Nanocrystalline FCC Thin Films Are More Prone to RT Creep Than What We Think

Although nanocrystalline (Nc) metallic thin films are excellent candidate materials for Microelectromechanical Systems (MEMS) and microelectronics due to their outstanding yield strength, serious reliability concerns arise from their increased room temperature creep rates. A comprehensive experimental investigation was carried out to extract the strain-rate dependent mechanical behavior of Au (38 nm grain size) and Ni (20 nm grain size) micron-thin films conducted for the very first time at strain rates in the broad range of 10−6 – 10 /s which spans time scales from ms to hours. Nc-Au films demonstrated a clear bi-linear change in their inelastic properties, i.e. the elastic limit and its yield strength, while the Nc-Ni films showed a linear increase in their inelastic properties over the same loading rates. This unexpected trend for the Au films emphasized the significant contribution of room temperature (RT) creep at strain rates between 10−6 – 10−4 /s, at which rate, larger grain size materials are not prone to creep at RT. This realization prompted a series of novel microscale creep experiments, the first of their kind, at time scales of 104–105 s. An important finding was that the first stage of creep, primary creep, proceeds at a very fast rate, of the order of 10−7 /s, lasting for 5–6 hours after the application of a stress. Furthermore, multi-stage creep experiments revealed that the primary creep rate decreased with the order of creep cycle, while the steady state creep response remained the same in all creep cycles. This creep response of nanocrystalline FCC films was modeled via a non-linear viscoelastic creep model that captured the effect of applied stress on both primary and steady-state creep regimes.