WE-NET Home Page/Summary of Annual Reports 1997

8.5 Development of Ultrahigh-Temperature Materials

8.5.1 R&D Goals

In the World Energy Net-Work(WE-NET) project, a hydrogen combustion turbine, which is expected to have good preservation of the environment and achieve greatly high efficiency, will be 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 inlete temperature(TIT) of 1700°C. Also, study and conceptional design of auxiliary equipment will be made. Furthermore, aiming at the TIT of 2000°C, a fundamental study on ultrahigh-temperature materials will be 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'97 which is the fifth 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, based on the technical subjects pointed out in FY'93. Also, some technical problems to be solved in future were found.

8.5.2 Results in fiscal year 1997

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

In addition to the single-crystal(SC) Ni-base superalloy(Ni_-5.3Al_-0.5Ti_-6.0Mo_-4.8W_-6.0Re) developed up to the last fiscal year, two other alloys were selected with addition of 0.15%Si and 2.0%Hf for precipitation hardening, respectively. Using these three alloys, their high-temperature tensile and creep-rupture properties were characterized. As a result, the Hf-added alloy was found to be excellent in strength.
Using an experimental material of FRC(Fiber-Reinforced Ceramic) made by the CVI method, its mechanical and thermal properties as well as porosity distribution were obtained.

(2) Development of Thermal Barrier Coating (TBC) for Cooled Blade of Superalloy

Material design, development and selection were made for the TBC with a porous ceramic. As a result, selecting the existing SC superalloy, CMSX-4, as a substrate material, the following combination was found to be excellent; CoCrAlY as a bond-coating material and spray coating layer by zirconia(YSZ)-coated carbon and YSZ powders as a ceramics coating material.
From a measurement result of the coefficient of penetration of the transpiration cooling model devised in this study, the compositional ratio of C and YSZ powders was found to have the optimum value between 1:2 and 2:1.

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

In order to improve the heat and environment-resistances of a CMC material(SiC(f)/SiC(m)), employing BN coating and high heat-resistant SiC fiber(Hi-Nicalon), the HIP(Hot Isostatic Pressing) method was applied as a process for increasing the density of SiC matrix. As a result, porosity rate was reduced less than 10% from about 20% in the previous non-HIPed material, and flexural strength at room temperature was increased more than twice.
The oxidation-resitance, thermal degradation and bending creep properties of this material were examined at high temperature more than 1300°C.

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

For improving the heat-resistance of a CMC core material(SiC(f)/S3iN4(m)), the composition of Al₂O₃ in matrix was reduced from 5% to 2%. As a result, its 4-point bending strength at 1500°C was increased about twice. Also, crystallization of grain-boundary phase was observed by a heat treatment(1400°C x 50h in nitrogen gas environment), and then the strength at 1500°C was increased about 3 times.
From the corrosion test result of a polycrystal layer of Al₂O₃ which is one of the candidates of a CMC surface material in a 1600°C steam(partial pressure:0.5atm) environment, it was found that the mass transfer of Al₂O₃ was accelerated in the high-temperature steam environment.

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

A fabrication method of 3-D FRC materials was established on a specimen level for carbon/carbon(C/C) composite and CMC materials.
The tensile properties were obtained by making tensile tests at room temperature in an air environment on both the C/C and CMC materials, and also at 2000°C and 1200°C in an Ar gas environment on the C/C and CMC materials, respectively. Also, stress distributions in a tensile specimen of the materials were analyzed by the finite element method. As a result, the relationships of fiber/matrix materials and fiber orientations to the strength were found.

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

An in-situ observation of specimen surface during tensile tests was successful at ultra-high temperature(1600°C)for a C/C composite material in both air and vacuum environments by using a laser microscope.
The measurement method of thermal diffusion coefficient for a thin sample by using a ring-laser beam was examined, and then the experimental result was found to be nearly consistent with a theoretical one. Also, a trial measurement of the emissivity on metals with high melting-point such as W and Mo was made. As a result, the measurement method could see far into the future.
An oxidation test method in a steam environment(test piece:SiC, test temperature:1700°C, steam concentration:60%) was examined by using an existing apparatus and some test data were obtained. Also, subjects to be studied in future were obtained for increasing test pressure in material testing equipment in ultrahigh-temperature steam/hydrogen environments.

8.5.3 Research Plan for fiscal year 1997

Focusing on a superalloy, ceramic-matrix and curbon-curbon composite materials which are expected to be applicable to ultrahigh-temperature components in the hydrogen cimbistion turbine, materials design and experimental production as well as materials tests and evaluation will continue in consideration of technical subjects which have come out to date, and then the fundamental properties of these materials will be obtained.