Advanced Characterization of Coatings and Thin Films

Thursday, May 2, 2013 8:00 AM in Room Royal Palm 1-3

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8:00 AM TS2-1-1 Correlative Analysis of Phase and Microstructural Evolution of Rapidly Solidified Metallic Multilayers by Transmission Electron Microscopy and Atom Probe Tomography
Peter Leibenguth, Isabella Schramm, Frank Mücklich (Saarland University and Materials Engineering Center Saarland, Germany)

Understanding the processes taking place at the interfaces of heat treated multilayered thin films requires high spatially resolved information on local chemical composition and phases present. By this, the explanation of the microstructural modifications resulting from the treatment can be facilitated. Especially the combination of structural and compositional data on the submicron and nano-scale can yield valuable insights. To this end, correlating selected area diffraction in a transmission electron microscope (TEM) and near-atomically resolved atom probe tomography (APT) is a powerful approach for such examinations.

Our study is focused on demonstrating the benefits of the combined use of these complementary techniques in elucidating the microstructural modification of binary metallic multilayers induced by ns-pulsed-laser treatment (Laser Interference Metallurgy). As material combination, technically and thermodynamically well-known binary systems, namely Ni/Al, Ti/Al and Ni/Ti were chosen.

Depending on substrate material and local stoichiometry in the heat affected zones, different crystalline and amorphous metastable phases were formed and could be kept frozen-in to room temperature. Even long-range ordered intermetallics were observed, which is in accordance with literature on pulsed-laser induced rapid solidification and partitionless transformations from the melt. The combination of site-specific crystallographic analyses by TEM and compositional data gained by APT with finite element based thermal simulations proved useful in clarifying the processes of phase selection and microstructure formation during the pulsed laser processing of multilayered thin films.

8:20 AM TS2-1-2 In situ Transmission Electron Microscopy Studies of Metal Diffusion on Ceramic Coatings
Isabelle Jouanny, Chilan Ngo (University of California, Los Angeles, US); Justinas Palisaitis (Linköping University, Sweden); Paul Mayrhofer (Vienna University of Technology, Austria); Lars Hultman, Per Persson (Linköping University, Sweden); Suneel Kodambaka (University of California, Los Angeles, US)
Using in situ transmission electron microscopy (TEM), we investigated the diffusion kinetics of metals on ceramic coatings. All our experiments were carried out in 200 kV CM20 and 300 kV Titan3 TEMs using sputter-deposited polycrystalline, 50-nm-thick, ZrB2 films on Al2O3(0001). A cross-sectional TEM sample was prepared via focused ion beam (FIB) milling using Ga ions. In this process, an amorphous carbon film is first deposited on the ZrB2 surface to serve as a protective layer and the sample is attached to a molybdenum TEM grid by depositing Pt at one end of the sample. During annealing at temperatures < 500 oC, we observed the formation and Ostwald ripening of liquid Ga droplets, formed due to the FIB, on the surface of carbon layer. From the measured rates of coarsening/decay of Ga droplets, we determine that surface diffusion is the rate-limiting mechanism. At temperatures > 800 oC, we observed a diffusion front moving unidirectionally along the ZrB2 film from the end attached to the TEM grid. Using energy dispersive spectroscopy, we identify the diffusing material to be a Pt0.95Mo0.05 alloy, presumably formed due to the intermixing of the FIB-deposited Pt and the Mo from the TEM grid. We followed the diffusion of this alloy front as a function of annealing time and temperature. From the measured rates of diffusion as a function of temperature, we extract an activation barrier of 3.8±0.5 eV.
8:40 AM TS2-1-3 Advanced Transmission Electron Microscopy Methods: Going beyond Imaging
Christina Scheu (LMU Munich, Germany)

Advanced transmission electron microscopy (TEM) is a powerful tool to investigate thin films and coatings where the grain size, type of phases present and their spatial distribution play an important role and where the constituents often possess nanometer dimensions. In addition, such nanostructured materials possess numerous interfaces which determine the functionality and which require atomic scale characterization.

