Coatings for Compliant Substrates

Thursday, May 2, 2013 1:30 PM in Room Sunrise

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1:30 PM F5-1-1 Deformation Domains of Nanostructured Metallic Thin Film onto Polyimide Substrate under Controlled Biaxial Deformation
PierreOlivier Renault, Eric Le Bourhis (University of Poitiers, France); Damien Faurie (University of Paris 13, France); Soundes Djaziri, Philippe Goudeau (University of Poitiers, France); Dominique Thiaudière, Cristian Mocuta (Synchrotron SOLEIL, France); Guillaume Geandier (University of Lorraine, France)

This paper reports on the mechanical behaviour of nanostructured W/Cu thin films deposited on a polyimide substrate under controlled biaxial loadings thanks to a biaxial testing device developed on DiffAbs beamline at SOLEIL synchrotron (Saint-Aubin, France). The elastic-plastic-failure behaviour of the composite metallic film - polymeric substrate can be investigated under equi-biaxial and non-equi-biaxial loading conditions. The in-situ tensile tests were carried out combining synchrotron X-ray diffraction (XRD) and digital-image correlation (DIC) techniques. The combination of these two techniques can accurately measure deformations at two different scales, namely the in- grain scale and the macroscopic scale. The results show that the two strain measurements, i.e. lattice strain in the crystalline part of the W component of the film measured by XRD and macroscopic strain in the substrate measured by DIC, match to within 1x10-4 in the linear elastic domain. This result clearly demonstrates that the applied strain in the elastic domain is transmitted unchanged through the film-substrate interface, and thus the W/Cu thin film elaborated by magnetron sputtering exhibit a good adhesion to the polymeric substrate without adhesion layer. The second part of the paper deals with higher strains response under equi-biaxial tensile tests. The elastic limit of the nanostructured W/Cu thin films was determined at the bifurcation point between the XRD lattice strain and the DIC macroscopic strain. Deformation mechanisms such as film fragmentation are proposed.

1:50 PM F5-1-2 Stress Measurement in Thin Films: Micro-focus Synchrotron X-ray Diffraction Combined with Focused Ion Beam Patterning for do Evaluation
Nikolaos Baimpas (University of Oxford, UK); Eric Le Bourhis (Université de Poitiers, France); Sophie Eve (ENSICAEN, CRISMAT, France); Dominique Thiaudière (Synchrotron SOLEIL, France); Christopher Hardie, Alexander Korsunky (University of Oxford, UK)

Nanocrystalline metallic coatings of sub-micron thickness are widely used in modern microelectronic applications. Their deformation properties differ considerably from those of thicker and more coarse-grained counterparts, due to obstacles to dislocation motion presented by the grain boundaries, and the proximity of the free surface and the interface.

In X-ray diffraction experiments to determine both the residual and ‘live’ stresses in nanocrystalline coatings, one difficult challange that comes up invariably is the determination of the strain-free lattice spacing do. The present study addresses this challenge as described below.

Previously, detailed experimental analysis by Digital Image Correlation and Finite Element modelling have been used to demonstrate that Focused Ion Beam (FIB) ring-core drilling can produce full stress relief of circular and rectangular “islands” [1]. We used this approach to generate a built-in strain-free reference by patterning a 50×50μm2 region of the coating by FIB milling to produce an array of small stress-relieved “islands” ~0.4×0.4μm2 each.

Transmission X-ray diffraction setup was used for data collection at DIFFABS beamline (Synchrotron Soleil, France). A 400nm-thick nanocrystalline gold coating on PMMA substrate [2] and a 240nm-thick multi-layered W-Cu nano-composite thin films on Kapton substrate [3] were studied. The samples were loaded incrementally using a compact uniaxial loading device, and micro-beam diffraction data were collected on and away from the reference array. It was shown experimentally that the “island” array remained strain free throughout the experiment, providing an on-board do lattice spacing reference. The changing lattice spacing d in the coating was also monitored away from the array, to deduce the elastic strain evolution during deformation. The results and their implications are presented and discussed.

[1] Korsunsky, A.M., (2009), Materials Letters, Vol. 63, p.1961-1963.

[2] Girault, B. et al. (2011), J. of Applied Physics, (109) 014305.

[3] Eve, S. et al. (2011), Int. J. Theoretical and Applied Multiscale Mechanics, Vol. 2, No. 1, p.38–45.

