Mechanical Properties and Adhesion

Monday, April 29, 2013 10:00 AM in Room Golden West

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10:00 AM E2-1-1 Time Resolved Synchrotron X-ray Strain Measurement in Biaxially Loaded Au Thin Films
Damien Faurie (LSPM-CNRS, Université Paris 13, Sorbonne Paris-Cité, France); PierreOlivier Renault (Institut P' - Universite de Poitiers, France); Guillaume Geandier (Institut Jean Lamour, France); Eric Le Bourhis (Institut P' - Universite de Poitiers, France); Cristian Mocuta, Dominique Thiaudière (Soleil Synchrotron, France)

Synchrotron x-ray radiation was used for in situ strain measurements in gold films on polyimide substrate during biaxial deformation tests. We have used an area detector that allows inspecting multiple directions in the polycrystalline thin film without serial sectioning during straining. We show in this paper the configuration used and the attainable orientations on a pole figure for which the x-ray strains are measured. Moreover, we show how to detect the onset on plasticity, by comparing x-ray strains and macroscopic strains measured by Digital Image Correlation.

This experimental Setup offers several possibilities that will be described in this talk:

(i) Continuous biaxial tensile tests

(ii) Continuous biaxial cyclic tests

(iii) Complete strain pole figure measurements during biaxial loading steps

A few examples of studies about elastic-plastic behavior of Au thin films with thicknesses below 100 nm will be shown.

10:20 AM E2-1-2 Grain Growth in Nanocrystalline Copper During Indentation at Very Low Temperatures
Corbett Battaile, Brad Boyce, Stephen Foiles, Khalid Hattar (Sandia National Laboratories, US); Elizabeth Holm (Carnegie Mellon University, US); Eric Homer (Brigham Young University, US); Henry Padilla, Garritt Tucker (Sandia National Laboratories, US)
The properties of most engineering materials are strongly influenced by the characteristics of their internal structures. A material's internal structure greatly affected not only by thermal and/or mechanical processing during fabrication, but also during service or storage through the influence of temperature or stress. Temperature influences a material's evolution partly through the modification of the migration kinetics of its internal interfaces, and stress does so by storing energy in the material inhomogeneously. Nanocrystalline metals exhibit properties that are advantageous to a wide variety of applications, but the relatively high energy (per unit volume) and curvature of the internal interfaces render these materials relatively unstable. Thus, it is important that we achieve an understanding, and thus enhance our ability to control, the microstructural stability of nanocrystalline metals. In this presentation, we will discuss experiments and simulations on grain growth in pure, nanocrystalline copper. Vickers indentation of thin films at various temperatures - including 4K, 77K, and 273K - demonstrates that substantial grain growth can occur even at very low temperatures. Precession TEM characterization and in-situ TEM cryo-indentation help elucidate the nature and character of the interfaces responsible for this anomaly. Molecular dynamics simulations of the migration of individual grain boundaries suggest that the mobilities of some special boundaries can increase with decreasing temperature, contrary to conventional wisdom. Coupled continuum simulations of grain growth and mechanical deformation demonstrate that the acceleration of grain growth at low temperatures might be explained by the mitigation of plastic deformation as a stress relief mechanism.

