UHV friction of tribofilms derived from metal dithiophosphates

The friction-reduction mechanisms of Modtp and Zndtp were highlighted by su bmitting tribofilms to friction in ultra-high vacuum (UHV). The use of an U HV tribometer to understand these phenomena is justified by the fact that t he friction coefficient recorded in UHV is close to the friction coefficien t obtained in traditional tests in oil. After UHV friction, the transfer fi lms on the pin were analyzed by in situ AES, XPS and AES mapping. Low frict ion is associated with the transfer to the pin of a sulfur-rich film. In th e case of Modtp, we observe a very thin MoS2 film. The UHV friction coeffic ient approaches 0.04. In the case of Zndtp, the transfer film contains ZnS together with some phosphates. Because of the poor capacity of ZnS to reduc e friction, the UHV friction coefficient recorded is near 0.15. A global mo del of the action of dithiophosphates in reducing friction is described on the basis of the hard and soft acids and bases (HSAB) principle.