Keywords: gold thin film, W-Cu, polymer substrate, stress relaxation, Synchrotron X-ray Diffraction, Focused Ion Beam

2:10 PM F5-1-3 Wrinkling and Delamination of Thin Films on Compliant Substrates
Rui Huang (University of Texas at Austin, US)
When the surface of a soft material is coated with a stiff thin film, compression induced buckling instability of the thin film results in surface wrinkles, and wrinkling of the thin film may lead to fracture and delamination. In this talk, I will present a study on the mechanics of wrinkling and buckle-delamination for an elastic film on a very compliant substrate. First, with no delamination to begin with, an analytical solution is developed to predict onset of wrinkling, which takes into account the effect of Poisson’s ratio of the substrate. In comparison with a nonlinear finite element analysis, an approximate formula is derived to estimate the normal traction at the interface and to predict initiation of wrinkle-induced interfacial delamination. Next, with a pre-existing delamination, the critical strain for onset of buckling instability is predicted by finite element analysis, showing a smooth transition from wrinkling to buckle-delamination. For an intermediate delamination size, the critical compressive strain is found to be lower than previous solutions for both wrinkling and buckle-delamination. Post-buckling analysis by the finite element method shows a significant shear-lag effect with an effective load transfer length over three orders of magnitude greater than the film thickness. Finally, concomitant wrinkling and buckle-delamination is simulated to illustrate the interaction between the two buckling modes, and the results are discussed in view of predicting failure mechanisms as well as other applications of thin film materials.
2:50 PM F5-1-5 Interfacial Failure in a Model Polymer-metal Thin Film Structure
Raymond Friddle, David Reedy, Edmundo Corona, David Adams (Sandia National Laboratories, US); Marian Kennedy (Clemson University, US); Megan (M.) Cordill (University of Leoben, Austria); David Bahr (Washington State University, US); Neville Moody (Sandia National Laboratories, US)
Interfaces are the critical feature governing performance of polymer-metal thin film structures where differing properties between adjacent films can induce strong interlaminar normal and shear stresses and catastrophic failure. We are studying these effects in a model system created by spin coating PMMA films with thicknesses ranging from 10nm to 650nm onto copper coated silicon substrates followed with a sputter deposited overlayer of highly stressed tungsten. The high film stresses triggered spontaneous delamination and buckling along the PMMA-tungsten interface accompanied by intense deformation in the PMMA layers. In this presentation we will show recent AFM images that convey PMMA plastic deformation varied markedly between each system studied and from model elastic behavior. We will also show our recent AFM and nanoindentation work in understanding the mechanism of failure of the PMMA polymer near the tungsten interface. This work was supported by Sandia National Laboratories through USDOE NNSA under Contract DE-AC04 94AL85000.
3:10 PM F5-1-6 Fatigue-corrosion Behavior of Flexible Optoelectronic Device Electrodes
Theodros Bejitual, Konstantinos Sierros, Darran Cairns (West Virginia University, US)

The electrical, optical, and structural integrity of flexible transparent electrodes is paramount in the design and fabrication of optoelectronic devices, such as organic light emitting diodes, liquid crystal displays, touch panels, solar cells, and solid state lighting applications. The electrodes may corrode due to acrylic acid containing pressure sensitive adhesives. In addition, structural failure may occur due to external applied loading. The combined action of mechanical loading and corrosion can aggravate the failure of the electrodes.

In this study we investigate the effects of acrylic acid concentration, film thickness, number of bending cycles, and applied strain on the electrical and structural integrity of carbon nanotube and indium tin oxide films on polyethylene terephthalate substrates.

In situ electrical resistance measurements are conducted during corrosion, bending, corrosion-bending, fatigue, and fatigue-corrosion experiments in order to determine the crack onset strain. Crack density calculation is performed on images acquired using optical microscopy. In addition, scanning electron microscopy is conducted in order to determine failure mechanisms.

