ICMCTF2013 Session TS2-1: Advanced Characterization of Coatings and Thin Films

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.

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.

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.

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.

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.

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.