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Eingeladener Vortrag
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Automotive brake pads consist of many components but it is still not entirely clear which role each of the elements of this complex composition plays to provide the specified regimes of sliding. This is due to the mutual interaction of multiscale mechanisms, realized during the friction.
In this work we have attempted to partly answer this question using computer simulations. Since the simulation allows us to consider various combinations of the structure of the system being simulated ceteris paribus, it becomes possible to understand the role of each constituent sequentially. The main attention is paid to the structure and composition of the thin film that forms on the surface of both bodies as a result of compaction of the wear product, its chemical composition and oxidation. This layer, also named a third body or friction film, differs in composition and microstructure from the two first bodies. We considered a single contact for the steady state sliding when the structure and composition of friction films already are formed. As a modelling tool we used the method of movable cellular automata, which has well proven itself in solving of such tasks. We investigated the influence of modification of the structure and composition of the third body on the features of system behaviour at friction. To assess the adequacy of the numerical model, experimental studies with an artificial third body were also carried out. The Simulation results are in good agreement with experimental data.
Das tribologische Verhalten wird neben den chemischen und physikalischen Eigenschaften der Reibpartner auch von deren Oberflaechentopographie bestimmt. Um deren Beitrag auf die Reibkraft zu untersuchen, wurden daher definierte Rauheitsstrukturen in Siliziumwafer geaetzt. Reibversuche mit einem Mikrotribometer [1, 2] gaben Aufschluss ueber den Einfluss dieser Strukturen auf das Deformationsverhalten im Kontaktbereich. Mit Hilfe der gewonnenen Daten wurde ein Deformationsmodell entwickelt, das eine Abschaetzung der Reibkraft in Abhaengigkeit von der Rauheit erlaubte. Treten starke Adhaesionskraefte im Reibkontakt auf, ist ein inverter Einfluss der Rauheit auf die Reibkraft zu verzeichnen. Um diesen Verlauf weiter zu charakterisieren, wurden mit einem UHV-AFM Adhaesionsmessungen an strukturierten Siliziumwafern durchgefuehrt. Diese zeigen den Verlauf der Adhaesionskraft in Abhaengigkeit von der Groesse der Kontaktflaeche auf. Ein Vergleich dieser experimentell gefundenen Korrelation mit den klassischen Kontaktmodellen machte dabei den Einfluss der Oberflaechenrauheit auf die Groesse der Adhaesionskraft deutlich. Danach ergab sich, dass bei gleicher Kontaktflaeche, aber unterschiedlicher Rauheit sich die Adhaesionskraefte um bis zu einem Faktor 3 unterscheiden. [1] K. Meine, T. Schneider, D. Spaltmann, E. Santner, Wear, 253, 2002, 725-732 [2] K. Meine, T. Schneider, D. Spaltmann, E. Santner, Wear, 253, 2002, 733-738
Wear mechanisms, as adhesion, abrasion, fatigue and tribochemical wear, are complex in their physical and chemical nature. A theoretical description and prediction of wear are in most cases still far from reality. It is, therefore, important to use test rigs before a practical application of a given material combination under tribological loading.
On the other hand, model equations can be helpful for wear description, if a single wear mechanism is dominant.
Under oscillating and continuous sliding contact conditions, equations for wear calculation are presented to describe running-in and stationary wear behaviour of metals and ceramics. By using shear energy density, real area of contact, flash temperature, activation energy and numerical simulations with the method of movable cellular automata (MCA), wear data were calculated and compared to experimental laboratory results.
Tribofilms formed during dry sliding usually exhibit a nanocrystalline structure and complicated composition. In the present study, tribofilms consisting mainly of a solid lubricant, namely graphite nanoparticles, are considered. Systems providing such tribofilms are candidates for anti-friction applications. Since sliding action always leads to mixing of the materials at both sides of the tribological interface, it was of major interest to study the impact of different amounts of a hard constituent, SiC in the considered case, within the soft matrix systematically. Furthermore, the impact of normal pressure was considered. A mechanically mixed layer was observed for the whole range of normal pressures and SiC volume fractions. The calculated coefficient of friction decreased significantly with increasing thickness of this layer but was only marginally affected by SiC volume fraction, which is good news for anti-friction applications.