Gerhard Dehm

Electron-energy-loss spectroscopy studies of Cu-α-Al2O3 interfaces grown by molecular beam epitaxy

Abstract The electron-energy-loss near-edge structure (ELNES) of a Cu-α-Al2O3 interface grown by molecular beam epitaxy has been studied using spatial difference electron-energy-loss spectroscopy in the scanning transmission electron microscope. Interpretation of the interface-specific Cu L2,3, Al L2,3 and O K ELNES components implies the existence of Cu-O bonding at the interface together with the retention of a local octahedral oxygen coordination of aluminium atoms and hence their non-participation in the interface plane. There is significant evidence for interfacial charge transfer from the copper layer to the oxygen layer, resulting in the existence of one monolayer of copper at the interface nominally in the Cu+ oxidation state. Furthermore, the ELNES studies reveal that the interfacial Cu-O bonds are of mixed ionic-covalent character. This becomes apparent when considering the O K ELNES where the presence of hybridized Cu 3d-O 2p states is indicated. The basal plane of α-Al2O3 at the interface is t...

Microstructure and Tribological Properties of Ni-Based Claddings on Cu Substrates

Abstract Cu substrates were laser-clad with a Ni-based powder using a CW–CO 2 laser. In order to obtain sufficient absorption of the laser irradiation by the substrate a Ni–B–Si plasma-sprayed layer was deposited on the Cu substrate and remelted prior to the cladding process. High hardness values in the range of 5 to 5.4 GPa were achieved for the laser-clad Cu surfaces, compared with a Cu substrate hardness of ∼1.3 GPa. The high hardness of the Ni-based cladding is due to a microstructure consisting mainly of Ni dendrites and Ni and Ni 3 B lamellae. However, the large number of brittle boride phases at the interfaces between the cladding, the plasma-sprayed layer, and the substrate causes cracks during cooling owing to high thermal stresses. Tribological tests of the claddings reveal poor adhesive friction and wear properties with a tendency to seizure when sliding against steel under dry conditions. Abrasive wear coefficients of ∼0.1 were obtained for the claddings when abraded by #320 emery paper. Some correlation was found between the cladding thickness and the tribological properties due to thickness induced defects in the cladding.

Microstructure and Phase Evolution of Niobium‐Aluminide–Alumina Composites Prepared by Melt‐Infiltration

The microstructure of an alumina aluminide alloy prepared by infiltration of a Nb 2 O 5 containing precursor with liquid Al was studied. X-ray diffraction analysis shows that A1203, A13Nb and AlNb2 have formed. Scanning electron microscopy and transmission electron microscopy reveal a dense and fine grained intermetallic reinforced alumina composite consisting of Al 2 O 3 , Al 3 Nb and AlNb 2 grains, which is in accordance with the ternary phase diagram. Additionally, nanometer sized inclusions of AlNb 2 form in the Al 2 O 3 and Al 3 Nb grains and at grain boundaries. While the particles are assumed to precipitate in Al 3 Nb, an occlusion mechanism is considered for the AlNb2 particles located in Al 2 O 3 grains. Furthermore, a change in lattice parameters was observed for the AlNb 2 phase.

Formation and interface structure of TiC particles in dispersion-strengthened Cu alloys

Abstract TiC-dispersion-strengthened Cu alloys were prepared by mechanical alloying and subsequent hot extrusion. The evolution of the microstructure with respect to the preparation process is analysed by transmission electron microscopy techniques. The TiC dispersoids are formed in situ by the reaction of Ti and graphite. Ti diffuses from the pre-alloyed CuTi matrix to C inclusions which are embedded in the matrix after high-energy milling. Heat treatment of the powder mixtures at 400°C leads to heterogeneous nucleation of TiC at the C/Cu interface. Thereby a well defined cube-on-cube orientation relationship is established between TiC and the Cu matrix. A study of the morphology of the TiC dispersoids shows that they are faceted on {111}TiC {110}TiC and {100}TiC planes and possess ledges on the atomic scale. The TiC/Cu interfaces are atomically abrupt and free of interface phases. The {100}TiC∥{100}Cu and ⟨110⟩TiC∥⟨110⟩Cu topotaxy leads to a misfit of 17.6% between the adjacent lattices. This misfit is ...

Solid state dewetting phenomena of aluminum thin films on single crystalline sapphire

................................................................................................................................. I Kurzzusammenfassung ...................................................................................................... III Preface ................................................................................................................................... V Table of contents ............................................................................................................... VII List of Abbreviations and Symbols ................................................................................... IX 1 Introduction .................................................................................................................. 1 1.1 Aim of the thesis .............................................................................................. 4 2 Theoretical background ............................................................................................... 5 2.1 Solid state dewetting ........................................................................................ 5 2.1.1 Thermodynamics ...................................................................................... 5 2.1.2 The process of solid state dewetting ........................................................ 7 2.2 Al thin films on (0001) α-Al2O3 substrates ..................................................... 9 2.2.1 Material properties ................................................................................... 9 2.2.2 Defined model systems for solid state dewetting..................................... 9 3 Experimental details and characterization methods ............................................... 11 3.1 Thin film growth conditions .......................................................................... 11 3.2 Annealing treatment conditions ..................................................................... 12 3.3 Characterization techniques ........................................................................... 12 3.3.1 X-ray diffraction .................................................................................... 12 3.3.2 Electron microscopy .............................................................................. 13 3.3.3 Scanning electron microscopy ............................................................... 14 3.3.3.1 Electron backscatter diffraction ......................................................... 15 3.3.3.2 In-situ environmental scanning electron microscopy ........................ 16 3.3.4 Focused ion beam microscopy ............................................................... 17 3.3.5 Transmission electron microscopy ........................................................ 20 4 Microstructural evolution and solid state dewetting of tetracrystalline Al films . 27 4.1 Introduction ................................................................................................... 27 4.2 Results ........................................................................................................... 29 4.2.1 As-deposited film ................................................................................... 29 4.2.2 Microstructural evolution with annealing time by solid state dewetting 34 4.2.3 Texture evolution results by in-situ annealing XRD pole figures ......... 38 4.3 Discussion ...................................................................................................... 39 4.3.1 Microstructural and texture evolution .................................................... 39 4.3.2 Void formation and void evolution ........................................................ 42