Carsten Gachot

Contact Mechanics of Laser-Textured Surfaces

We study numerically the contact mechanics of a flat and a curved solid. Each solid bears laser-induced, periodic grooves on its rubbing surface. Our surface topographies produce a similar load and resolution dependence of the true contact area as nominally flat, but randomly rough, self-affine surfaces. However, the contact area of laser-textured solids depends on their relative orientation. The estimated true contact areas correlate with kinetic friction measurements.

Manufacturing and Tribological Investigation of Hot Micro-Coined Lubrication Pockets

Friction and wear of lubricated machine elements can be reduced by the introduction of lubrication pockets produced by surface texturing. Different manufacturing methods can be taken into consideration whereas a forming process offers the possibility for mass production. Hot micro coining is a forming process, which allows manufacturing of surface textures with different shapes and dimensions into a flat and deformable material, e.g. seals made of steel. In this work hemispherical and ellipsoid pockets with a maximum depth of 100 µm have been embossed into stainless steel (AISI 304). To ensure that the used process parameters will not lead to tool damage, Finite Element simulations were performed and experimentally verified. First tribological experiments were conducted on a ball-on-disk tribometer in order to study the tribological properties of hemispherical structures with pockets depths of 50 and 100 µm. Different sliding velocities were applied to study the coefficient of friction and wear volume for different lubrication conditions. A comparison between coined and not‑coined specimens demonstrates that the micro coined surface textures lead to a significant reduction in the wear volume at boundary and mixed lubrication conditions.

Long-lasting solid lubrication by CNT-coated patterned surfaces

The use of lubricants (solid or liquid) is a well-known and suitable approach to reduce friction and wear of moving machine components. Another possibility to influence the tribological behaviour is the formation of well-defined surface topographies such as dimples, bumps or lattice-like pattern geometries by laser surface texturing. However, both methods are limited in their effect: surface textures may be gradually destroyed by plastic deformation and lubricants may be removed from the contact area, therefore no longer properly protecting the contacting surfaces. The present study focuses on the combination of both methods as an integral solution, overcoming individual limitations of each method. Multiwall carbon nanotubes (MWCNT), a known solid lubricant, are deposited onto laser surface textured samples by electrophoretic deposition. The frictional behaviour is recorded by a tribometer and resulting wear tracks are analysed by scanning electron microscopy and Raman spectroscopy in order to reveal the acting tribological mechanisms. The combined approach shows an extended, minimum fivefold longevity of the lubrication and a significantly reduced degradation of the laser textures. Raman spectroscopy proves decelerated MWCNT degradation and oxide formation in the contact. Finally, a lubricant entrapping model based on surface texturing is proposed and demonstrated.

Synergetic effect of laser patterning and micro coining for controlled lubricant propagation

In this study, the anisotropic spreading behavior of Poly-(alpha)-olefin oil (kinematic viscosity of 7.8 cSt at 100 °C) on stainless steel samples (AISI 403) having periodic, channel-like structures produced by hot micro-coining (periodicity of 400 μm and depth of 40 μm) as well as multi-scale structures (coining and laser patterning) was investigated. These results were compared to the behavior of periodic channels fabricated by direct laser interference patterning (periodicity of 5 μm and depth of 1 μm). The spreading behavior of a droplet (3 μl) was studied for a polished reference as well as for all modified surfaces and recorded by a digital light microscope. From this study, it can be concluded that the polished reference leads to an isotropic spreading behavior resulting from the stochastic surface roughness without any preferential orientation whereas all structured samples induce an anisotropic spreading behavior but with different degrees of anisotropy. The observed behavior can be well correlated with pinning induced by the grooves thus hindering the droplet propagation perpendicular to the grooves and the generation of capillary forces which favor the droplet movement along the grooves. It could be proved that the structural depth is a very desicive parameter with regard to the resulting spreading behavior. The multi-scale surface combining large structural depths and the steeper pattern geometry of the micro-coined surface with much smaller grooves of the laser-structure shows the largest anisotropic spreading behavior due to a stronger pinning and increased capillary forces.

Characterization of Frictional Stressed White Etching Layers out of Cutting Tools by Means of Transmission Electron Microscopy (TEM)

Abstract White etching layers have been found at the surface of water submerged cutting tools and have been analyzed by transmission electron microscopy (TEM). For high resolution investigations, samples had been prepared by sandwich and lift-out preparation. The investigations have shown a grain refinement at the surface. The origin martensitic microstructure is transferred into a nano-crystalline microstructure of domain sizes down to 50 nm. The crystallographic structure was identified by electron diffraction as martensite. Moreover, the depth of the grain-refined layer is not as deep as the white etching phenomenon as it is found in conventional optical micrographs. Thus, the white etching layer is not only caused by the grain refinement.