10:40 AM E2-1-3 Inhomogeneous Stresses, Texture Transformations and Anomalous Grain Growth in Thin Metal Films
Shefford Baker (Cornell University, US)
Thin metal films on substrates often form fiber textures during deposition and processing. Since film properties depend strongly on texture, it is important to be able to predict what texture components will form. However, existing models are incomplete. For example, a simple thermodynamic model predicts that FCC films should form (111) texture to minimize interface energy when sufficiently thin, and (100) texture to minimize strain energy when sufficiently thick. While this texture trend is observed experimentally, evidence suggests that the driving forces are not so simple—e.g. films transform even when no substrate is present to provide the requisite strain energy. We have developed a novel high-throughput test and used it to study the thermodynamics and kinetics of texture transformations. Up to 100 samples with a range of thicknesses and different interface energies are produced in a single deposition run, eliminating variations due to fluctuating impurity levels. Texture and stress levels are determined using x-ray diffraction and grain structure is characterized by EBSD. We find that texture transformation occurs by anomalous grain growth, resulting in stable, thickness-dependent mixed (111)/(100) textures over a wide range of thicknesses. The final stress state does not correlate well with the strain energy, or the presence or absence of an adhesion layer. Transformation kinetics are strongly thickness dependent, with maximum transformation rates occurring at intermediate film thicknesses. These results suggest that variations in initial nucleus density with film thickness control the transformation kinetics. An explanation based on inhomogeneous stresses is proposed to account for the thickness-dependent mixed texture.
11:20 AM E2-1-5 Microstructure and Mechanical Properties of Nanodiamond Enhanced Diamond-like Carbon Thin Films on Ti Alloys
Chunzi Zhang, Hamid Niakan, Lezhi Yang, Yuanshi Li, Qiaoqin Yang (University of Saskatchewan, Canada)

Diamond nanoparticles (DNP) have been proven to be effective in enhancing adhesion between DLC thin film and Ti6Al4V substrate. In this research, the effect of DNP density on the adhesion and mechanical properties of DLC on Ti alloy were investigated in order to optimize the conditions. Initially, DNP with different density from separate particles to semi-continuous thin film were deposited on Ti6Al4V substrates by microwave plasma assisted chemical vapor deposition. A DLC thin film was then deposited on them by direct ion beam deposition. Scanning electron microscopy, Atomic force microscopy, Raman spectroscopy, synchrotron near-edge X-ray absorption fine structure, Nano analyzer and Rockwell indentation were used to evaluate the microstructure, mechanical properties and adhesion of the deposited films. Results show that the density of DNP has significant effect on the adhesion and other mechanical properties of the films: higher density resulted in higher adhesion, higher hardness and lower friction coefficient.

11:40 AM E2-1-6 Residual Stress Analysis in Thin Films using Focused Ion Beam and Digital Image Correlation - Stress Analysis by Raman Spectroscopy on Diamond Films
Furqan Ahmed, Markus Krottenthaler, Christoph Schmid, Karsten Durst (University Erlangen-Nuremberg, Germany)
The residual stresses in thin films are caused by thermomechanical mismatch and deposition process. These stresses affect the in-service mechanical performance and can reduce the lifetime of a coated component. The analysis and control of residual stresses is important for understanding the fracture and delamination behaviour of the coatings and to improve the adhesion.

In this work, crystalline diamond thin films on titanium substrate were used, which in general have compressive residual stresses after deposition. In order to evaluate these stresses at sub-micron scale, a semi-destructive trench cutting method based on focused ion beam (FIB) milling was employed. Using FIB tool, some rectangular bars were milled with trenches along the longer sides. These trenches introduced the strain relief in the coating perpendicular to the longer sides of the bar. To evaluate the strain change in film by digital image correlation (DIC), high resolution images of the concerned area were recorded with FIB microscope before and after the milling. The displacement produced in the film was determined with DIC and plotted against pixel positions on the image to determine the strain change. To get the magnitude of residual stress from this strain change, FIB-milled bar was modelled using finite element method (FEM) in three dimensions. A residual stress of -5 GPa (determined with Raman spectroscopy) was used as input material property for the coating. The resulting strain change measured on the simulated bar was very close to the DIC value. This result validated the stress value measured by Raman spectroscopy. Afterwards, the FIB-milled bars were scanned with micro-Raman spectroscopy to analyse the stress relief along the edges of the bar. The results showed a full relaxation of compressive stress from ~ -5 GPa to zero value. Furthermore, the stress profiles made with Raman spectroscopy, along the edges of the milled bar, were compared to the FEM based stress profiles and these showed the similar stress relaxation trend.