I. A. Podchernyaeva

Thermionic emission properties of refractory compounds and materials based on them (a review)

Conclusions1.On the basis of an analysis of the thermionic emission properties of solids, it is concluded that a correlation exists between a surface characteristic (electron work function) and volume properties reflecting the thermodynamic stability of their crystal structure. The correlation is attributable to the fact that the level of physical properties is strongly influenced by the strength of interatomic bonds, which detemine the potential energy of the electronic interaction in the lattice.2.As a result of systematic investigations into the thermionic emission properties of refractory carbides in their homogeneity regions and materials based on them, it has been established that the work function depends on the degree of defectiveness of such carbides, growing with increase in the strength of the Me-C and Me-Me bonds.3.It has been found that the electron work functions of boride and suicide phases of the Groups IV and V transition metals increases with rise in the nonmetallic component content of these phases, which is a consequence of an intensification of the electronic interaction between the metal and nonmetal atoms.

Refractory boride base electrode materials for electric-spark alloying

One of the main tasks in the development of tungsten-free electrode materials based on refractory borides must be to increase the contribution from the liquid-phase erosion component, which will result in higher electric-spark alloying (ESA) efficiency. This paper presents the results of differential thermal analysis of oxidation of type 45 steel with boride spark-deposited coatings of TiB/sub 2/, TiB/sub 2/ + 5% Mo, TiB/sub 2/ + 15% Mo, and TiB/sub 2/ + 20% Mo. The frictional and wear characteristics of coating and the effectiveness of ESA with alloys of TiB/sub 2/-Mo system are presented. The results show that it is possible to reach the following conclusion: when determining the optimum amount of the ductile binder in the alloying material, it is necessary not only to aim to attain the maximum alloying effectiveness but also to take into account the proposed application of resultant coatings.

Characteristics of coating formation on steel during electric-spark alloying with heterophase TiB2 + Mo materials in air

ConclusionsIt has been established that a decisive influence on AL formation in ESA with a TiB2 base alloy is exerted by the microstructure of the material, which depends on its composition and sintering temperature. The best results are obtained with an “annular” alloy structure of low porosity, in which fine grains of the hard phase are separated by interlayers of the lower-melting-point component, whose concentration does not exceed 20%. This ensures that the electrode erosion products consist mainly of particles with low-melting-point component “shells,” which firmly adhere to the cathode surface. The specific erosion of the electrode diminishes with treatment time owing to the formation on its surface of a secondary structure as a result of changes in the porosity and phase composition of its surface layer. The main phase constituents in the coating and anode material at all Mo binder contents are TiB2 andβ-MoB.

ELECTRIC-SPARK ALLOYING OF METALLIC SURFACES WITH GROUP IV TRANSITION METAL NITRIDES

ments a more objective assessment of the effectiveness of several magnetic-abrasive powders for different materials and to find a functional dependence between the MAP optimization parameters, e.g., the mass removed from the part being polished, and the process parameters. The experiments can be completed in a much shorter time, i/i0-i/i00 of the time needed for traditional methods (depending on the number of factors studied).

Thermionic emission from complex alloys containing lanthanum hexaboride

Conclusions1.A study of the thermionic emission properties of alloys of the LaB6-“ScB6” and LaB6−YB6 systems revealed that emission from these alloys is due to a film mechanism. Values ofθT are given by the expressions 2.66 + 2.0·10−4 T, for LaB6, and 3.57 + 3.5·10−4 T, for YB6.2.The monotonic increase in work function observed on passing from LaB6 to YB6 is a consequence of changes in the chemical composition of the surface layers of the alloys. After high-temperature heating alloys of the LaB6-YB6 system are characterized by a stable state of emission in the temperature range investigated. Prolonged heating at 1780‡K increases the work function of YB6 to a value close to those of YB12 and B as a result of enrichment of the specimen surface layer with boron liberated through the decomposition of part of the YB6 into YB4 and B.3.In the La-rich range alloys of the LaB6-“ScB6” system have been found to exhibit an unstable state of emission. The electron work function of alloys ranging in composition from equimolar to that of “ScB6” remains virtually constant at a level corresponding to the composition “ScB6”; under these conditions, emission becomes stable.

Differences in emission parameters and adsorption properties of single-crystal faces

Published data on the work function and adsorbtion properties of faces of metal and nonmetal single crystals are considered from the viewpoint of a model of solids based on configurational localization. An explanation is proposed for the different values of work function yielded by different faces of a crystal of the same metal.

Influence of electrospark alloying on the heat resistance of steels

ConclusionsThe high-temperature oxidation of electrospark coatings on R9K5, 5KhNV, and 50 steels obtained by electrospark alloying with standard (T15K6, VK8, VK6M) and carbide-, nitride-, and boride-base tungsten-free alloys was investigated by the differential thermal analysis method. It was shown that in increase in heat resistance the refractory compound-base electrode materials used for production of coatings are in the order (by base component) boride-carbidenitride. A significant factor increasing the high-temperature oxidation resistance of the coatings is the increase in their coverage. With a high coating coverage on carbon steel its high-temperature oxidation resistance approaches that of die steel. Therefore, in a number of cases, replacement of expensive alloy steels with carbon steels is possible.

Some features of the electric-spark alloying of high-speed steel with hard metals by the local coating deposition method

ConclusionsAn investigation has been carried out into the processes of erosion and AL formation during the ESA of R9K5 steel with a standard and a tungsten-free hard metal by the LCD method. It is shown that the tungsten-free NTsKhN45 alloy is superior to the standard VK6M hard metal in ESA efficiency and comparable to it in AL surface finish. It has been established that AL formation is controlled by two opposed processes, viz., transfer of anode material to the cathode and its disintegration as a result of cathode erosion in the liquid-vapor and solid phases under the action of repeated spark discharges. The general features of the process — nonadditivity, a cyclic character of variation of specific values of anode erosion and cathode weight gain, and AL thickness limitation — remain the same as in nonmechanized vibration ESA. A “secondary” electrode heating effect has been observed, which can be utilized for increasing the efficiency of ESA. On the basis of considerations of maximum process efficiency and alloyed layer thickness and surface finish, optimum alloying conditions are proposed (a large Isc and a low capacitance at an electrode pass number n=4–5).

The electron nature of interaction of materials in electrospark alloying of iron with carbides

The problem of electrospark alloying, the establishment of the interrelationship of the effectiveness of the electrospark alloying process with the phase formation in the alloy layer and the thermodynamic stability of the phases occuring is studied. The materials investigated were the carbides TiC, NbC, and WC, which are of interest as model objects and as the base of the hard alloys used in electrospark alloying practice. X-ray diffraction analysis of the alloy layers was made in an RKD chamber. From the stable phases obtained by calculation and the quasibinary lines between them it follows that the quantity of intermediate phases and binary sections formed in the ironcarbide system increases in the order TiC-NbC-WC.

Tribotechnical characteristics of titanium diboride electrospark coatings

ConclusionsThe tribotechnical characteristics of an alloyed layer obtained by electroerosion hardening of 45 steel with TiB2-Mo sintered materials were studied. It was established that at increased temperature the maximum wear resistance is possessed by a hardened layer in alloying with a material containing the optimum quantity of hard and plastic phases. A higher content of hard phase leads to brittle failure of the hardened layer and an increased content of plastic phase to seizing of the contacting pairs.