Development of sintered bearings with minimal friction losses and maximum life time using infiltrated liquid crystalline lubricants Tobias Amann a,n , Andreas Kailer a , Susanne Beyer-Faiß b , Werner Stehr c , Björn Metzger d a Fraunhofer Institute for Mechanics of Materials IWM, Freiburg, Germany b Dr. Tillwich GmbH Werner Stehr, Horb am Neckar, Germany c Werner Stehr Tribologie GmbH, Horb am Neckar, Germany d SL Gleitlagertechnik GmbH, Marburg, Germany article info Article history: Received 1 December 2015 Received in revised form 15 February 2016 Accepted 16 February 2016 Available online 24 February 2016 Keywords: Sintered iron bearings Ultralow friction Computer tomography In-situ wear measurement abstract Sintered bearings are widely used as machine elements due to their low costs in combination with a stable tribological performance. Given the current and future challenge of global conservation of energy and reliability of mechanically systems, the increase of energy efficiency of sintered bearings would have strong effect due to the tremendous number of mechanical devices. The approach of this investigation is to develop a complete system consisting of a lubricant, which is adapted for an optimized sliding bearing. Friction tests with a rotating ball-on-3-plates tribometer (m 0.005) and an application oriented com- ponent test (m 0.01) show that extremely low friction values can be realized using a liquid-crystalline lubricant in contrast to standard oils. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction Friction and wear occur in almost all engineering systems and cause energy losses and consequently high costs [1–5]. Improving energy efficiency of mechanical systems is one of the key factors to reduce the energy consumption and thus the global carbon emissions despite an increasing demand for energy. It has been estimated that approximately 11% of the annual total energy consumption for turbomachinery, transportation, power genera- tion and industrial processes can be economized with tribological optimization [6]. To realize long-life stable and energy efficient technical sys- tems, friction and wear can be reduced by developing novel lubrication systems. These efforts to reduce friction losses and simultaneously regarding the ecological balance can be summar- ized in the topics ‘sustainable’ or ‘green’ tribology [7]. Due to the minimization and increase of energy density in tribological sys- tems the requirements for lubricants increase. For technical applications lubricants are additivated with special surface active substances (friction modifiers, extreme pressure additives) to improve the tribological performance [8–10]. But especially in terms of energy efficiency, standard lubricants on mineral oil basis and even synthetic oils have a limited potential to significantly reduce friction and wear. Therefore novel approaches for efficient lubrication are being developed on the basis of complex fluids like mesogenic fluids (MFs) or ionic liquids (ILs) [11–19]. This work focuses on the development of new lubricants on the basis of complex fluids with specific physical and chemical prop- erties [20,21]. The aim of this work was to realize ultralow friction in a technical system consisting of sintered iron bearings in com- bination with a specific adapted MF. In the scientific literature monomeric liquid crystals (MLCs) have been studied for more than two decades to explore their tribolo- gical properties. At the present time there is a variety of publica- tions, conference papers, books and patents on this issue. A sum- mary of the current state of research on liquid crystals for tribolo- gical applications is given from Carrión et al. [22]. The proceedings of the symposium "Tribology and the liquid-crystalline state" (Washington, 1990) published by Biresaw [23] highlight previous research on liquid crystals in tribology and their possible applica- tion in bearings which operate in the elastohydrodynamic range [20]. This application was identified for liquid crystals (LCs) because of their high loading capacity despite low viscosity. LCs can also be used as additives in lubricants to improve the tribological behavior [24–26]. In these papers it is shown that polar groups of the LCs interact with the surface of the testing specimens and build a tribo- layer in which the alkyl chains are orientated in direction of the motion of counter surface. For tribological systems which are lubricated with LCs the influence of shear and confinement due to the orientation of the molecules plays an important role because it influences the viscosity [27]. Fig. 1a illustrates the three possible alignments of LC molecules under shear. The corresponding Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International http://dx.doi.org/10.1016/j.triboint.2016.02.023 0301-679X/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. Tribology International 98 (2016) 282–291