P.V. Kotvis

The surface and tribological chemistry of chlorine- and sulfur-containing lubricant additives

Abstract A model for the operation of chlorinated hydrocarbon extreme-pressure additives is outlined in which the chlorinated hydrocarbon thermally decomposes at the hot (~1000 K) interface forming a chloride+carbon film, which is simultaneously worn from the surface. Assuming that seizure occurs when the chloride film thickness decreases to zero reproduces the experimentally measured seizure load vs. additive concentration results. A similar model is found to apply to sulfur-containing additives. The thermal decomposition of model chlorinated hydrocarbons is investigated in ultrahigh vacuum (UHV) using constant-flux molecular beams and reveals that these react with similar activation energies as at higher pressures, forming films of identical compositions to those found tribologically. This allows the surface reaction chemistry of model extreme-pressure additives and the tribological properties of the films to be investigated separately in UHV. In this vein, measurements of the tribological properties of thin KCl films grown on iron in UHV show that the friction coefficient drops from a clean-surface value of ~1.9 to ~0.27 after completion of the first monolayer.

The surface decomposition and extreme-pressure tribological properties of highly chlorinated methanes and ethanes on ferrous surfaces

Abstract Two distinct types of extreme pressure lubricant additive behavior using chlorinated hydrocarbons have been identified using a pin and v-block apparatus. The first, which has been observed previously (and referred to as Type I) and exhibited by 1,4-dichloro-butane, CHCl3, and CH2Cl2, shows an initial increase of seizure load as a function of additive concentration, but reaches a plateau at higher concentrations. In contrast, the seizure load continues to increase with concentration when CCl4 is used as an additive. This behavior is designated Type II. Both C2Cl6 and C2HCl5 exhibit both types of behavior, being Type I at lower concentrations, converting to Type II at higher concentrations. The rate of film growth on an iron foil measured with a microbalance is shown to be significantly faster for a Type II additive (CCl4) and a Type I additive (CH2Cl2, CHCl3). CCl4 is also shown to grow according to an essentially linear rate law with an activation energy of about 38 kJ mol−1. Two possible seizure mechanisms are postulated. First, seizure is considered to occur when film removal by abrasion exceeds the growth rate, thereby leading to complete removal of the protective film. Secondly, measurements of the temperature close to the interface indicate that seizure can occur at some critical temperature Tc. Measurements of the seizure load as a function of bath temperature corroborate this view and Tc appears to be about 950 K. X-ray photoelectron spectroscopy analysis of the films formed by CH2Cl2 decomposition shows substantial carbon depletion in the film at higher growth temperatures. Surface analysis of chemisorbed carbon and chlorine formed by CCl4 decomposition shows that carbon is removed from the surface in this temperature range. FeCl2 also thermally decomposes and melts at about 950 K and provides a possible alternative explanation for the onset of seizure at Tc.

An investigation of the tribological properties of thin KCl films on iron in ultrahigh vacuum: modeling the extreme-pressure lubricating interface

The frictional properties of thin KCl films deposited onto clean iron are measured in ultrahigh vacuum using a tungsten carbide tribotip, where the observed initial rapid decrease in friction coefficient with film thickness is proposed to be due to the formation of a complete KCl monolayer where the friction coefficient of this film is ∼0.27. A 1800 A thick KCl film shows a hardness and friction coefficient similar to those for bulk KCl when the width of the surface height distribution of the tribotip measured by atomic force microscopy (AFM) is 2000–3000 A. This implies that the KCl film behaves like the bulk material when the film thickness exceeds the roughness of the interfaces.

Surface chemistry and extreme pressure lubricant additive properties of chloroform on iron

Abstract The growth kinetics and composition of films formed by the thermal decomposition of chloroform are very similar to those for methylene chloride reported previously [P.V. Kotvis et al., Wear 147 (1991) 401; 153 (1992) 305; Appl. Surf. Sci. 40 (1989) 213; Langmuir 9 (1993) 467], both depositing films that consist of an iron halide and incorporating carbon. Raman spectroscopy suggests that less carbon is incorporated into the layer grown from chloroform, an effect that is proposed to reflect the initial carbon : chlorine ratio in the precursor reactants. The tribological properties measured using a pin and V-block apparatus from the seizure load as a function of the additive concentration are very similar in both cases, showing an initial increase in seizure load up to additive concentrations of ≈ 2 wt% of chlorine and remaining constant thereafter, the main difference being that the saturation seizure load is higher when using chloroform than when methylene chloride is the additive. This effect is explained since the interfacial coefficient of friction of films grown from chloroform are lower than those from methylene chloride so that, for a particular applied load, the interfacial temperature is lower in the former case. Since the seizure load in the plateau region (at high additive concentrations) corresponds to the load at which the interface reaches the melting point of FeCl2, this temperature is attained at a higher load with CHCl3 than CH2Cl2. It is possible that the amount of carbon incorporated into the iron halide anti-seizure film affects the resulting coefficient of friction.

