電着ニッケルナノ結晶薄膜の切欠きによる疲労強度低下の予測

抄録

Nickel thin films have been used for structural materials for MEMS devices made by LIGA process. Nanocrystallization of thin films will increase the fatigue strength, while fatigue strength may become more sensitive to defects or stress concentrations. In this paper, the influence of micronotches on fatigue strength was studied using electrodeposited Nickel thin films. Two types of Ni thin films were produced by electrodeposition using sulfamate solution without and with brightener: CC with the grain size of 384 nm and CC-ally with that of 16 nm. Micro-sized notches were introduced in thin films by FIB. Notches had 2μm in width and various depths from 8 to 150μm. Fatigue tests were conducted under the stress ratio of 0.1 and the crack initiation and propagation behavior were observed by video recording with microscope. The fatigue strength decreased with increasing depth of notches. Notches as small as 8μm did reduce the fatigue strength of CC-ally. A model of fictitious crack can predict the fatigue strength reduction by micro-notches in thin films. SEM observation was conducted on fatigue fracture surfaces to reveal mechanisms of fatigue crack initiation.