| AVS 64th International Symposium & Exhibition | |
| Nanometer-scale Science and Technology Division | Thursday Sessions |
| Session NS+SP+SS-ThA |
| Session: | Advances in Scanning Probe Microscopy |
| Presenter: | Celia Polop, Universidad Autónoma de Madrid, Spain |
| Authors: | C. Polop, Universidad Autónoma de Madrid, Spain E. Vasco, Instituto de Ciencia de Materiales de Madrid, CSIC, Spain A. Perrino, Instituto de Ciencia de Materiales de Madrid, CSIC, Spain R. Garcia, Instituto de Ciencia de Materiales de Madrid, CSIC, Spain |
| Correspondent: | Click to Email |
From aircraft to electronic devices, and even in Formula One cars, stress is the main cause of degraded material performance and mechanical failure in applications incorporating thin films and coatings. Over the last two decades, the mechanisms responsible for stress generation during film deposition and processing have generated intense conjecture and scientific activity. However, no consensus has been reached so far. The main difficulty is that current models of stress generation, most of which are atomistic in nature, are only supported by data with at best sub-micron resolutions. For example, techniques such as curvature-based measurements, Raman spectroscopy, and x-ray diffraction cannot reveal the stress distribution in films on nanometer scales.
Here, we present a novel method for mapping the stress at the surface of polycrystals with sub-10 nm spatial resolution. This method consists of transforming elastic modulus maps measured by atomic force microscopy (AFM) techniques, such as force modulation method and bimodal AFM, into stress maps via the local stress-stiffening effect. The validity of this approach is supported by Finite Element Modeling simulations. By applying the method to Au polycrystalline films, we show that the intrinsic stress is heterogeneously distributed along the grain diameter, being concentrated in narrow strips adjacent and parallel to the grain boundaries (not directed inside the grain boundary, as is usually assumed). Stress gradients as intense as 100 MPa/nm are detected in these regions. Note that these gradients, which are undetectable by the standard techniques and tests used for stress analysis, are in the order of magnitude of the mechanical strengths required for many applications. The heterogeneous spatial distribution of the intrinsic stress along the grain diameter is the result of the Mullins-type surface diffusion towards the grain boundaries, and would be the probable cause of the kinetic compression that appears in polycrystals under conditions of high atomic mobility. Consequently, we demonstrate that the nanoscale stress mapping has great potential to disclose the nature and origin of the stress in solids.
[1] C. Polop, E. Vasco, A. P. Perrino and R. Garcia, “Mapping stress in polycrystals with sub-10 nm spatial resolution”, submitted.
[2] E. Vasco, C. Polop, “The compressive intrinsic stress in polycrystals is not inside the grain boundary”, submitted.