High-resolution TEM (HRTEM) and so called Z-contrast images (Z stands for the atomic number) using a scanning TEM (STEM) allow to study the atomic structure of nanometer-sized grains and interfaces and these are well established methods in thin film and nanostructured materials analysis. However, besides the atomic arrangement, the chemical composition of individual nanometer-sized grains and interfaces is of great interest and this can be studied by analytical TEM measurements such as energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS). In addition to the chemical composition, EELS measurements can also be used to determine optical properties and to get insight into the electronic structure down to the nanometer regime or even below. These information are obtained by analyzing the spectral features occurring in the low-loss region (up to energy-losses of around 50 eV) or with the help of the element-specific ionization edges which are found in the core-loss region (above 50 eV). To investigate locally the optical properties such as the dielectric function or the band gap, a high energy resolution is required in the low loss region which can be realized by using a monochromator or advanced deconvolution methods. The ionization edges in the core-loss EELS region can be used to determine both, the chemical composition and the electronic structure of interfaces and nanostructures. This latter information is obtained by analyzing the electron energy-loss near-edge structure which is associated with each element-specific edge and which contains information on e.g. bonding characteristics and nominal oxidation states of the probed atoms. The data can be interpreted applying a fingerprint method or by performing ab-inito calculations.

In this talk, examples for the different techniques will be presented and discussed, which show that advanced TEM studies can provided information beyond imaging.

9:20 AM TS2-1-5 In Situ Transmission Electron Microscopy Studies of Thermochemical Stability of TiO2/C Core/Shell Nanocrystals
Isabelle Jouanny, Suneel Kodambaka (University of California, Los Angeles, US)
We studied the thermal stability and the effect of electron beam irradiation during annealing of TiO2/C core/shell nanocrystals up to 900 °C by in situ transmission electron microscopy (TEM). All of our experiments were carried out in a FEI Titan 300 kV S/TEM using rutile-structured TiO2 nanocrystals encapsulated in 5-10 nm thick carbon shells. Upon heating, we observed several interesting phenomena. At temperatures above 600 °C, we find that the carbon shells become more graphitic; the graphene layers become highly-ordered and appear to align along the surface facets of the TiO2 cores. The shape and size of the TiO2 cores change with increasing time and temperature. At temperatures above 800° C, we observed the formation of hollow core graphitic shells due to the disappearance of the TiO2 particles. Our observations indicate that the core/shell structure and composition are sensitive to electron beam dose and annealing temperature. In our experiments, electron irradiation at doses above 105 A/m2 induce damage in the core/shell particles at room temperature. At temperatures above 600 °C, 105 A/m2 dosage leads to hollow core formation. At doses > 1.15.106 A/m2, carbon shells reconfigure into onion structures, consistent with previous reports.
9:40 AM TS2-1-6 Evaluation of Laboratory and Synchrotron Nanobeam X-Ray Diffraction Methods for the Characterization of Residual Stress Gradients in Hard Coatings
Mario Stefenelli (Materials Center Leoben Forschung GmbH, Austria); Rostislav Daniel (Montanuniversität Leoben, Austria); Angelika Riedl (Materials Center Leoben Forschung GmbH, Austria); Matthias Bartosik (Montanuniversität Leoben, Austria); Manfred Burghammer (European Synchrotron Radiation Facility, France); Christian Mitterer, Jozef Keckes (Montanuniversität Leoben, Austria)

Nanocrystalline hard coatings usually exhibit pronounced depth gradients of microstructure and residual stress which decisively predefine their function and properties. The residual stress gradients are routinely evaluated using laboratory X-ray diffraction techniques in reflection geometry utilizing varying penetration depths of the X-ray beam, thus generating a stress profile in Laplace space. Inverse Laplace transformation is applied to refine unknown strain profiles in real space. Recently, however, a new synchrotron approach of stress gradient characterization based on cross-sectional synchrotron X-ray nanobeam diffraction was proposed [1]. The new approach allows the stress characterization directly in the real space. In this contribution, both laboratory and the new synchrotron approaches are evaluated when analysing monotonous and non-monotonous residual stress depth gradients in representative hard coatings based on TiN and CrN. The results reveal that the approaches based on the Laplace transformation are limited to a relatively simple stress depth profiles. On the other hand, the X-ray nanobeam approach provides excellent resolution even for very complex stress profiles, but is connected with more experimental effort and sophisticated data treatment.

[1] J. Keckes, M. Bartosik, R. Daniel, C. Mitterer, G. Maier, W. Ecker, J. Vila-Comamala, C. David, S. Schoeder, M. Burghammer, X-ray nanodiffraction reveals strain and microstructure evolution in nanocrystalline thin films, Scripta Mat. 67 (2012) 748.

