J. Lara

The surface and tribological chemistry of carbon disulfide as an extreme-pressure additive

Abstract The reaction of carbon disulfide with clean iron was investigated for temperatures between 623 and 776 K and pressures between 10 and 30 Torr. Film growth is limited by the thermal decomposition of CS 2 at the growing interface and the activation energy for this process is 12.4±1.0 kcal/mol. The nature of the resulting film is analyzed using Raman and Mossbauer spectroscopies and by X-ray diffraction, where it is found that the film consists of a non-stoichiometric ferrous sulfide and also incorporates a carbide. This result is in accord with the tribological data where the interfacial temperature in the plateau region of a plot of seizure load vs. additive concentration is ∼1460 K, the melting temperature of FeS. The seizure load increases substantially when the additive concentration exceeds ∼2 wt.% of sulfur and, since carbide formation was detected in the film, this is ascribed to the formation of an iron carbide at higher additive concentrations.

The nature of the lubricating films formed by carbon tetrachloride under conditions of extreme pressure

Abstract The temperature T between the lubricated surfaces in a pin and v-block apparatus is given, for any lubricant at an ambient temperature of T 0 , by the formula T = T 0 + KrωμL where r is the radius of the pin, ω its rotational angular velocity, K a constant that depends on the thermal conductivity in the region of the contact between the pin and the v-block and L the applied load. The product Krω has been determined previously for the pin and v-block apparatus and the coefficient of friction μ can be measured from a plot of the torque required to rotate the pin versus applied load. The rate of material removal can also be measured from the width of the wear scar formed on the face of the v-block by the rubbing motion of the pin. Asymptotes in the plot of removal rate versus applied load have been shown to correspond to melting of the material that forms the anti-seizure film at the interface, so that measurement of the corresponding interfacial temperature yields its melting point, and therefore indicates the nature of the film. Using this strategy, it is shown that FeCl 2 forms the lubricating layer at low loads when CCl 4 is used as the lubricant additive. In addition, films grown by the thermal decomposition of CCl 4 vapor on an iron foil also consist of iron chloride. Mossbauer analysis of films deposited from CCl 4 vapor at higher temperatures (∼ 1050 K) show the formation of iron carbide (consisting of Fe 3 C). The tribological behavior is in accord with this observation since at higher loads, and therefore higher surface temperatures, asymptotes in the rate of film removal correspond to the melting of iron carbide (Fe 3 C) which is therefore proposed to form the anti-seizure film under these conditions. Ultimately, at the highest attainable loads, the asymptote in the removal rate curve suggests that a carbon film acts as the solid lubricant.

A molecular beam study of the tribological chemistry of dialkyl disulfides

Molecular beam studies carried out in ultrahigh vacuum show that dimethyl disulfide reacts with initially clean iron to evolve methane. The reaction is proposed to proceed via a methyl thiolate intermediate. Reaction ceases at ∼600 K, an effect that is proposed to be due to the surface being blocked by an overlayer of sulfur and carbon. Reaction recommences above ∼950 K as sulfur diffuses into the iron. The activation energy for the film-forming reaction is 52.5±2.1 kcal/mol, in good agreement with the activation energy for the growth of FeS films from dimethyl disulfide at higher pressures measured using a microbalance. A depth profile of the film grown in ultrahigh vacuum shows that the sulfur-containing film grows on a Fe+C underlayer. Similar molecular beam experiments with diethyl disulfide suggest the formation of an intermediate ethyl thiolate species which decomposes via a β-hydride elimination reaction to evolve ethylene. The activation energy for film growth, in this case, is 60±2.4 kcal/mol. The results of tribological experiments using a pin and v-block apparatus are consistent with FeS forming the anti-seizure film.

The surface chemistry of chlorinated hydrocarbonextreme-pressure lubricant additives

Chlorinated or sulfurized hydrocarbons are commonly added to abase fluid to synthesize lubricants used under extreme-pressure(EP) conditions. It has been demonstrated that the interfacialtemperature in the EP regime varies linearly with the appliedload and that temperatures in excess of 1000 K can be attained.At these temperatures, both microbalance experiments carried outat high pressure as well as molecular beam experiments performedin ultrahigh vacuum reveal that chlorinated hydrocarbonsthermally decompose forming a film that consists of a layer ofiron chloride and which can also incorporate small (~50 Ådiameter) carbon particles. These particles may affect the coefficient of friction of the film. The lubricant fails andseizure takes place when the film is removed sufficientlyrapidly for metal-metal contact to occur so that EP lubricationis described as a dynamic phenomenon. Under appropriatecircumstances, sufficient carbon can be incorporated into theiron substrate that it becomes a carbide. In this case, seizureis prevented even when the halide layer is removed because ofthe hardness and high melting temperature of this carbide.Ultrahigh vacuum experiments also suggest that carbon diffusioninto the iron and presumably also ultimately carbide formation,is facilitated by co-adsorbed chlorine which may then explain the excellent extreme-pressure properties of carbon tetrachloride.Finally, a similar tribological model is successfully appliedto dimethyl disulfide where, in this case, FeS forms the anti-seizure layer.

