Tribofilm thermally generated film

The tribochemical and thermally generated films contained mostly a mixture of short-and long-chain phosphates. The sulfur in tribofilms is always present as sulfide (Canning et al., 1999 Fuller et al., 1997 Yin et al., 1997a). 

At lower temperature, thermally generated film has more similarity to tribofilm. The thermal films, as in the case of the tribofilms, contain mostly a mixture of short-and long-chain polyphosphates. The chemical states of phosphorus and sulfur in each neutral and basic pair were very comparable. The aryl phosphate films contain long-chain polyphosphate throughout the film, whereas the alkyl phosphate films are composed of long-chain polyphosphates in the bulk. Also, the aryl polyphosphate films contain more unchanged ZDDP. 

Table 5.13. The composition of tribofilms and thermally generated films from the combination of ZDDP and MoDTC. Interpretation of sulfur (S) L-edge XANES spectra recorded in TEY surface technique (Kasrai et ah, 1998)...

Tribochemically generated tribofilm-phosphorus characterization. Combined results of the phosphorus L-edge and K-edge XANES tribofilm spectra generated from a mixture of MoDTC and ZDDP correspond to medium-chain length polyphosphates. Short-chain polyphosphates were found for thermal antiwear films generated from oil containing MoDTC and ZDDP in various concentrations at 150°C. The XANES spectra clearly show that the counter ion in phosphates is zinc rather than iron. The XANES results do not support the formation of iron phosphate (Kasrai et al., 1998). 

A Cameron-Plint friction machine generated tribofilms with two-layer structure a zinc polyphosphate thermal film overlying a mixed short-chain phosphate glass, containing iron sulfide precipitates. A tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the hard and soft acid and base HSAB principle (Martin, 1999 Martin et al., 2001). 

The uncertainty in the mechanism of antiwear tribofilm formation derives in part from observations that exposing metal surfaces to heated ZDDP/oil solution forms films similar to those generated in a tribochemical way. From the utility standpoint, both thermal and tribochemical films seem to provide protection from wear. Thus, the current model involves both a tribochemical and thermooxidative component for the decomposition of ZDDP and tribofilm formation (Aktary et al., 2001 Bancroft et al., 1997 Fuller et al., 1997 and 1998 Martin, 1999 Willermet et al., 1995b Yin et al., 1997a). 

The (P) L-edge XANES spectra of tribofilms and thermal films generated from the neutral di-isopropyl ZDDP along with the model compounds (zinc metaphosphate and zinc pyrophosphate) are very similar and compare well with model compounds. The surface may also play an important role in catalysis of the thermal decomposition and provide oxygen for phosphate formation. There is also... 

Fig. 4.4. Phosphorus (P) K-edge XANES spectra of a thermo-oxidative tribofilm generated at 100°C (48-108 h heating time) for model compounds ZDDP, disulfide and Zn orthophosphate. The surface adsorbed species of the thermal film [B] is a rearranged ZDDP (linkage isomer, LI-ZDDP). A shoulder in the thermo-oxidative tribofilm indicates that there is some orthophosphate present (Fuller et al., 1998)...

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