Dirk Spaltmann

The influence of roughness on friction: Part I: The influence of a single step

Part I of this paper series presents experiments whose goal is to reveal a correlation between roughness and friction force for contact areas on the micrometer scale. An elastic deformation model was developed to describe this correlation. This model concentrates on friction processes which are dominated by elastic deformation. The experiments were performed on defined model tribosystems with a tribometer of our own design equipped with an atomic force microscope (AFM) and an optical microscope. The samples were silicon wafers whose roughness was created by lithography and the counter bodies were balls of different materials. In contrast to part II the roughness of the samples consists only of a single step. In part II the elastic deformation model will be tested on roughness with two and more steps inside the contact area. Finally, the model will be validated on a surface which has a statistical roughness resulting from various grinding processes.

The influence of roughness on friction: Part II. The influence of multiple steps

Aim of the present work is to derive a correlation between roughness and friction force. This part II of a paper series tests the elastic deformation model presented in part I on more general tribosystems. Where in part I the influence of a single artificially generated step on friction force was investigated, part II considers the influence of two and more structures inside the contact area on friction force. This artificial roughness was created by lithography on silicon wafers, as regular grooves and ridges. The distances between the steps of the grooves ranged from 100 to 0.1 μm. Finally, the correlation between roughness and friction force was validated on surfaces with statistical roughness created by various grinding processes.

Applications of laser-induced periodic surface structures (LIPSS)

Laser-induced periodic surface structures (LIPSS, ripples) are a universal phenomenon that can be observed on almost any material after the irradiation by linearly polarized laser beams, particularly when using ultrashort laser pulses with durations in the picosecond to femtosecond range. During the past few years significantly increasing research activities have been reported in the field of LIPSS, since their generation in a single-step process provides a simple way of nanostructuring and surface functionalization towards the control of optical, mechanical or chemical properties. In this contribution current applications of LIPSS are reviewed, including the colorization of technical surfaces, the control of surface wetting, the tailoring of surface colonization by bacterial biofilms, and the improvement of the tribological performance of nanostructured metal surfaces.

Femtosecond Laser Texturing of Surfaces for Tribological Applications

Laser texturing is an emerging technology for generating surface functionalities on basis of optical, mechanical, or chemical properties. Taking benefit of laser sources with ultrashort (fs) pulse durations features outstanding precision of machining and negligible rims or burrs surrounding the laser-irradiation zone. Consequently, additional mechanical or chemical post-processing steps are usually not required for fs-laser surface texturing (fs-LST). This work aimed to provide a bridge between research in the field of tribology and laser materials processing. The paper reviews the current state-of-the-art in fs-LST, with a focus on the tribological performance (friction and wear) of specific self-organized surface structures (so-called ripples, grooves, and spikes) on steel and titanium alloys. On the titanium alloy, specific sickle-shaped hybrid micro-nanostructures were also observed and tribologically tested. Care is taken to identify accompanying effects affecting the materials hardness, superficial oxidation, nano- and microscale topographies, and the role of additives contained in lubricants, such as commercial engine oil.

CO2-neutral fuels and lubricants based on second generation oils such as Jatropha.

For the production of raw materials, the generation of energy and for their mobility, the industrialized countries strongly depend on the import of fossil resources. Second generation resources, such as Jatropha, grow on arid soils and produce non-edible oils. Thus, their cultivation does not compete with food production for farmland. Oils of such plants are therefore suggested as sustainable, CO2-neutral and regenerative alternatives to fossil fuels and lubricants. The present work establishes one of the first functional profiles of second generation oils with properties relevant for the use as fuel or lubricants in order to validate the potential for substituting fossil resources. It concentrates on the characterization of oil gained from the Jatropha Curcas plant to be used as fuel and lubricant. Properties were determined such as pour point, flash point, lubricity of diesel-like fuel in high frequency reciprocating rig, high-temperature/high-shear viscosity, and viscosities as function of temperature, extreme pressure behavior, oxidation resistance as well as its toxicity and bio-degradability. These properties are compared to those of currently used plant oils and fossil oils. The fatty acid chain length distribution of Jatropha oil was determined and found to be close to that of palm oil. This qualifies Jatropha oil as a substitute, thus releasing the pressure on the prices of food based oils. First tribological characterizations were carried out and are presented here, showing impressive performance of the as pressed filtered pure Jatropha plant oil. In oil dilution tests carried out on piston-ring/cylinder-liner test rigs, the performance of oils processed from plants such as Jatropha are compared to ester-based, polyalkylene glycol based, and hydrocarbon-based oils. Finally, functional properties of Jatropha were compared to further possible second generation bio-oils and aspects of availability and costs are discussed.