Volume 99, Issue 8

Nanoindentation characterization of polycarbonate is considered for load-controlled creep testing and displacement-controlled load-relaxation testing using a pyramidal Berkovich tip. An elastic – viscoelastic correspondence approach is used to obtain closed-form analytical solutions for fitting experimental data to deconvolute spring coefficients and viscosity values independently from creep and from relaxation tests. The a priori assumption of linear viscoelasticity is tested by varying the peak load and displacement levels by a factor of twenty. Results demonstrate surprisingly good agreement between parameters computed from relaxation and creep tests and little variation with load or depth. This result supports a linearly viscoelastic analysis under the conditions utilized in the current experiment, namely a Berkovich indenter tip and a glassy polymer. Plastic deformation is not explicitly considered in the current analysis, as all time-independent elastic and plastic deformation was lumped into a single effective deformation resistance term with a quadratic dependence of load on displacement. As such, obtained numerical values of the shear modulus are not representative of the material's elastic modulus. However, the technique demonstrates predictive capability across the interface between load-control and displacement-control, motivating further research on displacement-controlled nanoindentation relaxation for polymeric and biological material characterization.

A method for the identification of model parameters describing the time-dependent material behavior by means of cyclic nanoindentation is presented. The complex shear modulus of the material sample is determined from the prescribed amplitude in the load history, the measured amplitude in the penetration history, and the phase shift between the peak values for the load and the penetration. The parameters for a specific viscoelastic model are obtained by comparing the experimentally-obtained storage and loss moduli – both depending on the frequency used during cyclic testing – with the analytical expressions for the respective viscoelastic model. The presented method is applied to low-density polyethylene, giving access to the parameters of the fractional dash-pot which is used to describe the viscoelastic behavior. The results are compared with results from nanoindentation (static) creep tests, considering different maximum loads. Finally, the performance of the presented method is assessed by comparing the creep-compliance functions corresponding to the model parameters identified by nanoindentation with the macroscopic creep-compliance function obtained from bending-beam-rheometer tests.

Most geomaterials exhibit both viscoelastic and plastic material response when indented by a sharp conical tip. Lacking analytical solutions for viscoelastic – plastic material response, a numerical approach based on the finite element method is proposed for backcalculation of model parameters from indentation data. Departing from the analytical solutions recently obtained by Vandamme and Ulm, viscoelastic – cohesive conical indentation of a homogeneous material halfspace is dealt with in this paper. Results from finite element analysis, i. e., the load – penetration history for a rigid, conical indenter with linearly increasing load, are presented in dimensionless form. From the results, scaling relations are constructed for application to materials exhibiting viscoelastic – cohesive behavior.

Hardness measurement of thin films is a major topic. At the nanoscale the interplay between substrate and coating during the indentation process is not well known. Atomic force microscopy nanoindentation has been used to address this problem. We prepared a set of five chromium nitride films (100 – 500 nm thick) deposited on a steel substrate. Using nanoindentation we measured the hardness of these coatings, varying the depth of indentation and thus observing the effect of the substrate – coating interaction on the hardness evaluation. The results show that in this case (hard film on soft substrate) it is not trivial to have measurements not affected by the substrate even at very small depths of indentation. Focused ion beam cross-sections on the indentation and numerical simulations confirm these results showing a relevant contribution of the substrate to the hardness evaluation.

Existing indentation models (both analytical models and numerical analysis) show a linear relation between δ r /δ m and H/E r , where δ r and δ m are the residual depth and maximum depth, and E r and H are the reduced Young's modulus and hardness of the test material, respectively. However, in this paper it is shown that the relationship between δ r /δ m and H/E r is actually nonlinear. Based on the analysis by Oliver and Pharr, a new relationship between δ r /δ m and H/E r has been derived in different ways without any additional assumptions and verified by finite element analysis for a range of bulk materials. Further, with the aid of finite element simulations a new relationship between the irreversible work over total work and hardness over Young's modulus for materials with different work hardening behaviour has been derived which shows excellent agreement with experimental observations.

In nanoindentation tests the pop-in effect is a sudden increase in penetration depth without an increase in load, caused by some special effect because of the smaller specific volume of the high-pressure phase in Si. It has been investigated in Si samples covered with 20, 50, 100 and 150 nm thick layers of amorphous Si or Ge in single and multiple load events. The impressions caused by the indentation were imaged by atomic force microscopy. The characteristics of the load curves and the shape of the indented impressions are correlated. Our experiments provide a consistent evaluation of the Young's modulus and hardness of the samples and consistent data on the exact shape of the indent impressions. The dependence of the variation of the magnitude and spatial distribution of the relaxed volume on the composition and thickness of the sputtered layer is discussed.

A thin circular film is clamped at the periphery. An external mechanical force is applied to the film center via either a shaft with a spherical cap or a ball bearing with a specific weight. The deformed geometry allows the materials parameters of the film to be assessed for a wide range of length scales. Solid mechanics models are derived for a multi-scale deformation range as the film experiences (i) indentation or Hertzian contact, (ii) plate-bending, (iii) membrane-stretching, and (iv) large deformation. The method caters to ultrathin membranes where conventional macroscopic tensile tests might potentially damage the sample films. Thin film adhesion-delamination can also be characterized by the same set-up. Experiments were conducted for a range of polymer films.

