Film-Thickness Effect on Fatigue Properties of Nano-Crystalline Nickel Films Made by Electrodeposition
Keisuke TANAKA, Masashi SAKAKIBARA, Hiroto TANAKA, Shunpei TAKESHITA and Hirohisa KIMACHI
Abstract:Two types of free-standing nickel thin films were produced by electrodeposition using sulfamate solution: CC films made under constant current without brightener and CCally films with brightener. The effect of film thickness on the tensile and fatigue properties was studied by using films with 10 and 30 ƒÊm in thickness. The grain size of CC films determined by X-ray line broadening was about 52nm on the substrate side and increased away from the substrate side resulting in coarser grain size of thicker films on the solution side. The grain size of CCally film was about 9 to 10 nm, and the size was slightly larger on the solution side. The fracture strength and yield strength in tension tests was higher in thinner films, and follow the Hall-Petch relation irrespective of film thickness when plotted in terms of the grain size measured on the solution side. The fatigue strength was also higher in thinner films, and the Hall-Petch relation was observed when plotted in terms of the grain size measured on the solution side. The resistance to fatigue crack propagation was higher in thicker films. The threshold stress intensity factor increased linearly with the square root of the grain size irrespective of film thickness. The thickness-effect of thin films on fatigue properties comes from the distribution of the grain size throughout thickness and the coarser grain size made on the solution side controlled the strength of the films. The fatigue fracture surface near the threshold consisted of granular features whose size was larger for films with coarser grains. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CCally films. Key Words:Fatigue strength, Nickel thin film, Nanocrystal, Elecrodeposition, Fatigue crack propagation, Thickness effect