Volume 92, Issue 9

In order to describe the hardness of substrates and layers, different definitions are used which consider the plastic or the elastic-plastic behavior of the material. For a chosen spectrum of substrate and layer materials the absolute hardness values according to these definitions are compared with each other. Moreover, relationships between the hardness values are derived. For the investigation of differences in the accuracy of the various hardness definitions the variance coefficients of different hardnesses were determined.

The applicability of nanoindentation methods for industrial characterisation of thin film properties is discussed. The goal is to define reasonable boundary conditions for serious hardness testing in a justifiable amount of time. Nanomechanical test methods for industrial applications are confined by some simple rules. Different fields of application, like process development or quality control, for coating industry, are discussed by means of examples.

Although the investigation of frictional phenomena is one of the oldest research topics in physics, a satisfying understanding of the related atomic-scale effects is still missing. In recent years, the scanning force microscope enabled new insight into the frictional properties of well-defined atomic-scale contacts. In this article, we review our experimental and theoretical investigations of atomic-scale friction and relate these results with the well-known macroscopic laws of friction.

The laser-acoustic technique based on surface acoustic waves has proved to be an interesting method for testing coatings and material surfaces. The central point of the laser-acoustic technique is to measure the dispersion curve of the surface acoustic wave that represents a so-called signature for microstructural gradients perpendicular to the surface. The dispersion curve is the spectrum of the phase velocity of the surface acoustic wave depending on frequency. For a completely homogeneous material, the phase velocity is constant, i. e. does not depend on frequency. For coated materials, the phase velocity varies with frequency. The dispersion spectrum may ascend or descend, depending on the combination of film and substrate material. Fitting a theory to the measured curve enables the Young moduli of both film and substrate to be deduced. The physical principles of the method are described. The capability of the laser-acoustic method is demonstrated for three examples.

Native oxides significantly modify surface mechanical properties of metals. Hardness and Young’s modulus derived from quasi-static nanoindentation are not sufficient to characterize thin oxide films due to substantial substrate effects. An in-situ acoustic emission (AE) monitoring of nanoindentation is introduced for thin oxide film characterization. The proposed method is a synergy of localized indentation contact AE monitoring and advanced digital signal processing algorithms. Experimental data on W(100), Cu covered by native oxides and Ni-37 at.% Al alloy that undergoes stress-induced martensitic tranformation are presented in terms of advanced statistical and energy methods. A new method is proposed that combines nanoindentation with in-situ AE monitoring and enables characterization of oxide fracture, film thickness and film delamination and phase transformation.

Mechanical properties of amorphous and crystalline Ta-silicide thin films were studied in cyclic depth-sensing nanoindentation experiments. At the first loading cycle (up to peak load P 1 ) an initial indent of h 0 in depth was formed. Then without shifting, the indenter was reloaded to a higher load of P 2 . Up to P 1 , the elastic loading of the initial indents took place. When the repeated load exceeded P 1 , the plastic deformation of the initial indent began. Using the initial indent reloading data, the deformation curves of Ta-silicide thin films at point loading have been found. Such deformation curves do not depend on the depth of the initial indent and characterize the mechanical behavior of materials under point loading. Crystallization of amorphous phase during annealing caused an appreciable change of the Ta-silicide thin film deformation curve. Both elastic and plastic properties of Ta-silicide improved due to transition from amorphous to crystalline state.

The load – displacement curves as obtained by nanoindentations on most metallic and intermetallic materials show discontinuities or pop-ins at load levels below or around 1 mN. These pop-ins mark a sharp transition from pure elastic loading to a plastic deformation of the specimen surface, thus correspond to an initial yield point. On smooth surfaces pop-ins are observed frequently, but not on surfaces with a high roughness. Step edges on the surface are discussed as dislocation sources for the initial yield events. Since the pop-ins mark the transition from elastic to plastic deformation, the elastic loading part can be described by the Hertzian contact model. Discontinuities in the load –displacement curves are observed on nearly all materials, examples are given for c-BN, TiAl, Ti 3 Al, Cu, and Mo 5 SiB 2 . On Mo samples, discontinuities are found sometimes only in the unloading part of the load – displacement curves. The origin of the different pop-in effects are discussed.

In this work, we have used a nanoindentation system inside a temperature chamber to investigate the time- and temperature-dependent deformation behavior of the cell walls in an Al-based foam. Constant indentation strain rates were used to study the deformation behavior between room temperature and 100 °C. The depth-sensing indentation experiments were significantly influenced by a very strong, nonlinear thermal drift. Nevertheless, it was possible to measure the hardness as a function of indentation depth by measuring dynamically the contact stiffness and assuming a constant indentation modulus for the tested material. In summary, it was found that the hardness of the Al foam is twice as high as of pure Al, and the hardness decreases with increasing temperature and decreasing strain rate, following a power-law dependence.

