Hiroshi Kaya

The application of ceramic-matrix composites to the automotive ceramic gas turbine

Abstract A Japanese 100 kW automotive ceramic gas turbine (CGT) project was started in 1990 and was concluded successfully in 1997. This project was supported by the Ministry of International Trade and Industry and was conducted by the Petroleum Energy Center to achieve the targets of this project such as higher thermal efficiency over 40% at a turbine inlet temperature of 1350°C, lower exhaust emissions to meet Japanese regulations, and multi-fuel capabilities. Ceramic-matrix composites (CMCs) are expected to become one of the most reliable materials for high-temperature use to make up for the deficient properties of monolithic ceramics and heat-resistant alloys. Carbon fiber, silicon nitride fiber, silicon carbide fiber, silicon carbide whisker, in situ silicon nitride, TiB 2 /milled carbon fiber were used as reinforcements for silicon carbide, SiNC, SiAlON and silicon nitride matrix composites. Higher mechanical properties tested by the developed testing standards, and reliability against thermal shock, particle impact damage and creep resistance were confirmed to apply these CMCs for engine components. Several screening test steps were performed before the engine tests and these confirmed that CMC had strong potential for actual engine components.

Creep and fatigue behavior of SiC fiber reinforced SiC composite at high temperatures

Abstract Tensile creep and tension-tension fatigue of SiC fiber reinforced SiC composite were investigated in argon at 1000 and 1300°C. Time-dependent (creep) strains under cyclic loading are much larger than those under constant load. However, the minimum creep strain rates under cyclic loading are similar to those under constant load at 1000°C and lower than those under constant load at 1300°C. All the data of the minimum creep strain rates versus time to rupture under both cyclic loading and constant load fall on the same line, i.e., the relations fit Monkman—Grant relationship. This means that creep controls the rupture life under cyclic loadings at high temperatures. At high stresses, the creep crack propagation paths are similar to fatigue crack propagation. However, at low stresses, more 0° fibers were broken in the way of flush with the matrix under creep load than that under fatigue load.

Tensile creep behavior of a SiC-fiber/SiC composite at elevated temperatures

Abstract The tensile creep behavior of a SiC-fiber-reinforced SiC composite has been investigated in argon at temperatures of 1000–1300°C. The apparent stress exponents for creep of the composite and the apparent activation energies for creep increase with decrease in stress. The threshold stress approach can be used to treat the data. Creep of the CVI–SiC matrix controls the creep of the composite. The relationship between creep rate and the time to rupture can be described by the Monkman–Grant equation which provides a method of life prediction. The Larson–Miller parameter can also be used for creep-life prediction of the composite when the appropriate constant is selected.

In situ observation of cyclic fatigue crack propagation of SiC-fiber/SiC composite at room temperature

Abstract In situ observation of cyclic fatigue crack propagation of SiC-fiber reinforced SiC composite at room temperature has been carried out by laser microscopy. Both smooth (unnotched) and notched specimens are used for tension-tension cyclic fatigue tests. Cracks initiate at the comers of large pores during loading in smooth specimens. In notched specimens cracks are formed at the interfaces between fibers and matrix that are connected to the notch. The balance between the fiber bridging in the wake of propagating crack tip and the breakage of bridged fibers by the degradation of interfaces maintains a steady cyclic crack propagation. Crack propagation rate gradually decreases with time after the maximum load being applied.

Stress, strain and elastic modulus behaviour of SiC/SiC composites during creep and cyclic fatigue

Abstract Creep and fatigue tests of Hi-NicalonTM/SiC (SiC matrix contains glass-forming, boron-based particulates), Standard SiC/SiC (SiC matrix is pure SiC) and Enhanced SiC/SiC (SiC matrix contains glass-forming, boron-based particulates) were carried out in air at 1300°C. The stress–strain hysteresis loops during fatigue and creep were studied. The change of Young’s modulus during creep and fatigue was analysed and compared among the three kinds of materials. Creep strain rates of Hi-NicalonTM/SiC in air were similar to those of Enhanced SiC/SiC, but much lower than those of Standard SiC/SiC. Consequently, the time to rupture at a given stress in Hi-NicalonTM/SiC was similar to that in Enhanced SiC/SiC, but much longer than in Standard SiC/SiC. Fatigue resistance of Hi-NicalonTM/SiC was similar to that of Enhanced SiC/SiC, but much better than Standard SiC/SiC.

Static and cyclic fatigue in SiC whisker-reinforced silicon nitride composite

Abstract Static and cyclic fatigue tests of 20 vol% SiC whisker-reinforced Si3N4 matrix composite processed by gas pressure sintering were carried out at room temperature (RT), 1000°C and 1200°C by four-point bending tests. Cyclic fatigue lives were equal to static ones at both RT and 1000°C, but higher than static ones at 1200°C. Crack propagation was a mixture of intergranular and transgranular modes at RT and 1000°C. However, fracture at 1200°C was characterized by a process of nucleation, growth and interlinkage of cavities in front of a major crack. The enhanced bridging stress under cyclic loading retards the cavity growth and therefore leads to longer fatigue life than under static loading.