電着ニッケルナノ結晶薄膜の疲労に及ぼすマイクロ切欠き形状の影響

抄録

In recent years, MEMS technology has been applied to sensor and electronic components. Nickel thin films have been used for main structural materials for MEMS devices made by LIGA process. Nanocrystallization of thin films will improve the fatigue strength, while the fatigue strength may become more sensitive to defects or stress concentration. In this paper, the influence of micro-notches on the fatigue strength was studied using electrodeposited nickel thin film with ultra fine grain (UFG) of 384 nm in grain size. Micro-notches were introduced to thin films with FIB. Micro-notches have four lengths ranging from 8 to 120 μm. The notch-tip shapes are two types: square type (MN type) and round type with radius of 1 μm (MNR type). The fatigue strength decreased with increasing notch depth. The fatigue strength is nearly equivalent between MN and MNR specimens in short-fatigue-life regime, while near the threshold, the fatigue strength is lower in MN than in MNR specimens because of smaller notch-tip radius. Below the fatigue limit, there is no non-propagation crack observed. The fatigue limit is controlled by the initiation of fatigue cracks. The influence of notch length on the fatigue limit of both type of specimens are predicted by the fictitious-crack model.