Lev N. Rabinskiy

Mechanical behavior of porous Si 3 N 4 ceramics manufactured with 3D printing technology

The paper focuses on experimental measurement and analytical and numerical modeling of the elastic moduli of porous Si3N4 ceramics obtained by 3D printing and pressureless sintering. The pores in such a material have complex irregular shape and porosity varies over a wide range (up to 50%), depending on the technological parameters used. For analytical modeling, we use effective field methods (Mori–Tanaka–Benveniste and Maxwell homogenization schemes) recently developed for pores of superspherical shape. For FEM simulation, we used microstructures generated by overlapping solid spheres and overlapping spherical pores. It is shown that elastic properties of ceramics are largely determined by the granular structure and the concave pore shape, which have been observed in the ceramics microstructure after sintering of the 3D-printed powder green bodies.

Fabrication of porous silicon nitride ceramics using binder jetting technology

This paper presents the results of the binder jetting technology application for the processing of the Si3N4-based ceramics. The difference of the developed technology from analogues used for additive manufacturing of silicon nitride ceramics is a method of the separate deposition of the mineral powder and binder without direct injection of suspensions/slurries. It is assumed that such approach allows reducing the technology complexity and simplifying the process of the feedstock preparation, including the simplification of the composite materials production. The binders based on methyl ester of acrylic acid with polyurethane and modified starch were studied. At this stage of the investigations, the technology of green bodys fabrication is implemented using a standard HP cartridge mounted on the robotic arm. For the coordinated operation of the cartridge and robot the specially developed software was used. Obtained green bodies of silicon powder were used to produce the ceramic samples via reaction sintering. The results of study of ceramics samples microstructure and composition are presented. Sintered ceramics are characterized by fibrous α-Si3N4 structure and porosity up to 70%.

Binder Jetting of Si3N4 Ceramics with Different Porosity

This article presents the results of the binder jetting technology application for the silicon nitride ceramics production. A modified version of the Plan-B 3D printer with an epoxy-based binder was used for silicon green bodies preforming. Silicon powder was pre-coated with epoxy resin, and the curing agent was added during 3-D printing of green bodies using a standard cartridge. Curing and removal of organics was carried out during the high-temperature vacuum drying of the printed preforms. Reaction-bonded silicon nitride was obtained by using pressureless sintering. An additional compaction of green bodies is proposed to reduce the porosity of green bodies and sintered ceramics. It is shown that the proposed methods allows to improve the mechanical properties of sintered specimens.

OPERATING CAPACITY OF ANTI-OXIDIZING COATING IN HYPERSONIC FLOWS OF AIR PLASMA

In this paper, we present the results of investigations of degradation processes that occur in the structure promising high-temperature anti-oxidizing of heat-resistant coating of the Si–TiSi2–MoSi2–B–Y system in hypersonic flows of air plasma. The coating is designed to protect a wide range of heat-resistant materials (carbon–carbon and carbon–ceramic composites, coal–graphite materials, alloys based on Nb, Mo, W, etc.). It is found that the coating operating capacity at surface temperatures Tw≤ 1820–1830∘C is provided by the structural-phase state of its microcomposite main layer and formation on the coating surface of a heterogeneous passivating protective film. It is based on borosilicate glass reinforced by rutile microneedles. The mechanism of coating destruction at Tw≥ 1850–1860∘C is erosion loss of oxide film as well as generation and growth of gas-filled cavities at the “coating main layer–oxide film” interface. As the pressure of saturated vapor of gaseous oxidation products (SiO, CO, MoO3 and B...

High Temperature Coatings for Oxidation and Erosion Protection of Heat-Resistant Carbonaceous Materials in High-Speed Flows

Modern approaches to the creation of single-layer and multi-layer high-temperature coatings for the protection of heat-resistant carbon-containing composite materials from oxidation and erosion in the high-speed fluxes of oxygen-containing gases are analyzed. Particularly have been outlined the heat-resistant coatings, the main components of which are either super refractory transition metal borides (ZrB2, HfB2, TiB2) with the addition of carbides (SiC, ZrC, HfC, TiC, TaC), silicides (MoSi2, TiSi2, ZrSi2, TaSi2, WSi2) and nitrides (HfN, ZrN, TiN), or refractory oxides (HfO2, ZrO2), or more complex synthetic compositions based on oxide ceramics. The results of fire gas-dynamic tests of coatings of perspective compositions are presented. The potential architecture of ultra-high-temperature coatings with high efficiency of protective action is justified.

Evaluation of the Thermal Processes and Simulation Methods for Additive Manufacturing Based on the Geometry Voxel Representation

Currently, the technological and hardware base additive production technologies are actively developing. The emergence of new developments in the field of materials science and their possibilities for the creation of products using layer-by-layer method promotes the development of innovative solutions that increase the efficiency of complex technical parts and improves their quality. This paper describes simulation methods based on the Model Voxel Representation part discretization allowing evaluating thermal processes during 3D printing process. The simulation model was extended taking into consideration the following criteria: heat conduction coefficient as a constant; temperature in the center of the elementary volume equals to the temperature at the volume border; temperature of the powder surrounding the item taken as a constant. It was determined that the convergence is affected by numerous factors: temperature in the machine chamber, material properties, heat exchange and the processes running into the chamber of the certain machines.

Mathematical Modeling of Thermal Association in a Porous Medium with a Capillary Effect in Anisotropic Composite Materials

Authors developed a mathematical model of heat and mass transfer in multidimensional bodies made of composite materials, taking into account porosity and capillarity. Continuity equations for pyrolysis gases were taken for the research: the pressure ratio at the outer boundary of the body. Based on the nonstationary heat conduction problem, under conditions of high-intensity heat transfer at the boundaries of bodies, taking into account the laws of decomposition and non-linear filtration of pyrolysis gases through a porous residue, the time distribution of the movable boundary of phase transformations and the distribution of the temperature field along the variable coordinate at different instants of time are obtained. It was found that, although the thermal load at the outer boundaries is symmetric with respect to the symmetry axis, there is an obvious asymmetry in the temperature field and the movable boundary of the phase transformations.

Semi-Inverse Solution of a Pure Beam Bending Problem in Gradient Elasticity Theory: The Absence of Scale Effects

The semi-inverse solutions of pure beam bending problems within the three-dimensional formulation of gradient elasticity theory as exact tests for the problem of estimating the efficient bending stiffness of so-called scale-dependent thin beams and plates due to the necessity of modeling sensing devices are presented. It is shown that the solutions within the gradient elasticity theory give classic beam bending stiffnesses and demonstrate the invalidity of the widespread results and estimates obtained in the past 15 years during study of scale effects within the gradient beam theories, according to which the relative bending stiffness grows by a hyperbolic law with decreasing thickness.