OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 low density, and high fluidity. Nickel-based Ni-Cr-B-Si alloys are most often used for this purpose; during plasma spraying of the alloys, B2O5-Cr2O3-SiO2 oxides form a fusible slag, which, when fusing, comes to the surface in the form of a thin glassy coating [6–8]. Coatings applied by plasma spraying have some disadvantages. First of all, its porosity ranges between 8 % and 15% [3–5]. In order to reduce porosity, special plasma surfacing techniques were developed, such as high-speed plasma surfacing, applying multilayer coatings, nanostructuring of sprayed coatings, rare-earth doping, repeated laser surface melting, and a combination of the techniques [9–11]. However, the presence of pores in a coating can be functionally advantageous in some cases. At high operating temperatures, oxygen easily penetrates through the pores deep into the coating, thus causing massive oxidation throughout its thickness, with the formation of a protective film of Cr2O3 or Al2O3 oxides [12–14]. A combination of several differently applied protective layers is successfully used to reduce the disadvantages of coatings [15– 19]. When a single application method is used, the layers may differ in the chemical and phase composition and perform different protective functions. In fact, functional gradient coatings are formed. Plasma spraying of multilayer protective coatings can be successfully used to increase the durability of piercing mandrels, which are the main tools used to produce hollow billets [20–22]. During operation, piercing mandrels are affected by cyclic heating (20 to 1100 °C) and by the pressure of the metal being deformed (up to 170 MPa) as it moves relative to the mandrel at a speed of up to 1 m/s [20, 23]. The durability of mandrels can be effectively increased by oxidizing the nose surface and the spherical surface in order to form an oxidized layer, which prevents the billet metal from sticking to the mandrel and is a heat-insulating layer [21–23]. The formation of an oxide layer on the surface increases the wear resistance of mandrels by a factor ranging from 1.5 to 2.0 [23]. The oxide layer provides additional thermal resistance, the value of which depends on the thickness of the scale layer, its thermophysical and mechanical properties [24–26]. As the temperature increases, the scale softens, and very soon it starts to act as a lubricant during contact. As is known [27, 28], under high-temperature oxidation (900-1000 °С), a layer consisting of FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite) is formed on a metal surface. The FeO film is an inner layer, and it peels off easily; therefore, when forming an oxide layer, one should reduce the amount of wustite as much as possible and make it turn into magnetite (Fe3О4). To increase the service life of piercing mandrels, protective coatings containing iron oxides can be successfully used. The development of various combinations of differently functioning layers is of interest. The aim of this paper is to study the chemical composition, structure, and microhardness of plasmasprayed multilayer high-temperature coatings of two different compositions, which are supposed to be used to increase the service life of piercing mandrels. Materials and research methods The coatings were applied by means of a UPN-60KMTSP2017 plasma powder spraying unit with contact initiation of an arc discharge (NPP TSP LLC, Ekaterinburg). All the layers of the multilayer coatings were sprayed onto 20CrNi4V (3310) chromium-nickel steel specimens under the same following conditions: a current of 310 A and a voltage of 57…60 V, with argon as the main plasma-forming gas and hydrogen as a high-enthalpy gas. Multilayer coatings with two different compositions were studied. The coatings were produced by successive overlaying of three layers. The first layer is meant for protection against hightemperature oxidizing and wear, it is produced by spraying self-fluxing powders of compositions 1 and 2 (see Table 1). This layer is required to prevent rapid fracture of mandrels in the case of the wear of the upper layers. It will enable one to remove the mandrel from operation in due time and to reapply the fractured outer layers, thus restoring the protective properties of the mandrel. The second layer is transitional, and it is produced by spraying a 50:50 mixture of high-temperature powders of compositions 1 or 2 with Fe powder. This layer is designed for a smooth variation of properties from the outer layer to the inner one, and it delivers iron to form an oxide film at high operating temperatures. The third metal-oxide layer is produced by spraying Fe powder in an oxidizing environment. It serves to restore the outer oxidized layer at the operating temperatures of piercing mandrels. The morphology of the particles of the sprayed powders is shown in Fig. 1.
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