A new technique to measure the true contact area using nanoindentation testing

Nanoindentation technique requires the determination of projected contact area under load for calculation of modulus and hardness of materials. This projected contact area is usually calculated by models which take into account the pile-up or sink-in phenomena around the tip. The most commonly used model was developed by Oliver and Pharr [1] which can precisely model the sink-in around the tip, but cannot account for pile-up. Another model developed by Loubet et al can be used [2]. It can take into account the pile-up and the sink-in phenomena and can precisely measure the projected contact area for a large range of materials, except for materials with high strain hardening exponent. Other techniques, like post mortem measurements, can be used. However these measurements do not take into account the elastic recovery during unloading. A new technique to estimate the true projected contact area will be presented. It consists of combining two models (The Dao et al. model and the Kermouche et al. model) that are used normally to calculate the representative stress and the representative strain in indentation. Consequently, the projected contact area calculation does not depend on any contact area model. Moreover, it can account for the pile-up or sink-in phenomenon and the strain hardening of the material, which is not possible with the actual models used. This new technique requires measuring indentations parameters like the maximum load, the contact stiffness and the loading curvature. It requires also the use of two tetrahedral indenters: a Berkovich tip and a tetrahedral tip where the included semi-angle is 50°. The method was tested on three different samples: glass, PMMA and 100C6 steel. For indentations on glass and PMMA samples, the projected contact area was precisely measured. For indentations on 100C6 steel sample, the method was adapted to take into account the Indentation Size Effect observed at small indentation depths. The projected contact area values measured with this new technique will be presented and compared to the values calculated with classical literature models. Also, the limits of the technique will be discusse

Population balance modelling for a flow induced phase inversion based granulation in a two-dimensional rotating agglomerator

A novel two-dimensional rotating agglomerator was developed to carry out the flow induced phase inversion (FIPI) based granulation. The process in this agglomerator shows that a continuous paste flow (mixed with liquid binder and primary particles) is extruded into the interstice of two relatively rotating disks, as the paste becomes solidified due to the loss of heat to the disks, it is then broken into granules by the shearing force imposed by the rotating disk. Experimental measurements have shown that the size of these granules is enlarged along the positive radial direction of the disks. It is also found that these granules contain approximately the same quantity of binder in terms of its volume fraction. The paper thus proposes a population balance (PB) model to describe the growth of the granules by considering a size independent agglomeration kernel. The PB simulated results are found to be well capable of describing the change of the particle size distribution (PSD) of the granules in the radial direction. This study also proposes a velocity profile for the paste flow and attempts to establish a quantitative relationship between the granulation rate and the deformation rate as this would help us understand the mechanism of the agglomeration. It is hoped that this study would be used to improve the design of the agglomerator and to assure the control of the process and the granular product quality

Antireflection coatings from analogy between electron scattering and spin precession

We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band

A new dynamic module for in-situ nanomechanical testing at high strain rate

In-situ nanomechanical testing is commonly used to probe surface mechanical properties of bulk materials or thin films, like hardness, Young’s modulus, Yield stress…Actually most of the instruments can measure these properties only statically, i.e. a low frequency, leading to property measurement only at low strain rate (usually 10-1s-1 by nanoindentation). This is mainly caused by the low resonance frequency of the system, preventing making tests at higher speed. Performing high dynamic measurements could bring new information on materials properties like deformation mechanism at high strain rate, or high dynamic fatigue properties. A new high dynamic module usable for in-situ mechanical testing has been developed. It is composed of a small piezotube attached directly behind the tip. Because of the small dimensions of the module, his resonance frequency is very high (higher than 50kHz) in comparison to classical nanomechanical testers, permitting to perform and measure precisely the signals at very high frequency. Moreover, it can be used as a sensor and as an actuator, in x, y and z directions which gives to this module a very large range of measurements. Firstly, the characteristics, the performances and the limits of the new high dynamic module will be presented. Secondly some indentations experiments performed at high strain rate on nanocrystalline nickel with the in-situ nanomechanical tester (Alemnis Gmbh) equipped with the high dynamic will be presented and discussed (Fig. 1). Finally, some micropillar compression at high strain rate on the same material will be described and discussed

Tube Models for Rubber-Elastic Systems

In the first part of the paper we show that the constraining potentials introduced to mimic entanglement effects in Edwards' tube model and Flory's constrained junction model are diagonal in the generalized Rouse modes of the corresponding phantom network. As a consequence, both models can formally be solved exactly for arbitrary connectivity using the recently introduced constrained mode model. In the second part, we solve a double tube model for the confinement of long paths in polymer networks which is partially due to crosslinking and partially due to entanglements. Our model describes a non-trivial crossover between the Warner-Edwards and the Heinrich-Straube tube models. We present results for the macroscopic elastic properties as well as for the microscopic deformations including structure factors.Comment: 15 pages, 8 figures, Macromolecules in pres