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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.
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.
Copper is one of the most important components in brake pads and its amount can reach up to 14%. In spite of a number of positive features copper usage in brake pad formulations has recently become the subject of considerable discussions, primarily due to concerns about potential risks related to environmental impacts of copper particles. So, for developing new pad formulations with possible replacements of copper content, it is very important to understand the functionality of copper additions to brake friction materials. In the paper theoretical investigation of the role of copper as a pad ingredient was carried out on the basis of modelling by the method of movable cellular automata (MCA). In the study the concentration of copper particles in a Fe3O4-matrix was varied. The sliding simulations were performed while assuming material properties at 500 degrees C in order to assess the beneficial role of copper during severe braking conditions corresponding to fading cycles during dynamometer testing.
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.
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.
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.
In part 1 it was shown that tribofilms usually are 100 nm thick and exhibit a multiphase nanocrystalline structure. The objective of our modelling efforts was to obtain a better understanding of the sliding behaviour and associated friction properties and to study the impact of internal and external parameters on these properties. The method of movable cellular automata (MCA) was used. The third bodies were considered as aggregates of linked nanoparticles which may decompose and form a layer of granulär material, the so-called mechanically mixed layer (MML), if certain fracture criteria are fulfilled. The basic model structure which consists of Fe3Ü4 nanoparticles with 13 % graphite inclusions was used. In order to assess the robustness of the model the following parameter studies were performed. The pressure ränge at an asperity contact was varied between 15 and 50 MPa. The mechanical properties of the oxide were varied between brittle and ductile behaviour corresponding to room temperature and high temperature behaviour. The mechanical properties of the soft ingredient were varied + 50 % of the properties of graphite. The influence
The method of movable cellular automata (MCA) and method of molecular dynamics (MD) were applied to simulate the friction and sliding behavior of model-tribofilms formed from a nanocomposite consisting of an epoxy matrix, 10 vol % micron-sized carbon fibers and 5 vol. % silica nanoparticles. Whereas MCA considered the tribofilm as an agglomerate of silica nanoparticles released from the composite and mixed with graphite particles, MD simulated the sliding behavior of an amorphous silica layer supported by stiff crystalline substrates on both sides. The MCA model provided reasonable quantitative results which corroborate experimental findings at moderate stressing conditions. The very low coefficient of friction observed experimentally under severe stressing conditions was not explained by this model. This could be attributed to the lack of mechanical data at the high temperature expected under these conditions. Although based on a simpler assumption of the tribofilm composition, MD-modelling could be easily applied to the expected high flash temperature and was able to predict friction reduction and smooth sliding under these conditions.
In the paper the method of discrete modeling (movable cellular automata method) and combined discrete-continuous description of the simulated medium are used to analyze processes occurring in the local contact of the automotive brake system. The characteristic size of the considered region is 1.5 ?m. The following contact situation is simulated: steel fiber coated by an iron oxide film as the brake pad and pearlitic steel also coated by an iron oxide layer as the disc. On the assumption of oxide layer wearing we simulate the iron oxide - iron oxide, iron oxide - metal and metal - metal contacts.
The calculation results for the friction coefficient for various contact situations give quite adequate values. For example, for the oxide - oxide system the calculated coefficient is approximately equal to 0.4, while for the metal - metal contact the obtained value varies from 0.7 to 0.9. Analysis of a set of the obtained results allows concluding that oxide is formed more rapidly than the sliding layer, which in turn makes the friction coefficient value stabilized.
Copper is one of the most important components in brake pads and its amount can reach up to 14%. In spite of a number of positive features copper usage in brake pad formulations has recently become the subject of considerable discussions, primarily due to concerns about potential risks related to environmental impacts of copper particles. So, for developing new pad formulations with possible replacements of copper content, it is very important to understand the functionality of copper additions to brake friction materials. In the paper theoretical investigation of the role of copper as a pad ingredient was carried out on the basis of modelling by the method of movable cellular automata (MCA). Our previous studies show that copper as a constituent of the tribofilm formed during braking provides smooth sliding by forming a granular layer of mechanically mixed materials from the friction layers. In the present study the concentration of copper particles in a Fe3O4-matrix was varied systematically in the range 5.5-28 vol. % and compared to mixtures with the same amount of graphite nanoparticles. The sliding simulations were performed while assuming material properties at 500°C in order to assess the beneficial role of copper during severe braking conditions corresponding to fading cycles during dynamometer testing.
In the paper, a model typical for contact situations of automotive brakes is established based on the method of movable cellular automata. The processes taking place at local contacts in an automotive brake system are analysed. Based on microscopic and micro-analytical observations, the following contact situations were simulated: (i) a couple of ferritic steel against pearlitic steel, both covered by an oxide layer mixed with graphite nanoparticles and (ii) the same situation but without oxide layers. The results of calculated mean coefficients of friction of the oxide-on-oxide contact correspond well to expected values for a real braking system, whereas steel-on-steel contact are twice as high. This allows one to make some conclusions; for example, oxide formation will take place more quickly than friction layer elimination, and finally this is responsible for the stabilisation of the coefficient of friction.
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 additive manufacturing of low elastic modulus alloys that have a certain level of porosity for biomedical needs is a growing area of research. Here, we show the results of manufacturing of porous and dense samples by a laser powder bed fusion (LPBF) of Ti-Nb alloy, using two distinctive fusion strategies. The nanostructured Ti-Nb alloy powders were produced by mechanical alloying and have a nanostructured state with nanosized grains up to 90 nm. The manufactured porous samples have pronounced open porosity and advanced roughness, contrary to dense samples with a relatively smooth surface profile. The structure of both types of samples after LPBF is formed by uniaxial grains having micro- and nanosized features. The inner structure of the porous samples is comprised of an open interconnected system of pores. The volume fraction of isolated porosity is 2 vol. % and the total porosity is 20 vol. %. Cell viability was assessed in vitro for 3 and 7 days using the MG63 cell line. With longer culture periods, cells showed an increased cell density over the entire surface of a porous Ti-Nb sample. Both types of samples are not cytotoxic and could be used for further in vivo studies.
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.
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.
A modified pin-on-disc test was applied to determine tribological properties of typical brake pad constituents. Ball-milling of these ingredients together with iron oxide and graphite provided model materials displaying the main features of real third bodies. Solid lubricants like graphite affected the friction and wear behaviour of Fe3O4 powders considerably whereas further addition of hard nanoparticles induced only minor effects. This was corroborated by comparison with modelling results. MoS2 played a dual role. Depending on special conditions, this ingredient either reduced or increased friction. The latter could be explained, after nanoscopic characterization, by oxidation and destruction of the wear-protecting tribofilm.