Real contact temperatures as the criteria for the reactivity of diamond-like-carbon coatings with oil additives

The operating conditions under which chemical reactions between diamond-like-carbon (DLC) coatings and oil additives occur and the main driving forces, i.e., the “activation criteria” for these chemical reactions, have not yet been defined. In order to clarify the difference between the “test” temperature and “real” contact temperature, and to determine the effect of the real contact temperature for these reactions, we have calculated the contact temperatures using two well-known models and compare them with results of tribological experiments and some state-of-the-art analyses of worn surfaces. The results suggest that the actual surface temperatures are 100−130 °C higher than the test temperatures. A contact temperature of about 250−260 °C was found to be the required key activation criterion for chemical reactions between the dialkyl dithiophosphate extreme-pressure (EP) additive and the DLC coating. Gradual formation of a tribochemical protective film from phosphates and organic sulphur/sulphates suggests a lower chemical reactivity and slower formation of the “optimal” tribochemical protective layer on DLC coatings than on steels. No tribological effect of anti-wear (AW) or EP additives could be found on the DLC coatings when the surface temperatures were below 120−140 °C. The temperature-induced graphitisation of the DLC that occurred in the contacts with the base oils, however, require about 250 °C of contact temperature. Lower surface temperatures or the use of additives suppressed this graphitisation.