Tribological properties of films formed by the reaction of carbon tetrachloride with iron F. Gao, O. Furlong, P.V. Kotvis and W.T. Tysoe* Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA Received 13 March 2005; accepted 21 August 2005 The tribological properties of halide films grown on iron by reaction with carbon tetrachloride vapor at a temperature of 617 K and a pressure of 1.7 Torr are compared, in ultrahigh vacuum, with FeCl 2 films evaporated onto the surface. It is found that the reactively formed film has a slightly lower limiting friction coefficient than the evaporated layer (0.06 compared to 0.08), which may be due either to the diffusion of some carbon into the substrate or the formation of a more oriented layer when this is formed reactively. The major difference between the reactively grown and evaporated film is that the evaporated layer attains the minimum friction when 40 A ˚ of FeCl 2 has been evaporated, while the reactively formed layer has a minimum friction coefficient when a film of 6±2 A ˚ has been deposited. In the case of the evaporated FeCl 2 film, the growth of second and subsequent layers proceeds before the first layer is complete. It has been shown that the friction coefficient reaches its minimum value after completion of the first monolayer, a process that is complete after the evaporation of 40 A ˚ of FeCl 2 . In the case of the film formed by reaction with CCl 4 , the halide film grows directly on the surface implying that the FeCl 2 monolayer thickness is 6A ˚ . This value is in good agreement with the layer thickness in bulk ferrous chloride. KEY WORDS: halide films, FeCl 2 , carbon tetrachloride, ultrahigh vacuum tribometer, friction coefficient, extreme-pressure lubrication 1. Introduction It has been demonstrated that reactively formed extreme-pressure films consisting primarily of FeCl 2 are produced when using model extreme-pressure lubricants consisting of methylene chloride, chloroform or carbon tetrachloride dissolved in a poly a-olefin [1,2] where fric- tion coefficients of 11 ± 1  10 )2 were found for a film formed from methylene chloride [3] and 7.1 ± 0.6  10 )2 for films grown from chloroform or carbon tetrachloride [4,5]. The mean contact pressure during these experiments was between 500 and 1000 MPa with a linear sliding speed of 9.6  10 )2 m/s. In order to gain a fundamental under- standing of the frictional behavior of thin inorganic films on iron, we have studied a number of alkali halides evaporated onto clean iron in ultrahigh vacuum [6-10] and found, in general, that the deposition of a few tens of A ˚ ngstroms of these halides substantially reduces the interfacial friction coefficient from 2 for clean iron to values less than 0.6 [7]. The formation of thicker films causes a slight increase in the friction coefficient [9]. It has also been shown that a ferrous chloride film evaporated onto iron in ultrahigh vacuum reduces the friction coefficient to 0.08, in good agreement with the values found above for model lubricants comprising small chlorinated hydrocarbons dissolved in a poly a-olefin [10]. Transfer of the FeCl 2 from the iron to the tungsten carbide tribopin was also observed. In the following, the tribo- logical properties of films formed by the reaction of carbon tetrachloride vapor with iron surfaces is investigated [5,11]. The growth kinetics of these films has been studied previously, where film growth rates were measured on an iron foil using a microbalance, from the change in sample mass as a function of time [12]. This resulted in the deposition of an FeCl 2 film, along with the rapid diffusion of carbon into the bulk of the iron sample [13]. It has been demonstrated that carbon diffusion into the bulk is more pronounced for carbon tetrachloride than for either methylene chloride or chloroform, an effect that was attributed to a lowering of the activation barrier for dif- fusion due to the presence of co-adsorbed chlorine on the surface [13]. Such an enhanced surface-to-bulk transport of carbon is anticipated to have several effects. First, it is expected to result in an increase in substrate hardness causing a decrease in contact area and thus a lower friction coefficient. Second, once conditions have been reached at which the halide film is removed (at a surface temperature of 940 K [14,15]), seizure is prevented by the presence of the underlying carbide layer resulting in CCl 4 being an extremely effective extreme-pressure additive [13]. 2. Experimental Experiments were carried out in a stainless-steel, ultrahigh vacuum chamber operating at base pressures *To whom correspondence should be addressed. E-mail: wtt@uwm.edu 1023-8883/05/1000–0171/0 Ó 2005 Springer Science+Business Media, Inc. Tribology Letters, Vol. 20, No. 2, October 2005 (Ó 2005) 171 DOI: 10.1007/s11249-005-8313-z