Tribofilm

The basic processes of dissolution, acid-base interaction, micellization, solubilization, oxidation and reduction take place in oil formulation. During engine operation, additives of the lubricant interact continuously with engine surfaces and themselves. Thus, there is a progressive change in the surface due to the lubrication, friction, and wearing processes, tribofilm formation, and oxidation. All these processes are presented and discussed throughout this book. Surfactant additives are fundamental to reverse micelles (RMs) formation in oil... 

Zinc poly(thio)phosphate thermal films have been recently recognized to be precursor reaction products in the formation of polyphosphate glasses tribofilm (Bascom et al., 1959 Bovington and Dacre, 1984 Fuller et al., 1997 Martin, 1999 Varlot et al. 1999 Willermet et al. 1991). 

The thermal film made of long-chain zinc polyphosphates is formed on the surface. When friction increases, the process of transformation of phosphorus compounds into short-chain phosphate glasses is observed and iron sulfide abrasive particles are eliminated by tribochemical acid-base reactions. Under very severe wear conditions (nascent metal surface creation), an iron sulfide is formed, which will be mixed with the phosphate glasses tribofilm. 

Tribochemistry Tribochemistry is the science concerned with the chemical reactions in mineral and synthetic formulations affecting the tribofilm formation on metal surfaces during the boundary lubrication processes. What are the differences in the concept of electron sharing in the liquid processes and on metal surfaces ... 

Zinc dialkyldithiophosphates (ZDDPs) function mainly as antioxidants and antiwear additives. Molecules of ZDDPs adsorb on metal surface to participate in surface tribofilm formation under conditions of boundary lubrication. The solid tribofilms are formed at the metal surface to protect even under conditions of coarse contact under load (Bom et al., 1992). 

Dispersant-ZDDP interactions at surfaces. The dispersant reduces the amount of ZDDP available for tribofilm formation by forming complexes to increase wear in 4-ball and valve train tests. The borated PIBS dispersants may participate in the formation of a borate component in the antiwear film. PIBS dispersants adversely affect the antiwear activity of ZDDP. The stronger the complexation, the greater the adverse effect on wear. It may well be that this effect is due largely to keeping ZDDP in suspension and away from the surface (Rounds, 1978 Shiomi et al., 1992 Shirahama and Hirata, 1989 Willermet, 1998). 

In XANES spectra of tribofilms, both the phosphorus and sulfur signals are very weak compared to ZDDP used alone. For all the concentrations (1%, 2%, 5%) of dispersant PIBS used, antiwear polyphosphate films were formed and unreacted ZDDP was not present in the film. These results imply that antiwear films are much thinner than either tribofilm generated by ZDDP alone or ZDDP with the detergent. This confirms that the PIBS dispersants compete for ZDDP adsorption on the surface (Yin, 1997b). 

The XANES spectra of the tribofilms were recorded in the bulk FY mode, and in the surface TEY mode from the ZDDP and calcium phenate. The (S) L-edge and (P) L-edge XANES spectra indicate that sulfur in sulfide form was only present on the topmost part of the film the phosphorus signaled that the topmost surface contained relatively long polyphosphates, and the bulk chemistry of the film contained mostly shorter-chain polyphosphates. Using a range of concentrations of two detergents, it was concluded that calcium phenate influences the film formation much more than calcium sulfonate, and thus calcium phenate solubilize excess ZDDP from film surfaces more effectively (Yin et al., 1997). 

ZDDP is known to interact with most other additives employed in these formulations e.g., ZDDP is solubilized by soft-core and hard-core micelles and has been considered for reduced antiwear performance (Inoue, 1993 Kapsa et al., 1981 Rounds, 1981 Shiomi etal., 1986 Willermet 1995a and 1995b Yin et al., 1997). Based on the tribofilm formation (polyphosphate) and the presence (or absence) of unchanged ZDDP in the film, we can conclude that the additives compete with the adsorption of ZDDP on the surface (Varlot et al., 2000 Yin et al., 1997a and 1997b). 

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. 

Esters stick to the surface better than mineral oils. Since ester groups are polar, they form physical bonds with metal surfaces. At high loads, esters will tend to form chemisorbed films. Under extreme boundary conditions, esters tend to break down to form acids which leads to wear protection and friction reduction (Randles, 1999). These acids readily react with freshly exposed metal surfaces to form metal carboxylates tribofilms. 

Calcium sulfonate RMs (c < 2%) + ZDDP - polyphosphate tribofilm and unreacted ZDDP ... 

PIBS-PAM surfactant RMs (c 1%) + ZDDP -> short chain polyphosphate tribofilm only. 

If less ZDDP is adsorbed on the surface, a greater percent is transformed as a short chain phosphate tribofilm. 

X-ray absorption study of tribofilms generatedfrom a combination of ZDDP and borate-sulfonate RMs was used to determine the chemistry of tribochemical films at the surface and the bulk The calcium phosphate content in the tribofilm generated from either ZDDP + borate-sulfonate RMs or from ZDDP + calcium sulfonate soft-core RMs is similar (Varlot et al., 2001). Calcium sulfonate S(+5) undergoes disproportion reaction to form sulfate S(+6) and sulfite S(+4), and the presence of ZDDP affects the disproportion process. Close to the steel surface, the... 

Table 3.11. The chemistry of tribofilm generated by multifunctional additives composed of soft-core and hard-core reverse micelles (RMs) in oil formulation. Evaluation of tribofilm by XANES spectroscopy (Varlot et al., 2001)...

Tribofilm composition Calcium phosphate present Calcium phosphate present... 

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

Additive status in tribofilm ZDDP => zinc polyphosphate PIBSI = iron oxide ZDDP > zinc polyphosphate Calcium borate-salicylate > calcium borate glass... 

Micelle status in tribofilm PIBSI identified, oxidized species, residual succinimide Salicylate not identified... 

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... 

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

B ZDDP + polyisobutylene succinimide dispersant + calcium carbonate-phenate [Ca] = 890 ppm Composed of inorganic, low molecular weight amorphous short chain ortho-(P043 ) and pyro- (P2074 ) phosphates ( 20% of zinc was replaced by calcium in phosphate tribofilm)... 

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