Papers by Author: Kotoji Ando

Abstract: Mullite reinforced by SiC whiskers, Mullite(W), was developed with the aim of improving fracture toughness and to add crack-healing ability. For determining the threshold stress during crack-healing, a composite having a semi-elliptical surface crack of 100 μm was crack-healed at 1473 K for 8 h in air under elevated static and cyclic stresses and the bending strength at 1473 K of the crack-healed composite under stress were also investigated. Mullite(W) crack-healed under static stress below 100 MPa were never fractured during crack-healing and had the same bending strength as the specimens crack-healed under no-stress. Therefore, the threshold static stress during crack-healing of Mullite(W) has been determined to be 100 MPa. The threshold cyclic stress has also been determined to be 150 MPa. Considering that the crack growth is time-dependent, the threshold stress for all conditions during crack-healing of Mullite(W) has been concluded to be 100 MPa.

Abstract: Si3N4/SiC composite ceramics were hot-pressed in order to investigate their crack-healing behavior under cyclic stress and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000 °C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that when the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.

Abstract: For improving fracture toughness as well as endowing with excellent self-crack-healing ability, mullite/SiC particles /SiC whiskers multi-composites were developed. Crack-healing ability and the mechanical properties of the specimens were investigated. Mullite/ 20 vol% SiC whiskers, mullite/ 25 vol% SiC whiskers and mullite/ 15 vol% SiC whiskers/ 10 vol% SiC particles composites can completely heal the pre-crack of 100 ım in surface length by heat-treatment at 1300 oC for 2 h in air. However, not completeness of the strength recovery was caused for mullite/ 15 vol% SiC whiskers composite by crack-healing. In spite of the same SiC content, the crack-healing ability of mullite/ 15 vol% SiC whiskers/ 10 vol% SiC particles composite is superior to that of mullite/ 25 vol% SiC whiskers composite. Admixing with SiC whisker was effective for improvement in fracture toughness. Additionally, the fracture toughness was not reduced by admixing with SiC particles.

Abstract: Four kinds of brittle materials were used to evaluate the bending strength under three-point bending and the characteristics of the elastic wave signal by Vickers indentation. The bending test was carried out under room temperature and high temperature. A crack was made at the tension side of the specimen with a Vickers indenter to investigate fracture strength. Fracture wave detector was used to evaluate characteristics of waveform and frequency of the elastic wave signal.