Vol 21, No 6 (2018)

An experimental and theoretical study of contact fatigue damage accumulation at the coating-substrate interface has been carried out for the case of frictional contact between a smooth coating and a rough counterbody. Coatings were synthesized via low-temperature thermal decomposition of metal (Al, Zr) carboxylate solutions, which resulted in the formation of nanoscale amorphous nanocrystalline oxide layers 20–400 nm thick (depending on the concentration of the film-forming solution and the number of loading cycles) on the substrate surface (quartz glass). The investigation included coating deposition, determination of coating mechanical properties by indentation, development of the friction test procedure, stress calculation at the coating-substrate interface by modeling high-cyclic frictional loading, and the choice of a damage accumulation model for the coating-substrate interface that relates the stress state to the number of cycles to coating spalling. Preliminary tests revealed the coating compositions and coating deposition techniques that provide the highest spalling resistance under cyclic contact loading. Parameters in the relation for contact fatigue damage accumulation were determined and the model was verified by analyzing the experimental load dependence of the number of cycles to coating spalling on the microscale. It has been shown that the linear damage summation model conventionally used for describing failure due to fatigue damage accumulation in some materials can be applied to investigate the coating-substrate interface whose properties depend not only on the properties of the interfacing materials but also on the coating deposition technique.

The formation and propagation of localized nanobands with elastic lattice distortion in nickel crystallites have been studied within the molecular dynamics framework. Such nanobands are formed due to the presence of regions with tensile and compressive stresses on the free surface. The nanoband propagation region is characterized by a collective vortex motion of atoms. The effect of different-type grain boundaries on nanoband propagation was investigated. It was shown that grain boundaries do not significantly affect the reorientation angle in the nanoband, but the nanoband propagation direction changes after crossing a grain boundary in accordance with the different crystallographic orientation of grains.

A system of equations for studying the fracture of solids has been formulated using a physico-mathematical model that describes the heat and mass transfer in heterogeneous media. The derived relations are used to analyze the energy balance equation and to study the fracture of a concrete sample under uniaxial compression. The analysis is performed by a quantitative interpretation of experimental data on two acoustic emission amplitude and frequency spectra recorded during sample loading. The spectra provided data on the time evolution of the structural characteristics of the material and were used to evaluate the relationship between the rates of change of surface energy and acoustic emission intensity per unit volume of the compressed concrete sample at the time of recording the spectra. Conditions for the occurrence of possible fracture scenarios by the time of recording the second spectrum were investigated. With experimental data on acoustic emission amplitude-frequency spectra for more than two time points, it is possible to study in more detail the changes in the structural characteristics, surface energy, and acoustic emission intensity per unit volume of a solid. The beginning of particular fracture stages can be predicted with higher accuracy.

The paper presents a molecular dynamics study into the temperature and size effects in nanostructures on their mechanical characteristics and fracture. The study shows that among the cross-sectional areas studied, the least one measuring ny × nz = 5 × 5 lattice cells is boundary for perfect nanostructures, and at larger areas, these characteristics tend to those of macrostructures. dor all systems considered, a linear decrease with increasing temperature is observed in Young's modulus and in critical applied stress at which fracture occurs.

Problems associated with a qualitative increase in the selectivity of fluid filtration remain the major challenge in a variety of areas such as fluid transport through porous materials and media, ion separation, water desalination and purification, and many others. A promising way to solve these problems is to design and develop membranes with slit-shaped nanopores. In the paper, we studied the selectivity and permeability of slitshaped nanosized pores in the natural mineral (hydroxyapatite) with the use of the nonequilibrium molecular dynamics approach with all-atom models. We showed that the subnanometer-wide slit-shaped pores in hydroxyapatite are able to demonstrate both good salt rejection and relatively high water permeability. In particular, the numerically predicted water permeability of hydroxyapatite with 0.4 nm thick slit-shaped nanopores reaches about 200 L/(m² h bar) that is higher than that of commercial membranes and has the same order of magnitude as the theoretically predicted water permeability through single-layer MoS₂ nanoporous membranes. An increase in the nanopore thickness is accompanied by a multiple growth in permeability, which is comparable with advanced 2D-CAP (2D-conjugated aromatic polymer) membranes, but in so doing the filtration selectivity is lost. The results show that nanoporous materials with the connected network of slit-shaped nanopores is a promising filter material for water treatment including seawater desalination and other important technical and environmental applications.