WE-NET Home Page/Summary of Annual Reports 1998

8.5 Development of Ultra-high Temperature Materials

8.5.1 R&D Goals

In the WE-NET project, a hydrogen combustion turbine, which is expected to have good preservation of the environment and achieve greatly high efficiency, has been developed as one of the utilization techniques of hydrogen. In Phase I(FY'93-FY'98) program, the optical cycle, combustion control methods and cooling structures for turbine blade and vane will be determined to achieve the high efficiency over 60% at the turbine inlet temperature(TIT) of 1700°C. Also, study and conceptual design of auxiliary equipment are being made. Furthermore, aiming at the TIT of 2000°C, a fundamental study on ultrahigh-temperature materials is being conducted. In a field of ultrahigh-temperature materials technology, machining techniques, property evaluation and structural design application of the materials are fundamentally studied to select the ultrahigh-temperature materials for their applications to hydrogen combustion turbine components through improvements and advances of existing materials.

In FY'98 which is the final year of Phase I program in this project, focusing on heat-resistant alloys, ceramic and carbon/carbon composite materials which are expected to be applicable to the hydrogen combustion turbine, design and experimental production of these materials were conducted and then their basic properties were obtained. Also, some technical problems to be solved in future were found.

8.5.2 Results in FY1998

(1) Development of Advanced Single-Crystal Super alloy and Materials for Hybrid Cooled Blade

A single-crystal(SC) Ni-base super alloy was experimentally made by adding Hf to the alloy with the highest high-temperature strength among those previously developed. As a result, composition control method and heat treatment condition were selected which are expected to improve the strength of the alloy. Also, its high-temperature mechanical properties and others were characterized.
Making a FRC(Fiber-Reinforced Ceramic) material experimentally by the precursor method which is expected to provide the material with good heat-resistance, the manufacturing process conditions were determined. The mechanical and thermal properties of machined test pieces as were characterized (Fig. 8-5-1)

(2) Development of Thermal Barrier Coating (TBC) for Cooled Blade of Super alloy (Fig. 8-5-2)

The cooling performance of the transpiration cooling model based on the TBC with a porous ceramic was obtained by heat transfer calculation, and then its applicability was examined.
In order to optimize spraying conditions for the TBC with a porous ceramic, several conditions were examined by using a mixed powder of zirconia(YSZ)-coated carbon and YSZ at its compositional ratio of 1:1. As a result, its porosity and adhesion intensity were found to be dependent on the spraying conditions.

(3) Development of Ceramic-Matrix Composite (CMC) Material

In order to improve the heat and environment-resistance of a CMC material (SiC(f)/SiC(m)), the CMC material was experimentally made by the precursor method together with the HIP (Hot Isostatic Pressing) method. Another material was also made experimentally by forming its high-density matrix with the reaction sintering method. Using these materials, their long-term environment-resistance was evaluated at 1300°C and 1500°C. As a result, the high-density material was found to have a superior property (Fig. 8-5-3).
From experimental manufacture of a cylindrical model part, the future possibility of processing techniques was shown at a part level.
Using a test piece with multi-layers consisting of oxidation-resitance coating (Al₂O₃) and thermal barrier coating (ZrO₂ and HfO₂), its long-term environment-resistance was demonstrated at 1300°C and 1500°C.

(4) Development of Multi-Structure Ceramic Material and Evaluation Method of Fracture Behaviours at Ultra-high Temperatures under Mixed-Mode Conditions

A integrated CMC material of a core part (SiC(f)/Si₃N₄(m)) and a intermediate one (SiC(f)/SiC(m)) was experimentally made and examined (Fig. 8-5-4). As a result, its Young's modules was found to be improved at high temperatures by means of heat treatment.
Using the intermediate material coated with oxides (YAG and Al₂O₃) which were a candidate for a surface material, its damage was analyzed in a steam environment at high temperatures. As a result, the protecting effect of the oxides on corrosion was found to be remarkably reduced due to liquid-phase formation by reaction between corrosives and the oxides.

(5) Development of 3-Dimensional Fiber-Reinforced Composite Materials

Four kinds of carbon/carbon (C/C) composite materials in which fiber material and orientation were different were experimentally made by using a 3-D texture. As a result, the future possibility of processing a part of the 3-D fiber-reinforced composite materials could be shown.
Conducting tensile tests at RT and 2000°C by using a test piece machined from the block material, the correlation's of fiber material and orientation with tensile strength were examined. As a result, useful and fundamental findings of the material were obtained for future blade and materials designs (Fig. 8-5-5).

(6) Development of Experimental Evaluation Method for Ultrahigh-Temperature Materials

Using a plate specimen of C/C composite material with a slit, tensile tests were carried out at high temperatures. As a result, an in-situ observation of initial fracture near the tip of slit was found to be successful during the tests.
After a black body layer which was used for improving the absorptivity of laser beam in the laser flash method was formed on the surface of a SUS304 stainless steel, its emissivity was measured. As a result, the measurement was found to be possible. It was also found that the process of solid phase reaction around 2000°C could be observed by means of a high-temperature X-ray diffraction method.
A study was made on the development of an oxidation test equipment in a high-temperature and -pressure steam environment.

8.5.3 Subject in Future

(1) At the present stage, the materials development has been made mainly as one of the elemental studies. It was concluded from the Phase I studies that these materials were applicable at a material level. However, it is necessary for the practical use of the materials to assess their environment-resistance, durability and reliability as a component material. Also, a study on the possibility of practical parts including large sizing should be made.

(2) It is essential for all materials to develop adequate coating materials for maintaining their heat- and environment-resistance's as well as their high-temperature and long-term durabilities. Consequently, a functionally gradient material together with some kinds of coating forms the component.