Introduction. In modern production, the requirements for reliability and durability of steel products are constantly increasing, particularly those that operate under difficult operating conditions. To address this issue, traditional methods involve forming diffusion coatings on the surfaces of these products with increased hardness and wear resistance [1, 2]. This includes carbide-type coatings obtained by diffusion saturation of steel with chromium [3], tungsten [4], molybdenum and other carbide-forming elements [5]. A significant disadvantage of this technology is that it is time-consuming, taking more than 8 hours to complete. However, it is possible to accelerate diffusion saturation by applying high-energy effects on the material, for example, plasma [6], ion plasma [7], laser [8, 9], electric spark [10], as well as heating using thermionic effects [11]. These technologies are effective, but require complex and expensive equipment. In this regard, the method of microarc surface alloying [12] has an undeniable advantage. In this process, heating and diffusion saturation of steel products occur in a metal container filled with coal powder. Heating is caused by microarcs that result from the passage of an electric current through the circuit: power source – container – coal powder – steel product. Acceleration of the diffusion saturation process is achieved by exposing the material to microarc discharges that occur between the surface of the product and the coal powder. The obvious simplicity of this technology, combined with its low energy consumption, does not require additional evidence of its advantages.

Carbide-type coatings containing molybdenum are widely used in mechanical engineering. The process of molybdenum steel saturation involves heating of chemical compounds based on molybdenum or ferromolybdenum in powders, as well as in a gaseous medium of molybdenum halides, or in melts based on sodium molybdate. This process is conducted at temperatures ranging from 1000 to 1200°С for at least six to seven hours. The use of the microarc alloying method to obtain such coatings can significantly reduce time required for this process, making it an urgent area of research [13, 14]. The main factor that determines the properties of these coatings is the presence of carbide phase particles in their structure. In this study, we aimed to investigate the features of the fine structure of the surface layer of steel after microarc molybdenum steel saturation.

Materials and Methods. Microarc molybdenum steel saturation was performed using a coating of ammonium molybdate (NH4)2MoO4 powder in an electrically conductive gel, in a volume ratio of 1:1. The coating was applied to the surface of steel 20 samples, each with a diameter of 12 mm and length of 35 mm. These samples were then immersed in a metal container containing carbon powder with a dispersion of 0.4–0.6 mm, and an electric current was passed through the source – container – coal powder – sample chain for six minutes. To achieve the desired temperature for the molybdenum plating process, a current density of 0.53 A/cm² was maintained on the surface of the samples.

After diffusion saturation, a transverse microplate was created by pouring epoxy resin into cylindrical mandrels, ensuring strict perpendicularity of the sample's surface to its longitudinal axis. The samples were then ground on abrasive papers with grain sizes ranging from P480 to P2500 and polished first with Cr2O3 oxide grade ОХА-0 according to GOST 2912–79, and finally with AM diamond paste with a 3/2 powder grain size according to GOST 25593–83. After removing any remaining paste residue with ethyl alcohol, chemical etching was performed using Rzheshotarsky reagent (a 4% solution of nitric acid in ethyl alcohol).

The microstructure was studied on a Neophot-21 microscope with a ToupCam Xcam0720P-H HDMI digital console. X-ray phase analysis was performed on an ARL X'TRA-435 diffractometer in Cu-Kα radiation. The hardness of the diffusion layer was measured with a PMT-3 microhardness meter at loads of 0.490 and 0.196 N. The molybdenum content in the layer was determined using a ZEISS CrossBeam 340 scanning electron microscope with an Oxford Instruments X-max 80 X-ray microanalyzer. The relief of the transverse section of the diffusion layer was studied on an atomic force microscope (AFM) NanoEducator in constant force mode.

Results. Metallographic analysis revealed a weakly etching coating with a thickness of 50–55 µm on the surface of the samples after microarc molybdenum steel saturation. A carbonized layer with a pearlitic structure about 200 µm thick was found under it, followed by the initial structure. The coating consisted of a dispersed ferrite-carbide mixture containing carbide inclusions up to 5 µm in size. The microhardness of the base layer was 8–9 GPa, and the carbide inclusions were up to 21 GPa (Fig. 1).

The results of measuring the mass fraction of molybdenum (Fig. 2) are presented in Table 1. From the data obtained, it could be seen that the molybdenum content at different points of the coating differed, and the coating itself had a heterogeneous composition.

There was no molybdenum in points 1 and 2. Next, a transition zone of a solid molybdenum solution was formed, containing about 3% Mo. The thickness of the diffusion layer was 50–55 µm. It consisted of a base (spectra 5, 6) with rounded inclusions located in it (spectra 7, 8). As can be seen from Table 1, the base contained approximately 47% Mo, and therefore it could be an intermetallic Fe3Mo2 or carbides (Fe,Mo)3C [15, 16]. The inclusions (spectra 7, 8) contained approximately 94% Mo, which corresponded to the carbide phase Mo2C [16, 17].

The formation of such carbides in the surface layer was confirmed by X-ray phase analysis (Fig. 3).

High microhardness of the coating base could be explained by the formation of nanoscale carbide particles in it, which was confirmed by the results of atomic force microscopy (AFM) (Fig. 4, 5).

Thus, the coating structure contained carbide inclusions up to 5 µm in size, as well as multiple nanoscale inclusions that protruded above the surface plane of the sample. Such inclusions had a higher hardness compared to other structural components.

To quantify the strengthening effect of these particles, it was advisable to use the hardness additivity rule, according to which HAB hardness of a two-phase alloy could be represented as the sum of the hardness HA and HB of the constituent phases A and B, taken in their volume fractions VA and VB:

 (1)

The dispersed ferrite-carbide mixture acted as phase A, the base of the diffusion layer, and nanoscale carbide inclusions acted as phase B. For the calculation according to formula (1), the following initial data were used: HA = 3,000 MPa, HB = 23 GPa, the values of VA and VB were determined according to method [18] and assumed to be equal to: VA = 0.73; HB = 0.27. From where it was obtained: HAB = 8,400 MPa, which is consistent with the measurement results of the integral microhardness of the diffusion layer.

Discussion. The data obtained confirmed the possibility of accelerated production of a high-hardness molybdenum coating on steel using the microarc surface alloying method. With a total coating thickness of 50–55 µm, it has a complex structure in depth and consists of a dispersed ferritocarbide mixture with a microhardness of 8–9 GPa, with inclusions of relatively large particles of the carbide phase up to 5 µm in size (microhardness up to 21 GPa), and multiple nanoscale inclusions. This is followed by a carbonized layer with a pearlitic structure about 200 µm thick, which transforms into the initial structure of steel 20. A calculated assessment of the strengthening effect of such nanoscale inclusions confirmed that their presence determines the high microhardness of the coating base. It should be noted that the obtained value of the microhardness of the coating base exceeds its value, which is achieved using traditional molybdenum plating methods. It can be assumed that the formation of nanoscale inclusions of the carbide phase during microarc molybdation occurs under the influence of numerous microarc discharges that occur between the surface of the steel and the adjacent coal powder during passage of electric current. However, the physical processes taking place under such conditions require separate consideration and could be one of the areas for future research.

Conclusion. Microarc surface alloying can be used to create high-hardness coatings on steel using the method of molybdenum plating. Investigation of the fine structure of the coating has shown that it has a complex phase composition: a dispersed ferritic-carbide mixture with numerous small and nanoscale carbide inclusions, which gives the coating high microhardness. Underneath this, there is a carbonized layer with a pearlitic structure, followed by the original steel structure. The results obtained from these studies can be useful for the development of technologies to harden the surfaces of steel products such as tools and machine parts that operate in difficult conditions.