Initiation and stagnation of room temperature grain coarsening in cyclically strained gold films

Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening

Initiation of fatigue damage in ultra-fine grained thin films: Schmid, Taylor or Hall-Petch?

The evolution of fatigue damage in metallic films is usually described by several subsequent stages. First, the dislocation slip induces formation of slip steps and slip bands. Application of further cyclic mechanical load leads to formation of extrusion/intrusion couples and, finally, crack propagation. Although this general description of the fatigue damage development is well established and can be generally applied to different materials, surprisingly little is known about the very early stage of fatigue damage initiation especially for ultra-fine grained (UFG) and nanocrystalline thin films. From the point of view of classical plasticity theory, the plastic slip should occur first within most favorably oriented grains with the highest resolved shear stress that corresponds to the lowest Taylor factor or the highest Schmid factor. The core question which this presentation will try to address can be formulated as following: is it possible to predict where the fatigue damage will be initiated for a given UFG thin film with a given microstructure? Polymer-supported thin Au and Cu films with UFG microstructures were subjected to cyclic strain with different amplitudes. By means of in-situ resistance measurements the cycle numbers corresponding to the very early stage of damage initiation were deduced. The surface and the microstructure of the films were then analyzed by the scanning electron microscopy and electron backscatter diffraction (EBSD). Please click Additional Files below to see the full abstract

Size effect in polymer-supported ultrathin metallic glass films

Although metallic glasses (MGs) exhibit a unique combination of mechanical and chemical properties, their application as structural or functional materials is hindered by the lack of ductility which leads to catastrophic brittle-like fracture. When the size of a MG sample is reduced below some critical value, typically of the order of a few hundred nanometers, then considerable ductility can be observed. However, this size effect was demonstrated so far mostly by nanomechanical testing inside a transmission electron microscope using samples prepared by focused ion beam (FIB) milling. Whether the ductile-like behavior of submicrometer-sized metallic glasses is a real “intrinsic” size effect or it is rather caused by extrinsic factors like sample shape, ion beam effect or parameters of the testing setup is currently a subject of extensive discussions in the community. In this contribution the tensile properties of thin film Pd82Si18 MGs grown by sputter deposition on a polymer substrate are considered. The integrity of the MG films during stretching was monitored by in-situ measurements of the electrical resistance. An overview of electro-mechanical behavior of considered films is demonstrated in Fig. 1.The 250, 100, and 60 nm thick films fail in a brittle manner at 2% strain through propagation of long cracks perpendicularly to the straining direction. The rapid crack propagation in these films results in rapid increase of in-situ resistance signal. The size effect on the deformation behavior appears when the film thickness drops below 15 nm. The 7 nm thick films with the same composition show a crack-free deformation up to a strain of 7%. Even at higher strains no brittle-like failure but rather short and isolated cracks are observed. Cyclic tensile loading revealed extreme fracture resistance of ultrathin amorphous films showing no cracks after 30000 stretching cycles with a strain amplitude of 3%. Since all tests are performed at ambient conditions on films deposited using an industrially scalable process, the demonstrated size effect can be directly utilized for applications, such as protective coatings, nanoelectromechanical devices or half-transparent conductive layers for flexible electronics. Please click Additional Files below to see the full abstract

Estimation of the in-situ elastic constants of wood pulp fibers in freely dried paper via AFM experiments

Atomic force microscopy-based nanoindentation (AFM-NI) enables characterization of the basic mechanical properties of wood pulp fibers in conditions representative of the state inside a paper sheet. Determination of the mechanical properties under different loads is critical for the success of increasingly advanced computational models to understand, predict and improve the behavior of paper and paperboard. Here, AFM-NI was used to indent fibers transverse to and along the longitudinal axis of the fiber. Indentation moduli and hardness were obtained for relative humidity from 25 % to 75 %. The hardness and the indentation modulus exhibit moisture dependency, decreasing by 75 % and 50 %, respectively, over the range tested. The determined indentation moduli were combined with previous work to estimate the longitudinal and transverse elastic modulus of the fiber wall. Due to the relatively low indentation moduli, the elastic constants are also low compared to values obtained via single fiber testing

Electromigration in Gold Films on Flexible Polyimide Substrates as a Self-healing Mechanism

<p>The study of electromigration (EM) in metallisations for flexible thin film systems has not been a major concern due to low applied current densities in today's flexible electronic devices. However, the trend towards smaller and more powerful devices demands increasing current densities for future applications, making EM a reliability matter. This work investigates EM in 50 nm Au thin films with a 10 nm Cr adhesion layer on a flexible polyimide substrate at high current densities. Results indicate that EM does occur and could be used as a self-healing mechanism for flexible electronics.</p