The Chemistry of Tribofilm Formation

In contrast to TEY detection which probes a 5 nm depth of the film, FY mode probes 50 nm (Kasrai et al., 1996). In Fig. 4.3 it was shown that the spectra measured for the model polyphosphates in TEY and FY modes were similar indicating that surface and bulk species are the same. Examining spectra F-H, it is immediately clear that the intensity of peaks a and b in (FY) spectra is much weaker than that in the corresponding TEY spectra A-C. 

This suggests that the polyphosphate chain length is much shorter in the bulk than on the surface. But the broad shoulder indicates that the spectra still contain small components from ZDDP, which can be trapped during rubbing when the wear rate is high. Spectra I and J in all aspects are very similar and show that these bulk films are ortho- or pyrophosphate (Yin et al., 1997a). 

Zinc dialkyldithiophosphate ZDDP A linkage isomer (LI-ZDDP) of ZDDP 

The XANES data suggest (for a thermal film generated at 100°C, 48-108 h) that it is more likely that the surface adsorbed species in Fig. 4.4, spectrum [B], is a rearranged ZDDP, known as LI-ZDDP (linkage isomer of ZDDP). 

Peak a of the thermal film (spectrum B) is shifted by 1.2 eV to higher energy from peak a of the ZDDP (spectrum A) and peak a of the disulfide (spectrum C), and yet peak a of the thermal peak (spectrum B) does not correspond to peak b of the zinc orthophosphate (spectrum D), revealing that this species is a phosphate or the original ZDDP. 

---