M. S. Kovalchenko

Elasticity and Viscosity of Isotropic Porous Materials

Theoretical dependences of elasticity moduli, Poissons ratio, and the shear and volumetric viscosity of porous isotropic materials on their relative density are generalized. An approximate description is suggested for the elasticity and viscosity of isotropic porous materials on the basis of analyzing these dependences for spherical, needle- and disk-shaped pores, and also experimental data for the elasticity moduli of sintered nickel, iron, and boron carbide taking account of mesostructural imperfection of powder bodies, and also of data for amorphous glass sintered under pressure in a vacuum.

Densification kinetics of boron carbide in hot pressing

The densification of a pure boron carbide powder, produced with and without activating iron additions, was analyzed. Pressing was performed in graphite dies. Microstructural changes were measured by a potentiometer. Temperature was measured with an optical pyrometer. Boron carbide densification with and without an activating iron addition was due primarily to a dislocation climb mechanism leading to creep. Creep rate was a quadratic function of stress.

Hot pressing of binary titanium-chromium diboride

ConclusionsAn experimental study was made of the process of densification of a binary titanium-chromium diboride powder. The dependence of the relative density of hot-pressed specimens on temperature, applied pressure, and pressing time was determined. An analysis of experimental data carried out within the framework of the theory of volume viscous flow of a porous solid has shown that densification in the hot pressing of a binary titanium-chromium diboride powder is effected through creep of its particles. The creep is controlled by the dislocation climb mechanism. Determinations were made of specific rates of steady-state creep for the particles of the binary titanium-chromium diboride powder; the specific rate of steady-state creep was found to be proportional to the square of stress. The energy of activation for steady-state creep of the binary titanium-chromium diboride powder was calculated.

Dynamic hot pressing of a porous non-Newtonian solid

Conclusions1.An analysis is made of the densification of porous G13 steel to demonstrate that the rheological theory of the dynamic densification of a porous non-Newtonian solid can be employed for describing the behavior of real porous solids during dynamic hot pressing. The limits of applicability of the theory are examined.2.A method is proposed for the processing of experimental data upon the densification of materials during dynamic hot pressing. Using this method, it is possible to extract from such data useful information concerning the densification process and the mechanism of the deformation of the solid phase in a porous solid and also to distinguish between a static and a dynamic hot pressing process.3.From the results of the analysis it follows that, during dynamic hot pressing, high pressures — exceeding by more than an order those employed in static hot pressing — are generated within a short densification time (of the order of 10−3 sec). Under the action of these pressures, the material experiences intense plastic flow. This process, whose rate is a nonlinear function of stress, is less sensitive to temperature than creep, which is linked with diffusional mass transport.

Nonisothermal Pressure Sintering Kinetics of the B4C–SiC Powder Mixture, Structure and Fracture Behavior of Sintered Composite

The kinetics of nonisothermal pressure sintering of boron carbide powder mixed with 20 wt.% silicon carbide in the controlled heating mode is studied. The isothermal sintering kinetics of the mixture at temperature of 2240 K under applied pressures of 36.1, 49.6, 63.2, and 72.2 MPa was analyzed to determine the Laplace pressure. It is found that the kinetics is controlled by steady-state creep mechanism in the matrix forming the porous body, with the viscous flow rate being proportional to the square of stress. The relatively low value of the evaluated Laplace pressure (5.6 MPa) explains the difficulties in producing boron carbide composites with pressureless sintering. The current values of temperature and height of the samples during pressure sintering were used to determine the heating rate and the temperature derivatives of relative density, which enabled to describe the pressure sintering kinetics in the terms of the theory of bulk viscous flow of the porous bodies in a die. The evaluated activation energy of the intermediate and late stages of pressure sintering of the composite for different heating rates ranges from 670 to 710 kJ/mol. These values indicate that the sintering kinetics is controlled by dislocation climb mechanism. The structure and fracture behavior of the sintered samples are shown to depend on the heating rate. The higher the heating rate during B4C–20% SiC sintering, the more heterogeneous is the distribution of powder components and the larger the portion of transcrystalline fracture of sintered samples.

Rheological models of pressure sintering of powders

Rheological models of deformable bodies are used to develop a dynamic approach to the problem of mechanical actions on porous bodies during pressure sintering. Solutions of dynamic systems describing the force acting on porous viscoelastic bodies, either strain-hardenable or not, are presented depending on the control parameters of the systems. These parameters are determined by the inertial properties and rigidity of the machine and the rheological properties of the deformable bodies. The intensity of densification during the pressure sintering of porous bodies mainly depends on the ratio of rigidity of the system to the viscous resistance of the body. The simulation and analysis of the pressure sintering of porous bodies using the obtained solutions enable prediction of their densification conditions and functional properties depending on the machine characteristics and the sizes and rheological properties of deformable bodies.

An investigation into the external friction of niobium and tantalum carbides at high temperatures in vacuum

Conclusions1.An investigation was carried out into the temperature dependence of external friction between specimens made from NbC0.98 and also between specimens made from TaC0.97 in the temperature range extending from room temperature to 1800°C in a vacuum of 10−5 mm Hg.2.The coefficient of friction of niobium carbide has its minimum at 1200°C and that of tantalum carbide at 1300°C. Above this temperature a sharp increase in the coefficient of friction is observed. The reason for this increase is an intense adhesion interaction between contact surfaces.3.It was found that the loading of a static contact surface between the materials under investigation at high temperatures produces creep processes controlled by transverse slip of dislocations with a considerable contribution from the diffusion mechanism. The creep activation energy in the temperature range 1300–1500°C is 1.3 eV for NbC and 1.4 eV for TaC.4.It was shown that the slip activation energy is determined mainly by the presence of convalent bonds in pure materials and compounds.

Effect of substrate temperature on the electrospark deposition, structure, and mechanical properties of coatings. II. Coating deposition features

The relationship between deposition temperature, coating inhomogeneity, and temperature of cathode spots is studied. It is shown that the spark deposition of coatings proceeds in the area of cathode spots; therefore, the substrate temperature has an essential effect on the deposition process. A simple model is proposed to evaluate the relationship between the temperature of cathode spots and deposition temperature and coating inhomogeneity.

Effect of temperature on the wear and character of rupture of titanium and niobium carbides

Conclusions1.A study was made of the effect of temperature on the wear of titanium and niobium carbides. It was established that, within the whole temperature range investigated, the carbides exhibit excellent wear resistance. The maximum increase in wear intensity is observed in the range 500–1200° C.2.An electron-microscopic investigation was made into the character of rupture of the surface of niobium carbide under friction conditions in the range 20–1600° C. It was found that, at temperatures in excess of 1200° C, all signs of brittle rupture disappear from the surface layer.

The Production and Properties of High-Temperature Electrical-Insulation and Heat-Resistant Aluminum Nitride Materials

The effect of activating additives on the formation of aluminum nitride composites in the sintering process has been studied. The production and properties of the sintered composites have been optimized. The properties of the aluminum nitride composites with an optimum ratio of components allow them to be used as electrical insulators in radioisotope and electrical equipment, as thermal shields in vacuum furnaces, and as corrosion-resistant refractories operating in contact with steel, aluminum, zinc, copper, and cast iron melts.