Determination of interfacial temperatures under extreme pressure conditions

Interfacial temperatures attained in a pin and v-block apparatus under extreme pressure (EP) conditions were measured using pins made from either copper or an aluminum alloy from the asymptotes in the curve of removal rate versus applied load since these have been shown to correspond to the temperatures at which the interfacial material melts. The interfacial temperature rise was proportional to the applied load, where the proportionality constant α = Aμ where μ is the interfacial friction coefficient and A a geometrical constant which has been previously measured for steel pins and v-blocks lubricated by chlorinated hydrocarbons dissolved in a poly α-olefin as 2.3 ± 0.3 K/N. Values of A measured when using the aluminum alloy (2.4 ± 0.1) and for copper (2.1 ± 0.2) were in good agreement with this measurement and indicated that interfacial temperatures in excess of 1000 K can be attained during EP lubrication. Finally, the rate of material removal in the pin and v-block apparatus can be related to the metallurgical properties of the pins.

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.

Surface Chemistry of Chlorinated Hydrocarbon Lubricant Additives–-Part I: Extreme-Pressure Tribology

Chlorinated hydrocarbons are commonly added to a base lubricating fluid when it is used for extreme-pressure (EP) lubrication of ferrous metals. It is demonstrated here that the interfacial temperature in the EP regime varies linearly with the applied load in a pin and v-block testing apparatus and that temperatures in excess of ˜ 1000 K can be attained. Thermally decomposing chlorinated hydrocarbon vapors on iron heated to these temperatures (1) shows that a film consisting of an iron chloride which incorporates small carbon particles (˜50A) is formed. In this paper, tribological measurements at extreme pressures and the corresponding analyses of the rubbing surfaces and wear particles also indicate that this film, formed from the chlorinated lubricant reacting with these surfaces, is the critical antiseizure material at less severe EP loads and interfacial temperatures less than ˜1000 K.

Low-temperature, shear-induced tribofilm formation from dimethyl disulfide on copper.

The frictional properties of a sliding copper-copper interface exposed to dimethyl disulfide (DMDS) are measured in UHV under conditions at which the interfacial temperature rise is <1 K. A significant reduction in friction is found from the clean-surface values and sulfur is found on the surface and below the surface in the wear scar region by Auger spectroscopy. Because the interfacial temperature rise under the experimental conditions used to measure friction is very small, tribofilm formation is not thermally induced. The novel, low-temperature tribofilm formation observed here is ascribed to a shear-induced intermixing of the surface layer(s) with the subsurface region as suggested using previous molecular dynamics simulations. Although the tribofilm contains predominantly sulfur, a small amount of carbon is also found in the film.

Surface Chemistry of Chlorinated Hydrocarbon Lubricant Additives–-Part II: Modeling the Tribological Interface

In Part I (1), the concept of “Type I” antiseizure behavior for chlorinated hydrocarbons in extreme-pressure (EP) lubrication of ferrous metals was introduced; interfacial temperature measurements and surface analyses revealed that a solid lubricating layer consisting of ferrous chloride (FeCl2) and carbon prevents seizure and acts as a solid lubricant at less than ˜1000 K. In this paper, careful measurement of the film growth and removal rates successfully rationalizes this tribological behavior. Thermodynamic calculations also show that iron carbides are favored at higher decomposition temperatures. Analysis of films formed from the thermal decomposition of carbon tetrachloride (CCl4) and chloroform (CHCl3) at ˜1000 K using Mossbauer spectroscopy demonstrates that iron carbide is indeed formed in this case; tribological measurements also confirm this material as critical antiseizure material at high loads in “Type II” tribological behavior for chlorinated hydrocarbons with ferrous metals.

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.