Tribofilm composition

Tribofilm composition Calcium phosphate present Calcium phosphate present... 

Tribofilm composition Calcium and zinc borophosphates with oxides, sulfonates and nitrates Zinc polyphosphates... 

The effect of ZDDP, dispersant and carbonate-phenate RMs on the tribofilm composition is given in Table 3.13. 

Some fundamental aspects of tribochemistry of ZDDP have been investigated using an ultrahigh vacuum UHV analytical tribometer on chemisorbed films previously formed on a steel surface (Martin et al., 1996). The steel surface was immersed in a solution of ZDDP at a concentration of 2 wt.% in PAO synthetic lubricant base at 130°C for 24 hours. The chemistry of the treated steel surfaces was investigated by XPS and AES, and friction tests were carried out in UHV just after the analysis. At the end of the friction tests, AES microanalysis was performed both inside and outside the wear scar. The AES analysis showed that sulfide was present inside the wear scar and phosphorus was eliminated from the surface, presumably as wear debris. Summary data of the tribofilm composition after ZDDP degradation are presented in Table 4.1. 

Table 4.1. The tribofilm composition after ZDDP degradation based on results of surface techniques...

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... 

Calcium sulfonate or Calcium phenate (wt%) Tribofilm composition (%) unreacted ZDDP Chain length, number of (P). Surface (TEY) 5 nm Calcium sulfonate Chain length, number of (P) Bulk (FY) 50 nm Calcium phenate... 

The tribofilm composition On the basis of Table 4.1, what analytical surface techniques were used to evaluate the tribofilm composition after ZDDP degradation The tribofilm conditions include the following (a) the tribofilm consists of Zn, P, S, O and Fe and (b) the tribofilm contains mostly of a mixture of short-and long-chain phosphates and sulfur is present as zinc sulfide. What analytical technique has the ability to identify elements and their chemical state, e.g., Fe° or Fe2+, in case (a), and compounds in case (b) More about analytical techniques for evaluation of the metal surfaces can be found in chapter 4.3. 

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). 

The next chapter is a review of current practice in lubrication of internal combustion engines and lubricant design. The role of individual lubricant components and their use in mineral and synthetic formulations is covered. This is followed by a discussion of the tribochemical effects of additive interactions. The heart of the manuscript is chapters, "Tribochemical nature of antiwear film , "Surface tribochemistry and activated processes", and "Analytical techniques in lubricating practices". Topics covered include tribofilm formation, organomolybdenum compounds in surface protection, catalytic activity of rubbing surfaces, introduction of some techniques for evaluation of tribofilms composition and analytical techniques for evaluation of lubricant degradation. Examples of the application of basic concepts are introduced, eg., acidity and basicity in the process of lubricant deterioration. 

Innumerable tribologieal researeh papers have demonstrated the eflfeetiveness of the derivatives of thiophosphorie aeids, sueh as the elass of zine dithiophosphates, as lubri-eant additives for ferrous materials. In the last 20 years, great progress has been made in determining the surfaee eomposition of the films formed—even if the meehanism of film formation from mixed-element additives is poorly understood and sometimes eontradictory. This might be due to differences in temperature, test duration, applied load, and, last but not least, different in situ and ex situ analytical techniques and their individual effects on tribofilm composition. The aim of the present study is to assess the respective roles of physical and instrumental parameters. 

Heuberger, R., Rossi, A., Spencer, N.D. XPS study of the influence of temperature on ZnDTP tribofilm composition. Tribol. Lett. 25, 185-196 (2007)... 

Conceming the test with the two additives (test C), our results show the presence of both nitrogen and boron in the tribofilm composition. 

Thus, the h-BN evidenced in the tribofilm composition is effectively in situ generated by the friction process (so-called tribochemical reaction). 

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 case study. The composition of the surface tribofilms formed by ZDDP and of carbonate-phenate RMs in a cam and tappet friction apparatus were examined using a combination of surface analysis techniques. Adding carbonate-phenate RMs to ZDDP resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight phosphates in favor of ortho- and... 

Our objective in this chapter is to derive a chemical mechanism for the formation, composition and structure of surface tribofilms. These tribofilms result from interactions among additives, the base stock, the surface and the ambient... 

The tribofilms consist of amorphous, short-chain phosphates (ortho- and metaphosphates), with some evidence of sulfur incorporated into the phosphate chain structure. Tribofilm structure depends not only on the lubricant composition of the substrate material, but also on the severity of the rubbing contact. The ZDDP typically forms phosphates under antiwear conditions, whereas sulfides and sulfates are typically formed under severe or very severe conditions, li > 0.8 (Willermet et al., 1997). 

XANES spectroscopy has been used to study the composition and mechanism of antiwear tribofilm formation. The absorption XANES spectra were recorded in total electron yield (TEY) versus fluorescence yield (FY) detection to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. The application of XANES surface TEY mode which analyzes the top 5 nm layer, and the FY technique which analyzes the 50 nm layer of the bulk, taken together, give a marvelous opportunity to study nondestructively the antiwear tribofilms. Both techniques can be used under a wide variety of conditions e.g., the formation of tribofilms at different rubbing times, load, concentrations, temperatures and surface roughness (Kasrai et al., 1993 and 1996 Koningsberger and Prins, 1988 Martin et al., 2001 Yin et al., 1997a). 

Several groups found from adsorption tests that tribofilm became thicker with time sulfur and phosphorus composition on the surface changed with rubbing... 

Additives8 Chemical nature of antiwear tribofilm and composition (unreacted %)... 

X-ray spectroscopy On the basis of interpretation of phosphorus and sulfur L-edge tribofilm XANES spectra in Table 4.4 and 4.5, what is the chemical nature of antiwear tribofilm and composition of the following pairs (a) ZDDP + detergent, (b) ZDDP + dispersant ... 

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)...

Zinc dialkyl dithiophosphates (ZnDTPs) are widely used as extreme pressure and antiwear additives in many different kinds of engine and industrial lubricants. It is known that ZnDTP forms tribological films on rubbing metal surfaces it has been proposed that these films consist of amorphous polyphosphates, but the exact chemical composition of the different polyphosphates in the ZnDTP tribofilm is not known, and a generally accepted reaction mechanism has not emerged to date. Most authors believe that thermal decomposition is the major mechanism of ZnDTP tribofilm formation as a result only tribological experiments conducted at elevated temperatures (60-200 °C) are, typically, reported in the literature. 22 All the evidence obtained so far for substantiating the amorphous polyphosphate model has been based on ex situ experiments. 

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