Mechanical spectroscopy of nanocrystalline aluminum films: effects of frequency and grain size on internal friction

Energy dissipation by internal friction is a property of fundamental interest for probing the effects of scale on mechanical behavior in nanocrystalline metallic films and for guiding the use of these materials in the design of high-Q micro/nanomechanical resonators. This paper describes an experimental study to measure the effects of frequency, annealing and grain size on internal friction at room temperature in sputter-deposited nanocrystalline aluminum films with thicknesses ranging from 60 to 120 nm. Internal friction was measured using a single-crystal silicon microcantilever platform that calibrates dissipation against the fundamental limits of thermoelastic damping. Internal friction was a weak function of frequency, reducing only by a factor of two over three decades of frequency (70 Hz to 44 kHz). Annealing led to significant grain growth and the average grain size of 100 nm thick films increased from 90 to 390 nm after annealing for 1 h at 450?(°)C. This increase in grain size was accompanied by a decrease in internal friction from 0.05 to 0.02. Taken together, these results suggest that grain-boundary sliding, characterized by a spectrum of relaxation times, contributes to internal friction in these films.