Performance of CrN coatings under boundary lubrication

In the case of metallic surfaces, the action of extreme-pressure (EP) and anti-wear (AW) additives is well understood and has been described in detail. However, this is not the case for coated surfaces. The aim of the present investigation was to determine the influence of additive type and concentration on the tribological behaviour of boundary-lubricated CrN coatings operating under different contact conditions. The results show that like for steel and diamond-like-carbon coatings, also in the case of CrN-coated contacts, the additive type, concentration and contact conditions have an influence on the tribological behaviour under boundary lubrication. The anti-wear additive has the smallest influence and results in complete protection of the CrN coating with respect to wear. On the other hand, the friction modifier gives the lowest friction, but high wear, while extreme-pressure additive concentrations above 1% are found to be too aggressive, even for the CrN/steel contact, and might lead to the formation of Cr-S tribofilms.