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Stress Assisted Tribofilm Growth: A New Model
Under extreme conditions the lubricant film fails to separate the rubbing surfaces and solid-to-solid contact occurs. To prevent excessive wear of the base materials anti-wear additives are used in these cases. The additives create a protective layer which is worn instead of the base materials and allows to control the friction and wear developed in the system. The key here is to ensure that the growth rate of the tribofilm is larger than the rate of its removal. On the other hand, the tribofilms do not grow infinitely and stabilize at a certain thickness. These two effects were addressed in the current study.
One of the most frequently used anti-wear additives is the Zincdialkyldithiophosphate (ZDDP). Recent Atomic Force Microscopy study clearly showed that the growth rate of ZDDP tribofilm is determined by the shear stress and temperature according to the following equation:
where are the reaction constants, are the shear stress and temperature, is the Boltzmann constant.
In the recent study in Surface Technology and Tribology group of University of Twente, equation (1) was combined with a simple wear equation to predict the growth and wear of the tribofilm, its self-limitation and wear of the base materials in a rough sliding contact. The wear equation had the following simple from:
where is an empirical wear coefficient. Overall, the tribofilm growth equation give the equation to calculate the tribofilm growth rate at given contact shear stress and temperature according to the simple equation:
The model was validated using experimental data at various temperatures found in the literature:
The model was also applied to simulate base material removal and a good agreement with the experimental data was observed. As can be seen from the Fig. 2, the substrate material removal is maximum at 60C, while the overall tribofilm wear rate is maximum at 100C (see Fig. 1, right). This feature emphasizes the sacrificial nature of the tribofilm: at 100C the tribofilm grows faster and even though the overall wear rate is larger, the base material removal is less.
Further details can be found in the original article: A. Akchurin, R. Bosman, A Deterministic Stress-Activated Model for Tribo-Film Growth and Wear Simulation, doi:10.1007/s11249-017-0842-8.