Microstructural and temperature-dependent deformation mechanisms governing the cyclic creep of freestanding, submicrometer gold thin films

A detailed understanding of the fatigue behavior of freestanding metallic films is crucial for extending the lifetime of microsensors and actuators. Previous investigations have pointed at cyclic creep as the dominant deformation mechanism in columnar gold thin films. Here, we investigate the widespread ultra-fine grained (UFG) and nanocrystalline (NC) microstructures, at typical operational temperatures from ambient to 100 °C. While the UFG films show low failure strains, the NC samples can withstand several percents strain, which was attributed to their high initial dislocation density. The combination of strength and ductility in NC films results in superior fatigue lifetimes compared to the UFG films for both low-cycle and high-cycle fatigue regimes. At the highest temperature, NC films even evidence an unexpected fatigue limit. Cyclic creep was evident for all microstructural conditions and temperatures. Hence, creep and diffusion processes are dominant, especially in the low dislocation density UFG gold films. In addition, NC films experience a reduction of the high-angle grain boundaries after cycling at 23 °C and local grain coarsening at 100 °C. • Comparison of the fatigue behavior of freestanding gold thin films of different microstructural conditions by stress-controlled bulge fatigue tests • The nanocrystalline (NC) films contain high initial dislocation density compared to the ultra-fine grained, columnar (UFG) condition, leading to improved strength and ductility • Enhanced LCF and HCF fatigue lifetime of NC films at all temperatures, and evidence of previously unreported fatigue limit at 100 °C • Transmission electron micrographs of samples cycled at 100 °C evidence locally coarsened grains with newly formed dislocations inside NC films and grain boundary grooving in UFG specimens • Cyclic creep is present in all films, with the activation energy pointing at creep and diffusion mechanisms controlling the fatigue deformation in both microstructures