ナノ結晶ニッケル電着薄膜の疲労特性

抄録

Nickel nanocrystalline thin films were produced by electrodeposition using sulfamate solution. Three types of thin films with different grain sizes were produced : CC films were made under constant current, PC films under pulse current, and CC-ally films under constant current with grain refinement additive. The grain size gets smaller in the order of CC, PC, and CC-ally films down to nanometers. The fracture strength and yield strength in tension tests follows the Hall-Petch relation, and the elongation was largest for CC-ally films. The fatigue strength increased with decreasing grain size, following the Hall-Petch relation down to about 10nm. The resistance to fatigue crack propagation decreased for nano grain-sized films. The threshold stress intensity factor was the smallest for PC and CC-ally films. In the intermediate-rate range, the propagation rate increased with decreasing grain size when compared at the same stress intensity factor. The fatigue fracture surface near the threshold consisted of granular features whose size decreased with decreasing grain size. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CC-ally films. On the specimen surface near the fatigue fracture surface of CP and CC-ally films, the small grain boundary fracture facets were observed at high stress intensity factors, while not observed near the threshold. For CC films, slip bands were seen together with the grain boundary fracture facets.