Volume 97, Issue 9

Feedback-controlled nanoindentation with a Berkovich diamond tip has been used to perform creep and stress relaxation tests on polycarbonate at room temperature for a wide range of loads (10 – 30000 μN) and indentation depths (30 – 3000 nm). The creep compliance J ( t ) and relaxation modulus G ( t ) have been calculated from experimental data as a function of time in the range t = 0.1 – 100 (1000) s. The data are analysed by different rheological models which are compared: (1) the Burgers model, (2) the generalised Maxwell/generalised Kelvin model, and two empirical approaches: (3) a logarithmic model, and (4) a power law model. The Burgers model gives a poor description of the material behaviour since it assumes a steady-state flow of material which is not observed in the experimental time range. The generalised models yield a set of discrete relaxation- and retardation time constants. It is shown that these time constants do not correlate with specific molecular moving processes in the polymer, but are only one of several possible parameterisations of the creep and relaxation curves. Numerical differentiation of G ( t ) and J ( t ) shows that polycarbonate has continuous relaxation- and retardation time spectra, respectively, and the dynamic viscosity η ( t ) of the material increases linearly with time. The behaviour of polycarbonate is best represented by the empirical power law model, which allows optimum fit of creep/relaxation curves, relaxation and retardation time spectra and time-dependent viscosity.

We report on an investigation of the suitability of SiO 2 thin films for use as calibration standards for the measurement of elastic modulus by laser-acoustic measuring devices. Film thicknesses for a range of thin film standards, originally developed for the calibration of ellipsometers, were obtained by X-ray reflectometry, ellipsometry, and metrological scanning force microscopy. Using the calibrated values of film thickness, a laser-acoustic method was used to obtain the elastic moduli of the thin films. Nanoindentation of the films, performed at two different laboratories was used to provide an independent measure of the elastic modulus values. The modulus values determined by nanoindentation and the laser-acoustic method were found to agree within 4.6 %, which was within the calculated uncertainty of the measurements performed. As a result of their high homogeneity with respect to film thickness and elastic modulus, their low surface roughness and the good laser-acoustic signal quality, the thin films investigated appear to hold promise as standards for laser-acoustic measuring devices.

The comprehensive mechanical characterisation of thin films and surfaces is very important for a suitable optimisation of protection layers and layer systems. The Young’s modulus and the yield strength are widely accepted material parameters which can be determined by spherical nanoindentation in normal direction. This article reports on the determination of a further parameter, the critical tensile stress, by spherical nanoindentation with additional lateral forces. The critical tensile stress was determined for sapphire. It will be shown that the results obtained with two different nanoindentation devices equipped with spherical diamond indenters of different radii agree with less than 10% deviation.

Four-point-bending and strain-controlled fatigue experiments were performed on electrodeposited nanocrystalline nickel specimens at room temperature. For the first time, the deformation behaviour of bulk nanonickel specimens was studied in an atomic force microscope with a newly developed in situ bending machine. Results show that grain boundary sliding plays an important role during deformation, and the fatigue crack propagates along grain boundaries.

The hydrogen effect on dislocation nucleation in Ni single crystals with (111) surface orientation has been examined with the aid of a specifically designed nanoindentation setup for in situ electrochemical experiments. The effect of the electrochemical potential on the indent load – displacement curve, especially the unstable elastic-plastic transition (pop-in), was studied in detail. The experiments allowed the exclusion of the surface from hydrogen effects. The observations showed a pop-in load drop from an average value of 250 to 100 μN due to in situ hydrogen charging, which is reproducibly observed within sequential hydrogen charging and discharging. Clear evidence is provided that hydrogen atoms facilitate homogeneous dislocation nucleation.

(011) thin films of GaAs and InP (220 and 250 μm thick, respectively) were deformed by a Vickers indenter at elevated temperatures (350 – 400 °C) under loads in the range 0.4 – 2.9 N. For loads higher than 1.9 N, symmetric punching through InP was detected at the opposite face while it was asymmetric through GaAs. In InP, when the punching mechanism took place, pile-up around the indent site progressively disappeared. Moreover, the measured plastic volumes reveal that plastic flow throughout the specimens is volume conservative, implying that pile-up and punching events are related. In GaAs, the anisotropic pile-up plasticity was studied using transmission electron microscopy. Models describing the material flow through the samples and reverse flow of matter are proposed.

We report on mechanical properties of multiwall carbon nanotubes (MWNTs)-polymer composites using the nanoindentation technique. The nanoindentation experiments conducted on thin films containing MWNTs revealed that the presence of nanotubes does not affect the nanomechanical properties of the composites. Even a layer-by-layer assembly of MWNTs with a high concentration and a homogeneous distribution of carbon nanotubes does not ensure reinforcement of the composites. For that reason, we synthesized and utilized carbon nanotubes with a silica shell. Nanohardness and Young’s modulus have been found to increase strongly with increasing content of these nanotubes in the polymer matrix. The silica shell on the surface of a nanotube enhances its stiffness and rigidity. Our composites, at 4 wt.% of the silica-coated MWNTs, display a maximum hardness of 120 ± 20 MPa and a Young’s modulus of 9 ± 1 GPa. These are, respectively, 2 and 3 times higher than those for the polymeric matrix.

