On Stress-Induced Tribochemical Reaction Rates

A number of recent experiments have demonstrated that normal and shear stresses are able to accelerate the rates of chemical reactions of adsorbates on surfaces, yielding an approximately exponential increase in reaction rate with force. It seems that such mechanochemical reactions play a central role in tribochemistry and the consequent formation of anti-wear or friction-reducing films from lubricant additives. It is thus appropriate to summarize the theories that can be used to analyse such tribochemical data. The models have a strong parallel with those used to analyse sliding friction, the Prandtl/Tomlinson model. It is found that the force-dependent activation barrier for chemical reactions varies as E-act(F) = E-act + AF + BF2. The linear variation in activation barrier with force, the so-called Bell equation, and the quadratic dependence, the so-called Hammond effect, are analysed using model reaction energy profiles, which reveal a dependence of the parameters A and B on the shape of the energy profile. The influence of contact pressure and sliding velocity is also discussed. The above models assume that the normal and shear stresses are coplanar with the thermal reaction coordinate for the tribochemical processes, and the effects of this not being the case are also discussed. Finally, the existing results of tribochemical reactions of oxygen-functionalized graphene and zinc dialkyl dithiophosphates are also analysed using these models.