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- Third body (8)
- Tribofilm (8)
- Sliding simulation (6)
- Movable cellular automata (5)
- Friction (4)
- Automotive braking (3)
- Dry friction (3)
- MCA-modelling (3)
- Mechanically mixed layer (3)
- Nanocomposite (3)
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The sliding behaviors of two simplified tribofilms with amorphous structure consisting either of SiO₂ molecules or C atoms were simulated by molecular dynamics modeling. The objective was to identify mechanisms explaining the experimentally observed lubricating properties of the two amorphous films. The impacts of layer thickness, normal pressure, temperature and different substrate materials were studied systematically, while the sliding velocity was kept constant at 30 m/s. While the layer thickness was not critical, all the other parameters showed special effects under certain conditions. Normal pressure impeded void formation and could even eliminate voids if applied at high temperature. Stick-slip sliding was changed to smooth sliding at high temperature due to void healing. Considering the carbon film, high friction forces and shearing of the entire film was observed with diamond substrates, whereas interface sliding at low friction forces and an amorphous layer of iron mixed with carbon was observed if the supporting substrates consisted of α-Fe. Both films show a decrease of friction forces and smooth sliding behavior at elevated temperature, corresponding well to the tribological behavior of and advanced nanocomposite sliding against a steel disc under severe stressing conditions when high flash temperatures can be expected.
A model based on movable cellular automata (MCA) is described and applied for simulating the stress–strain and sliding behavior of a nanocomposite consisting of an epoxy matrix and 6 vol.% of homogeneously distributed silica nanoparticles. Tensile tests were used for verification of the model. It was realized that a slight modification of epoxy properties due to the addition of silica nanoparticles had to be taken into account in order to obtain good correlation between experimental and modeling results. On the other hand, sliding simulations revealed no susceptibility of results to slight modifications of matrix properties, but a significant impact of nanoparticles on the interface structure and smoothness of sliding mechanism. Furthermore, assuming both possibilities, bond breaking and rebinding of automata pairs, can explain different friction levels of polymer materials.
The method of movable cellular automata (MCA) was applied to simulate the stress-strain behavior of a nano composite consisting of an epoxy matrix and 6 vol. % silica nano particles. The size of the elements used for modelling was fixed at 10 nm, corresponding approximately to the diameter of the filler particles. Since not only the stress-strain response of the two constituents but also debonding of neighboring particles and granular flow was taken into account, plastic deformation as well as crack initiation and propagation could be simulated with the model. Modelling results were compared with tensile test results of both, pure epoxy as well as the epoxy-6 vol. % SiO2 composite. Since assuming bulk properties of the two constituents did not yield satisfactory results, slight modifications of the nanoparticle response functions and nanostructures were tested numerically. Finally, it was observed that only the assumption of slightly increased strength properties of the epoxy yielded good correlation between experimental and modelling results. This was attributed to an increased cross linking of the epoxy caused by the presence of silica nano particles.
In the mechanical friction and wear process, normal and tangential (shear) forces cause elastic and plastic deformations, generate cracks and form loose wear particles through fracture processes. Depending on the working conditions, the material properties and the real contact Situation (topography), a third body is formed as a mechanical ly mixed layer, which determines different running-in and steady-state behaviours.
Two dimensional (2D) MCA Simulation results will be described via empirical equations, which are related to the incubation time (period without wear) and the following running-in wear rate (kinetic concept of strength, fatigue wear model).
In order to understand the running-in and steady-state wear, kinetic model equations for mass balance of the third body are useful and are therefore to be included in the investigation.
For a more complex material behaviour, numerical Simulation results are also presented for friction at the pad-disk interface of automotive brakes.
The impact of pressure, sliding velocity and property variation of constituents on the sliding behaviour of a model tribofilm was studied with the method of movable cellular automata (MCA). Whereas a clear pressure dependency of the coefficient of friction (COF) was always observed and could be correlated with the structure formation in terms of varying thickness of a mechanically mixed layer, the impact of the other parameters was either negligible or rather weak. Only if a brittle-to-ductile Transition of the oxide-based tribofilm was assumed, a significant decrease in the COF level was predicted.
Temperature-dependent property changes can be neglected during MCA modelling, unless this transition takes place. For magnetite-based tribofilms, the transition temperature is beyond 800 °C, i.e. a temperature leading to fading effects during braking anyway. Thus, it could be concluded that, except for very severe braking conditions, sliding simulations with the MCA method yield meaningful results without considering temperature-dependent mechanical properties.
The sliding behavior of an amorphous silica sample between two rigid surfaces is in the focus of the present paper. Molecular Dynamics using a classical Tersoff’s potential and a recently developed ReaxFF potential was applied for simulating sliding within a thin film corresponding to a tribofilm formed from silica nanoparticles. The simulations were performed at different temperatures corresponding to moderate and severe tribological stressing conditions. Simulations with both potentials revealed the need of considering different temperatures in order to obtain a sound interpretation of experimental findings. The results show the striking differences between the two potentials not only in terms of magnitude of the resistance stress (about one order of magnitude) but also in terms of friction mechanisms. The expected smooth sliding regime under high temperature conditions was predicted by both simulations, although with Tersoff’s potential smooth sliding was obtained only at the highest temperature. On the other hand, at room temperature Tersoff-style calculations demonstrate stick-slip behavior, which corresponds qualitatively with our experimental findings. Nevertheless, comparison with a macroscopic coefficient of friction is not possible because simulated resistance stresses do not depend on the applied normal pressure.
In the paper by using molecular dynamics method we investigate behavior of nickel-phosphorus compound in amorphous-like state under conditions of shear loading at the constant velocity. Samples with an amorphous layer of pure nickel and nickel-phosphorus compound were considered. The analysis showed that forces of shear resistance in the sample with an amorphous layer containing phosphorus in about 3 times less than the sample with a layer of pure nickel. Thus, it was shown that nickel-phosphorous coating in amorphous-like state may exhibit low friction properties, and, therefore, serve as the solid lubricant material.