Yu. G. Tkachenko

Structure and Properties of Wear-Resistant Spark-Deposited Coatings Produced with a Titanium Carbide Alloy Anode

The paper examines the phase and structure formation during hot pressing of titanium carbide electrode materials as well as the structurization and properties of spark-deposited coatings on steel substrates. The influence of the operating current used for erosion processing on the phase composition and hardness of the coatings is established. It is shown that the electrode materials with a certain composition and structure enable the electrospark deposition of 100 μm-thick coatings with hardness to 14 GPa. The mass of the material deposited on a steel substrate is three times higher than that formed using standard titanium carbide alloy TN20. The wear resistance of the coatings in abrasive and dry sliding friction is high, hence they may be recommended for protection of steel parts for severe wear applications.

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.

Temperature dependence of the coefficients of friction of boron and silicon carbides in a vacuum

The carbon-boron compound B~C and the carbon-silicon compound SiC are widely used in manyfields of engineering [1, 2]. One of the most remarkable characteristics of boron and silicon carbides is their wear resistance. However, the external friction mechanism of these compounds has not yet been studied adequately. There are virtually no data on the friction of B~C and SiC under high-temperature conditions. The authors are aware of only three works containing information on the effects of temperature on the coefficients of friction of SiC [3, 4] and B4C [4, 5] during vacuum heating. It has been established that, in the rubbing of pairs composed of similar specimens from SiC and B4C single crystals, their coefficients of friction steadily decrease with rise in temperature [4]. In the friction of polycrystalline B iC on B~C, heating to 1800~ has been found to produce a slight increase in its coefficient of friction (from 0.2 to 0.35) [5]. The values quoted in the literature for the room-temperature coefficient of friction of boron carbide vary quite appreciably, ranging from 0.6 [4] to 0.2 [5]. In none of the works cited have the characteristics of the materials investigatedcomposition, method of preparation, and specimen porositybeen given.

Optimization of the composition, structure, and properties of electrode materials and electrospark coatings for strengthening and reconditioningof metal surfaces

The structure and phase composition of Ni-Cr-Al alloys doped with Si, Ti, Mn, and Co have been studied. An eutectic three-phase structure was found to be in the doped alloys. Doping with Si and Ti increases the microhardness and wear resistance of the alloys. The highest coefficient of the mass transfer (0.75) during the electrospark alloying is observed for Co-containig alloys. The coatings with the doped alloys have a higher wear resistance than those with the Ni-Cr-Al basic alloy. Steel 45’s heat resistance is increased after the electrospark doping with Si-, Ti-, Mn-, and Co-containing alloys by 4, 4.3, 5.1, and 4.6 times, respectively. The electrode materials have been developed for the electrospark reconditioning of workpieces based on PE8418 (Ni-Ni3B-Cu-Si) with the additions of titanium carbide, chromium carbide, and tungsten carbide, which make it possible to manufacture coatings up to 5-mm thick. The results of the investigation of the erosion properties of B4C-TiB2 alloys manufactured using the method of reactive sintering under hot pressing of B4C-TiO2 powder blends that were used as the electrode materials for the electrospark hardening of titanium surfaces are presented. The tests show that, in the surface layers of the electrode materials, under the impact of the electric discharge, the boron carbide content substantially decreased, while the quantity of titanium borides increased and new phases of TiCxNy, TiO2, and Ti appeared. Only these components are transferred onto the surface of the titanium alloy and form there a protective coating up to 100 μm thick with high hardness (32–43 GPa) and wear resistance. The materials developed are promising for their application as the electrodes in the electrospark alloying of construction steels and titanium alloys.

DEFORMATION BEHAVIOR OF REFRACTORY METAL CARBIDES DURING RUBBING IN A WIDE TEMPERATURE RANGE

Conclusions1.It is shown that the processes of strengthening (and strength loss) take place in the surface layers of titanium, niobium, and tungsten carbide specimens during high-temperature rubbing in a vacuum are linked with charges in fine crystal structure and phase composition experienced by these carbides in their surface layers.2.It has been established that the strengthening of the surface layers of these carbides observed in the range from room temperature to 400–800°C increases their wear resistance. The strength loss exhibited by the surface layers of the carbides in the range 900–1400°C is a result of the effect of the temperature factor becoming stronger than that of the deformation factor.

Influence of addition of calcium hexaboride on the structure and properties of hot-pressed titanium boride ceramic

In our study of the microstructure of hot-pressed TiB2—x CaB6 specimens we have found that the amount of CaB6 added affects the grain size. The TiB2 grain size decreases when the CaB6 content in the composite exceeds 5%. The retardation of the grain growth during hot pressing of powder with the composition TiB2—10% CaB6 is attributed to dissolution of Ca in it. A composite with that composition has a high microhardness and bending strength.

Frictional characteristics and contact-zone deformation behavior of TiC in its homogeneity range

Conclusions1.It has been established that with fall in carbon content the Youngs modulus, σtr, and microhardness of titanium carbide decreases.2.In vacuum friction tests the coefficient of friction of the carbide is directly proportional to its carbon content at temperatures of 20 and 400°C and inversely proportional to it at 1200°C. At 800°C the effect of composition on the coefficient of friction is negligible.3.The rate of wear of the carbide is proportional to the area of true contact.4.An x-ray structural analysis has demonstrated that the maxima on curves of physical interference line broadening observed after friction tests at all temperatures correspond to the carbide TiC0.68.

High-temperature friction and wear of eutectic carboboride alloys of group V metals

ConclusionsIt is shown that curves of coefficient of friction and wear intensity plotted against temperature for the alloys investigated pass through minima at 800-900° C. An x-ray structural analysis has yielded data on the deformation behavior of the carbide and boride components during the rubbing of the eutectic TaC-TaB2 alloy. The level of high-temperature bearing properties of the alloys investigated is much higher than that of their starting components.

Effect of temperature and porosity on the external friction of carbides in vacuum

Conclusions1.The friction coefficient of like couples of TiC, ZrC, and HfC decreases with increasing temperature and then rapidly increases. The minimum is at 950°C for TiC, 1100° for ZrC, and 1250°C for HfC.2.The friction coefficient of refractory carbides increases with the porosity. The effect of porosity diminishes with increasing temperatures.3.The friction coefficient, other conditions being equal, decreases with increasing percentages of stable configurations of the localized portion of the valence electrons of the atoms in the carbide.

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.