The surface chemistry of chloroform as an extreme-pressure lubricant additive at high concentrations

Carbon tetrachloride is an extremely good extreme-pressure (EP) lubricant additive at low concentrations (<3 wt% chlorine) since it can react to form a high-melting-point Fe3C antiseizure layer. In contrast, small hydrogen-containing additive molecules (CH2Cl2, CHCl3) decompose to form FeCl2 which melts at ~940 K and limits the maximum seizure load to ~3500 N as measured hi a pin and v-block apparatus. However, both thermodynamic calculations and results of a Mössbauer analysis of an iron foil heated in CHCl3 at 830 K indicate that iron carbide can be formed from chloroform. In addition, it is also found in that case that a plot of seizure load versus concentration, after initially forming a plateau, once again increases with higher additive concentrations (>4 wt% chlorine) in accord with the idea that a higher melting point carbide film can be formed. It has been shown previously that asymptotes in the plot of removal rate versus applied load correspond to melting of the interfacial anti-seizure film. When using 9.0 wt% chlorine from chloroform as the additive, a drastic increase in removal rate is found at an interfacial temperature of ~940 K corresponding to the melting of FeCl2 and an additional asymptote is evident at ~1500 K due to the melting of Fe3C in accord with the thermodynamic and Mössbauer results.

A molecular-beam study of the tribological chemistry of carbon tetrachloride on oxygen-covered iron

Dc molecular-beam methods are used to examine the reactivity of carbon tetrachloride with oxide films grown on iron in ultrahigh vacuum. The incident CCl4 beam flux is sufficiently low that the nature of the surface oxide is dictated by the annealing temperature allowing the reactivity of Fe2O3, Fe3O4 and FeO films to be examined. Carbon tetrachloride reacts rapidly with Fe2O3 and reaction with Fe3O4 commences at ∼620 K to evolve CO. The activation energy for this process is 20.6±1.0 kcal/mol. CCl4 reacts with FeO above ∼790 K, also to evolve CO, and the activation energy for this reaction is 5.7±0.4 kcal/mol. X-ray photoelectron spectroscopy shows the formation of a halide after reaction at 900 K. These results are in accord with film-growth kinetics measured using a microbalance at high pressures, where it was found that it was not necessary to remove the oxide layer prior to reaction. This contrasts with the behavior of sulfur-containing molecules, where the oxide layer had to be removed before a film would grow. This effect may contribute to the additive synergies commonly found in extreme-pressure lubricant additives where one of the roles of the chloride may be to reduce the oxide layer.

The surface and tribological chemistry of carbon tetrachloride on iron

The thermal decomposition of carbon tetrachloride on clean iron was studied in ultrahigh vacuum using molecular beam strategies, where it is found that carbon tetrachloride thermally decomposes on the surface to deposit iron and carbon with exactly identical kinetics as found at high pressures. No gas‐phase products are detected and the activation energy for the reaction (14.2 ± 0.5 kcal/mol) is in good agreement with the value measured at high pressures. Little carbon is detected on the surface using Auger spectroscopy following reaction and it is found that this diffuses into the surface much faster when formed from CCl4 than from CH2Cl2. This effect is ascribed to the effect of co‐adsorbed chlorine on the adsorbed carbon, which is proposed to decrease the activation energy for diffusion into the bulk of the sample. This effect explains the increased tendency for carbon tetrachloride to form carbides under extreme‐pressure tribological conditions.

The Surface Chemistry of Chlorinated and Sulfurized Extreme-pressure Lubricant Additives

Chlorinated or sulfurised hydrocarbons are commonly added to a base fluid to formulate lubricants used under extreme-pressure (EP) conditions. It has been demonstrated that the interfacial temperature in the EP regime varies linearly with the applied load and that temperatures in excess of 1000 K can be attained. At these temperatures, both microbalance experiments carried out at high pressure as well as molecular beam experiments performed in ultra-high vacuum, reveal that chlorinated hydrocarbons thermally decompose on iron forming a film comprising a layer of iron chloride which can also incorporate small ( 50 A diameter) carbon particles. These particles may affect the coefficient of friction of the film. The lubricant fails and seizure takes place when the film is removed sufficiently rapidly for metal-metal contact to occur so that EP lubrication is described as a dynamic phenomenon. Under appropriate circumstances, sufficient carbon can be incorporated into the iron substrate that it becomes a carbide. In this case, seizure is prevented even when the halide layer is removed because of the hardness and high melting temperature of this carbide. Ultra-high vacuum experiments also suggest that carbon diffusion into the iron and presumably also ultimately carbide formation is facilitated by co-adsorbed chlorine which may then explain the excellent extreme-pressure properties of carbon tetrachloride. Finally, a similar tribological model is successfully applied to dimethyl disulphide where, in this case, FeS forms the anti-seizure layer.