Laser interference metallurgy of metallic surfaces for tribological applications

Tribological phenomena play a decisive role in diverse systems. For many years, researchers have sought to alleviate these problems and to understand their origin. There are many potential solutions to manipulate friction. In particular, the rapidly growing field of laser surface texturing has attracted a lot of attention in the last decades and shown to be an effective means of improving tribological properties. A possible approach of laser surface texturing to scrutinize the effects of various pattern geometries and lateral feature sizes in one single laser shot is the so called Laser Interference Metallurgy (LIMET) which will be applied within this thesis. The aim is to study the microstructural and topographic possibilities of LIMET concerning the tribological performance of laser-patterned thin film systems (Au and TiAl multilayer) and bulk aluminium as well as steel surfaces. It will be shown that depending on the laser fluence for example, distinct grain size arrangements and intermetallic phase composites can be created with superior tribological properties compared to the unpatterned reference situation. Moreover, a successful process combination of micro-coining and LIMET will be presented with an enhanced oil retainment capability under lubrication. Finally, the results of laser-textured steel surfaces and their ability to geometrically interlock will be shown. Depending on the relative alignment between the textured sliding surfaces and the selected pattern line-spacing, the frictional response can be significantly influenced. Most of the experimental results will be directly correlated to simulations in order to reveal the underlying phenomena.Reibung spielt eine zentrale Rolle in vielen Bereichen. Insbesondere die Steuerung von Reibung ist dabei von enormer Bedeutung. Zur Minimierung von Reibung sind in den vergangenen Jahrzehnten bereits unzählige Methoden für trockene und geschmierte Bedingungen entwickelt worden. Besonders laserstrukturierte Oberflächen scheinen hierbei vielversprechend für tribologische Anwendungen zu sein. Ein Ansatz, mikrostrukturell und topographisch ma geschneiderte Werkstoffe zu erzeugen, ist die Laserinterferenzmetallurgie (LIMET). Das Ziel der vorliegenden Arbeit ist die Untersuchung der erzielbaren mikrostrukturellen und topographischen Effekte durch LIMET und deren Auswirkungen auf die tribologischen Eigenschaften von metallischen Schichtsystemen (Au und TiAl-Multilagen) sowie massiven Aluminium- und Stahloberflächen. Hierbei wird gezeigt, dass es z.B. abhängig von der gewählten Fluenz möglich ist, Korngrößenarchitekturen oder intermetallische Phasenkomposite definiert zu erzeugen, deren Reibeigenschaften denen der unbehandelten Ausgangssituation überlegen sind. Des Weiteren wird die erfolgreiche Kombination des Mikroprägens mit der LIMETMethode vorgestellt. Die Ergebnisse zeigen, dass insbesondere die Ölspeicherfähigkeit in den hierarchischen Kavitäten unter geschmierten Bedingungen deutlich gegenüber einer unstrukturierten Oberfläche gesteigert ist. Schlie lich werden die tribologischen Auswirkungen von beidseitig strukturierten Stahloberflächen in Abhängigkeit von deren gegenseitiger Orientierung und den gewählten Strukturabständen näher untersucht. Je nach Ausrichtung und den genannten Strukturperiodizitäten lässt sich das Reibverhalten reproduzierbar in gewissen Grenzen manipulieren. Die experimentellen Ergebnisse werden mit Simulationen verknüpft, um dadurch die Wirkmechanismen näher zu beleuchten

Laser Surface Texturing of TiAl Multilayer Films—Effects of Microstructure and Topography on Friction and Wear

Laser surface texturing is an efficient way to control the friction and wear properties of materials. Although described in many papers, most previous work relates to a pure topographic view of laser-textured surfaces. As lasers are heat sources, their thermal impact during treatment can be high enough to modify the material’s microstructure or surface chemistry and affect tribological properties as well. This research took a closer look at the microstructure of laser-textured TiAl multilayers, besides topographic aspects. Direct laser interference patterning was used to create well-defined line-like surface textures in TiAl multilayers with differing lateral feature sizes in the micron range. High-resolution techniques such as TEM and XRD highlighted the effect of this method on microstructure, and in particular, the phase situation of the TiAl multilayer. Thermal simulations demonstrated that the maximum achievable temperatures were around 2000 K, thus being high enough to melt Ti and Al. Cooling rates on the order of 109 K/s depended on the lateral feature size, potentially leading to metastable microstructures. Finally, ball-on-disk tests on as-textured TiAl specimens showed a reduction in wear under dry conditions depending on the periodicity of the line-like textures used

Effect of Low Depth Surface Texturing on Friction Reduction in Lubricated Sliding Contact

