Andrey I. Dmitriev

METHOD OF MOVABLE CELLULAR AUTOMATA AS A TOOL FOR SIMULATION WITHIN THE FRAMEWORK OF MESOMECHANICS

ConclusionThe proposed MCA method is based on mesomechanics of heterogeneous media [4, 5, 9]. It is connected first with the ability to describe the material as a set of structural elements of deformation [9]. The role of the structural unit in the MCA method is played by the element (movable cellular automaton). The expressions of interparticle interactions used, as well as the rules of changing the state of the elements, allow us to simulate a wide range of phenomena including melting, chemical reactions, and phase transformations. The characteristic size of the element and its properties are defined based on the features of the model constructed in the framework of mesomechanics as described in [9]. Therefore the MCA method as a computational technique allows us to realize the principles of mesomechanics when simulating material response to external loading of different types. This method is highly recommended in computer-aided design of new materials.

Investigation of surface film nanostructure and assessment of its impact on friction force stabilization during automotive braking

Abstract The unique nanostructure formed during severe as well as moderate braking on the surface of brake discs was investigated by conventional and analytical Transmission Electron Microscopy. In both cases nanocrystalline magnetite mixed with carbon nanoinclusions and minor amounts of other pad constituents were identified. On the basis of these observations the friction performance of a single micro-contact was simulated with the method of Movable Cellular Automata. Inspite of a simplified nanostructure which was examined in two dimensions only, the calculated mean coefficient of friction fitted well to the value usually demanded for automotive braking. Furthermore, the model predicts that oxide films without soft nanoinclusions are not capable of providing smooth velocity accommodation at the pad–disc interface and thus lead to unstable friction behaviour.

Numerical Simulation of Mechanically Mixed Layer Formation at Local Contacts of an Automotive Brake System

Processes taking place at local contacts in an automotive brake system are analyzed on the basis of computer simulation with the method of movable cellular automata. The conditions of a mechanically mixed layer (MML) formation on the tribosurfaces and the influence of the layer on the friction coefficient are investigated. The results show that the MML formation leads to the stabilization of the coefficient of friction at a convenient range (0.3–0.4) for brake application. The presence of graphite particles in the MML decreases the critical value of local normal stress for switching from stick-slip to smooth relative motion with MML formation.

Local structural transformations in the fcc lattice in various contact interaction. Molecular dynamics study

The work is a molecular dynamics study of the peculiarities of local structural transformations in a copper crystallite at the atomic level in contact interaction of various types: shear loading of perfectly conjugate surfaces, local shear loading and nanoindentation. Interatomic interaction is described in the framework of the embedded atom method. It is shown that initial accommodation of the loaded crystallite proceeds through local structural transformations giving rise to higher-rank defects such as dislocations, stacking faults, interfaces, etc. In further plastic deformation, the structural defects propagate from the contact zone to the crystallite bulk. The egress of structural defects to a free surface causes deformation of the model crystallite. The deformation pattern can evolve, depending on the loading conditions, with a change in crystallographic orientation of the crystallite near the contact zone, generation of misoriented nano-sized regions, and eventually formation of a stable nanostructural state. The obtained results allow conceptually new understanding of the nature of defect generation in a crystalline structure during the nucleation and development of plastic deformation in loaded materials.

Does ultra-mild wear play any role for dry friction applications, such as automotive braking?

Nanostructured third body films and/or storage of wear debris at the surfaces of the first bodies are deemed as prerequisites of sliding under ultra-mild wear conditions. Since such features have been observed experimentally on brake pads and discs, attempts were undertaken to study their sliding behaviour by modelling on the nanoscopic scale with an approach based on Movable Cellular Automata (MCA). The model rendered the possibility to study the influence of different nanostructures systematically and to assess the impact of different brake pad ingredients on the sliding behaviour, velocity accommodation and friction force stabilization at a sliding contact. Besides providing a review on previously published modelling results, some additional new graphs enabling better visualization of dynamic processes are presented. Although ultra-mild wear conditions were considered to be essential for achieving the desired tribological properties, transitions to mesoscopic and macroscopic wear mechanisms were studied as well. The final conclusion is that ultra-mild wear and corresponding smooth sliding behaviour play an important role during automotive braking, even though temporarily and locally events of severe wear may cause friction instabilities, surface damage and release of coarse wear particles.

Effect of adhesion transfer on the surface pattern regularity in nanostructuring burnishing

In the paper the influence of friction-induced adhesion of metal to the tool on the formation of surface topography under nanostructuring burnishing was studied. A comprehensive approach, including both experimental (optical microscopy and profilometry) and theoretical (computer-aided simulation) methods was used. The results showed a direct connection between values of adhesion strength of materials in contact with the workpiece surface pattern quality caused by the tool movement. Results of the experimental and theoretical study are in good agreement and allow us to identify the reason of regular profile forming during surface burnishing.

Molecular dynamics study of slip mechanisms of nickel with amorphous-like Ni-P coating

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.

Calculation of mechanical properties of BCC Ti-Nb alloys

We have calculated mechanical properties of bcc Ti-Nb alloys in the framework of the first-principles approach using the exact muffin-tin orbital method. The results obtained quantitatively correlate well with known experimental data and can be used in order to design new materials based on of Ti alloys intended for various applications, e.g. for bio-medical applications.

Calculation of the effective diffusion coefficient for random wear surface migration on different scales

The paper considers the contact interaction of crystalline solids under shear deformation in the context of molecular dynamics. The interatomic interaction is specified by a potential calculated using the embedded atom method. The peculiarities of structural changes in a contact zone are studied for various materials of the contact pair. Based on the data extracted, the effective diffusion coefficient was estimated for random migration of the contact zone in a direction perpendicular to applied shear strains. The calculation results agree well with data of a microscopic contact model built around the method of movable cellular automata.

About vortex-like atomic motion in a loaded single crystal

The paper presents a molecular dynamics study of internal stress and atomic displacement redistributions in a preliminary loaded solid. The study demonstrates the possibility of self-organized vortices as dynamic defects of typical size 1–5 nm due to atomic motion in the elastic region at the stage of relaxation. The simulation results agree well with experimental data on strain localization in elastic distortion regions which gives rise to nanodipoles of partial disclinations.