Thin films are popular objects for investigating mechanical size effects. Most of the knowledge about freestanding films has been gathered from bulge tests and concerns the increase of the strength with decreasing film thickness.
In Erlangen, this technique was further developed in order to allow the investigation of the fracture, time-dependent, and fatigue properties of thin films. Experiments were performed both ex-situ to provide accurate quantitative measurements, and in-situ in an AFM to qualitatively investigate the corresponding mechanisms.
The new methods were most notably applied to PVD gold films with thicknesses ranging from 30 to 400 nm. These films exhibited strong size effects. In detail, different behaviors were evidenced for freestanding films and films supported by a SiNxsubstrate. The freestanding gold specimens showed a dramatic decrease of their fracture toughness, a large increase of their strain-rate sensitivity, and a strong ratcheting at low film thickness. Beside the nanocrystalline microstructure, a likely explanation for this is the grain boundary sliding repeatedly observed at the surface during all kinds of tests. Grain boundary sliding most likely occurs in these films because of their columnar microstructure, which results in a low geometrical constraint on the grains for out-of-plane displacement.
Gold films on SiNx exhibited an even higher strength than their freestanding counterparts. Their strain-rate sensitivity was on the opposite much lower, and seemed exclusively dependent on their nanocrystalline microstructure. The effects occurring during the creep, strain-rate jump, and fatigue tests were weaker than for freestanding films, presumably because the good adhesion to the substrate suppresses grain boundary sliding in these films.