Ex situ surface analytical investigations

The solid wear particles from ZDDP films produced in lubricated contacts contain phosphorus, sulfur and zinc from the ZDDP molecule and oxygen mainly from the surrounding air environment. They also have a low iron content. Ex situ examination by XAS, EELS and CEMS of these particles (configuration shown in Fig. 5a) has been carried out to provide local analysis of the iron atoms, since their localized environment in the surface film is of great interest it is related directly to the wear of the steel surface and can also play a key role in the adhesion mechanisms of the film. In order to investigate this aspect, a study was made by XAS (EXAFS plus XANES) of a collection of wear debris from two lubricated tests, with and without ZDDP in the lubricant base, respectively. The processed EXAFS data presented in Fig. 8 show the RDFs of iron atoms (noncorrected phase shifts) in four samples data from the standards, pure crystalline iron and iron oxide have been included for comparison. From this EXAFS study, some important results can be deduced [4]  

Compared to the metallic substrate, iron atoms have become bound to oxygen in the wear debris, irrespective of the lubricant. The main peak in the RDF is attributed to an Fe-O separation of 0.19 nm. In the sample without ZDDP, the. second peak corresponds to Fe-Fe separations of 0.29 nm. 

In the presence of the additive, and during the steady-state friction regime, iron atoms in the wear debris are found to be isolated from each other, because the second Fe-Fe peak in the RDF has decreased considerably, indicating a high level of crystallographic disorder. 

To complete these investigations. XANES experiments undertaken by recording the Fe-K near-edge structure and ELNES experiments in the TEM at the O K-edge indicated, respectively, the presence of a fully oxidized Fe + state in tKtahcdral symmetry and the presence of some residual iron oxide Fe.iOj [4. Additional CEMS measurements confirmed the presence of octahedrally coordinated ferric cations [28.  

Based on these results, the transition from the abrasive wear to the low-wear regime has been explained in terms of velocity accommodation mechanisms in the interface film. The most important phenomenon is a complex polyphasic tri-bochemical reaction responsible for the formation of a solid transition-metal phosphate glass material as an adherent thin film. This acts as a protective layer against wear due to its superplastic behavior when it is formed in the contact 

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