Role of nitrogen in tribochemical interaction between Zndtp and succinimide in boundary lubrication

Tribochemical interactions between zinc dithiophosphate (Zndtp) and polyisobutene succinimide (PIBSI) were studied in the mild wear regime by a dual analysis approach. Both TEM analyses of wear fragments and inside wear scar XPS on tribofilms were obtained in the same location of the wear track. In presence of the succinimide, there are notable modifications in the composition of the zinc polyphosphate tribofilm which is classically formed in presence of Zndtp alone. Apart from amorphous zinc polyphosphate glass, the film contains also large amounts of oxidised species (oxides, sulphates and nitrates) together with residual succinimide function groups. The atomic ratio N/P is about 0.3. Moreover, compared to Zndtp, less tribofilm is produced due to some competition in adsorption process. The results highlight the role of the tribochemical reaction to explain the antagonism between the two additives. The presence of some iron oxide in the tribofilm material indicates that the anti-abrasive wear mechanism of Zndtp is hindered by the presence of the succinimide. The lower content of the tribofilm in sulphide species also suggests a lower efficiency to control the anti-seizure properties of the mixture of the two additives.