OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 Introduction Austenitic stainless steel AISI304 is used in the chemical and aerospace industries, atomics, medicine and other fields, due to its high corrosion resistance [1]. However, it is highly susceptible to wear due to its relatively low hardness (~200 HV) [2]. In addition, AISI304 steel is also subjected to pitting corrosion in electrolytes and begins to oxidize actively at temperatures above 800 °C in air [3]. The application of hard and anticorrosive coatings on the surface of AISI304 stainless steel is designed to reduce these disadvantages and expand its scope of application. Metal-ceramic (MC) materials are a kind of metal matrix composites, and combine in its composition a metal matrix reinforced with ceramic particles. Ceramic phases provide high hardness, and a relatively soft matrix holds the ceramics and gives such a composition high crack resistance and strength. MC materials are highly resistant to abrasion [4]. There are two ways to obtain a MС structure: the introduction of ceramic particles into a metal matrix or the crystallization of solid phases from the melt [5]. MC coatings attract a lot of attention of researchers because of its high hardness, wear resistance and corrosion resistance [6]. One of the promising reinforcing compounds is titanium silicide Ti5Si3, which has a high melting point, high-temperature resistance and oxidation resistance [7, 8]. Thus, in [9] it was shown that the coating with Ti5Si3 was preserved under cyclic oxidation conditions at 900 °C for 1,000 hours. Due to the strong covalent atomic bond, Ti5Si3 has high hardness and is stable under conditions of abrasive and adhesive wear [10]. Earlier, we showed the possibility of single-stage coating based on titanium silicide Ti5Si3 by electrospark deposition (ESD) of a titanium alloy with a non-localized anode made of titanium granules with the addition of crystalline silicon powder [11]. The deposited coatings had advanced high-temperature resistance at 900 oC and high wear resistance. It was shown that the Ti 5Si3 phase is formed by the interaction of silicon with titanium melt in the discharge microbath. Moreover, this interaction can occur both on the surface of the granules and on the titanium cathode. In the case of processing steel AISI304, only the first option can be implemented, so the transfer of Ti-Si material from titanium granules to the stainless steel surface plays a key role. As is known, the ESD is based on the phenomenon of polar material transfer from the anode to the cathode. It consists in the release of very hot microparticles of the electrode material into the melt microbath at the cathode, mixing of these materials and rapid solidification after the discharge [12]. Polar transfer is observed if the erosion of the anode exceeds the erosion of the cathode during the ESD process. The proportion of anode elements in the coating and its thickness depend on the polarity degree. The polarity criterion positively correlates with the thermophysical properties of the materials of the electrode pair; however, it is influenced by many factors [13]. Therefore, the establishment of polar transfer and its degree is achieved empirically. The purpose of this work is to study the structure of AISI304 stainless steel surface layer after ESD in a mixture of titanium granules with silicon powder, and to study the hightemperature resistance, corrosion and tribotechnical properties of the deposited coatings. Materials and methods Preliminary experiments have shown that in the case of an anode mixture of titanium granules with silicon powder, a positive cathode weight gain was not observed. To achieve a positive cathode weight gain, titanium powder was gradually added to the anode mixture, since it has better electrical conductivity compared to silicon. Titanium particles act as contact bridges among silicon particles, reducing the resistance of the system. The content of titanium powder in the mixture was gradually increased until a stable of the cathode weight gain began to be observed. So, the silicon concentration in the powder mixture was 31.6 vol.%. Granules from titanium alloy VT1-00 and powder mixture in various ratios (Table 1) were poured into a metal container connected to the positive output of the pulse generator; therefore, the granules acted as an anode. The silicon powder had an average particle size of 10 µm. The coatings were applied to a stainless steel AISI304 substrate in the form of a cylinder with a diameter of 12 mm and a height of 10mm(Table 2). The substratewas connected to the negative of pulse generator. The IMES-40 pulse generator
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