Energy-based inelastic hardness of soda-lime silicate glass is obtained from the work of indentation and the residual impression volume, which is measured by using a scanning probe microscope. The indenter shape dependence and the indentation load dependence of energy-based hardness are investigated. Physical meanings of the energy-based hardness and the apparent hardness of glass are discussed in terms of the mechanisms of inelastic deformation for brittle and glassy materials.

a-C, a-SiC and Ti-C: H coatings were deposited on two steel substrates with different hardness by magnetron sputtering. Their mechanical properties were investigated by depth sensing indentation and dry sliding testing. Load – displacement curves were obtained using a NanoTest™ NT600 instrument equipped with a Berkovich indenter. The depth profiles of the mechanical properties (indentation hardness H, effective modulus E eff , and also H/E eff and H 3 / E 2 eff ratios) on both substrates are presented. The goal of this article is to compare the dependence of the real response of a coating/substrate system on the substrate. Regardless of substrate type the highest and the lowest values of hardness and modulus belong to a-C and Ti-C: H films, respectively.

The major research topic presented in this study concerns nanoindentation experiments of thin soft films, showing high adhesion forces. The aim of this research was to evaluate the nanomechanical properties of a 200 nm thick thermoplastic polymer layer mr-I 7000E devoted to nanoimprint lithography. The experiments were carried out using an atomic force microscopy instrument equipped with a modified 14 N m − 1 silicon cantilever tip (5 μm in diameter). The forces exerted during film indentation varied from 0 up to 2.5 μN. The Hertz and Johnson – Kendall – Roberts (JKR) models were used for analysis of the force – distance curves in order to extract the Young's modulus value. Contrary to the Hertz model, in the JKR model the adhesion force between tip and sample was considered as an additional load during the nanoindentation. The final results of these estimations were 0.20 and 0.25 GPa for the Hertz and JKR models, respectively. It shows that neglect of the adhesion force may cause an error of 20 %.

The nanomechanical behaviour of micromembranes for high temperature applications of a microelectro mechanical system pressure sensor has been studied by use of atomic force microscopy. A method was elaborated and used for measure nanometre deformation of the membrane under application of high loads. This method was also useful for identifying failures of a membrane, e. g., undesired position against the substrate of the structure. The tests were performed for a series of membranes. The characteristics of the mechanical behaviour were plotted for different geometrical features of the membranes to identify the influence of geometry and dimension. The method was found to be very effective for the process characterisation of the fabricated MEMS micromembranes devoted, e. g., to high temperature pressure sensors.

Carbon nanotube brushes of micrometre range height were deposited on a silicon substrate and tested by use of atomic force microscopy. The cantilever's tips were made of silicon or steel. Firstly, the effect of the structure of carbon nanotubes on indentation (mechanical behaviour) in normal direction to the substrate was studied. Moreover, to examine nanotribological properties, sliding in the lateral direction to the substrate was studied. We also studied how the resonance frequency of vibration of one kind of microfork changes with the depth, to examine the connection between friction force and depth of the measurement. Finally, a mathematical model of the mechanical and tribological properties of carbon nanotube brushes was elaborated.

K 2 O introduced with manganese ore is redistributed by evaporation and condensation within submerged arc ferromanganese smelting furnaces. To characterise the phase equilibria relevant to these slags experimental studies have been carried out to determine pseudo-ternary sections of the form “MnO” – CaO – (SiO 2 + Al 2 O 3 ) with a fixed Al 2 O 3 /SiO 2 weight ratio of 0.65 and 6.0 wt.% K 2 O for temperatures in the range 1523 – 1673 K. It was found that the presence of K 2 O in the slag decreases liquidus temperatures relative to those of K 2 O-free slags in the anorthite and gehlenite primary phase fields at low MnO concentration; at higher MnO concentration however the KAlSiO 4 and KAlSi 2 O 6 phases are formed maintaining the liquidus above 1500 K. The manganosite primary phase field is expanded in the direction of increased (Al 2 O 3 + SiO 2 ) and the liquidus temperatures in the manganosite primary phase field are increased significantly with the presence of K 2 O in the slag.

The microstructures and dry sliding wear behavior of Al-12Si and Al-12Si-3Cu cast alloys were studied after various melt treatments such as grain refinement and modification. Results indicate that combined grain refined and modified Al-12Si-3Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al–Si and fine CuAl 2 particles in the interdendritic region. These alloys exhibited better wear resistance in the cast condition compared with the same alloy subjected to only grain refinement or modification. The improved wear resistances of Al-12Si-3Cu cast alloys are related to the refinement of the aluminum grain size, uniform distribution of eutectic Al–Si and fine CuAl 2 particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of the microstructural changes in the Al-12Si and Al-12Si-3Cu cast alloys by grain refinement, modification and combined action of both on the dry sliding wear behavior.

This work focuses on the experimental investigation of the ternary Al – Mg – Ni system at 700 and 427 °C by means of X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy and electron probe microanalysis. Only one ternary phase τ 1 (Ni 2 Mg 3 Al) with a composition of about 15.5 at.% Al, 49.5 at.% Mg, and 35 at.% Ni was observed at both 700 and 427 °C. At 700 °C, the binary NiAl phase dissolves about 17 at.% Mg and the Ni 2 Mg phase dissolves up to about 10 at.% Al. At 427 °C, NiAl and Ni 2 Mg were found to dissolve the third element in appreciable quantities. Ni 2 Al 3 was also observed dissolving up to about 2.5 at.% Mg. The isothermal sections at 700 and 427 °C are presented.