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 T0, by the formula T = T0 + 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 FeCl2 forms the lubricating layer at low loads when CCl4 is used as the lubricant additive. In addition, films grown by the thermal decomposition of CCl4 vapor on an iron foil also consist of iron chloride. Mössbauer analysis of films deposited from CCl4 vapor at higher temperatures (∼ 1050 K) show the formation of iron carbide (consisting of Fe3C). 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 (Fe3C) 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.