OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 2 2022 hardness of stainless steel, but at the same time reduces its ductility. Application of hardening coatings can increase the surface hardness of stainless steel and improve its tribological behavior [3-4]. Cermet materials (MC) are a composite of ceramic phases embedded in a metal matrix [5]. Due to ceramic inclusions, MC coatings have a high hardness, and a plastic metal bond provides high strength and adhesion to the substrate, which together leads to high wear resistance [6–7]. Transition metal borides have high hardness and, therefore, are considered as a ceramic component of MC coatings [8]. Thus, it was shown in [9] that FeCrB coatings improve the microhardness and wear resistance of ASTM 283-C steel. It was shown in [10] that an increase in the content of boride ceramics in MC coatings leads to an increase in its microhardness. According to the paper [11], the microhardness of AISI 304 borated steel can reach 17 GPa. Electrospark deposition (ESD) is widely used to form MC coatings on metal base [12–14]. ESD is based on the phenomenon of polar metal transfer from the anode to the cathode in the process of exposure tomultiple microarc discharges [15]. Due to the high cooling rate of the material, a coating with an exceptionally fi negrained structure is formed after the termination of the discharge [16]. In addition, ESD is characterized by high adhesion of the formed layer to the base without thermal infl uence on the bulk characteristics of the substrate material [17]. The modifi ed ESD method with a non-localized electrode in a mixture of granules with ceramic powder has a number of advantages over traditional ESD, since it does not require additional operations for the preparation of MC electrodes and allows coating parts with a curved surface in automatic mode [18]. In addition, the method of ESD with a non-localized electrode is characterized by a low cost of equipment compared to other methods of deposition of MC coatings. In this work to obtain Fe-CrB MC coatings, AISI 304 stainless steel was processed in a mixture of iron granules with different concentrations of chromium diboride powder in order to establish the effect of the CrB2 powder concentration in the anode mixture on the structure, wear behavior, oxidation resistance and corrosion properties of the formed ESD coatings. Methods Three anode mixtures of steel granules (St3 steel) in the form of cylinders (d = 4±0.5 mm, h = 4±0.5 mm) and CrB2 powder were used as a non-localized electrode (Table 1). The diameter of the powder particles was signifi cantly smaller than the diameter of the area affected by the discharge ~0.8 mm, and was in the range from 25 to 134 μm with a median of 62 μm (Fig. 1). The substrate (cathode) of stainless steel AISI 304 (Table 2) was made in the form of a cylinder (d = 12 mm, h = 10 mm). The layout of the installation for the deposition of coatings with a non-localized anode with the addition of powder is described in detail in [19]. The IMES-40 discharge pulse source generated rectangular current pulses with amplitude of 110 A, duration of 100 μs, and frequency of 1000 Hz at voltage of 30 V. To prevent oxidation of the samples surface the argon was supplied to the working volume of the container with rate of 10 L/min. The mass transfer kinetics was studied by successively weighing the cathode every 120 s of ESA on a Vibra HT120 analytical balance with an accuracy of 0.1 mg. The total processing time for one sample Ta b l e 1 The content of CrB2 in the anode mixture, designation and characteristics of coatings CrB2 concentration, vol.% 5 10 15 Designation of samples Cr5 Cr10 Cr15 Coating characteristics Thickness, μm 35.7±2.3 33.5±5.7 30.7±6.1 Roughness (Ra), μm 7.1±0.88 7.4±1.14 9.1±0.60 Water contact angle, o 70.2±8.6 58.1±5.8 57.6±10.6
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