DOI: 10.1002/adem.201400007 Ultra-Low Friction on 100Cr6-Steel Surfaces After Direct Laser Interference Patterning** By Matthias Bieda,* Cindy Schm € adicke, Teja Roch and Andr  es Lasagni This study discusses the effects of surface topographies on the frictional behavior of 100Cr6 bearing steel. A solid state laser with nanosecond pulses is used to produce one- and two-dimensional periodic micropatterns using direct laser interference patterning. Line-, cross-, and dot-like patterns with pitches of 5 mm, and aspect ratios (AR) between 0.02 and 0.17 are fabricated. The friction tests of the surface textured samples are performed under lubricating conditions using a ball-on-disk configuration in rotating mode. The results show that through the surface structure a reduction of the friction coefficient from 25 to 65% can be achieved compared to unstructured surfaces. The smallest coefficients of friction are obtained for ARs between 0.07 and 0.11. 1. Introduction The efficiency, reliability, and durability of machine components depend on friction occurring at the sliding contact interface. [1] Inefficient lubrication may result in high friction and wear losses. This can adversely affect the fuel consumption of engine systems and powertrain compo- nents. [2–4] Due to stricter regulations to reduce carbon dioxide (CO 2 ) emission from cars, innovative solutions must therefore be developed to improve the tribological performance of mechanical components, for example, by using low friction materials, coatings, and lubricants. [5–7] Surface texturing is also found to be beneficial for tribological properties of many mechanical components. Positive results for textured surfaces in components were reported for mechanical seals, piston rings, and thrust bearings. [8–11] Depending on the application, surface texturing can provide several advantages, e.g., minimum contact area, and thus the reduction of adhesive forces providing reservoirs for lubricants and wear particles as well as supporting hydrodynamic effects. [12–17] The latter are strongly influenced by the geometry of the artificially generated surface pattern. [18–21] Tribological investigations on patterned metallic surfaces showed that the edge steepness Da is one of the most decisive parameter under lubricating conditions. This parameter can be calculated by considering all slopes between successive points of the pattern profile: [18,22] Da ¼ 1 n  1 X n1 i¼1      dy i dx i      ð1Þ where n is the number of slopes, and dy i /dx i the derivatives at each point of the slope. The coefficient of friction (COF) typically increases for high values of Da. [18,22] However, there is an optimum slope, especially for microstructures, where maximum hydrodynamic pressure occurs. [21] The most established techniques to generate controlled surface textures can be divided into three main methods: mechanical, lithographic, and laser-based processes. [13,14,23–26] Among laser beam techniques, laser surface texturing (LST) has already been studied by other authors to improve sliding properties. [27–30] Likewise, direct laser interference patterning (DLIP) is one technology that is capable of producing surface structures on metals. DLIP allows the large area fabrication of periodic line-, cross-, and dot-like structures with feature sizes in the micro- and submicrometer length scale in a single step at high speeds (several square centimetres per second). [31–36] By controlling the angle between the interfering laser beams, periodic structures with different sizes and shapes can be fabricated under regular ambient conditions. [*] M. Bieda, A. Lasagni Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstrasse 28, 01277 Dresden, Germany E-mail: matthias.bieda@iws.fraunhofer.de C. Schm€ adicke Currently at Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische Universit€ at Dresden, 01062 Dresden, Germany T. Roch, A. Lasagni Chair for Laser Structuring in Manufacturing Technology, Technische Universit€ at Dresden, 01062 Dresden, Germany [**] This work was financially supported by the Fraunhofer- Gesellschaft under Grant No. Attract 692174. The European Union and the Free State of Saxony are also acknowledged for financial support of the (Dresdner) Innovationszentrum Energieeffizienz – DIZEeff (Grant No. 14262/2423). 102 wileyonlinelibrary.com © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2015, 17, No. 1 FULL PAPER