Since 2008, Japanese boiler, turbine and valve manufacturers, research institutes and utility companies have been working together to develop 700V A·USC technology, with support from the Japanese government. The key areas of discussion are technology development of high temperature materials such as nickel-based alloys and advanced 9Cr steels, and their application to actual power plants. At the EPRI conference in 2013, our report mainly focused on the development of fundamental material and manufacturing technology during the first five years of the project, and the preparation status of the boiler component test and turbine rotor test for the latter four years of the project. The boiler component test, using a commercially-operating boiler, began in May 2015 and is scheduled to be finished by the end of 2016. The turbine rotor test at 700°C with actual speed will be carried out from September 2016 to March 2017. At this year’s conference, we will: l) briefly summarize the development of fundamental material and manufacturing technology and 2) provide an update on the progress of the boiler component test and the turbine rotor test.

In this study, a variety of yttria-stabilized zirconia (YSZ) coatings have been obtained by plasma spray physical vapor deposition with fine powders at high power. The coating structures were found to change significantly with powder feed rate, but less with substrate temperature and rotation speed. Porous YSZ coatings with a feather-like structure and low thermal conductivity were deposited at rates as high as 220 μm/min. These results demonstrate the potential of plasma spray physical vapor deposition for producing thermal barrier coatings.

Air plasma sprayed thermal barrier coatings, which reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of an industrial gas turbine. For TBC systems, the adherence of the top coating is one of the most important parameter for the durability of TBC system. In this work, the thermal fatigue behaviour of an air plasma sprayed thermal barrier coating was investigated. In addition, the residual interfacial strength was also evaluated by means of the 4-point bending test. From the measurement of the AE signals during the thermal fatigue tests, micro-cracking occurred in each cooling stage of the thermal fatigue cycles and then such damage depends on the number of thermal cycle. In addition, TGO grew at the interface with the exposed time at elevated temperature (the time dependent damage). Thermal barrier coating undergoes both time dependent damage and cycle dependent one under thermal fatigue condition. The life of thermal cycle with high temperature dwell time is shorter than not only that of isothermal exposure but also that of thermal cycle without dwell time.

The development of novel thermal barrier coatings (TBCs) by a 300 kW twin hybrid plasma spraying system was conducted as a case study of the integrated and comprehensive thermal plasma deposition technique, namely, the combination of thermal plasma powder spray (TP-PS) and the thermal plasma physical vapor deposition (TP-PVD). Microstructures of the coatings deposited from either vapor or melted droplets were tailored to compose peculiar functional layered Al 2 O 3 /YSZ and YSZ/YSZ coatings in the comprehensive deposition processes, aiming to meet the severe requirements of the next-generation TBCs. Even for the vapor deposition, ultrafast deposition rate over 150 µm/min was achieved. These results showed that the comprehensive spraying process has successfully integrated the potentials of different thermal plasma deposition techniques, and the effectiveness and flexibility of this process made it very promising to be applied in future coating industry.

It has been said that plasma-sprayed ceramics particles are often supercooled before the impact on substrate. Some numerical models of the droplet impact actually included the supercooling effects. However, there is no report that has experimentally confirmed the effects on splat morphology. Therefore, in this research, we have mainly investigated the supercooling effects on splat morphology as well as splat microstructure. To achieve this, we developed an in-situ measurement technique utilizing radiation from a melt particle to monitor the impact of single particle successively under plasma spraying. The system was able to identify each single particle, which enabled us to correlate the splat morphology with impact velocity and thermal history of each particle during the impact. Yttria-stabilized zirconia powders were sprayed onto quartz glass substrate by the argon-hydrogen dc-rf hybrid plasma under atmospheric pressure. Waveforms of emissions and thermal history obtained during the impact were precisely analyzed. Especially, we closely examined thermal history during particle spreading to find the recalescence. In addition, splat morphologies were examined statistically in relation to their thermal histories. Based on the measurement, we also evaluated the viscosity of zirconia, cooling rate, and thermal contact resistance experimentally.

A new plasma spray process was developed for the rapid deposition of very dense electrolyte layers for solid oxide fuel cells (SOFCs). The dense yttria-stabilized zirconia (YSZ) film was prepared by a center-injection low pressure plasma spraying (CI-VPS) process on various substrates in a triple-torch reactor. For deposition on porous substrates, an intermediate layer was applied using conventional atmospheric plasma spraying (APS) to close the large pores in the substrate. The films were characterized by XRD, SEM, and EMPA. The porosity of the film was analyzed by computerized image analysis of the micrographs. The film was also fractured by four-point bending to characterize the nature of bonding of layer-to-layer and within the deposit. The film analysis showed that YSZ layers with porosities of 0.3 % could be obtained at very high deposition rates with the CI-VPS process, with a very good functional performance of the layer as an electrolyte. Building of a complete SOFC by successive deposition of an atmospheric pressure sprayed porous cermet film, the dense YSZ electrolyte layer, and a porous perovskite film is discussed.