Tribochemical

Synthesis of chelates (including those with heterocyclic ligands) by direct interaction of ligands with metals or metal oxides, particularly using electrochemical or tribochemical activation 97MI41. 

Infrared, X-ray diffraction, DTA, TG, electrical conductivity and solubility measurements have been used to investigate the mechanisms of tribochemical reactions between solids [385,386],... 

In summary, for metal surfaces in boundary lubrication, complex tribochemical reactions occur along with the physical/chemical adsorptions, which lead to the formation of surface hlms, consisting of reaction products, oxide layer, the mixture of particles and organometallic polymer, and perhaps a viscous layer. The surface hlms operate as a sacri-... 

First-order instabilities may not only involve the translational motion of atoms confined within contacts, but they may also involve chemical reactions within the confined fluid itself. This has been demonstrated recently in first-principles studies of zinc phosphates, which are found in protective films formed in automobile engines.19,83 Here, we focus on simulations of systems containing phosphate molecules in which pressure-induced chemical reactions lead to hysteresis and energy dissipation. The reactions involving zinc phosphates are discussed below along with other tribochemical reactions. 

An emerging subdiscipline of tribological simulation involves the study of tribochemical reactions—that is, reactions that are activated by pressure and shear. These reactions alter the structure of lubricants and films that are used to protect surfaces from wear. Understanding the effects of these reactions on the intended behavior of these films is of utmost importance. However, simulation studies of tribochemical reactions have been impeded by the difficulty in accurately describing changes in chemical bonding. In a limited number of cases, this can be achieved with the use of reactive FFs, as noted above, whereas in other cases, one must resort to expensive quantum chemical calculations. In this section, we will describe two studies where such methods were used to examine tribochemical reactions. 

Reactive FFs can only be applied to a few specific cases for which they have been developed, such as the hydrocarbon systems discussed in the first part of this section. For other systems, describing tribochemical reactions requires the use of quantum chemical methods. In recent studies, such methods have been applied to investigate the behavior of zinc phosphates (ZPs) in response to high pressures. ZPs form the basis of anti-wear films derived from zinc dialkyldithiophosphates (ZDDPs), which are additives that have... 

Overall, this work highlights how quantum chemical methods can be used to study tribochemical reactions within chemically complex lubricant systems. The results shed light on processes that are responsible for the conversion of loosely connected ZP molecules derived from anti-wear additives into stiff, highly connected anti-wear films, which is consistent with experiments. Additionally, the results explain why these films inhibit wear of hard surfaces, such as iron, yet do not protect soft surface such as aluminum. The simulations also explained a large number of other experimental observations pertaining to ZDDP anti-wear films and additives.103 Perhaps most importantly, the simulations demonstrate the importance of cross-linking within the films, which may aid in the development of new anti-wear additives. 

Carboxylated polymers can be prepared by mechanical treatment of frozen polymer solutions in acrylic acid (Heinicke 1984). The reaction mechanism is based on the initiation of polymerization of the frozen monomer by free macroradicals formed during mechanolysis of the starting polymer. Depending on the type of polymer, mixed, grafted, and block polymers with a linear or spatial structure are obtained. What is important is that the solid-phase reaction runs with a relatively high rate. For instance, in the polyamide reactive system with acrylic acid, the tribochemical reaction leading to the copolymer is completed after a treatment time of 60 s. As a rule, the mechanical activation of polymers is mainly carried out in a dry state, because the structural imperfections appear most likely here. 

Other theories of initiation, not based on the initial action of a hot spot, have been proposed. One of these, the tribochemical mechanism, postulates the direct breaking of chemical bonds without the conversion of the mechanical energy to heat. Reference 49 discusses this mechanism more thoroughly... 

M. G. Jacko, "Tribochemical Conveision of Asbestos in Brake Friction Materials," 5th Annual Colloquium on Brakes, Atlantic City, N.J., Oct. 1987. 

Due to the continuous input of thermal energy necessary to maintain mechanical work, tribological systems are in progressive equilibrium. In the tribosystem, the flow of energy is accompanied by an increase in entropy of the total system and is reflected by the tribochemical reactions and deterioration of lubricant quality. Our understanding of tribosystems has been seriously limited by a lack of kinetic information on critical reactions in hydrocarbon formulation and critical reactions at interfaces. 

A "Tribochemical Tree" shown in Fig. 1.1 summarizes our knowledge of some of the most important processes occurring in the bulk oil and the effects of those processes on the mechanically activated surfaces under the boundary lubrication conditions (Dowson, 1998 Kajdas, 2001 Martin, 1999 Pawlak, 2001 Sakurai, 1981 Spikes, 2001 Willermet, 1998 . Yinetal., 1997a and 1997b). 

The additive mixtures interact in a variety of ways, both in the bulk oil and on surfaces. Tribochemical interactions of additives in the oil formulation are discussed in Chapter 2. Surfactant molecules, when dissolved in base oil, are capable of self-organization to form aggregates such as soft-core reverse micelles (RMs). The polar or charged head groups of these molecules with the counter ions form the interior of the micelle (core), and the hydrocarbon chains made up its external shell. The most important factor governing the tribochemical reactions under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles discussed in Chapter 3. 

As a result of tribochemical reactions and the presence of oxygen or hydroperoxide in oil, adsorbed ZDDP and LI-ZDDP on the surface are thermo-oxidatively decomposed to give long-chain polyphosphates Zn(P03)2. 

Fig. 1.4. The cycle of tribochemical film formations during the tribological mild, more severe, and very severe wear conditions initiated by thermooxidative decomposition of the ZDDP additive in the steel-on-steel combination (not to scale)...

The role of organic sulfur species (other than those in thiophosphate form) in the tribochemical process is to react immediately with the nascent iron surfaces and ZnO to form metal sulfides (FeS, ZnS) embedded in the short chain phosphate matrix. 

The tribochemical reactions based on the hard and soft acids and bases (HSAB) principle linked to the friction coefficient (p) are summarized above. The effect of tribological processes consists of the formation of mixed short-chain phosphate glasses containing sulfide precipitates. 

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. 

A multifunctional additive of carbonate-detergents RMs retarded the decomposition of ZDDP in the ISOT test (Yamada et al., 1992). Mixtures of ZDDP plus carbonate-detergents RMs additive have been reported to have synergistic effects on detergency, see Chapter 3.3 on tribochemical interactions of hard-core RMs and ZDDP (Inoue, 1993 Ramakumar et al., 1994 Willermet, 1995a and 1995b Yin et al., 1997)... 

The antiwear and antioxidant additive, zinc dialkyldithiophosphate, is a key ingredient in the great majority of engine oil formulations, and other lubricant applications such as hydraulic fluids and gear oils. The ZDDP-derived tribochemical films have been studied by a number of laboratories, but their mode... 

Lubricity - wear protection and fatigue. The coefficient of friction is a measure of lubricity of a lubricant. The esters, being more polar, are attracted to the metal surfaces and form monolayers. These thin layers reduce the coefficient of friction at the surface. The surface phenomena that determine the behavior of boundary lubricants can be described in the following terms physically and chemically adsorbed layers and tribochemically formed films. 

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