Grain-size effect on fatigue properties of nanocrystalline nickel thin films made by electrodeposition

Nickel nanocrystalline thin films were produced by electrodeposition using sulfamate solution. Free-standing thin films with various grain sizes ranging from 9 to 67 nm were produced under a constant current with changing the content of brightener and temperature of electrodeposition. The grain-size effect on the fracture strength and the yield strength in tension tests can be divided into two regions: region A with the grain size larger than about 15 nm and region B with the grain size between 9 to 15 nm. In region A, the tensile strength and yield strength increase following Hall-Petch relation, and the tensile elongation decreases with decreasing grain size. In region B, the tensile and yield strength increases following a different relation of Hall-Petch type, and the tensile elongation also increases with decreasing grain size. The fatigue strength also increases with decreasing grain size following different Hall-Petch relations between region A and B. The resistance to fatigue crack propagation decreases for nanocrystalline films. Scanning electron microscopic observation of deformation near cracks shows two different features depending on the grain-size region A or B. In region A, ordinary slip deformation within the grain is predominant, while grain boundary sliding is more important in region B for fatigue damage formation. © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11