Yasuhiko Kohtoku

A ductile ceramic eutectic composite with high strength at 1,873 K

Monolithic ceramics are not widely used as structural materials because of their brittleness. Ceramic matrix composites, in which whiskers 1-3 or fibres 4-7 of strong ceramics such as silicon carbide or silicon nitride are embedded in a ceramic matrix, offer improved toughness and strength because the energy of cracks may be dissipated at the whisker/matrik interface. Here we describe a strong ceramic composite with a different kind of microstructure, made by unidirectional solidification of an Al 2 O 3 / GdAlO 3 eutectic mixture. This composite has a microstructure in which continuous networks of single-crystal Al 2 O 3 and single-crystal GdAlO 3 interpenetrate without grain boundaries. Rather than brittle fracture, the material displays plastic deformation at 1,873K owing to dislocation motion, as in metals. The high strength and resistance to brittle failure of this material at such high temperatures augurs well for applications in mechanical engineering.

High-strength alkali-resistant sintered SiC fibre stable to 2,200 °C

The high-temperature stability of SiC-based ceramics has led to their use in high-temperature structural materials and composites. In particular, silicon carbide fibres are used in tough fibre-reinforced composites. Here we describe a type of silicon carbide fibre obtained by sintering an amorphous Si–Al–C–O fibre precursor at 1,800 °C. The fibres, which have a very small aluminium content, have a high tensile strength and modulus, and show no degradation in strength or change in composition on heating to 1,900 °C in an inert atmosphere and 1,000 °C in air — a performance markedly superior to that of existing commercial SiC-based fibres such as Hi-Nicalon. Moreover, our fibres show better high-temperature creep resistance than commercial counterparts. We also find that the mechanical properties of the fibres are retained on heating in air after exposure to a salt solution, whereas both a representative commercial SiC fibre and a SiC-based fibre containing a small amount of boron were severely degraded under these conditions. This suggests that our material is well suited to use in environments exposed to salts — for example, in structures in a marine setting or in the presence of combustion gases containing alkali elements.

High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite

A unidirectional solidification method was investigated to manufacture Al2O3/YAG eutectic composites with high-temperature resistance that would make them usable at very high temperatures. We were successful in manufacturing a single-crystal Al2O3/single-crystal YAG eutectic composite with a dimension of 40 mm in diameter and 70 mm in length containing no colonies or pores. This composite also displayed excellent high-temperature strength characteristics. The flexural strength was in the range 350∼400 MPa from room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. During tensile tests above 1923 K, the eutectic composite showed evidence of plastic deformation occurring by dislocation motion, and a yield phenomenon similar to many metals was observed. In addition, the microstructure of the composite was extremely stable: after 1000 h of heat treatment at 1973 K in an air atmosphere there was no growth. The above superior high-temperature characteristics are caused by such factors as the eutectic composite having a single-crystal Al2O3/single-crystal YAG structure, the formation of a compatible interface with no amorphous phase and thermal stability, and the combined effect of a YAG phase with superior high-temperature characteristics.

Sapphire matrix composites reinforced with single crystal YAG phases

An investigation of fabrication technology on eutectic composites consisting of Al2O3 phases and YAG (Y3Al5O12) phases was carried out by applying the unidirectional solidification process. Unidirectionally solidified eutectic composites consisting of 〈110〉 sapphire phases and 〈420〉 single crystal YAG phases could be fabricated successfully by lowering a Mo crucible at a speed of 5 mm h−1 under a pressure of 10−5 mmHg of argon. These eutectic composites have excellent high-temperature properties up to 1973 K. For example, the flexural strength is 360–500 MPa independent of testing temperature from room temperature to 1973 K. Oxidation resistance at 1973 K in an air atmosphere is superior to SiC and Si3N4 and the microstructure of these eutectic composites is stable even after heat treatment at 1773 K for 50 h in an air atmosphere.

The creep and thermal stability characteristics of a unidirectionally solidified Al2O3/YAG eutectic composite

Compressive creep characteristics at 1773, 1873, and 1973 K, oxidation resistance over 1000 h at a temperature of 1973 K in ambient air, and the thermal stability characteristics at 1973 K in ambient air of a unidirectionally solidified Al2O3/YAG eutectic composite were evaluated. At a test temperature of 1873 K and a strain rate of 10−4/s, the compressive creep strength of a eutectic composite manufactured by the unidirectional solidification method is approximately 13 times higher than that of a sintered composite with the same chemical composition. The insite eutectic composite also showed greater thermal stability, with no change in mass after an exposure of 1000 hours at 1973 K in ambient air. The superior high-temperature characteristics are closely related to such factors as (1) the in-situ eutectic composite having a microstructure, in which single crystal Al2O3 and single crystal YAG are three-dimensionally and continuously connected and finely entangled without grain boundaries and (2) no amorphous phase is formed at the interface between the Al2O3 and the YAG phases.

