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Schwarzer N.,Saxonian Institute of Surface Mechanics SIO
Philosophical Magazine | Year: 2012

It will be shown how relatively simple models simulating bond interactions in solids using effective potentials, such as Lennard-Jones and Morse, can be used to investigate the effect of pressure-induced stiffening or enhancement in these solids. The value of the current study is the possibility of deriving relatively simple dependencies of the bulk-modulus B on the pressure P in a way that is completely free of microscopic material parameters wherever the solid bond interaction can be described, or at least partially described, by Lennard-Jones potential approaches. Instead of bond energies and length, only specific integral constants, such as Young's modulus and Poisson's ratio, are required. The influence of the pressure-induced Young's modulus change is discussed, especially with respect to mechanical contact experiments. © 2012 Taylor & Francis. Source

Kohl J.G.,University of San Diego | Randall N.X.,CSM Instruments Inc. | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Ngo T.T.,University of San Diego | And 2 more authors.
Journal of Applied Polymer Science | Year: 2012

Silicone elastomer coatings are currently being investigated as foul release coatings on ships hulls. Previous tests on silicone duplex elastomer coatings used a progressive load scratch test. It has been shown that the durability of uniform silicone duplex elastomer coatings is a function of thickness, indentation modulus, and stylus and that the failure mechanism depended on coating thickness and stylus. When applying silicone coatings to a ship's hull, there are regions on the ship where the coating is not uniform. This article investigates the effect of a thickness gradient on the durability of a single layer silicone elastomer coating. In these tests, a constant normal load was used as the stylus moved transversely to the surface. It was found that when the scratch test started in the silicone coating and proceeded in the direction of decreasing coating thickness ("Elastomer to Metal"), there was first a scratch tract followed by the initiation of detachment of the coating, then by gross detachment of the coating. When the scratch started on the exposed aluminum surface and proceeded into the silicone in the direction of increasing coating thickness ("Metal to Elastomer"), there was first gross detachment of the coating, followed by recovery (i.e., silicone coating is intact) and a scratch tract into the silicone. It was also found that the coefficient of friction was much higher in the silicone when the scratch test was going in the direction of decreasing coating thickness as opposed to the scratch test going in the opposite direction. © 2011 Wiley Periodicals, Inc. Source

Gies A.,Balzers Ag | Chudoba T.,ASMEC GmbH | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Becker J.,Oerlikon Balzers Coating Germany GmbH
Surface and Coatings Technology | Year: 2013

In this work, homogeneous as well as gradient a-C:H:W coatings were deposited on a steel substrate. The mechanical properties of both coating systems were determined using nanoindentation. In addition, a multiaxial, 3-dimensional nanoindentation test (reciprocating wear test with nanometre resolution) was carried out in order to analyse the wear-performance of the two different coating systems.By comparing the wear-performance of the gradient coating system to the non-graduated one we were able to show that according to a theoretical approach proposed a few years ago the gradient coating systems lead to a better performance in micro-wear tests as well as in macro-scale wear tests.In addition, a new wear model taking into account the changing stress fields during the wear tests is proposed in order to simulate the wear behaviour of both coating systems. © 2013 Elsevier B.V. Source

Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Heuer-Schwarzer P.,ESAE and ZumKranich
ECCOMAS Special Interest Conference - SEECCM 2013: 3rd South-East European Conference on Computational Mechanics, Proceedings - An IACM Special Interest Conference | Year: 2013

Recently developed completely analytical tools for the modelling of contact problems on thin film structures are adapted to allow the investigation of arbitrarily mixed purely isotropic and transversally isotropic laminate structures under impact loads. The new tool is applied to model a variety of load problems resulting in the failure of windsurfing boards consisting of a relatively thin laminate shell and a soft polymer foam core. It is shown that local impact and distributed bending loads due to "bad landing" after high jumps or contact with parts of the sailing gear (the so called rig) especially the front part of the boom are leading to the most critical stress distributions resulting in failure. So, most of the investigated boards were damaged because the rider (windsurfer) landed flat and thus produced a sudden impact force under his feet (impact defect). Other overloading occurred due to overturning of so called loop movements or the landing of the board exactly on respectively between two waves and this way producing high bending moments. Some of those typical loads are analysed in detail and the stresses occurring in the complex structure of the windsurfing boards are evaluated. Source

Fischer-Cripps A.C.,Fischer Cripps Laboratories Pty. Ltd. | Bull S.J.,Northumbria University | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO
Philosophical Magazine | Year: 2012

Claims for ultra-hardness (H≈100GPa) in nanocomposite coatings are critically examined in terms of the experimental evidence first presented in 1999 and theoretical support published over the past 10 years. It is shown that the results of experimental work cannot be validated, and that there are many unresolved issues associated with the supporting theoretical arguments. Using the methods outlined by the authors, whose work is reviewed here, but with more precise application of the equations involved, and reading directly from their reported relationships between Y and H, the best estimate of the hardness for the materials under consideration appears to be of the order of ≈55GPa. This estimate is validated by actual measurements on a diamond sample and super-hard coatings, and finite element computations in comparison with experimental results for ultra-hard coatings. It is shown that the conclusions of the work being reviewed do not stand up to scrutiny and that the hardness of the ultra-hard coatings is most likely over-estimated by a factor of ≈2. © 2012 Taylor & Francis. Source

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