Tribochemical interactions

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. 

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)... 

Tribochemical interactions of acid-base in tribosystem have been observed at two levels (i) acid-base reaction in oil formulation and (ii) acid-base reaction on metal surfaces. 

Here we discuss the properties of solids that are to be polished during CMP. Surface properties are important for CMP because they are related to the real area of contact, friction, wear, and tribochemical interactions between surfaces of wafers, slurry particles, and polishing pads. Upon contact, there are four structural elements involved in the wear mechanisms. They are, as shown in Figure 2.5,4 surface films that are present < 1 pm from the surface, near-surface structure occurring between 1 and 150 pm from the surface, subsurface structure that occurs between 50 and 1000 pm from the surface,... 

Previous studies have indicated that no hydrodynamic lubrication occurs during CMP.28 3la There is always a physical contact between the wafer and the polishing pad asperities. In the following section, we will see that there is enough evidence to prove interactions between a wafer and a pad. The boundary lubrication associated with tribochemical interactions plays a dominant role. In order to understand the mechanisms of boundary lubrication in CMP, the physical, electrochemical, and mechanical processes of interfaces must be considered. The mechanisms can be classified into the following categories based on the surface physical chemistry of materials involved during CMP. 

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. 

The lubrication system is extremely complex. The mechanism of lubrication is partly dictated by the nature of interactions between the lubricant and the solid surface. Additives blended into lubricating oil formulations either adsorb onto the sliding surfaces, eg., fatty alcohols, fatty amines, amides, phosphoric acid esters (friction modifiers), or react with the surface, eg., ZDDP, MoDTC, MoDDP organic phosphates (extreme pressure). Some interactions affecting the surfaces of metals include adsorption, chemisorption, and tribochemical reactions-these form new compounds on the surface and lubrication by reaction products (Bhushan and Gupta, 1991 Briscoe et al., 1973 Briscoe and Evens, 1982 Heinicke, 1984 Hsu and Klaus, 1978 and 1979 Klaus and Tewksbury, 1987 Lansdown, 1990 Liston, 1993 McFadden et al., 1998 Studt, 1989). 

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. 

Unlike chemical reactions, tribochemical reactions are triggered by frictional force, resulting in wear. Tribochemical wear can be seen as one type of corrosive wear. When the corrosion is activated by mechanical interactions between the contacting surfaces, it produces activated surface sites and localized high temperatures sufficient for chemical reaction. Tribochemical wear involves surface charging of electrons, surface passivation, and surface film removal processes. 

The specificity of corrosion protection of the inhibited plastic-metal friction pair is liable to tribochemical transformation of the polymer matrix. The products of tribochemical reactions enter into interactions with the elements of the corrosion system and Cl liberated from the plastics ... 

P, M and L are the pol3mier, metal and lubricating medium P, 1/, Cl are the frictional factors, i.e. pressure, velocity and the presence of Cl in the friction zone TCRP are the tribochemical reaction products. The latter can fulfill the function of wear inhibitors (WI) during physical-chemical interactions with the inhibitor on the metal friction surface or form neutral wear products (NWP) affecting neither corrosion nor friction. The task is how to transform these products into useful ones during friction. 

In its true sense, any component of a tribosystem promoting suppression of unfavorable tribochemical processes in the friction zone, irrespective of whether it was impregnated or formed internally, can be related to WI [34]. WI can be formed both as a result of physical-chemical interactions of polymers and metals, or physical fields and energy effects on a part or friction joint. This can be, e.g. thermochemical or radiation-thermal treatment, ion implantation, superposition of electrical and magnetic fields, shifts of electrode potentials, passage of electrical current, etc. 

Elementary stages of tribochemical transformations. Tribochemical reactions in polymers usually follow a free-radical mechanism [78,80]. As a result of mechanical interactions pol3mier or low-molecular radicals appear, taking part in substitution, addition, decomposition or other reactions. On breakage of a macromolecule, two primary radicals with free valence at the chain ends are formed. In the vinyl series these radicals are CH2 and CXY (where X and Y are hydrogen atoms and other side substitutes) in... 

The combinatorial approach, especially when combined with imaging surface spectroscopies, has proven itself to be useful in characterizing the lubricant-additive interaction. It allows the tribochemical reaction occurring under a variety of tribological conditions to be characterized, and this approach shows promise as a tool for the screening of new antiwear additives. 

Elastohydrodynamic lubrication (EHD) occurs when the extent of surface deformation is comparable with the lubricant film thickness, so that a heavy load causes local elastic deformation of the contacting surfaces, but without any significant a.sperity interaction. Taking into account the low lubricant thickness (typically from 0.01 to 10 pm) and the high contact pressures (typically in the GPa range), the lubricant properties differ from tho.se of a traditional bulk liquid. since a strong viscosity increase may occur, when the lubricant behaves more like a solid than a liquid. Moderate temperature rises may occur, thus inducing some thermochemical reactions between the surfaces and the lubricant additives, but no tribochemical phenomena are involved, unlike the next lubrication mode. 

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