On the relationship between surface roughness evolution and size effects in free-standing Cu films with fine microstructure

In situ surface roughness measurements are used to elucidate size effects on plastic deformation mechanisms in free-standing Cu films with a fine microstructure. Spectral analysis of the surface roughness distribution showed the competing influences of microstructure- and free surface-induced effects on strength and ductility. The measurements show that the free-surface effect on intergranular interactions underlies the reduction in strength and ductility seen with thickness reduction. We find that while microstructure refinement increases film strength, it also serves to limit ductility.