Surface tribofilm formation

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). 

Boundary lubrication - Nascent surfaces — Tribofilm formation... 

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... 

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 ... 

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). 

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). 

Formation of the tribofilm layer on friction surfaces occurs under the effect of the field in the electrochemical metal,-lubricant-metal2 system, owing to formation of electro-potential (emf), forming free copper tribofilm (Shpenkov, 1995a). Since the process of tribofilm formation takes place during the friction process, disintegration of the reverse micelles takes place in a tribochemical reaction, where a redox reaction occurs, and copper oxide reduces to free copper. 

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 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). 

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). 

Considering present understanding of the reactions of ZDDP in solution and on surfaces (Hastie et al., 1993 Martin 1986a Martin et al., 2001 Willermet et al., 1995a and 1995b Yin et al., 1997a), a possible mechanism for the tribofilm formation is presented ... 

Table 5.3. The adhesive wear (mild wear, more severe wear, very severe wear) properties and tribofilm formation on metal surfaces (Hsu et al. 1997)...

When surfaces of tribological systems are involved in the mechanical activity of rubbing, direct reactions of surface adsorbed films with solid surfaces take place. The mechanically activated clean surface (nascent surface) of the metals and alloys is extremely reactive. Tribofilm formation is caused by the interaction between the metal (M, substrate) nascent surface under high energy and chemisorbed molecules of additive (adsorbate) (Buckley, 1981). 

It has to be considered that the formation of reactive intermediate negative ions and free radicals on the rubbing surfaces follows tribofilm formation from most... 

The science concerned with all the all chemical reactions in mineral and synthetic formulation affecting the tribofilm formation on metal surfaces. It is a branch of chemistry different types of energy and catalysis. Tribochemistry deals with the relations between tribosystem and chemical changes of the surface layer. 

This volume is a comprehensive text that attempts to deal with the tribochemical reactions in hydrocarbon formulations affecting the tribofilm formation on metal surfaces. The most important factor governing the tribochemical processes under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles, RMs. The book covers a very broad spectrum of topics, e.g., additives interactions, acid-base processes in lubricating formulations and the importance of solubilization. Emphasis is on chemical interpretations of the phenomena of tribochemistry of reverse micelles, surface tribochemistry, and current analytical techniques of metal surfaces. 

The book starts off with an introduction to tribochemistry, the science concerned with chemical reactions affecting the tribofilm formation on activated metal surfaces. The "Tribochemical Tree" summarizes our knowledge of some of the most important processes proceeding in the bulk lubricant and on mechanically activated surfaces. 

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. 

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. 

One of the prerequisites for an antiwear additive is to form a surface film in the region where mechanical stress is applied. Some authors have reported that the thermal decomposition of ZnDTP is the key step in the mechanism of tribofilm formation [13], whereas other researchers have suggested that the tribofilm can also be formed at room tem-... 

While the reactivity of new ashless additives with metal surfaces has been investigated quite extensively, less is known about the thermal degradation of these compounds in oil solution. As was the case for ZnDTPs [1, 3], investigating the reactions that take place in oil solution at high temperature could significantly contribute to elucidating the mechanism of thermal film and tribofilm formation on metal surfaces. 

Erdemir A., Method to improve lubricity of low sulfur diesel and gasoline fuels, U.S. Patent 6,783,651 (2004). Komvopoulos, K., Chiaro, V., Pakter, B., Yamaguchi, E. S., and Ryason, P. R., Antiwear tribofilm formation on steel surfaces lubricated with gear oil containing borate, phosphoms, and sulfur additives, Tribol. Trans., 45 (2002) 568-575. 

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 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. 

Other processes responsible for formation of a fine copper tribofilm are mentioned in (Shpenkov, 1995a). In particular, copper ion deposition on steel is explained by contact substitution of iron for copper in solution. If sufficient metallic iron is in contact with the copper micelle, iron dissolution (friction will saturate the surface quicker) and copper deposition will continue until the activity ratio of their ions satisfies the equation ... 

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... 

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