Multi-scale analysis of hydrogen-induced buckling in Pd films

Hydrogen loading causes a significant volume expansion, which is isotropic in free-standing bulk materials. Contrary to bulk samples, thin films are clamped to an elastically stiff substrate, which prevents in-plane expansion. Hence, volume expansion of a thin film is strongly anisotropic because it expands only in the out-of-plane direction. High internal stresses introduced during hydrogen loading may lead to a situation when detachment of film from the substrate is energetically favorable. In the present work, we studied hydrogen-induced buckling of thin Pd films using a multi-scale approach. Defects in buckled films were characterized on the atomic level by positron annihilation spectroscopy combined with microstructure studies by transmission electron microscopy. Meso-scale measurements were performed by acoustics emission. Observations at the macroscopic level were performed by optical microscopy. It was found that buckling of thin films occurs at hydrogen concentrations xH>0.1. Defect studies of buckled Pd films revealed a significant increase of dislocation density in agreement with acoustic emission studies which demonstrated a correlated movement of dislocations with a well-defined threshold coinciding with the onset of buckling.