It is well known that materials of same indentation hardness do not necessarily exhibit the same wear behaviour. A nanomechanical test is required, to describe the behaviour more accurate. The Mohs minerals define a relative hardness scale by scratch testing samples. They are used as a reference. This set of standard minerals is tested by conventional quasistatic nano-indentation testing as well as by nanoscratch testing. The objective is to define whether the scratch resistance and wear behaviour of the minerals is better tested by indentation testing or rather by scratch testing.

Certain disagreement is observed in the binary Cd–Ge system; in our opinion this is due to the fact that cadmium presents a high vapour pressure and a low temperature of vaporisation. In this case, if experiments are performed at constant volume, variation of pressure must be taken into account with the variation of temperature. The Cd–Ge binary system was experimentally investigated by differential thermal analysis (DTA), differential scanning calorimetry (DSC), and X-ray diffractometry. As it is difficult to realise a good measurement of the pressure in a sealed ampoule of silica, we chose to estimate the pressure by using thermodynamic data. The optimised model parameters for the Gibbs energy functions of the system were obtained by using the optimisation/calculation programs [1, 2]. The experimental results were reproduced correctly by calculation using the thermodynamic description established in this work. In order to explain the disagreement, a calculation of the phase diagram at different pressures has been made by using the Thermo-Calc program [3].

The enthalpy of formation of YAM (Y 4 Al 2 O 9 ) and Y 2 Si 2 O 7 (γ) has been derived using high-temperature drop solution calorimetry at 1073 K in molten alkali borate. The enthalpy of formation from oxides at 298 K is -6.24 ± 6.21 and -67.08 ± 5.98 kJ/mol for YAM and disilicate, respectively. The thermodynamic functions in the Y 2 O 3 -Al 2 O 3 and Y 2 O 3 -SiO 2 systems are assessed from phase equilibrium data and calorimetric measurements for YAG (Y 3 Al 5 O 12 ), YAP (YAlO 3 ), YAM (Y 4 Al 2 O 9 ), Y 2 SiO 5 and Y 2 Si 2 O 7 phases. The calculated phase diagrams of the Al 2 O 3 -Y 2 O 3 and Y 2 O 3 -SiO 2 systems are in reasonable agreement with the experimental data. Suggestions for further experimental measurements on solid phases are given. Using limited phase equilibrium data for the ternary system Y 2 O 3 -Al 2 O 3 -SiO 2 , the thermodynamic functions are assessed. The liquidus surface, an isothermal section at 1673 K and isopleths in the systems Y 3 Al 5 O 12 -SiO 2 , YAlO 3 -SiO 2 , Y 4 Al 2 O 9 -SiO 2 , Y 2 SiO 5 -Al 6 Si 2 O 13 and Y 2 Si 2 O 7 -Al 6 Si 2 O 13 are presented.

The constitution of alloys and the liquidus surface in the Al-rich corner of the Al –Si–Sm ternary system were determined by the examination of controlled heated and cooled specimens, as well as heat-treated specimens by means of optical and scanning electron microscopy, energy-dispersive X-ray spectroscopy, differential thermal analysis and X-ray diffraction. The Al-rich corner of the Al –Si–Sm ternary system comprises five regions of primary crystallisation (α Al , β Si , Al 3 Sm, Al 2 Si 2 Sm and AlSiSm) with following characteristic invariant reaction sequences: ternary eutectic reaction L → α Al + β Si + Al 2 Si 2 Sm, and two liquidus transition reactions, i. e., L + Al 3 Sm → α Al + AlSiSm, and L + AlSiSm → α Al + Al 2 Si 2 Sm. Along with the position of ternary eutectic and both interstitial points in the Al-rich corner of the Al-Si-Sm ternary system, the temperatures for each reaction were determined.

The microstructure and phase composition of metal matrix composites on the base of Al and Al–Si alloys, containing particles or fibres (MMCp, MMCf) of silicon carbide up to 20 wt.%, have been studied. The character of this interaction depends only slightly on the type of reinforcing elements. Using the literature and experimental data concerning the microstructure and phase composition of composite materials after prolonged holding at 700, 800, and 900 °C, the Al–Si–C phase diagram was corrected. The polythermal sections constructed both for equilibrium and nonequilibrium conditions allow to analyse the structure formation of MMCp and MMCf composite materials. The irreversible character of the chemical interaction in composite materials is often related to the retardation or the complete suppression of some reactions realized only under equilibrium conditions.

A simple statistical mechanics model has been used to study the temperature dependence of the surface properties of Fe –Cu liquid alloys. The calculated values for the composition dependence of surface segregation indicates that Cu atoms segregate to the surface and, in addition, that the surface tension in Fe –Cu alloys decreases with increasing copper content. Furthermore, our formalism is able to predict the temperature dependence of the surface properties for the liquid alloy based only on knowledge of the results at the melting point. This illustrates that the model used provides a simple recipe to extract information in the undercooled region about the dependence of surface properties in liquid binary alloys. In addition, close to the decomposition temperature, the surface concentration of Cu increases sharply in the undercooled region.