Papers by Author: Yoshio Harada

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Authors: Noriyuki Mifune, Yoshio Harada
Abstract: The applicability of 2CaO·SiO2-CaO·ZrO2 ceramic coatings as thermal barrier coatings (TBCs) was investigated. Coatings consisting of various ratios of 2CaO·SiO2-CaO·ZrO2 bond-coated with NiCrAlY were prepared using the plasma spray process. The structure of the coatings was characterized by scanning electron microscopy and X-ray diffraction analysis. The resistance of the coatings to thermal shock was evaluated with acoustic emission techniques under a thermal cycle from 1273 K to room temperature, and the hot corrosion resistance of the coatings was investigated with V2O5 and Na2SO4 at 1273 K for 3 h. The 2CaO·SiO2-10~30mass%CaO·ZrO2 coatings had excellent thermal shock resistance, because the coatings contained a vertical micro-crack in a single flattened ceramic particle. These coatings possessed excellent corrosion protection preventing direct contact between the corrosive ashes and a NiCrAlY bond coating. The CaO in the coating reacted with vanadium compounds and inhibited the penetration of corrosive ashes to the bond coating. The developed 2CaO·SiO2-20mass%CaO·ZrO2 thermal barrier coating on stationary vanes was evaluated in an actual gas turbine. The ceramic coating did not separate from the bond coating and reacted with SOx in combustion gas to produce a stable sulfate (CaSO4), which fixed in the coating. The TBC effectively protected the metal substrate of the vanes in practical operating condition for 25,000 h.
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Authors: Satoru Takahashi, Masayuki Yoshiba, Wataru Kakuta, Sayuri Matsuoka, Yoshio Harada
Abstract: In order to clarify the failure behavior of plasma sprayed thermal barrier coating (TBC) systems under the complicated modes of thermal-mechanical-chemical loadings, the stress rupture property evaluation and failure analysis were conducted for Y2O3-ZrO2 (YSZ) and CaO-SiO2-ZrO2 (C2S-CZ) TBC systems in air and two kinds of high-temperature corrosive environments. Static creep loading was found to bring about the typical creep failure for TBC systems even in the aggressive environment so called hot corrosion almost in similar manner to the case in air. On the contrary, it was revealed that the dynamic fatigue loading tends to cause a significant failure life reduction of TBC systems both in air and in corrosive environments. For YSZ TBC system, the penetration crack preexisting through the top-coat layer tends to provide a nucleation site for the fatigue crack even in air, and more significantly a short circuit path for the corrosive species in hot corrosive environment. For C2S-CZ system, on the contrary, the top-coat / bond-coat interface tends to provide easily the nucleation site for a main crack to propagate thereafter toward both the alloy interior and outer surface. Under lower stress level at 950°C, however, the oxide-induced crack closure together with crack tip blunting attributed mainly to the high reactivity of Ca compounds as a major constituent of the TC is effective to suppress substantially the crack propagation, so as to cause the prolonged failure life as compared to YSZ system even in aggressive gaseous environment.
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Authors: Satoru Takahashi, Masayuki Yoshiba, Raito Kawamura, Yoshio Harada
Abstract: In situ observation of the mechanical failure behavior was conducted for different kinds of the plasma sprayed thermal barrier coating (TBC) systems by means of an optical microscopy under the static loadings at room and elevated temperatures; as the fundamental aspect, in order to clarify the thermomechanical failure mechanism of TBC system in connection with various coating characteristics. Mechanical tensile or compressive loading was applied progressively to the TBC specimen by an axial loading mode. It was found that the failure behavior of TBC system depends strongly on the testing temperature under both the tensile and compressive loadings. At the elevated temperature which is higher than the ductile-brittle transition temperature (DBTT) of metallic bond-coat (BC), in particular, the ceramic top-coat (TC) spallation can be prevented by virtue of the stress relief induced by the enhanced plastic flow in the BC layer. At the room temperature which is lower than the DBTT of BC, on the contrary, the TC spalling was inevitably induced, but the initiation site of TC spalling is closely related with the magnitude of local plastic deformation in the alloy substrate. Furthermore, an influence of thermally grown oxides (TGO) layer developed at the TC / BC interface on the crack initiation and propagation behavior was investigated in some detail.