We tested the nanomechanical properties of various materials (Si, SiO 2 , Si 3 N 4 , AlCu, TiN) for the design of stamps used in nanoimprint lithography. Nanoindentation was used on both insulated and non-insulated areas. The test enabled an estimation of the nanohardness and Young’s modulus of the tested materials (films). The results are very useful for selecting the material for a protective film on the stamps used in nanoinprinting lithography.

New experimental results on the liquid + graphite + M 7 C 3 equilibrium are presented. The solubility of Co in the M 7 C 3 carbide has been measured at 1523 and 1723 K. These measurements together with recently published measurements on the fcc + graphite + M 7 C 3 equilibrium show that the solubility of Co in the M 7 C 3 is much higher than predicted from thermodynamic calculations. A new thermodynamic description of the Co–Cr –C system is presented which accurately describes the solubility of Co in the M 7 C 3 carbide in the temperature range 1373 – 1723 K.

Published measurements of the diatomic sulfur fugacity were used to make a thermodynamic analysis of high-temperature cuprous sulfide between 789 K and 1321 K. Gibbs–Duhem integration of the fugacity data was used to determine the enthalpy of formation of stoichiometric Cu 2 S as – 123.8 kJ mol –1 . Statistical thermodynamics was applied to find partition functions, interaction energies, and free energies to characterize the configurational aspects of the phase. Theoretical calculations indicated that Cu vacancies might be localized in one half of the Cu lattice sites under quasi-chemical equilibrium.

This paper deals with the densification and phase transformation of Si 3 N 4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080 °C. Upon sintering at 1500 °C the bulk density increases to more than 95% of the theoretical density without phase transformation from α-S 3 N 4 to β-Si 3 N 4 , i. e. the phase transformation lags behind the densification process. Above 1500 °C the secondary phase is completely converted into a glass and the α-to-β transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.

The influence of the sintering additive LiYO 2 (5– 15 wt.%) on sintering behavior, microstructure and mechanical properties of Si 3 N 4 ceramics was investigated. Since LiYO 2 enables densification of Si 3 N 4 at extraordinarily low temperatures, sintering was carried out in the range from 1200– 1700 °C. Densification was found to be enhanced with increasing additive content due to an increasing volume fraction of the liquid. The phase transformation and grain growth occurred through a solution-reprecipitation mechanism, where the precipitation took place preferentially on pre-existing β-Si 3 N 4 nuclei (of which the starting powder already contained 20 wt.%). The indentation fracture toughness increased with both sintering time and additive content as a result of the growth of elongated grains.

This paper reports the results of mechanical investigations (tensile and relaxation tests), geometrical measurements as well as macro- and microscopic (slip traces) observations of zinc single crystals in the ‘hard’ orientation (from the line [11 2 0] – [10 1 0] of the standard triangle) tested in the temperature range 523– 673 K. Consideration of the experimental data lead to the conclusion that the quasi-viscous flow of crystals results from localized coarse slip in the basal system occurring in the structure of uniformly distributed slip in the 1st order prismatic (and/or the 1st order pyramidal) systems. The proposed mechanism explains the high strain-rate sensitivity and the ‘jerky’ character of the flow of the crystals.

In pure metals or homogeneous alloys, the number of electrons per atom as well as the pressure are constant in a temperature gradient, and the electrochemical potential of the electrons is a unique function of the temperature. That is why it is possible to obtain the thermopower of these materials not only in the conventional evaluation of the Onsager equations, which involves the calculation of two transport coefficients, but also with an alternative approach, which involves the calculation of the temperature dependence of the electrochemical potential. The method is illustrated with the calculation of the thermopower of Cr, Mo, and W from the density of states of these metals.

A computational model is developed to include heat transfer, effects of resistance of the metal –mould interface, and pressure in the simulation of the solidification process. The simulation of the interface resistance is based on the Zero Thickness Element, utilizing the Finite Element Method. The solid boundary conditions, including contact resistances, have been modified by a pressure gradient in each of the Zero Thickness Elements. The pressure gradient has been modelled on experimental data. In order to verify the computational results, an Al-11.5 wt.% Si alloy has been poured into a permanent mould and the temperature of the interface measured. Then we modelled the effect of metallostatic pressure on the overall heat transfer coefficient in the metal–mould interface. The comparison between the experimental and the simulation results during the solidification process shows a good consistency that confirms the accuracy of the model for the effect of the interface resistance on solidification time.