Strain rate and creep response of Au and Ni thin films

Metallic films for MEMS and microelectronics are often subjected to strains near the elastic limit and over a wide spectrum of strain rates. In this study, comprehensive uniaxial tension experiments were carried out to extract the strain rate response of thin nanocrystalline Au and Ni films over eight orders of applied strain rate, i.e. 10^-6 - 20 s^-1. Full-field strains were obtained from MEMS-scale samples to determine their elastic and inelastic mechanical behavior. The microscale tension experiments on Au and Ni films showed monotonic increase of the elastic limit, yield stress, and ultimate strength with increasing strain rate. Furthermore, the fracture strain decreased with increasing strain rate with a sharp transition at 10^-4 s^-1, implying enhanced creep at rates slower than this rate. The failure of Au films was predominantly ductile with different damage mechanisms at the slow (through-thickness damage) and fast (film mid-plane damage) strain rates. In creep experiments conducted over the span of days the primary creep response was significant followed by a steady-state response, which behavior repeated itself under periodically applied fixed stress (period of one day).