TAKAHASHI Koji Yokohama National University, Faculty of Engineering, Associate Professor, 大学院工学研究院, 助教授 (90334630)
NAKAO Wataru Yokohama National University, Faculty of Engineering, Assistant, 大学院工学研究院, 助手 (60361870)
|Budget Amount *help
¥15,400,000 (Direct Cost: ¥15,400,000)
Fiscal Year 2006: ¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2005: ¥11,800,000 (Direct Cost: ¥11,800,000)
Ceramics have been used in various fields as machined components. In particular, Alumina and Silicon Nitride is expected to be employed in structural components operating at high temperatures in various atmospheres, because of its excellent mechanical properties and oxidation resistance. However, ceramics has low fracture toughness, resulting in numerous surface cracks due to machining. Post machining operations, such as polishing and lapping, together with nondestructive inspection, are generally performed to remove the non-acceptable flaws. Although these techniques are very costly, they cannot eliminate minute flaws. Thus, the reliability of the ceramic components cannot be secured. If such cracks can be healed, ceramic components can offer improved reliability without costly final machining. In this study, silicon nitride / SiC particle, alumina/ SiC particle and alumina/ SiC whiskers composites were sintered and the crack-healing behavior for the cracks caused by machining was inv
estigated. Small bending specimens (22×4×3 mm) were made. These specimens were machined hardly into various machined figuration so that they have various stress concentration factors replicate those of components. The machined specimens were crack-healed under various conditions. The fracture stress and the cyclic fatigue limit of these specimens after crack-healing were evaluated systematically. The conclusions were obtained as followed :
(1)Numerous cracks were introduced during heavily machining, as a result, the strength of Silicon Nitride / SiC particle, alumina/ SiC particle and alumina/ SiC whiskers composite reduced by 50 〜 80% compared to the polished samples. However, complete strength recovery was attained by crack-healing at 1400℃ for 10 h.
(2)In the case for alumina/ SiC particle and alumina/ SiC whiskers composite, Crack-healed zones exhibited the same strength as base material up to 1300 ℃, and exhibited the same levels of cyclic fatigue strength as the pristine, base material at 900 ° and 1100 ℃.
(3)In the case of silicon nitride / SiC particle composite, the bending strength of healed machined sample exhibited approximately 700 MPa from RT to 1400 ℃. Moreover, cyclic fatigue strength at 800℃, 1000℃, 1200℃ and 1300℃ were found to be 700MPa, 650MPa, 600MPa and 550MPa. Thus, it was found that healed machined silicon nitride exhibit high cyclic fatigue strength and reliability at high temperature.
These results have demonstrated that crack-healing is a highly effective technique which may potentially reduce machining costs and improve the structural integrity of machined ceramics Less