This content will become publicly available on December 1, 2024
 Award ID(s):
 2020525
 NSFPAR ID:
 10535335
 Publisher / Repository:
 Springer
 Date Published:
 Journal Name:
 Tribology Letters
 Volume:
 71
 Issue:
 4
 ISSN:
 10238883
 Page Range / eLocation ID:
 121
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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This paper shows how the effect of combined normal and shear stresses on the rates of tribochemical reactions can be calculated using EvansPolanyi (EP) perturbation theory. The EP approach is based on transitionstate theory, where the rate of reaction is taken to be proportional to the concentration of activated complex. The equilibrium constant depends on the molar Gibbs free energy change between the initial and transitionstates, which, in turn, depends on the stresses. EP theory has been used previously to successfully calculate the effects of normal stresses on reaction rates. In this case, ln(Rate) varies linearly with stress with a slope given by an activation volume, which broadly corresponds to the volume difference between the reactant and activated complex. An advantage of EP theory is that it can calculate the influence of several perturbations, for example, the normal stress dependence of the shear stress during sliding. In this paper, EP theory is used to calculate shearinduced, tribochemical reaction rates. The results depend on four elementary activation volumes for different contributions to the Gibbs free energy: two of them due to normal and shear stresses for sliding over the surface and two more for the surface reaction. The results of the calculations show that there is a linear dependence of ln(Rate) on the normal stress but that the coefficient of proportionality between the ln(Rate) and the normal stress now has contributions from all elementarystep activation volumes. Counterintuitively, the analysis predicts that the ln(Rate)normal stress evolution tends, at zero normal stress, to an asymptotic rate constant that depends on sliding velocity and differs from the thermal reaction rate. The theoretical prediction is verified for the shearinduced decomposition of ethyl thiolate species adsorbed on a Cu(100) single crystal substrate that decomposes by C‒S bond cleavage. The theoretical analyses show that tribochemical reactions can be influenced by either just normal stresses or by a combination of normal and shear stresses, but that the latter effect is much greater. Finally, it is predicted that there should be a linear relationship between the activation energy and the logarithm of the preexponential factor of the asymptotic rate constant.more » « less

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