10:00 AM TS2-1-7 Cross-Sectional X-ray Nanodiffraction on a Graded Multiphase Cr-N Thin Film
Matthias Bartosik (Christian Doppler Laboratory for Application Oriented Coating Development at Montanuniversitat Leoben and Vienna University of Technology, Austria); Jozef Keckes, Rostislav Daniel, Christian Mitterer (Montanuniversität Leoben, Austria); Manfred Burghammer (European Synchrotron Radiation Facility, France); Liangcai Zhou (Vienna University of Technology and Montanuniversität Leoben, Austria); David Holec (Montanuniversität Leoben, Austria); Paul Mayrhofer (Vienna University of Technology, Austria)
Wide angle X-ray nanodiffraction performed along the cross-section of nano-crystalline thin films allows for in-depth characterization of microstructure and residual strain with submicron resolution along the film-growth direction [1]. The position-resolved analysis is performed using synchrotron X-ray nanobeams with diameters focused down to 100 nm.

Here, we demonstrate that the nanodiffraction approach opens the possibility to analyze not only single-phase but also complex multiphase coatings with gradients in phases, microstructure and strain at the sub-micron scale along the growth direction when using cross-sectional investigations. The new position resolved technique [1] allows for a simultaneous evaluation of depth gradients of X-ray elastic strain, lattice parameters, microstructure and phases as performed and demonstrated on 6 µm thick Cr-N films consisting of hexagonal Cr2N and nonstoichiometric cubic CrNx. The studies are furthermore corroborated by ab inito investigations. Cross-section transmission and scanning electron microscopy were additionally applied for comparison and complementation of the results obtained from the nanodiffraction experiments. The results reveal how growth conditions and chemical composition correlate with structural gradients. The new approach represents a significant step forward in depth-resolved microstructural characterization of thin films.

[1] J. Keckes, M. Bartosik, R. Daniel, C. Mitterer, G. Maier, W. Ecker, J. Vila-Comamala, C. David, S. Schoeder, M. Burghammer, X-ray nanodiffraction reveals strain and microstructure evolution in nanocrystalline thin films, Scripta Mater. 67 (2012) 748.

10:20 AM TS2-1-8 Smart Approach of Surface Characterizations of Engineered Diamond-like Carbon (DLC) Coatings
Daniela Caschera, Barbara Cortese, Giuseppe Gigli, Alessio Mezzi, Marco Brucale, Gabriel Ingo, Tilde De Caro, Giuseppina Padelletti (CNR, Italy)

The ability to control the specific wettability, mechanical stability and bio-inertness of Diamond-like Carbon (DLC) films is of increasing awareness in both academic sciences and industrial technologies. Tuning of growth parameters and incorporated hydrogen and oxygen, influences the amount of sp2/sp3 electronic hybridizations imparting attractive properties such as extreme hardness, low friction coefficients and high corrosion resistance. We report the deposition of DLC films by Plasma Enhanced Chemical Vapour Deposition (PECVD) on different substrates. The influence of different precursors plasma pre-treatments of H2, Ar or O2 on the properties of DLC coatings is evaluated and analysed in terms of structure and mechanical properties. The electronic configuration of the DLC specimen are investigated combining Raman and XPS measurements. The I(D)/I(G) ratio, the disp(G) and the FWHM of G band have been calculated using a micro-Raman apparatus. The sp2/sp3 ratio was calculated using the first derivative of C KLL spectra by means of an ESCALAB apparatus. Contact angle (CA) measurements were measured and compared. Based on roughness measurements, the differences in the pre-treatment of the substrate showed an influence on the hydrophilic/hydrophobic behaviour as well as mechanical properties of the DLC functionalized substrates. Additionally, the use of the different reactive elements in the pre treatment coating was investigated, demonstrating the flexibility and viability of this low cost coating concept.