3:30 PM F5-1-7 Load Bearing Capacity of Hydrogenated Amorphous Carbon Coatings on Ultrafine Grained Al Substrates
Christoph Schmid, Christopher Schunk (University of Erlangen-Nürnberg, Germany); Sven Meier (Fraunhofer Institute for Mechanics of Materials, IWM, Germany); Mathias Göken, Karsten Durst (University Erlangen-Nuremberg, Germany)

Thin hydrogenated amorphous carbon (a-C:H) coatings suffer from insufficient load bearing capacity for high load applications when they are deposited on relatively soft substrates like Al-alloys. In this work, the contact damage behaviour of an a-C:H coating system deposited on Al-alloy sheets was investigated by indentations with different indenter geometries and scratch tests. The investigated coating system consists of a silicon rich adhesion layer, an adjacent ramp layer with a graded chemical composition and a 2 µm thick a-C:H top coat deposited by PECVD. Using an accumulative roll bonding (ARB) process the flow stress as well as the grain size of the Al-alloy has been modified. Additionally shot peening was applied to increase the load strength and residual stresses in the Al-alloy sheet. Doing so, the mechanical properties of the substrate were varied systematically without changing its chemistry. The aim of this treatment was to enhance the load bearing capacity of the coating-substrate system. During indentation with a spherical tip, several discontinuities (pop-ins) occurred in each load displacement curve. Using Focused Ion Beam (FIB) cross-sections, these events were attributed to the formation of cracks in the coating allowing for an analysis of the fracture toughness of the a-C:H layer. The pop-in behaviour also depends on the mechanical properties of the substrate and reflects the different load bearing capacities. Finally, the concepts for increasing the load bearing capacity are discussed with the help of FE models.

3:50 PM F5-1-8 Three-dimensional Finite Element Analysis of Adhesive Failure on Coated Systems under Uniaxial Tensile Tests
Newton Fukumasu (University of São Paulo, Brazil); Felipe Silva (Federal University of ABC, Brazil); Roberto Souza (University of São Paulo, Brazil)

Tensile tests are an important tool to improve the understanding of the mechanical behavior of coatings and their interaction with compliant substrates. When submitted to an increasing uniaxial tensile load, the coating presents nucleation and propagation of transversal cracks, up to a critical load in which the space between two consecutive cracks remains predominantly constant. Beyond this load, the stress state in the film, which is affected by the difference in lateral contraction of the film and the substrate, may lead to an adhesive failure and possible spallation of the coating. This work aims improving the understanding of the mechanical behavior of the coating after the saturation of inter-crack spacing and before spallation. A set of 3D finite element analyses was conducted to evaluate the influence of the stress state at the coating/substrate interface on the adhesive failure. Results showed non-uniform stress distributions on film-strips between two consecutive cracks, even when cracks were distributed uniformly. This non-uniformity led to distinct patterns of stress distribution that are function of the mechanical properties of the system components.

4:10 PM F5-1-9 Annealing Induced Structural Evolution and Optical Properties of Block Copolymer Templated Nanostructured Tungsten Oxide Films
Ching-Lin Wu (National Cheng Kung University, Taiwan, Republic of China); Chung-Kwei Lin (Taipei Medical University, Taiwan, Republic of China); Chun-Kai Wang (National Cheng Kung University, Taiwan, Republic of China); Sheng-Chang Wang (Southern Taiwan University of Science and Technology, Taiwan, Republic of China); Jow-Lay Huang (National Cheng Kung University, Taiwan, Republic of China)
The effect of microstructure on the optical and electrochemical properties of nanostructured tungsten oxide films has been evaluated as a function of annealing temperature. The films using block copolymer as the template were prepared from the peroxotungstic acid (PTA) by spin-coating onto the substrate and post-annealed at 200-500 oC to form the tungsten oxide films with nanostructure. The microstructure of the films were measured by the X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The films annealed at temperatures below 300 oC are characterized by amorphous or nanocrystalline structures with the pore size less than 20 nm. The evaluated annealing temperature caused the monoclinic crystalline structure and larger pores. The cyclic voltammetry measurements were performed in the LiClO4-propylene carbonate electrolyte. The results showed that the exchange charges are maximized for films annealed at 200-300 oC and decrease with the increasing of annealing temperature. The electrochromic properties of the nanostructured tungsten oxide films were evaluated simultaneously by potentiostat and UV-Vis spectroscopy. The films annealed at 300 oC exhibit the high transmission modulation (∆T ~ 60 %) at λ= 633 nm and good reversibility for the lithium insertion-extraction process in cycling. As a result, the correlation between the microstructure and electrochemical properties was established, and the electrochromic properties have been demonstrated.