The influence exerted by electric-spark spraying on the kinetics of mass transfer and the physicomechanical properties of coatings is investigated. It is shown that electric-spark spraying is determined by the dynamic properties of the cathode jets, which depend on the electrical parameters of the spark discharge, the size of the interelectrode interval, and the physical properties of the coatings, which vary during spraying.Keywords: rate of spark transfer, pulse duration, interelectrode interval, cathode jet.Requirements ensuring stable service properties are currently set forth for protective and wear-resistant coatings. Coatings should exhibit one-hundred-percent continuity and uniform thickness, as well as high adhesive strength to the substrate [1]. Coatings produced by spark spraying fully satisfy only the last requirement. Discontinuity of coatings [2], their high nonuniformity and roughness [3], and tendency to embrittlement [4] are basic deficiencies of the spark method of spraying, which inhibit its broader use.As will be demonstrated below, these disadvantages of spark-applied coatings are dictated by production features of the spraying process. Local heating and melting of the substrate being treated occur with a lengthy spark discharge [5]; this results in vigorous reverse transfer of material, contamination of the alloyed layer and the surface of the anode by substrate material, and an increase in the roughness of the coatings. The material becomes embrittled as a result of multiple recurrent and rapid melting and cooling. Due to embrittlement, the coating begins to self-destruct during spraying, as a result of which its further growth is curtailed. Thus, one of the basic problems of spark spraying is minimization of substrate melting to a level sufficient to ensure adhesion and reliable cohesion with the coating, but insufficient for occurrence of the above-described negative processes.According to Mesyats [6], a spark discharge consists of cathode jets (flares) which develop as a result of a thermal burst of cathode spots. Experiments in which cathode flares were separated from the discharge channel using dielectric capillaries indicated that the portion of anode erosion due to cathode jets is 75% of the overall erosion on average [7,8]. At the present time, basic efforts of researchers are directed toward study of single spark discharges. As a result, the velocity, temperature, and pressure of the plasma in the spark has been determined [6], and the dependence of anode erosion on the size of the interelectrode interval established [9]. It is noted that the rate of the spark erosion of electrodes depends heavily on their service life. To eliminate the effect of the prior history of electrodes on results of experiments, their surface is, as a rule, ground prior to the start of investigations. Let us stress that experiments with single pulses has not, in principle, made it possible to establish characteristic features of the formation of coatings, since the effect of millions of spark discharges is...
The effect of electrospark spaying on the hardness, wear resistance, and phase composition of hard-alloy coatings is investigated. A coating mechanism is proposed to consider change in dynamic properties of cathode jets depending on the interelectrode interval and the effect of pulse duration on the local temperature of the substrate.This study deals with the mechanical properties of coatings based on solid alloy VK16, which are sprayed using a high-frequency electrospark laboratory device. The experimental device and coating technique are described in [1]. In addition to practical application, the mechanical properties of coatings are also of scientific interest. Electrospark spraying is a complex nonlinear process. Electrospark coatings formed in alloying by different electrode materials (from pure metals to multicomponent alloys) have been studied in detail. There are extensive data on physicochemical transformations in altered layers, accumulation of mechanical stresses, reverse mass transfer, and effect of alloy environment [2,3]. The interrelation between the composition and properties of coatings and electrospark alloying (ESA) characteristics, such as the electric discharge length in a short-pulse range and interelectrode interval, have been far less studied.This study attempts to develop this poorly investigated ESA area. The minimization of the duration and power of electric pulses is assumed to decrease the thermal effect of the discharge on the electrodes. This will somewhat inhibit the diffusion and physicochemical transformations to make the effect of the process parameters more pronounced.The electrode is so connected to the specimen that the interelectrode interval (IEI) is determined, on the one hand, by the repulsive force in the electrospark discharge and, on the other hand, by the force pressing the electrode against the specimen. Therefore, an increase in the pressing force should lead to a decrease in the IEI, and vice versa. In this connection, the pressing force F can be used to quantify the IEI as it is technically difficult to measure it directly. An x-ray analysis (DRON-3M, Cu-K α radiation) shows that the phase composition of coatings is greatly dependent on the alloying conditions. Coatings formed under the minimum pulse duration and interelectrode interval showed the greatest compliance with the initial composition of the electrode. Such coatings mainly consist of hexagonal tungsten carbide and cobalt. They have low roughness (R a ≈ 1 μm) and transfer rate of 0.05 to 0.1 mg for 30 min of alloying. The phase composition of coatings changes when the interelectrode interval increases with simultaneous increase in the transfer rate (Δ c ≈ 0.3 to 0.6 mg, where Δ c is the cathode weight increment). The cubic tungsten carbide WC cub (50 vol.%) is the main component of the coating; other phases (in descending order of volume fraction) are as follows: WC hex , W 2 C, WO 3 , Co (6-8%), Co 2 O 4 (traces). Noteworthy is the presence of high-temperature cubic tungsten
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