Laser surface texturing is an interesting possibility to tailor materials’ surfaces and thus to improve the friction and wear properties if proper texture feature sizes are selected. In this research work, stainless steel surfaces were laser textured by two different laser techniques, i.e., the direct laser interference patterning by using a nanosecond pulsed Nd:YAG laser and additionally by an ultrashort pulsed femtosecond Ti:Sa. The as-textured surfaces were then studied regarding their frictional response in a specially designed linear reciprocating test rig under lubricated conditions with a fully formulated 15W40 oil. Results show that dimples with smaller diameter lead to a significant reduction in the coefficient of friction compared to the dimples with a larger diameter and surfaces with a grid-like surface pattern produced by direct laser interference patterning

Direct Laser Interference Patterning: Tailoring of Contact Area for Frictional and Antibacterial Properties

Surface functionalization by topographic micro- and nano-structures in order to achieve unique properties, like super-hydrophobicity or ultrahigh light absorption, is a common strategy in nature. In this paper, direct laser interference patterning (DLIP) is presented as a promising tool allowing for the generation of such surface patterns on technical surfaces in order to mimic these biological surfaces and effects. Friction optimization and antibacterial effects by DLIP are exemplarily described. Topographic surface patterns on the micro- and nano-scale demonstrated a significant reduction in the coefficient of friction and bacterial adhesion. It was shown that in both cases, the control of the contact area between surfaces or between surface and bacteria is of utmost importance

APPLICATION OF MXENE NANOSHEETS FOR IMPROVING MACHINE ELEMENTS PROPERTIES

Due to a growing energy problems in the last decades, different ideas and methods for saving energy and efficiency improvement become more and more requested. Taking into account that almost half of the energy losses in machines and mechanisms occur in frictional processes thus the importance of improving tribology performances in different machine elements is rather clear. Machine elements such as gears, rolling and sliding bearings in various applications are usually operating with different lubricants in order to reduce friction and wear of their contacting surfaces. Besides surfaces in contact cannot be conventionally lubricated due to legal and environmental restrictions or physical limitations, in applications in food, textile, paper or medical industry as well as limitations due to ultra-clean conditions, extreme temperatures, radiation or vacuum, solid lubricants gain more and more attention due to the possibility to maintain essential lubrication properties. MXenes are a novel class of new two-dimensional materials, derived from well established MAX phases such as Ti3AlC2, which have the ability for a wide use in fields and applications due to their unique structure and properties. The distinct structure, low shear resistance, and easy-to-modify ability endow MXenes with particularly superior lubrication potentials. Authors of the paper present an overview synthesis and variety of MXenes, their mechanical and frictional properties dealing with ideas for possible applications in several machine elements aimed to reduce friction, wear rate and improve their performances making longer working life

APPLICATION OF MXENE NANOSHEETS FOR IMPROVING MACHINE ELEMENTS PROPERTIES

Due to a growing energy problems in the last decades, different ideas and methods for saving energy and efficiency improvement become more and more requested. Taking into account that almost half of the energy losses in machines and mechanisms occur in frictional processes thus the importance of improving tribology performances in different machine elements is rather clear. Machine elements such as gears, rolling and sliding bearings in various applications are usually operating with different lubricants in order to reduce friction and wear of their contacting surfaces. Besides surfaces in contact cannot be conventionally lubricated due to legal and environmental restrictions or physical limitations, in applications in food, textile, paper or medical industry as well as limitations due to ultra-clean conditions, extreme temperatures, radiation or vacuum, solid lubricants gain more and more attention due to the possibility to maintain essential lubrication properties. MXenes are a novel class of new two-dimensional materials, derived from well established MAX phases such as Ti3AlC2, which have the ability for a wide use in fields and applications due to their unique structure and properties. The distinct structure, low shear resistance, and easy-to-modify ability endow MXenes with particularly superior lubrication potentials. Authors of the paper present an overview synthesis and variety of MXenes, their mechanical and frictional properties dealing with ideas for possible applications in several machine elements aimed to reduce friction, wear rate and improve their performances making longer working life

Microstructural and Chemical Characterization of the Tribolayer Formation in Highly Loaded Cylindrical Roller Thrust Bearings

Zinc dithiophosphates (ZDDP) have been widely applied in automobile industry for over 70 years as a lubricant additive for wear protection. Tribolayers have been described as blue- and brown-colored layers on surfaces observed by microscopical observation or even bare eye presumably as a consequence of layer thickness or chemical composition. However, the reaction pathways of ZDDP tribolayers are still not yet fully understood. In the present study, the difference between the blue- and brown-colored tribolayers has been revealed by high resolution methods in cylindrical roller thrust bearings at relatively high contact pressures of around 1.92 GPa. After running a FE8 standard bearing test with a normal load of 80 kN and a temperature of 60 °C, said tribolayers could be identified on the bearing surfaces. By using Raman spectroscopy, it could be shown that the blue-colored layers are enriched by FeS and ZnS whereas the brown-colored layers show a significant amount of Fe3O4. This is an interesting finding as it clearly shows a correlation between the color appearance of the films and the chemical composition besides potential film thickness variations. Finally, transmission electron microscopy verified the amorphous nature of the formed tribolayer which is in a good agreement with literature

