Taiji Torigoe

Novel Low Thermal Conductivity Ceramic Materials for Thermal Barrier Coatings

The increase of the efficiency for gas turbines leads to the increasing combustion-chambertemperatures. Rapid degradation of the conventional yttria-stabilized zirconia coatings does not fulfill therequirements at these temperatures for a reliable thermal barrier coatings (TBCs) due to the phasetransformation of zirconia and the sintering behaviour. Therefore, it is very important to develop novelceramic materials for TBCs with low thermal conductivity and long-term stability at high temperatures.In this paper, the developments of potential novel ceramic materials for TBCs with low thermalconductivity are reviewed.

Thermal Conductivity of the New Candidate Materials for Thermal Barrier Coatings

Rare-earth zirconate ceramics (Gd2Zr2O7, Sm2Zr2O7, Nd2Zr2O7, Dy2Zr2O7, Er2Zr2O7 and Yb2Zr2O7) were successfully prepared by pressureless sintering at 1550oC for 10 hours. The thermal conductivities of these ceramics were measured and the results indicated that the thermal conductivities of rare-earth zirconates were much lower than that of YSZ in the temperature range 20-800oC.

Preparation and Characterisation of Gd2Zr2O7 Ceramic by Spark Plasma Sintering

Rare earth Gd2Zr2O7 ceramic was prepared by spark plasma sintering from Gd2O3 and ZrO2 powders. The powders were sintered at 1400°C for 10min. The synthesized ceramic was annealed at 800°C for 2h under air atmosphere. XRD structural and SEM microstructural characterization showed the formation of a single phase material with pyrochlore crystal structure. The relative density of Gd2Zr2O7 ceramic was measured by the Archimedes method with an immersion medium of water and the results revealed that the relative density of the ceramic was 92%. The thermal conductivity of the ceramic was tested by laser flash method from room temperature to 700°C. The result shows the thermal conductivity of Gd2Zr2O7 ceramic is lower than that of 7YSZ.

Synthesis of Gadolinium Zirconate by Coprecipitaiton and Its Properties for TBC Application

The increase of the efficiency for gas turbines leads to the increasing combustion-chambertemperatures. Rapid degradation of the conventional yttria-stabilized zirconia coatings does not fulfill therequirements at these temperatures for a reliable thermal barrier coatings (TBCs) due to the phasetransformation of zirconia and the sintering behaviour. Therefore, it is very important to develop novel ceramic materials for TBCs with low thermal conductivity and long-term stability at high temperatures.In this paper, the developments of potential novel ceramic materials for TBCs with low thermalconductivity are reviewed.

Fundamental Coating Development Study to Improve the Isothermal Oxidation Resistance and Thermal Cycle Durability of Thermal Barrier Coatings

Thermal barrier coatings(TBCs) are used in high temperature gas turbines to reduce the surface temperature of cooled metal parts such as turbine blades[1]. TBC consist of a bondcoat (e.g. MCrAlY where M is Co, Ni, CoNi, etc.) and a partially stabilized zirconia ceramic topcoat. Usually, the MCrAlY bondcoat is applied by LPPS (low pressure plasma spray) or HVOF(high velocity oxi-fuel spray). The topcoat is applied by APS (atmospheric plasma splay) or EB-PVD (electron beam-physical vapor deposition). High temperature oxidation properties, thermal barrier properties and durability of TBC are very important to increase the reliability in high temperature service. In this study, new TBC has been investigated. The new TBC consists of a two-layered bondcoat (LPPS-MCrAlY plus dense PVD overlay MCrAlY) and the EB-PVD type YSZ columnar structure topcoat. As a result of evaluation tests, it was confirmed that the new TBC had better oxidation properties and durability than a conventional TBC system.

The microstructures of yttria-stabilized zirconia deposited with ion-plating apparatus

The thermal barrier coating, typically yttria-stabilized zirconia (YSZ), with a columnar microstructure possesses superior tolerance against thermal expansion mismatch. This structure can be obtained with the electron-beam physical vapor deposition at high substrate temperature over 1000 °C. A low temperature process is proposed using an ion-plating apparatus equipped with ionization and bias power supplies for the deposition of YSZ. In this primary study, we focused on the effects of bias voltage on the crystal structure as well as the microstructure of the YSZ coatings. As the results, columnar structured YSZ coating can be formed at lower substrate temperature of 400 °C when bias voltage is over 500 V.

Development of the Advanced TBC for High Efficiency Gas Turbine

Since 2004, Mitsubishi has been pursuing a 1,700°C gas turbine as part of the Japanese National Project [1][2]. One of the most important key technologies for the target is thermal barrier coatings (TBCs) which are capable of improving cooling efficiency of hot parts. With increasing the turbine inlet temperature, TBCs surface temperature is also rising up. In addition, the temperature gradient through TBCs thickness must steepen as a result of keeping metal temperature. Both have a significant effect on durability such as spallation and erosion of TBCs. To evaluate these issues, thermal cycle test and hot erosion test were introduced. After the screening of those component tests, the advanced TBCs coated in the first unit of M501J were verified at our pilot plant called T-point. Sound condition for row 1 blades and vanes had been confirmed after over 3 year operation.