Tribological Properties of Films Formed by the Reaction of Carbon Tetrachloride with Iron

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 FeCl₂ 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 Å of FeCl₂ has been evaporated, while the reactively formed layer has a minimum friction coefficient when a film of 6±2 Å has been deposited. In the case of the evaporated FeCl₂ 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 Å of FeCl₂. In the case of the film formed by reaction with CCl₄, the halide film grows directly on the surface implying that the FeCl₂ monolayer thickness is ~6 Å. This value is in good agreement with the layer thickness in bulk ferrous chloride.