Article pubs.acs.org/Langmuir
Ultralow Friction Induced by Tribochemical Reactions: A Novel Mechanism of Lubrication on Steel Surfaces Ke Li,† Tobias Amann,‡ Michael Walter,§ Michael Moseler,‡ Andreas Kailer,‡ and Jürgen Rühe*,† †
Department of Microsystems Engineering, University of Freiburg, IMTEK, Georges-Koehler-Allee 103, Freiburg, Germany Fraunhofer Institute for Mechanics of Materials IWM, Woehlerstraße 11, Freiburg, Germany § Freiburger Materialforschungszentrum, University of Freiburg, FMF, Stefan-Meier-Straße 21, Freiburg, Germany ‡
S Supporting Information *
ABSTRACT: The tribological properties of two steel surfaces rubbing against each other are measured while they are in contact with 1,3-diketones of varying structure. Such systems show after a short running-in period ultralow friction properties with a coefficient of friction of as low as μ = 0.005. It is suggested that the extremely favorable friction properties are caused by a tribochemical reaction between the 1,3-diketones and the steel surfaces, leading to formation of a chelated iron−diketo complex. The influence of temperature and the molecular structure of the 1,3 diketo-lubricants onto the friction properties of the system is elucidated under both static and dynamic conditions. With progression of the tribochemical reaction, the sliding surfaces become very conformal and smooth, so that the pressure is greatly reduced and further wear is strongly reduced. All iron particles potentially generated by wear during the initial running-in period are completely dissolved through complex formation. It is proposed that the tribochemical polishing reaction causes a transition from boundary lubrication to fluid lubrication.
1. INTRODUCTION
When the sliding surfaces are in direct contact with each other, contact between asperities can induce locally very high temperatures, sometimes called asperity flash temperature.7 It is known that the local temperature caused by the dissipation of frictional energy could be 100−150 °C higher than bulk fluid temperature.8 The large energy input through temperature or strong mechanical shear can induce tribochemical reactions in the sliding surfaces themselves, in the lubricant or between the lubricant molecules and the rubbing surfaces.9 Typical tribochemical reactions are for example oxidation reactions, mechanochemical fragmentation of organic high viscosity oils, or the formation of high molecular weight compounds through linking together of molecules present at the boundary of the sliding bodies. All of these reactions are caused by the input of large amounts of mechanical and/or thermal energy.10−12 Hsu et al.11 investigated the tribochemical reactions between hydrocarbon-based oils like di-2-ethylhexyl sebacate with various metals (copper, brass, aluminum, lead, and steel), which suggested that thermochemistry and organometallic chemistry dominate the interfacial chemical reaction in the sliding contact. Akbulut et al.12 studied tribological and tribochemical properties of nanothin silver films sliding on various metal and inorganic substrates, which found that
The energy efficiency of mechanical devices with moving components like bearings or gears is strongly reduced through friction. Additionally, friction causes wear and finally leads to mechanical failure of devices with moving parts.1 Accordingly, an important goal of research and development in tribology is to develop new strategies for lubrication in order to minimize friction and wear. Lubricants are used to separate the sliding bodies and avoid direct solid−solid contacts. Common examples are oils2 and greases,3 for example mineral or silicon oils4 or perfluoropolyethers.5 Depending on the operating conditions, lubrication can be divided into three different regimes (boundary, mixed and fluid lubrication), often depicted by the classical Stribeck curve.6 The main parameters which determine the friction are the load, the sliding speed, and the viscosity of the lubricating fluid. In the case of high load, low speed, and low viscosity the friction behavior is dominated by boundary lubrication. In this regime little fluid is located at the interface between the two friction partners, and the load is mainly carried by solid contacts. With decreasing load or increasing speed the surface separate, and more and more load is carried by the lubricant (mixed lubrication regime). At even lower loads/higher speed a full fluid film can be formed so that no solid contact occurs. In this regime the load is entirely supported by the fluid film (fluid lubrication), and typically low friction and little wear are observed.6 © 2013 American Chemical Society
Received: January 24, 2013 Revised: March 19, 2013 Published: April 1, 2013 5207
dx.doi.org/10.1021/la400333d | Langmuir 2013, 29, 5207−5213
Langmuir
Article
initially rough surfaces are chemically more reactive than initially smooth ones. The resulting changes of the properties of sliding surfaces and lubricant fluids after the reaction may have strong impact to their lubrication performance.13,14 In prior work15,16 on the tribological performance of steel surfaces, which are lubricated with certain 1,3-diketones, a very low coefficient of friction (≈0.005) has been observed after a short running-in period. The coefficient of friction for this system was much lower than that when a variety of standard oil-based lubricants were employed. A possible chemical reaction between the 1,3-diketones and steel surface was proposed.17 However, so far it is yet not well understood how the chemical properties of the 1,3-diketones influence the reaction with the steel surfaces under different reaction conditions and why such unusually low friction coefficients are obtained in these specific friction systems. The aim of this work is to elucidate the nature of the tribochemical reactions of the 1,3-diketones with steel surfaces and to obtain a better understanding of the lubrication mechanism. IR and UV/vis spectroscopy are used to investigate changes in the chemical composition of a series of 1,3diketones caused by the tribochemical process as a function of the molecular structure of the respective diketone. The reaction rate of the tribochemical reactions are compared to the friction properties as a function of time.
Table 1. Notation of the Used 1,3-Diketones, Their IUPAC Names, Melting Point (mp), and Dynamic Viscosity η at 90 °C at a Shear Rate of 1000 s−1 notation MF-m/n MF-02/01 MF-02/06 MF-06/08
IUPAC names 1-(4-ethylphenyl)butane-1,3dione 1-(4-ethylphenyl)nonane-1,3dione 1-(4-hexylphenyl)undecane-1,3dione
mp/°C
