2015 Volume 64 Issue 11 Pages 918-925
Three types of free-standing copper thin films were produced by electrodeposition using sulfate solution under constant current: CC made without brightener addition, CC-B with brightener and CC-T with thiourea. The grain size of CC, CC-B, CC-T measured by transmission electron microscopy was 421, 237, and 28.5nm. Those films did not have any strong texture. The effect of grain size on the tensile and fatigue properties was studied. The tensile yield stress of CC and CC-B were slightly higher than the Hall-Petch relation published for bulk wrought copper, while that of CC-T was lower than the relation. The fatigue life increased with decreasing grain size especially for longer fatigue life region. The fatigue strength gets higher with decreasing grain size. The fatigue limits of CC-B and CC-T approximately follow the Hall-Petch relation, while the fatigue limit of CC was lower than the relation. The fatigue crack propagation rate for R = 0.5 was higher than that for R = 0.1 when compared at the same stress intensity range. The threshold stress intensity for fatigue crack propagation increased with increasing grain size. A fatigue crack of CC was accompanied with shear bands and striations were observed on the fatigue fracture surface made at high stress intensities. There was no shear band near fatigue cracks in CC-T, and only few in CC-B. The granular feature was observed on the fatigue fracture surface of CC-B and CC-T, and the size of the granular feature was much smaller in CC-T than in CC-B.
