Rolf Merz

Friction and wear mechanisms of tungsten-carbon systems: a comparison of dry and lubricated conditions.

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.

Modular modelling of thermal building behaviour using Modelica

This article presents a modular model library describing various processes of thermal building behaviour. The library is implemented in the modelling language Modelica emulating thermal building dynamics in a comprehensive way. This includes the heating, ventilation and air-conditioning system as well as solar radiation and user behaviour. The way in which the dynamic behaviour of many coupled subsystems can be investigated in only one universal modelling and simulation environment will be illustrated. At the same time, the object-oriented and acausal language Modelica ensures a high degree of reusability. The library is free and can be downloaded from the website of the Modelica Association: http://www.modelica.org/libraries.shtml

Surface softening in metal-ceramic sliding contacts: an experimental and numerical investigation.

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.

Removing biofilms from stainless steel without changing surface properties relevant for bacterial attachment

The influence of oxygen (and argon) plasma cleaning and a base-acid cleaning procedure on stainless steel surfaces was studied. The main aim was to clean stainless steel samples from Paracoccus seriniphilus biofilms without changing the surface properties which are relevant for bacterial attachment to allow reuse in a biofilm reactor. It is shown that oxygen plasma cleaning, which very successfully removes the same kind of biofilm from titanium surfaces, is not suitable for stainless steel. It largely influences the surface chemistry by producing thick metal oxide layers of varying compositions and changing phenomenological surface properties such as wettability. A promising method without changing surface properties while cleaning satisfactorily is a combination of base and acid reagents at elevated temperature.

Solvent cleaning and wettability of technical steel and titanium surfaces

The ability of the cleaning agents such as cyclohexane, acetone, isopropanol and hydrogen peroxide to remove oil from technical steel and titanium samples was studied in systematic experiments. The composition of the organic residues and the layer thickness on the differently cleaned surfaces were determined via X-ray photoelectron spectroscopy analysis, and the corresponding water contact angles were measured. The ability of the cleaning agents to remove oil was found to increase in the following order: acetone, isopropanol, cyclohexane and hydrogen peroxide. The relationship between the chemical composition of the residues and the observed contact angles is investigated, and a quantitative correlation is developed.

On the Role of Oxidation in Tribological Contacts under Environmental Conditions

Oxidation processes in tribological steel contacts are investigated, which are treated in a dry sliding, linear reciprocating model tribometer, by EDX (energy dispersive X-ray spectroscopy), AES (Auger electron spectroscopy), and HREFTEM (high resolution energy filtered transmission electron microscopy). Typical for steel contacts under environmental conditions is the feature that the counterparts are separated by oxide layers, which influence the tribological properties. And vice versa the tribological load will influence and change the oxide layers. The interaction of this dynamically coupled system was resolved by focussing the postexperimental surface analysis to long time stable balance states. As special challenge for the analyst of the tribological experiment under environmental conditions a postexperimental grown oxide layer covers the tribological induced changes and has to be distinguished from the tribological induced changes. Thick oxide layers, formed during the tribological load, were observed, which start to grow in form of islands and at the end separate the metallic bulk materials of the counterparts completely and avoid direct metal-metal contact. Thicknesses up to some microns strength, exceeding native oxide layers by magnitudes, were reached. Ploughing under fresh surface oxide and compacting and embedding of fresh oxidized debris particles were identified as main mechanisms responsible for the growing of these thick oxide layers.