Se Nanopowder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions

: Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2 , even though selenides and tellurides have demonstrated superior tribological properties. Here, an innovative in operando conversion of Se nanopowders into lubricious 2D selenides, by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films, is described. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in the shear-induced synthesis of selenide monolayers from nanopowders. The use of Se nanopowder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nanopowder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in operando and exploit their friction- and wear-reducing impact

Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures: An Experimental and Machine Learning Approach

This work critically scrutinizes and compares the tribological performance of randomly distributed surface pores in sintered materials and precisely tailored laser textures produced by different laser surface texturing techniques. The pore distributions and dimensions were modified by changing the sintering parameters, while the topological features of the laser textures were varied by changing the laser sources and structuring parameters. Ball-on-disc tribological experiments were carried out under lubricated combined sliding-rolling conditions. Film thickness was measured in-situ through a specific interferometry technique developed for the study of rough surfaces. Furthermore, a machine learning approach based on the radial basis function method was proposed to predict the frictional behavior of contact interfaces with surface irregularities. The main results show that both sintered and laser textured materials can reduce friction compared to the untextured material under certain operating conditions. Moreover, the machine learning model was shown to predict results with satisfactory accuracy. It was also found that the performance of sintered materials could lead to similar improvements as achieved by textured surfaces, even if surface pores are randomly distributed and not precisely controlled

Laserinterferenzmetallurgie an metallischen Oberflächen für tribologische Applikationen

Tribological phenomena play a decisive role in diverse systems. For many years, researchers have sought to alleviate these problems and to understand their origin. There are many potential solutions to manipulate friction. In particular, the rapidly growing field of laser surface texturing has attracted a lot of attention in the last decades and shown to be an effective means of improving tribological properties. A possible approach of laser surface texturing to scrutinize the effects of various pattern geometries and lateral feature sizes in one single laser shot is the so called Laser Interference Metallurgy (LIMET) which will be applied within this thesis. The aim is to study the microstructural and topographic possibilities of LIMET concerning the tribological performance of laser-patterned thin film systems (Au and TiAl multilayer) and bulk aluminium as well as steel surfaces. It will be shown that depending on the laser fluence for example, distinct grain size arrangements and intermetallic phase composites can be created with superior tribological properties compared to the unpatterned reference situation. Moreover, a successful process combination of micro-coining and LIMET will be presented with an enhanced oil retainment capability under lubrication. Finally, the results of laser-textured steel surfaces and their ability to geometrically interlock will be shown. Depending on the relative alignment between the textured sliding surfaces and the selected pattern line-spacing, the frictional response can be significantly influenced. Most of the experimental results will be directly correlated to simulations in order to reveal the underlying phenomena.Reibung spielt eine zentrale Rolle in vielen Bereichen. Insbesondere die Steuerung von Reibung ist dabei von enormer Bedeutung. Zur Minimierung von Reibung sind in den vergangenen Jahrzehnten bereits unzählige Methoden für trockene und geschmierte Bedingungen entwickelt worden. Besonders laserstrukturierte Oberflächen scheinen hierbei vielversprechend für tribologische Anwendungen zu sein. Ein Ansatz, mikrostrukturell und topographisch ma geschneiderte Werkstoffe zu erzeugen, ist die Laserinterferenzmetallurgie (LIMET). Das Ziel der vorliegenden Arbeit ist die Untersuchung der erzielbaren mikrostrukturellen und topographischen Effekte durch LIMET und deren Auswirkungen auf die tribologischen Eigenschaften von metallischen Schichtsystemen (Au und TiAl-Multilagen) sowie massiven Aluminium- und Stahloberflächen. Hierbei wird gezeigt, dass es z.B. abhängig von der gewählten Fluenz möglich ist, Korngrößenarchitekturen oder intermetallische Phasenkomposite definiert zu erzeugen, deren Reibeigenschaften denen der unbehandelten Ausgangssituation überlegen sind. Des Weiteren wird die erfolgreiche Kombination des Mikroprägens mit der LIMETMethode vorgestellt. Die Ergebnisse zeigen, dass insbesondere die Ölspeicherfähigkeit in den hierarchischen Kavitäten unter geschmierten Bedingungen deutlich gegenüber einer unstrukturierten Oberfläche gesteigert ist. Schlie lich werden die tribologischen Auswirkungen von beidseitig strukturierten Stahloberflächen in Abhängigkeit von deren gegenseitiger Orientierung und den gewählten Strukturabständen näher untersucht. Je nach Ausrichtung und den genannten Strukturperiodizitäten lässt sich das Reibverhalten reproduzierbar in gewissen Grenzen manipulieren. Die experimentellen Ergebnisse werden mit Simulationen verknüpft, um dadurch die Wirkmechanismen näher zu beleuchten