Microstructural characterization of laser-irradiated bulk copper under dry sliding conditions

The functionalization of surfaces for tribological applications is still a challenging task. There are numerous techniques for improving the friction and wear behaviour of metallic surfaces ranging from different coatings to textured surfaces. A promising approach for textured surfaces is the usage of laser interference patterning by a pulsed nanosecond Nd:YAG laser. Copper samples have been laser patterned using the laser interference metallurgy process. The resulting topographies were evaluated by white light interferometry and scanning electron microscopy in order to study the homogeneity of the structures at both macro- and micro-scales. Electron backscattering diffraction and transmission electron microscopy were performed in order to study the microstructural and oxide layer modifications induced by laser irradiation. Electron backscattering diffraction measurement did not show fine grains but a small proportion of misorientation due to the high quenching rate. Transmission electron microscopy did not allow to observe oxide layers. The friction and wear behaviour of bulk copper samples structured by laser interference metallurgy have been studied under dry sliding conditions using a tribometer in linear reciprocating sliding mode. Friction coefficient measurements revealed that patterned surfaces present a reduction in friction of up to 70% compared to untreated samples due to their good bearing properties and the reduction of the contact area.

Direct laser interference patterning, 20 years of development: from the basics to industrial applications

Starting from a simple concept, transferring the shape of an interference pattern directly to the surface of a material, the method of Direct Laser Interference Patterning (DLIP) has been continuously developed in the last 20 years. From lamp-pumped to high power diode-pumped lasers, DLIP permits today for the achievement of impressive processing speeds even close to 1 m2/min. The objective: to improve the performance of surfaces by the use of periodically ordered micro- and nanostructures. This study describes 20 years of evolution of the DLIP method in Germany. From the structuring of thin metallic films to bulk materials using nano- and picosecond laser systems, going through different optical setups and industrial systems which have been recently developed. Several technological applications are discussed and summarized in this article including: surface micro-metallurgy, tribology, electrical connectors, biological interfaces, thin film organic solar cells and electrodes as well as decorative elements and safety features. In all cases, DLIP has not only shown to provide outstanding surface properties but also outstanding economic advantages compared to traditional methods.

Wear behavior of micro-coined steel surfaces under mixed lubrication:

Purpose The purpose of this study is to investigate the influence of hemispherical structures fabricated by hot micro-coining on the resulting wear performance. Hemispherical structures with different area densities (20 and 30 per cent), depths (50 and 100 µm) and diameters (100 and 200 µm) were fabricated by hot micro-coining on stainless steel samples. Design/methodology/approach The wear performance of these samples was studied using a ball-on-disk tribometer in rotational sliding mode using a normal load of 30 N and a fixed sliding velocity of 2 cm/s. Two different poly-(alpha)-olefin (PAO) oils without any additive having a kinematic viscosity of 4 and 40 cSt, were used to study the influence of the oil viscosity on the wear behavior. Findings Concerning the polished reference, an enlarged wear volume with an increase in the cycle number and the oil viscosity was observed. In the case of the micro-coined surfaces, all samples demonstrate a pronounced reduction in the wear volume (up to a factor of 100 for PAO 40) compared to the polished reference irrespective of the oil viscosity used. Originality/value This study details new research work studying the wear behavior of hot micro-coined surfaces.

Frictional Performance Assessment of Cemented Carbide Surfaces Textured by Laser

Cemented carbides are advanced engineering materials often used in industry for manufacturing cutting tools or supporting parts in tribological system. In order to improve service life, special attention has been paid to change surface conditions by means of different methods, since surface modification can be beneficial to reduce the friction between the contact surfaces as well as to avoid unintended damage. Laser surface texturing is one of the newly developed surface modification methods. It has been successfully introduced to fabricate some basic patterns on cemented carbide surfaces. In this work, Direct Laser Interference Patterning Technique (DLIP) is implemented to produce special line-like patterns on a cobalt (Co) and nickel (Ni) based cemented tungsten carbide grade. It is proven that the laser-produced patterns have high geometrical precision and quality stability. Furthermore, tribology testing using a nano-tribometer unit shows that friction is reduced by the line-like patterns, as compared to the polished one, under both lubricated and dry testing regimes, and the reduction is more pronounced in the latter case.

Tuned wettability of material surfaces for tribological applications in miniaturized systems by laser interference metallurgy

Innovative surfaces are successful, if we succeed to put in the correct place the correct property with technological efficiency. Until now, material surfaces can be systematically structured in different ways in order to fulfil chemical or mechanical requirements such as corrosion protection or wear resistance for example. Moreover, the properties of materials are strongly related to their microstructure as well as to their spatial distribution. For that reason, the design of materials with tailored microstructures is a key for the functionalization of surfaces. This is possible by an artificial fabrication technique called Laser Interference Metallurgy. In this context, textured or functionalized surfaces are beneficial in overcoming stiction and adhesion in MEMS devices. With regard to tribological applications, a systematic study of the effect of geometrically differing laser interference patterns on the wetting behaviour of metallic gold thin films with a thickness of about 300 nm and 125 μm thick polyimide foils should be presented. It could be shown that in case of gold films, a laser interference patterning reinforces the hydrophilic sample behavior whereas the polyimide foils reveal a significant increase in hydrophobicity after the laser patterning process. Both wetting regimes are advantageous under dry or lubricated friction conditions. The corresponding geometrical limits of the abovementioned method concerning the structure depth, periodicity and pattern form has been determined. All the samples have been characterized by scanning electron and focused ion beam microscopy and white light interferometry. Additionally, IR spectroscopy has been applied to the polyimide samples in order to separate topographic and chemical influences.