. The spalling in the thermal cycle test of each TBC specimen occurred at the same position and when the thickness of the oxidation layer was 11-13 μm. The mechanism of spalling of the coating in the thermal cycle test was discussed in terms of stress in the coating. Stress in the direction of spalling occurs by an uneven interface between the bond coat and top coat, and increases with growth of the oxidation layer. It is thought that the high durability of the triplex TBC in the thermal cycle test is derived from suppressing the growth of the oxidation layer and decreasing the stress due to the addition of the aluminized layer.