OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 3 2023 a b Fig. 4. Microstructure of borides in the coatings obtained by surfacing the powder mixtures with different CrB content: a – 20 % CrB; b – 30 % CrB An increase in CrB amount in the surfacing mixture to 20 % or more resulted in a significant variation in the coating structure (fig. 4). The main difference from the previously considered coatings is the change in the type of formed eutectic from lamellar to skeleton. The structure of these coatings was also represented by primary borides with eutectic areas between it. It should be noted that chemical etching does not reveal the interphase boundaries between primary borides and eutectic colonies. Thus, the eutectic coalesced with the primary borides, which is clearly seen in Fig. 4b. Ta b l e 3 Elemental composition of eutectic borides Amount of CrB in the mixture, % B, % Cr, % Mn, % Fe, % Co, % Ni, % 5 6.4 24.1 18.5 12.3 19.7 19.0 10 7.2 43.6 15.3 10.2 12.1 11.6 20 4.6 3.4 26.2 5.2 27.0 33.7 30 4.9 3.7 22.2 12.6 24.3 32.3 The aforementioned change in the morphology of eutectic borides is presumably associated with a change in its composition and the type of crystal lattice. Table 3 shows the results of the elemental analysis of eutectic borides. It can be seen that in the coatings obtained from mixtures with the appending of 5 % and 10 % CrB, the main element in the composition of the borides is chromium (Table 3 and fig. 3). An increase in the fraction of borides in the surfacing mixture over 10 % led to a change in the elemental composition of eutectic borides. Ni was its main constituent element, while the chromium content decreased to 3–4 % (Table 3, fig. 5). These changes in the elemental composition led to a change in the type of crystal lattice, which will be discussed later along with results of X-ray diffraction analysis. In addition, darker areas were found in the central part of the primary borides formed in the coating obtained by cladding the mixture with 30 % CrB (fig. 4b). Elemental analysis (fig. 5) revealed that in such areas, the chromium concentration is highest, while the mass fraction of other metals does not exceed 3 %. Thus, these regions presumably correspond to CrB borides. Crystals of primary borides formed around it contained manganese and iron, in addition to chromium. The elemental analysis of the surfaced layers revealed the formation of a transition zone with a thickness of 50–150 µm enriched with iron (compared to the main part of the coating) at the boundary with the base material (fig. 6). The structure of this zone was heterogeneous; in a layer with a thickness of up to 10 µm, which was formed directly above the boundary, borides were not found (fig. 6a, c). Above the mentioned layer, eutectic borides appeared in the structure, but the large primary borides were not formed. The transition layer should have a favorable effect on the complex of the mechanical properties of materials since it decreases the alteration of properties between the surfaced layer and the base material.
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