Catalytic activity of PdO/ZrO2 catalyst for methane combustion

Abstract Catalytic activity of ZrO2 supported PdO catalysts for methane combustion has been investigated in comparison with Al2O3 supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO2 support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al2O3 catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO2 catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO2 catalyst. In order to confirm the formation of the solid solution of PdO and ZrO2, X-ray diffraction (XRD) analyses of the mixtures of ZrO2 and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO2 in the mixture was larger than that of ZrO2. Furthermore, the Pd thin film on ZrO2 substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO2 support might be stabilized by the formation of the solid solution of PdO and ZrO2.

Production mechanism of polyzirconocarbo- silane using zirconium(IV)acetylacetonate and its conversion of the polymer into inorganic materials

The reaction of polycarbosilane with zirconium(IV)acetylacetonate proceeded at 573 K in nitrogen atmosphere by the condensation reaction of the Si–H bonds in polycarbosilane and the ligands of zirconium(IV)acetylacetonate accompanied by the evolution of acetylacetone, and then the molecular weight increased by the cross-linking reaction with a formation of Si–Zr bond. The obtained polyzirconocarbosilane showed higher ceramic yield than the polycarbosilane. Zirconium contained in the pyrolysed polyzirconocarbosilane was furthermore found to have the effect of inhibiting crystalline grain growth of β-type SiC up to high temperature, so Si–Zr–C–O fibre, which was obtained by the use of polyzirconocarbosilane as precursor, showed high tensile strength up to high temperature.

Structure and properties of Si-Ti-C-O fibre-bonded ceramic material

Si-Ti-C-O fibre-bonded ceramic material was synthesized from pre-oxidized Si-Ti-C-O fibre with an oxide layer 400–600 nm thick, by hot-pressing at 2023 K under 50–70 MPa. The interstices in the Si-Ti-C-O fibre-bonded ceramic material were packed with an oxide material which existed on the surface of the pre-oxidized Si-Ti-C-O fibre, and the oxide material formed a small amount of the matrix phase (⩽10 vol%). At the fibre-matrix interface, aligned turbostratic carbon, which was oriented around the fibre, was formed during hot-pressing. The existence of the interfacial carbon layer indicated the Si-Ti-C-O fibre-bonded ceramic material to have a fibrous fracture pattern with high fracture energy. The Si-Ti-C-O fibre-bonded ceramic material showed excellent durability even at 1773 K in air, because a protective oxide layer is formed on the surface at a high temperature (above 1273 K) in air. Moreover, the Si-Ti-C-O fibre-bonded ceramic material almost maintained its initial strength in the bending and tensile tests, even at 1773 K in air.

Improving the fracture toughness of MgO–Al2O3–SiO2 glass/molybdenum composites by the microdispersion of flaky molybdenum particles

The flake-forming behaviour of powders of molybdenum, niobium, nickel, BS 316 S 12, Ni–17Cr–6Al–0.6Y, iron, titanium and Ti–6Al–4V, using a wet ball mill, was investigated. MgO–Al2O3–SiO2 (MAS) glass composites reinforced with these flaked particles were fabricated, and improvements in flexural strength evaluated. The MAS glass composites reinforced with flaky metallic particles such as molybdenum, niobium, iron, nickel and Ni–17Cr–6Al–0.6Y, showed an improvement. The effect of molybdenum particle size on the flake-forming behaviour of molybdenum, flexural strength and fracture toughness of MAS glass/molybdenum composites, were investigated. The flake-forming behaviour shows a high degree of dependence on molybdenum particle size and, upto a size of 32 μm, becomes conspicuous with increasing particle size. At 32 μm, the aspect ratio reaches a value of 17 and, above 32 μm, flake forming saturates. Fracture toughness is closely related to flake-forming behaviour and the more marked the flake forming, the greater is the increase in fracture toughness. A composite of MAS glass with flaky molybdenum particles has a greater improvement effect on fracture toughness than composites with SiC whiskers, SiC platelets or ZrO2 particles. This is closely linked to plastic deformation of the flaky metallic particles at the crack tip at the time of fracture.

Simultaneous improvement of the strength and fracture toughness of MgO-Al2O3-SiO2 glass/Mo composites by the microdispersion of flaky Mo particles

The effect of shape and volume percent of Mo particles on theflexural strength and fracture toughness of MgO-Al2O3-SiO2(MAS) glass/Mo composites was investigated. The flexural strengthand fracture toughness of composites depends heavily on Mo particleshapes, and there is greater improvement in composites reinforcedwith flaky rather than massive Mo particles. In the compositesreinforced with flaky Mo particles, fracture toughness increases withvolume percent of Mo and, at 50 vol% Mo, is 11.6 MPa√m,which is approximately 6.7 times higher than that of the matrix. Increases in fracture toughness of composites reinforced with flakyMo particles is greater than with SiC whiskers, SiC platelets, SiC particles or ZrO2 particles. Fabricating composites reinforcedwith flaky Mo particles is an effective toughening technique capableof simultaneously improving the strength and toughness of brittlematerials, such as monolithic Al2O3 and MAS glass, by utilizing plastic deformation of ductile phase.