Frictional behavior and interfacial adhesion of differently textured pyrolytic carbon layers on Si substrate were investigated by indentation and scratch testing. A large amount of elastic recovery and a low coefficient of friction (μ = 0.05 to 0.09) were observed. Elastic/plastic and frictional behaviors of the coatings are strongly influenced by the microstructure of the pyrolytic carbon films, especially by the texture. The critical load at which the first abrupt increase in the normal displacement occurs was used to characterize interfacial adhesive strength. A pyrolytic carbon film deposited at higher residence time from a gas mixture containing 3% oxygen exhibited higher critical loads than film deposited at lower residence time without oxygen. The results can be understood if one assumes that the gas phase composition during deposition significantly influences the bonding strength at the interface. Failure mechanisms are discussed for both types of films.

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

The paper is devoted to an experimental and numerical investigation of the problem of excitation of three-dimensional Tollmien–Schlichting (TS) waves in a boundary layer on an airfoil owing to scattering of an acoustic wave on localized microscopic surface non-uniformities. The experiments were performed at controlled disturbance conditions on a symmetric airfoil section at zero angle of attack. In each set of measurements, the acoustic wave had a fixed frequency fac, in the range of unstable TS-waves. The three-dimensional surface non-uniformity was positioned close to the neutral stability point at branch I for the two-dimensional perturbations. To avoid experimental difficulties in the distinction of the hot-wire signals measured at the same (acoustic) frequency but having a different physical nature, the surface roughness was simulated by a quasi-stationary surface non-uniformity (a vibrator) oscillating with a low frequency fv. This led to the generation of TS-wavetrains at combination frequencies f1,2=fac ∓ fv. The spatial behaviour of these wavetrains has been studied in detail for three different values of the acoustic frequency. The disturbances were decomposed into normal oblique TS-modes. The initial amplitudes and phases of these modes (i.e. at the position of the vibrator) were determined by means of an upstream extrapolation of the experimental data. The shape of the vibrator oscillations was measured by means of a laser triangulation device and mapped onto the Fourier space.

Surfaces with artificial roughness were generated by deposition of nanoparticles on single crystalline substrates. Nanoparticles with an average size ≈ 15 nm were produced by inert gas condensation and deposited in situ on the substrate mounted inside a modified ultra high vacuum (UHV) transmission electron microscope (TEM). We have investigated the smoothing behavior on annealing based on the difference in surface energies between cluster and substrate and their heat of mixing. The cluster substrate combination Co/Cu(100) was chosen as a model system in which the cluster has a significantly higher surface energy than the substrate. Upon deposition at 600 K, the clusters do not remain on the surface, but rather burrow into the substrate. This is confirmed by a detailed strain analysis of the particles. Nanoparticles in the system Ge/Si(100) in contrast have a lower surface energy than the substrate and are completely miscible. The particles assume the substrate orientation around 700 K. At 900 K coherent islands form which are arranged in clusters of 4 in the form of a square. The reason for this previously unobserved pattern is not yet understood.

The optical and electrical properties of InAs/GaInSb superlattice mesa photodiodes with a cutoff wavelength around 8 pim are investigated. The influence of the surface potential at the mesa sidewalls on the device properties was studied by fabricating gate-controlled diodes. At least two mechanisms determining the dark current in the reverse bias region can be identified. At high reverse biases bulk bandto- band tunneling dominates while the current at low reverse biases is most likely governed by surface effects. Bulk interband tunneling is further investigated by applying magnetic fields B up to 7 T parallel and perpendicular to the electric field E across the p-n junction.

We report on the structural characterization of InAs/(GaIn)Sb superlattices (SL) grown by solid-source molecular-beam epitaxy. SL periodicity and overall structural quality were assessed by high-resolution X-ray diffraction and Raman spectroscopy. Spectroscopic ellipsometry was found to be sensitive to the (GaIn)Sb alloy composition.

The process of mechanical alloying in binary metallic systems is discussed in a microscopic picture. Special emphasis is put on systems with large positive heat of mixing. In these systems, a competition between thermal diffusion and forced transport takes place. Based on results from Monte Carlo studies of the effect of combined shear and diffusion processes on an atomistic scale, a simple rate equation approach is developed. The approach can reproduce the phase evolution in the simulation and the main features of the temperature dependence of the alloy phase fraction in steady state. The results are further compared with experimental data.

Powder blends consisting of prealloyed FCC Ag-Cu solid solutions and elemental Fe powders were ball milled in overall compositions of Ag25Cu50Fe25, andAg40Cu20Fe40. The phase evolution with milling time was investigated by x-ray diffraction and differential scanning calorimetry. For the sample with higher Cu concentration, a ternary FCC alloy phase was formed, while milling the other sample resulted in a two phase mixture consisting of a Ag-rich FCC and an Fe-rich BCC solid solution. About the same amount of enthalpy between 12 and 13 kJ/g-atom is stored in the final states for the two different compositions. Two models based on kinetic and energetic considerations will be discussed and compared to the results. The kinetic model is based on the competition between forced atomic motion during shearing, which is driving the system towards a homogeneous alloy, and thermally activated diffusion, which favors phase separation. For the energetic model, the energy balance is calculated and a phase transformation is expected, if the required energy can be stored in phase boundaries.

Disordered clusters of the intermetallic compounds Ni3Al and Cu3Au have been produced using a high pressure sputtering technique. The clusters are either embedded in a film or studied, in-situ in an UHV electron microscope. The evolution of the ordered structure upon annealing is studied. Using a scanning transmission electron microscope, electron diffraction is obtained from individual clusters. Partial ordering is observed in Cu3Au clusters which have been annealed below the bulk order-disorder transition temperature. Under the experimental conditions, only clusters with sizes of 10–15 nm or larger show ordering.

A preparation method for mixtures of chemically different kinds of crystals on a nanometer scale is described. The method is based on inert gas condensation followed by the consolidation by means of high pressure. Since the size of the crystals is in the range of 6–20 nm, an appreciable volume fraction of the material is made up by interphase boundaries between the different crystals. Investigations of the morphological homogeneity of compacted mixtures consisting of Fe crystals and Cu or Ag crystals respectively have been carried out by means of electron microscopy and small-angle X-ray scattering. The atomistic structure in the vicinity of the Fe atoms has been studied by Moessbauer spectroscopy; indications for the formation of alloy phases have been found even for the Ag-Fe system which exhibits no miscibility in the liquid state. Alloy formation is proposed to take place at the interphase boundaries due to the special thermodynamic conditions in the boundary regions. These conditions are attributed to the free volume of the boundaries as well as to the presence of stresses.