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Stoyanov P.,Fraunhofer Institute for Mechanics of Materials | Stoyanov P.,Karlsruhe Institute of Technology | Stoyanov P.,Kennametal Inc. | Merz R.,IFOS Institute fur Oberflachen und Schichtanalytik GmbH | And 10 more authors.
ACS Nano | Year: 2015

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten-carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation. © 2014 American Chemical Society. Source


Stoyanov P.,Fraunhofer Institute for Mechanics of Materials | Stoyanov P.,Karlsruhe Institute of Technology | Romero P.A.,Fraunhofer Institute for Mechanics of Materials | Romero P.A.,Karlsruhe Institute of Technology | And 7 more authors.
Acta Materialia | Year: 2014

Macroscopic tribometry is linked to classical atomistic simulations in order to improve understanding of the nanoscale interfacial processes during sliding of hydrogenated DLC (a-C:H) against a metal (W) in dry and lubricated conditions. Experimentally, using an online tribometer, wear and roughness measurements are performed after each sliding cycle, which are then correlated with the frictional resistance. Ex situ analysis is also performed on the worn surfaces (i.e. plates and counterfaces) using X-ray photoelectron spectroscopy, Auger electron spectroscopy and cross-sectional transmission electron microscopy imaging of the near-surface region. Then, in order to elucidate the atomistic level processes that contribute to the observed microstructural evolution in the experiments, classical molecular dynamics are performed, employing a bond order potential for the tungsten-carbon-hydrogen system. Macroscopic tribometry shows that dry sliding of a-C:H against W results in higher frictional resistance and significantly more material transfer compared with lubricated conditions. Similarly, the molecular dynamic simulations exhibit higher average shear stresses and clear material transfer for dry conditions compared with simulations with hexadecane as a lubricant. In the lubricated simulations, the lower shear stress and the absence of a material transfer are attributed to hexadecane monolayers that are partially tethered to the a-C:H surface and significantly reduce adhesion and mechanical mixing between the sliding partners. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source


Stoyanov P.,Karlsruhe Institute of Technology | Stoyanov P.,Fraunhofer Institute for Mechanics of Materials | Stemmer P.,University of Duisburg - Essen | Jarvi T.T.,Karlsruhe Institute of Technology | And 13 more authors.
ACS Applied Materials and Interfaces | Year: 2013

The unfolding of a sheared mechanically mixed third-body (TB) in tungsten/tungsten carbide sliding systems is studied using a combination of experiments and simulations. Experimentally, the topographical evolution and the friction response, for both dry and lubricated sliding, are investigated using an online tribometer. Ex situ X-ray photoelectron spectroscopy, transmission electron microscopy, and cross-sectional focused ion beam analysis of the structural and chemical changes near the surfaces show that dry sliding of tungsten against tungsten carbide results in plastic deformation of the tungsten surface, leading to grain refinement, and the formation of a mechanically mixed layer on the WC counterface. Sliding with hexadecane as a lubricant results in a less pronounced third-body formation due to much lower dissipated frictional power. Molecular dynamics simulations of the sliding couples predict chemical changes near the surface in agreement with the interfacial processes observed experimentally. Finally, online topography measurements demonstrate an excellent correlation between the evolution of the roughness and the frictional resistance during sliding. © 2013 American Chemical Society. Source


Gross T.,BAM Federal Institute of Materials Research and Testing | Pippig F.,Fraunhofer Institute for Applied Polymer Research | Merz B.,IFOS Institute fur Oberflachen und Schichtanalytik GmbH | Merz R.,IFOS Institute fur Oberflachen und Schichtanalytik GmbH | And 5 more authors.
Plasma Processes and Polymers | Year: 2010

A first inter-laboratory comparison was conducted to demonstrate and document the capability of interested laboratories to measure the fraction of C-OH species on a plasma oxidised poly(propylene) sample by using a chemical derivatisation XPS approach. This report presents the results from that inter-laboratory comparison and includes the data received for the measured values and their associated standard deviations of laboratory means, at a 95% confidence level. The measurements were guided by a protocol developed within the group before and summarised shortly in the paper. Standard deviations that may characterise the stateof-the-art for the nominally simple and rather often practised case of TFAA chemical derivatisation XPS of C-OH species on a plasma oxidised polyolefin surface are calculated following ISO 5725-2:1994. The main conclusion is that the associated degree of equivalence reached by the participating laboratories in this comparison is still low. Further research to improve chemical derivatisation XPS protocols is mandatory. Figure Presented © 2010 WlLEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

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