Tribofilm polyphosphates chain length

ZDDP decomposes by a number of routes involving free radical and redox processes. Film composition varies from the iron-rich bonding layer, through the zinc phosphate layer to the outer surface, which contains organic material incompletely converted to precursor species. The polyphosphate chain length may vary as a function of depth into the film and the conditions under which the film is formed. Formation of polyphosphate tribofilms from simple ZDDP solutions is promoted by self-association of ZDDP molecules, which increases the local concentration of ZDDP. 

The phosphorus and sulfur L-edge XANES spectra of the tribofilms phenate-ZDDP were recorded in the bulk FY mode, and in the surface TEY mode for three concentrations of calcium phenate. The surface TEY spectrum assigned for the lowest concentration (0.5 wt%) of the calcium phenate has been assigned to the absence of (unreacted) ZDDP. The chain lengths (number of P) in the phosphates in the tribofilms were estimated they decreased from 44 in 0.5 wt% calcium phenate to 8 in 1 wt% calcium phenate and even further to 5 in 2 wt% calcium phenate. The same trend is followed in the bulk FY mode. This indicates that the polyphosphate chain lengths are shorter in the bulk without detergents (Yin et al., 1997a). 

Tribofilm composition Explain the polyphosphate chain length in the bulk and on the surface when ZDDP is in the presence of calcium sulfonate and calcium phenolate soft-core RMs (for solution see Table 4.4). 

In order to identify the chemical nature of phosphorus and sulfur in complex matrices such as antiwear tribofilms, it is essential to compare the spectra of films with different model compounds in which the local chemical environments of phosphorus and sulfur are known. The high resolution of the technique allows characterization of the chemical nature of phosphorus and sulfur in the tribofilm. Investigators have shown that the chain length of polyphosphate is related to the length of alkyl groups in ZDDP. By comparison of the L-edge XANES spectra of the tribofilms with the spectra of model compounds with known structures, it has been possible to speciate the chemical nature of phosphorus and sulfur in the antiwear tribofilms. 

The chain lengths in FY spectra are estimated at 12, 7 and 1 in for 0.5 wt%, 1 wt.% and 2 wt% calcium phenate, respectively. Thus the topmost surface measured by TEY contains relatively long chain polyphosphates, and the bulk chemistry of the film measured by FY contains mostly shorter chain polyphosphates. The bulk FY signal is relatively strong, indicating that the films formed with calcium phenate are relatively thick (several hundred angstroms). Summary of the tribofilm compositions is displayed in Table 4.4. The (S) L-edge... 

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). 

The chemical composition of the tribofilm can be expected to vary with the applied load [14]. Systematic investigations on the influence of the applied load are rare in the literature. The applied load probably affects the film-formation rate [21] by increasing the temperature in the contact area and may also change the length of the polyphosphate chains formed in the films [22]. In the present work, the effect of the applied load on the chemical composition of the surface layer in the contact region has been studied using... 

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