10:40 AM TS2-1-9 Multi-scale Residual Stress Analysis of AlN on (100)Si Substrate Deposited at Different Biases
Marco Renzelli, Edoardo Bemporad, Marco Sebastiani (University "Roma Tre" Rome, Italy)
The aim of the present work was to investigate the effect of substrate bias on AlN residual stress state at the micro and macro scale.Reactive magnetron sputtering with ion plating was used to coat silicon substrates with AlN. The applied bias on the substrates was changed in order to obtain different residual stress levels in the resulting film due to ion pinning of the growing surface. Several techniques have been used like wafer curvature method, TEM investigation, XrD and a recently developed focused ion beam (FIB) micron-scale ring-core method [1]. The effect of residual stress has been compared with mechanical properties such as adhesion and apparent elastic modulus by micro scratch, nano scratch and nanoindentation.In this way, the correlation between the micro- and macro-stress distributions with the adhesion of the coatings can be quantitatively assessed and related to the observed microstructure and growth mechanisms. The results clearly showed an effect of the surface defects and other microstructural features on the local residual stress field, thus confirming that a structured information can be achieved by the use of a multi-scale approach for residual stress assessment.A correlation between the macro and micro-stress in the coatings with the adhesion and observed failure modes during scratch is finally proposed. [1] Sebastiani, M., Eberl, C., Bemporad, E., Pharr, G.M. (2011) Materials Science and Engineering A, 528 (27), pp. 7901-7908.
11:00 AM TS2-1-10 Focused Ion Beam Milling for Localized Stress Measurement on Thin Films
Markus Krottenthaler, Fabian Haag, Christoph Schmid, Karsten Durst, Mathias Göken (University Erlangen-Nuremberg, Germany)

Focused ion beam (FIB) milling methods can be used for measuring residual stresses in thin films which has been shown in the literature for several coating systems. By FIB milling stresses are relaxed and the resulting displacement are tracked by means of digital image correlation (DIC), based on this deformation and by using finite element analysis (FEA), the residual stress can be reconstructed.

A newly developed geometry in form of an H-bar, as it is commonly used for transmission electron microscopy sample preparation, is used for analyzing the residual stress. The geometry has the distinct advantage that it can be adjusted by means of FEA to obtain a uniaxial relaxation and thus for the calculation of the residual stress only the relaxation strain and Young’s modulus are required.

To verify this new method, relaxation measurements were performed on a bending stage in a scanning electron microscope on bulk metallic glass (BMG). The BMG sample was loaded by four point bending and H-bars were cut on the compression side as well as on the tension side of the specimen. By using the bending beam theory, the residual stress is evaluated and used as a reference for the FIB-DIC analysis. Prior to the measurements platinum nanoparticles were sprayed onto the sample for surface patterning. The bar’s relaxation displacement was tracked by DIC and Hooke’s law is used to calculate the stress from the relaxation strain.

The resulting relaxation strain and stress over the cross section of the beam were in good correlation with the bending beam theory. The results can be used to further verify the sensitivity of the different approaches. Further, the FIB-DIC analysis was successfully applied to measure the residual stresses of an a-C:H coating system.

11:20 AM TS2-1-11 A New Methodology for the Analysis of Fracture Toughness and Residual Stress in Thin Hard Coatings
Marco Sebastiani, Edoardo Bemporad (University of Rome "Roma Tre", Italy); ErikGregory Herbert, GeorgeM. Pharr (University of Tennessee, US)
In this work, the effect of residual stress on fracture toughness and deformation modes of CAE-PVD TiN and CrN based coatings is analyzed and discussed.

A novel characterization methodology for the determination of surface elastic residual stress and fracture toughness in thin films is described. The new methodology is based on nanoindentation testing on focused ion beam (FIB) milled micro-pillars.

Finite element modeling (FEM) of strain relief after controlled material removal demonstrates that progressively increasing relaxation of pre-existing residual stress state can be achieved by milling of annular trenches of increasing depth and size at specimen surface, where the stress-free state is approached when the depth of the trench approaches the diameter of the remaining pillar.

Basing on FE modeling, the average residual stress present in the coating can be calculated by comparing the different sets of load-depth curves: the first obtained at the center of stress-relieved pillars, the second on the undisturbed (residually stressed) surface.

The implementation of in-situ SEM nanoindentation tests, using both a Berkovich and cube corner indenters, allowed to realize controlled fracture tests on the stress relieved pillars, thus giving quantitative information on fracture toughness by the use of analytical and numerical models, and allowing for the detailed analysis of the influence of the compressive residual stress on the mechanical behavior of the coating.

The proposed methodology can give further insights into the actual mechanisms that regulate deformation and fracture behavior of highly stressed ceramic coatings.