Anelasticity and scaling effects in thin metal films
E.Bonetti and E.G.Campari*
Dipartimento di Fisica e Astronomia Università di Bologna
Recent research on nanometer-submicrometer thin films demonstrate that in the low strain amplitude anelastic regime, scaling effects on mechanical properties can be observed.
The significant deviation of these properties from the ones of bulk samples have strong implications in technologies utilizing components in the sub-micrometer scale. Examples include applications based both on thin metal films free standing or supported on rigid substrates. In particular MEMS technologies are increasingly oriented to devices requiring the employment of structural films, with thickness of the order of microns, to increase performances in term of useful levels of force and power.
Among the different experimental methods to investigate time dependent elastic behavior of materials, mechanical spectroscopy (MS), offer specific advantages in comparison to tensile experiments.
In the present communication, after a brief overview of mechanical spectroscopy applied to the investigations of thin films, some preliminary results, obtained on polycrystalline pure Aluminum films prepared with different methods (sputtering on rigid substates; multiple rolling; nanoparticles consolidation), are presented.
The resuts obtained by measurements of the damping factor (internal friction ) and the dynamic elastic modulus clearly show an anelastic behavior strongly different from the one of polycrystalline bulk samples.
Two factors are responsible for the observed behavior: the small grain size, of the order of the film thickness; the presence of the substrate in supported thin films.
Some of the unexpected results such as the very low values of the elastic moduli in free standing thin films and the anelastic relaxation due to grain boundary sliding in supported thin films are discussed on the hypothesis of a dislocations and grain boundaries constrained dynamics.