Tribochemical energy

The good correlation between the exoelectrons emission intensity and the standard heat of formation (AHf) of the compounds strongly suggests that the electrons are excited by the exothermic reactions (Nakayama et al., 1995 Wei and Lytle, 1976). Exoemission intensity is related to the maximum possible kinetic energy of the electrons, which can be expressed as tribochemical energy (TribEn)... 

Table 5.11. The correlation of tribochemical energy (AHf - WF) between the heat of formation (AHf) and the work function (WF) for the stable compounds from abraded metal surfaces at25°C...

As seen in Table 5.11, oxidation and other surface reactions contribute to the emission of tribochemical energy when the negative AHf value exceeds the WF value of the surface. The tribochemical energy (AHf - WF), due to the chemisorption of adsorbates, e.g., oxygen or sulfur on zinc surface, is not of the same order of magnitude as the yield of chemisorption of ZDDP on zinc surface. The tribochemical energy comes from the energy released by chemisorption of the ZDDP on the fresh zinc surface. 

Thermodynamic parameters The tribochemical energy for the stable compounds of tribofilm comes from the heat of formation AHf and work function WF (AHf - WF). Using the data from Table 5.11, compare the tribochemical energy for iron compounds and explain formation of the tribofilms of FeS and FeO in spite of positive tribochemical energy. 

Catalytic activity of rubbing surfaces (a) By reference to Table 5.5, find the metal hydroxides, oxides and nitrides that illustrate their highest exoelectrons emission intensity (I, cps), (b) Calculate the tribochemical energy (TribEn) TribEn = (AHf - WF) for the listed compounds and correlate them with measured exoelectrons emission intensity (I, cps). Explain differences. 

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. 

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... 

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. 

Entrapped oxygen and water vapor in lubricants can act as anti-wear additives to form protective surface films. Metals are known to catalyze decomposition of certain lubricants, and decomposition temperatures may be reduced by 60°C or more. This is particularly true of bearing surfaces, on which the surface energy may be increased by stress-induced dislocations and by freshly exposed metal surfaces. An example of the way a tribochemical surface is affected by a polymerization process is vinyl chloride. If the load is increased from 0.1 to 0.5 kg in the presence of a vinyl chloride atmosphere, the Auger spectra of iron oxide surface shows a marked increase in the concentration of vinyl chloride on the surface (Buckley, 1981). 

Copp et al. believed that percussion sensitiveness (with the ball and disk apparatus) was very complex and involved, in addition to the formation of hot spots through friction, a tribochemical reaction in which there was a more direct transfer of mechanical energy to activation energy than was the case when the mechanical energy is first converted to heat. An example of this is shown with experiments using nickel and tin disks to confine Service lead azide in which the surfaces of the low melting point lessened the grit sensitivity but enhanced the impact sensitivity. 

Mechanical energy generated by the movement of the contacting bodies is converted by friction into other types of energy, leadings to triboelectric and tribochemical phenomena. 

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. 

Rupture of chemical bonds and formation of macro-radicals is observed when the energy of the mechanical action located on some link exceeds the energy of this link. The resistance of polymers to tribochemical transformations is evident from the value of the dissociation energy of the links (Table 4.10). 

In variable mechanical fields (the type of loading that occurs during friction) the slowest stage that defines the velocity of tribochemical transformations may turn out to be redistribution of elastic stresses in the polymer. In this case, the velocity constant Ki of the mechanical process will be proportional to the redistribution velocity j4(s ) and the probability of accumulating some critical deformation energy U on a given link sufficient for the reaction initiation [82] will be... 

Quinn, 1980 dA, exp[-Q/(R,TJ] 3eyvH Wcorr = wear rate p = density of material Aj, = Arrhenius constant Q = activation energy Rg = gas constant Tj, = contact temperature d = asperity contact diameter V = sliding velocity e = critical thickness of reaction layer asperity layers formed tribochemically are... 

Current explanations of tribochemical reactions state that the more obvious consequence of mechanical treatments, the increase of the surface area of a solid, is a minor factor, which contributes only to 10% of the reactivity increase. The more important effect is due to the accumulation of energy in lattice defects which can relax either physically by the emission of heat, or chemically by the ejection of atoms or electrons, formation of excited states on the surface, bond breakages, and other chemical transformations. Mechanical stress can be applied as single or periodic shocks, rapid loads, etc. An example is that of the mechanochemical decomposition of aluminum hydride, which increases with the frequency of the applied stress (Fig. 2).i ... 

The tribochemical interpretation takes into account a more specific effect, namely the excess energy received, i.e., a true activation process (p. 174 see also p. 110). In... 

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