Hydrogen effect on the strain rate sensitivity of nanocrystalline Pd thin films

Nanocrystalline (nc) materials are used in a large variety of micro- and nanoscale devices and sensors. The reliability of these devices requires maintaining adequate mechanical integrity of structural elements over time. This represents a real challenge due to their far from equilibrium microstructure. The stability of nanocrystalline materials is usually addressed in the literature in terms of thermal stability of a microstructure under heating or under severe plastic deformation. In this study, the mechanical stability will be evaluated under environmental perturbation using the rate sensitivity as a marker of stability. In the context of hydrogen technologies requiring sensors and catalyst applications, palladium in hydrogen environment is a good model system for investigating reliability issues. The objective of this work is thus to relate the hydrogen-induced microstructural changes in nc Pd thin film to its rate sensitivity, the latter being a signature of the viscoplastic mechanisms taking place. Pd thin films are characterized experimentally under different hydriding environment. The evolution of residual stress by defect recovery and creep is analyzed. On-chip microtensile testing technique and nanoindentation, coupled with advanced TEM microscopy, are used to quantify and understand the level of rate sensitivity of the specimens as well as the microstructural effects of hydrogen absorption. Finally, the mechanical perturbation applied via the indenter or using internal on-chip actuation of the material is evaluated and compared with respect to the different hydriding conditions.