A model based on movable cellular automata has been applied to study the sliding behaviour of tribofilms formed during automotive braking. Since it is not possible yet to determine the composition of real tribofilms quantitatively, final verification of modelling results is needed. This was done by preparing artificial third bodies with compositions and nanostructures matching the ones assumed for modelling. Pin-on-disc tests were performed while applying the artificial third bodies to the contact. The results revealed that not only the structure of the third body but also the amount of the applied normal pressure determines the COF obtained by modelling and that much better correlation between experimental and modelling results was obtained while assuming high normal pressures at asperity contacts.
This review article comprises of three parts. Firstly, reports of brake manufacturers on the beneficial impact of solid lubricants for pad formulations are surveyed. Secondly, since tribofilms were identified to play a crucial role in friction stabilization and wear reduction, the knowledge about tribofilm structures formed during automotive braking was reviewed comprehensively. Finally, a model for simulating the sliding behavior of tribofilms is suggested and a review on modelling efforts with different model structures related to real tribofilms will be presented. Although the variety of friction composites involved in commercial brake systems is very broad, striking similarities were observed in respect to tribofilm nanostructures. Thus a generalization of the tribofilm nanostructure is suggested and prerequisites for smooth sliding performance and minimal wear rates have been identified. A minimum of 13 vol.% of soft inclusions embedded in an iron oxide based tribofilm is crucial for obtaining the desired properties. As long as the solid lubricants or their reaction products are softer than magnetite, the main constituent of the tribofilm, the model predicts smooth sliding and a minimum of wear.