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Authors: Satoru Takahashi, Masaki Hatano, Yoshitaka Kojima, Yoshio Harada, Akira Kawasaki, Fumio Ono
Abstract: Thermal cycle resistance of Ni-20Cr, Ni-50Cr and CoNiCrAlY coatings produced by air plasma spraying was investigated according to Japanese Industrial Standard Testing method for thermal cycle resistance of oxidation resistant metallic coatings (JIS H 8452: 2008). The specimens were exposed to a cyclic heating and cooling regimen comprised of up to 100 cycles of 10 hours heating to 1000 °C or 1093 °C in air followed by cooling. The thermal cycle resistance of oxidation-resistant metallic coatings was found to depend strongly on testing temperature and on the chemical composition of the coating materials. In thermal cycle testing at 1000 °C, no remarkable failure was observed in any specimen. However, in thermal cycle testing at 1093 °C, spalling was observed over the entire surface of the Ni-20Cr coating, although the Ni-50Cr and the CoNiCrAlY coatings exhibited excellent thermal cycle resistance even upon exposure to 100 thermal cycles. The CoNiCrAlY coating showed mass gain with increasing number of thermal cycles due to preferential oxidation between thermal spray particle splats. Furthermore, the failure behavior of specimens was investigated in detail by SEM, XRD, EPMA, etc.
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Authors: Megumi Akoshima, Tetsuya Baba, Mitsue Ogawa, Takashi Tanaka, Yoshio Harada, Akira Kawasaki, Fumio Ono
Abstract: Ceramics-based thermal barrier coatings are used as heat and wear shields of blades of gas turbine. There are strong needs to evaluate thermophysical properties of coatings, such as thermal conductivity, thermal diffusivity and heat capacity of them. Since coatings are attached on substrates, it is not easy to measure these properties separately. In order to evaluate the thermal diffusivity of coating attached on substrate, we have tried to apply the multi-layer model based on the response function method and established a procedure for the measurement by the laser flash method. We verified the procedure by the measurements from room temperature to about 1000 K for two-layer ceramics sample prepared by the doctor blade tape casting method. The thermally sprayed CoNiCrAlY coating on the SUS304 substrate was also used for verification. The thermal diffusivity of coating attached on substrate approximately agreed with that of the single-layer coating removed from substrate. In the case of the ceramics sample, the thermal diffusivity of the coating including the interfacial thermal resistance determined within about 20 % uncertainty. We compared the laser flash measurement signals of the samples prepared by the thermal spraying with variant thickness and found the difference among them. It was found that the procedure has enough resolution to detect the heat shield effect caused by the change with about 200 m in thickness. The result shows that the procedure and analysis were practically effective for the thermal diffusivity estimation of coating attached on the substrate without remove from substrate.
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Authors: Kenta Takagi, Akira Kawasaki, Yoshio Harada, Masakazu Okazaki
Abstract: We are investigating the feasibility of measuring thermal expansion coefficient (CTE) of free-standing thermal barrier coatings with a practical thickness of several hundred micrometers by mean of a widespread thermomechanical analysis toward its industrial standardization. First of all, this study conducted the preliminary investigation for the accurate measurement of thin samples with dense yettria-stabilized zirconia (YSZ) ceramics. With a sample supporting jig and proper conditions, even the usual thermomechanical analysis could give the accurate CTE values of thin samples down to 0.3mm thick. Also it showed good reproducibility with small measurement error less than 5%. In actual, this modified method could provide the reasonable CTE values of plasma-sprayed YSZ samples with thickness of >0.3mm. Further investigation with this method found a slight monotonic decrease in the CTE with annealing. This decrease was estimated to arise from the continuous change of microstructure which still went on even after saturation of sintering shrinkage. All the results demonstrated the present method to be available for the industrial standard.
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