Papers

ICMCTF2008 Session F4: Applications of Analytical Electron Microscopy

Wednesday, April 30, 2008 8:00 AM in Room Sunset

Wednesday Morning

Start	Invited?	Item
8:00 AM		F4-1 Epitaxial TiN/VN Coatings: High-Resolution TEM and EELS Studies Compared to ab-Initio Calculations B. Rashkova (Austrian Academy of Sciences, Austria); C. Scheu (Montanuniversität Leoben, Austria); P. Lazar, J. Redinger (Vienna University of Technology, Austria); R. Podloucky (University of Vienna, Austria); S. Sturm (Jozef Stefan Institute, Slovenia); G. Kothleitner (Graz University of Technology, Austria); K. Kutschej, C. Mitterer (University of Leoben, Austria); G. Dehm (Austrian Academy of Sciences, Austria) TiN/VN coatings are technologically interesting due to a significant decrease in high temperature friction coefficient. In this study epitaxial TiN and VN single-layer as well as TiN/VN bi-layers were grown as a model system on single crystal MgO substrates using unbalanced magnetron sputtering in order to study the interface microstructure and in future diffusion and oxidation phenomena. The coatings and the interfaces were characterized by conventional transmission electron microscopy (TEM), high-resolution TEM, and electron energy-loss spectroscopy (EELS). The experimental studies reveal an epitaxial growth with a cube-on-cube orientation relationship between the film materials and the substrate. On the atomic level, the TiN/MgO and VN/MgO interfaces are abrupt. The TiN/VN interface structure and chemistry was analysed by high-resolution TEM and EELS. The results reveal nearly stoichiometric phases with Ti/N and V/N ratios of 1.1 ±0.1. The comparison of the energy-loss near-edge structure (ELNES) of the N-K, Ti-L_2,3, and V-L_2,3 spectra with the theoretically calculated partial density of states using density functional theory (DFT) revealed that the core hole effects must be included. The calculations indicate that the N-K edge is sensitive to vacancies in the sublattice of VN. Thus, the N-K edge may be used as a fingerprint when studying non-stoichiometric or diffusional phenomena. The electronic structure of the TiN/VN interface calculated using DFT is very similar to the respective bulk spectra. An experimental spectrum taken at the interface resembles an average of the simulated spectra for VN and TiN indicating that the interface region is partly intermixed with V and Ti atoms.
8:20 AM		F4-2 Impact of Low-Temperature Carburization on the Microstructure of Structural Alloys F.E. Ernst (Case Western Reserve University) Low-temperature gas-phase carburization -- a novel process originally developed by Swagelok Company for surface hardening of 316L austenitic stainless steel -- can significantly improve the surface-mechanical properties and corrosion resistance of structural alloys by generating a single-phase "case" with concentrations of interstitially dissolved carbon that exceed the equilibrium solubility limit by orders of magnitude. Upon prolonged treatment, however, carbides can precipitate and degrade the properties. Several sophisticated methods of microcharacterization, and in particular high-resolution and analytical transmission electron microscopy, have revealed important insights into the structural and microstructural changes that the materials undergo during low-temperature carburization and its ultimate limit: the precipitation of carbides.
9:00 AM		F4-4 High-Resolution Electron Microscopy Characterization of Nanocrystalline Grain Boundaries in Gold-Copper Alloys L.O. Nyakiti, J. Chaudhuri, A.F. Jankowski (Texas Tech University) The mechanical behaviours of nanocrystalline metals are intricately dependent upon the size, distribution, boundary structure, and stability of grains. Strengthening mechanisms dependent upon classic dislocation behaviour have a range of application to a lower bound in grain size. For example, numerous experimental studies indicate the Hall-Petch mechanism for strength may no longer apply in nanocrystalline metals with grain size below, approximately, 10-20 nm. However, an evaluation of the line tension between dislocations based on shear modulus and ideal strength suggests a lower bound in grain size, perhaps, down to 5 nm or less. Recently, nanocrystalline gold-copper alloys have been prepared with strengths that approach ideal values where Hall-Petch behaviour is still present at grain sizes of only 5 nm. Detailed examination of the grain boundary structure in these near perfect nanocrystalline specimens is now pursued using transmission electron microscopy to reveal how fundamental changes in the atomic arrangements present at grain boundaries affect the attainment of ideal strength. In addition to high-resolution lattice imaging, the inverse fast-Fourier transform (IFFT) imaging processing technique clearly shows the crystalline nature of the grain boundary and the presence of the extended defects near the grain boundary. Selected area diffraction pattern (SADP) will be employed to determine misorientation between different grains. Thus, differences will then be detailed so as to distinguish how other nanocrystalline alloys behave wherein free volume attributed at grain boundaries is responsible for loss in strength at typically, larger grain sizes.
9:20 AM		F4-6 Transmission Electron Microscopy and Electron Energy Loss Spectroscopy Study of C/Cr Coatings Z. Zhou, W.M. Rainforth (University of Sheffield, United Kingdom); A.P. Ehiassian, P.Eh. Hovsepian (Sheffield Hallam University, United Kingdom); M. Gass, A. Bleloch (SuperSTEM at Daresbury Laboratory, United Kingdom) Carbon based coatings have attracted great interest in a wide range of industries due to their excellent properties such as low friction coefficients and good wear resistance. Multilayer C/Cr coatings have been produced by unbalanced magnetron sputtering. Resultant wear rate of 10^-17m³3N^-1m^-1 were recorded with a friction coefficient of 0.17. The structure of the C/Cr coatings, and therefore the tribological properties, is strongly influenced by the deposition bias voltage. The coatings evolved from an amorphous (-65V to -95V) through an ‘onion-like’ structure (-120V) to nanoscale multilayer structure (-350V and -450V) through a self-organising mechanism, not related to substrate rotation, and finally a uniform fine grain structure (-550V). The change in structure was associated with friction coefficient variations from 0.22 (-65V) through 0.16 (-95V) to 0.31 (-350V). The structure evolution is believed to be a result of ion bombardment and elemental diffusion. In order to further understand the ion irradiation induced self-organising mechanism, multilayer structure (-350V) and amorphous structure (-75V) were examined using a dedicated aberration corrected scanning transmission electron microscope (STEM), coupled with electron energy loss spectroscopy (EELS), which is capable of sub angstrom high angle annular dark field (HAADF) resolution. The multilayer was composed of alternating C-rich layer (C-rich clusters) and uniform Cr-rich layer. Line scans across the multilayers and along the C-rich clusters were performed using a 0.1nm probe. Quantitative EELS analysis and profiles showed the variation of C and Cr. High resolution EELS near edge fine structure indicated that the C bonding was amorphous carbon, consistent with the high resolution electron micrograph. In addition, the ratio of Sp²bond and Sp³bond by using EELS are presented and discussed in relation to using Raman spectroscopy.