Min-Cheol Chu

Crack healing behaviour and high-temperature strength of mullite/SiC composite ceramics

Abstract Mechanical properties of sintered mullite/SiC ceramics related to its significant crack healing behavior are discussed in this paper. This investigation was made on four kinds of specimens such as as-received smooth, heat treated smooth, pre-cracked and pre-crack healed. Pre-crack sizes were 100 and 200 μm and they were semi-elliptical in shape. The main conclusions were obtained as follows: (a) mullite/SiC composite ceramic has ability to heal crack, (b) the best healing condition was found to be 1300xa0°C in air for 1 h, (c) maximum crack size able to be healed is semi-elliptical crack of 200 μm in diameter, (d) crack-healed zone has enough strength up to 1200xa0°C and most specimens failed outside the crack-healed zone.

Static and cyclic fatigue behaviour of crack-healed Si3N4/SiC composite ceramics

Abstract Si 3 N 4 /SiC composite ceramics were sintered in order to investigate their fatigue strength behavior after crack healing, Y 2 O 3 and Al 2 O 3 power was added as sintering additives to enhance its sintering property. Three-point bending specimen (sized 3×4×40 mm) was hewed out according to JIS standard from sintered compact obtained. About 100 or 200 μm semi-circular surface cracks were made on the center of the tension surface of the three-point bending specimen using Vickers indenter. After the crack-healing processing of 1200 and 1300°C, 1 h, in air, cyclic and static fatigue strength behavior of these crack-healed specimen were determined systematically at room temperature and high temperature of 800, 1000, 1200 and 1300°C. The results show that Si 3 N 4 /SiC composite ceramics possess remarkable crack-healing ability, and crack-healed specimens showed similar cyclic and static fatigue strength behavior as smooth specimens at high temperature, not only at room temperature. Crack-healed zones had sufficient fatigue strength even though at high temperature environment, and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.

Crack-healing behavior of Si3N4/SiC ceramics under stress and fatigue strength at the temperature of healing (1000°C)

Abstract Si 3 N 4 /SiC composite ceramics were sintered and subjected to three-point bending specimens made according to the appropriate JIS standard. A semi-circular surface crack of 100 μm in diameter was made on each specimen. We systematically studied crack-healing behavior, and cyclic and static fatigue strengths at the service temperature (1000xa0°C) by using three kinds of specimens (smooth, cracked and crack-healed). The main conclusions are as follows: (1) Si 3 N 4 /SiC composite ceramics have the excellent ability to heal a crack at 1000xa0°C; (2) this sample could heal a crack even under cyclic stress at 1000xa0°C; (3) a new crack-healing process was proposed. The sample crack-healed at 1000xa0°C by the process exhibited a sufficient static and cyclic fatigue strength at 1000xa0°C.

Effect of crack-healing and proof-testing procedures on fatigue strength and reliability of Si3N4/SiC composites

Abstract A Si3N4/SiC composite was hot-pressed. Using this material, fatigue tests on crack-healed and proof-tested specimens were conducted at 1000–1400xa0°C. A surface elliptical-crack of about 110 μm in diameter was introduced on the specimens using a Vickers hardness indenter. The crack-healing was performed at 1300xa0°C for 1 h in air, mainly. The fatigue limit of the crack-healed and proof-tested specimen (C.P specimen) decreased slightly with increasing test temperature. However, the crack-healed specimen is not sensitive to low-cycle fatigue up to 1400xa0°C, and the fatigue limit is almost equal to the minimum bending strength at each temperature. To investigate the reason, the crack-healing behavior under cyclic stress was carried out systematically at 1200xa0°C in air. A 110 μm surface crack could be healed perfectly at 1200xa0°C in air under cyclic stress with a frequency of 0.001–5 Hz. From this, it can be concluded that [crack-healing+proof test] and crack-healing during service are useful techniques for maintaining structural integrity of these ceramic components.

Crack-healing behavior and static fatigue strength of Si3N4/SiC ceramics held under stress at temperature (800, 900, 1000 °C)

Abstract Si3N4/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm in surface length was introduced on each specimen. The pre-crack was healed under constant bending stress of 210 MPa at 800, 900 and 1000xa0°C. Applied stress of 210 MPa is ∼70% of the bending strength of pre-cracked specimen. Bending strength and static fatigue strength of crack-healed specimens were systematically investigated at each crack-healing temperature. The bending strength of crack healed specimen showed almost the same value as smooth specimen. Thus, Si3N4/SiC composite ceramics could heal a crack even under constant bending stress of 210 MPa at 800, 900 and 1000xa0°C. Moreover, crack-healed zone had quite high static fatigue limit at each crack-healing temperature. These conclusions indicate that Si3N4/SiC composite ceramics has an ability to heal a crack under service condition, i.e. high temperature and applied stress.