Anti-wear properties of lubricants

  Polymerizable additives are compounds that can directly form a polymer protective film on the friction surface under certain conditions. They are closely related to the physical and chemical processes on the surface during the friction process. When the polymerizable additive is dispersed in the lubricating oil, due to the local high temperature generated by the friction of the friction pair, the catalysis of the newly exposed metal surface, and the exogenous electrons emitted by the metal surface, the polymerizable additives are condensed or added in the friction contact area to form a friction polymer protective film in situ. The polymer film is worn away by the friction at the peak of the contact surface, and a large amount of friction heat is released at the same time. Accompanied by the exposure of the new surface and the emission of exogenous electrons, the polymerizable additives in the lubricating oil are accelerated to polymerize. This in-situ friction polymer film is in a dynamic balance of continuous formation and destruction.

  The establishment of the boundary lubrication film model can more deeply reflect the inherent regularity of the friction lubrication process and predict the friction characteristics under new conditions. Therefore, the establishment of the boundary film model has received attention from many aspects. The key to boundary lubrication lies in the formation of a protective layer, including an adsorption film, an oxidation layer or a surface reaction film.

  There is a lateral mutual adhesion between the adsorption film and the surface inhomogeneity. When the surface temperature exceeds a certain critical value, the adsorption film begins to thermally desorb and the friction factor increases rapidly. After the thermal desorption of the adsorption film, the oxide layer or surface reaction film enters the contact stage, providing protection for the next micro-contact and reducing the friction and wear rate. Therefore, the failure of boundary lubrication depends largely on the generation and removal of the reaction film.

  From the above analysis, it can be seen that the generation rate and decomposition rate of the chemical reaction film depend on the concentration of the reaction film and the reaction rate constant (generation or decomposition reaction of the reaction film), and the reaction rate constant depends on the activation energy required for the reaction and the reaction temperature. According to the knowledge of chemical reaction kinetics, in the reaction process, the reactant molecules must absorb energy and cross an energy barrier to become activated molecules (molecules with higher energy and chemical reactions that can undergo collisions) before they can be converted into product molecules. The activation energy is the difference between the average energy of the activated molecules and the average energy of the ordinary reactant molecules (unactivated reactant molecules). The size of the activation energy of a chemical reaction is related to the temperature: Ea=Ec+mRT, where: Ec is the critical energy or threshold energy of the reaction (the minimum energy that the reactant molecules must have to undergo a reactive collision); m is a constant related to the reaction.

  Therefore, the chemical kinetic model of the formation and failure of the reaction film reported so far has two defects. It does not determine the order of the chemical reaction by experimental methods, but only assumes that the generation (or decomposition) reaction of the reaction film is a first-order reaction, that is, the generation (or decomposition) rate of the reaction film is proportional to the first power of the reaction film concentration, which is not universal.

  The effect of temperature on the activation energy is not considered, but the temperature changes greatly during the actual operation of the friction pair. Therefore, through the results of mechanical friction tests in lubricating oil systems containing additives, the chemical mechanism of boundary lubrication failure is explored, combined with the existing models, new parameters are introduced, and a model of boundary lubrication failure is established. The use of the model to provide a basis for mechanical design will have great theoretical significance and economic value.