Transient creep in free-standing thin polycrystalline aluminum films

We studied room-temperature transient creep in polycrystalline, free-standing Al films with a thickness between 220 and 550 nm using a high-resolution bulge test technique. A transient logarithmic creep strain is observed. The time and stress dependence of the creep strongly support the idea that dislocation glide, limited by forest dislocation cutting, is the prevailing rate limiting mechanism. This is in contradiction with the misfit dislocation model for thin-film strengthening but in agreement with recent work on plasticity in thin Ag and Cu films on a substrate. A comparison is made with data on bulk Al. Both the transient creep strain and the initial fast strain are at least three orders of magnitude smaller for the thin-film samples. We argue that the strain hardening coefficient is the key parameter distinguishing thin film from bulk creep.