Deformation behavior of micron‐sized polycrystalline gold particles studied by in situ compression experiments and frictional finite element simulation
Authors: Romeis Stefan , Paul Jonas , Herre Patrick , Peukert Wolfgang 
Published: December 2016

Gold particles in the micro‐ and nanometer size regime find wide spread technical applications. Especially for micro‐ and nanoelectromechanical applications the mechanical properties can determine device reliability and overall performance. Whereas studies on the compression of single crystal gold particles are quite abundant [1–3], the detailed deformation behavior of polycrystalline gold or other fcc metal particles is far less understood [4–5]. It has been shown for that the hardness of gold particles varies with strain and depends on the particle size: whereas hardness decreases for submicron particles (geometric softening) an increase in hardness is commonly observed for millimeter sized samples (work hardening). Geometric softening is driven by an ongoing geometric shape change during stressing and occurs for work hardened samples. Also an increase of yield strength has been observed: this effect is related to the particles' small dimensions and restricted dislocation activities. Particularly for metal particles it is known that the initial deformations are concentrated in proximity to the contact areas; only with increasing strain the deformed regions progresses deeper into the particles. Friction at the contact interfaces thus affects the plastic deformation inside the particles. In general frictional processes at the contact interfaces are difficult to access ‐ one possibility is the finite element method (FEM). So far mainly full stick (infinite coefficient of friction) or perfect slip (frictionless contacts) conditions have been modeled and the effect of friction on the deformation behavior of gold particles has not seen significant attention.

Within this contribution a combined experimental and finite element study on the deformation behavior of micron‐sized polycrystalline gold particles is presented [6]. In situ uniaxial compression experiments of single spherical polycrystalline gold particles in the size range of 1 µm were performed with an in situ scanning electron microscope supported custom built manipulation device [7]: stress‐strain data and information on particle morphology are thus accessible. A well reproducible stress‐strain behavior without plastic creep is observed. From the FEM modelling a detailed insight into the deformation and the influence of friction is obtained. The stress‐strain behavior and the observed geometric shape of the stressed particles can be modeled by an elastic‐perfectly plastic finite element model which accounts for frictional effects at the contact interfaces. Coefficients of friction are experimentally assessed by atomic force microscopy. A comparison to a frictionless finite element model reveals the necessity of considering the effects of friction: at small strains the particles appear to be softer due to a reduced dissipation of plastic energy, whilst at large strains the resistance to deformation is increased. The latter effect is found to be mainly due to the dissipation of frictional energy at the contact interfaces.