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The durability of thermal barrier coatings (TBCs) at gas turbine has been improved substantially by instituting more strain tolerant zirconia-ceramic top coatings with vertical cracks artificially, such as ZrO 2-Y 2 O 3 plasma-sprayed coating with purposely segmentation. Under the necessity of TBCs with thermal shock resistance and durability at high temperature, we have developed the top coating of 2CaOÁSiO 2-10$ 30 mass%CaOÁZrO 2 (C 2 S-10$30%CZ) with many vertical microcracks that is formed with the spraying process only, namely one vertical microcrack does not pierce through the sprayed layer but remains within a single flattened ceramic particle of the top coating. In order to discuss the mechanism of the vertical microcracking, the flattening-solidification behavior of the C 2 S-10$30%CZ sprayed splat was investigated. Microcrack was observed within the splat of C 2 S-10$30%CZ particle after solidification on the polished SUS304 stainless steel. It is assumed that spraying particle liquefied in plasma flame collides and strongly adheres on the substrate or the accumulated particles and change the phase from liquid to brittle solid due to rapid cooling, at the same time, the tensile stress is generated on the external surface of the particle and fine vertical microcrack occurs in the particle which could not withstand this stress. From strong correlation between the vertical microcracking and amorphous like behavior of the C 2 S-10$30%CZ coating, the liquid phase of C 2 S-10$30%CZ particle sublimated some CaO in plasma flame is considered to take an important role in vertical microcracking formation due to shrinkage during the cooling and solidifying process.

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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.

Hot corrosion behavior of Thermal Barrier Coatings (TBCs) has been studied by comparison between double layer coatings and graded coatings. Two types of oxide ceramics, 8 mass % Y2O3-ZrO2(8YZ) and 2CaOSiO2-15mass% CaOZrO2 (C2S-15 CZ), with a bond coating of NiCrAlY were applied to metallic substrates in this study. After a hot corrosion test by V2O5-Na2SO4 corrosive ashes, hot corrosion behavior of TBC has been investigated by visual inspection, metallography, X-ray diffraction and EPMA. The C2S-15%CZ coating reacted with V2O5 only where it was in direct contact with the material. The affected area from the reaction was limited to the coating surface where V2O5 existed. The coating showed adequate hot corrosion-resistance. It was found on the 8YZ coating that Y2O3, the stabilizing component, particularly reacts with V2O5 and loses its function; this led to partial spalling of the coating. It was observed that the durability of the double layer TBC was largely influenced by the performance of a corrosion resistant NiCrAlY undercoat which provided protection against corrosive components penetrating through the ceramic topcoat. It was observed that the graded coating degraded due to oxidation of NiCrAlY particles which independently existed near the coating surface and affected the durability of TBC.