The friction, mobility and transfer of tribological films: potassium chloride and ferrous chloride on iron

The tribological properties of FeCl2, measured using an ultrahigh vacuum tribometer, are compared with those of KCl. The friction coefficient of an FeCl2 film evaporated onto iron in ultrahigh vacuum decreases to ∼0.08 after the deposition of a film ∼40 Å thick. This value is in good agreement with that measured for reactively formed FeCl2 films in a pin and v-block apparatus. The contact resistance also increases at the same FeCl2 thickness as that at which the friction coefficient reaches its minimum value suggesting that this corresponds to the completion of the first monolayer as found previously for KCl. Comparison of the friction coefficients and contact resistances for a second pass suggests that the film is mobile during rubbing in both cases. It is found that repeated rubbing in the case of FeCl2 leads to a reduction in friction coefficient to ∼0.015 which is shown to be due to the formation of a transfer film on the tribotip. In contrast, no such reduction is found for a KCl film up to 40 cycles, implying that no transfer film is formed in that case. It is also found that, while friction reduction requires the surface to be completely covered by the halide film, suppression of damage in the iron requires the deposition of films thicker than ∼500 Å.