OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 2 2022 Figure 6 shows that the deposition of Fe-Cr-B coatings makes it possible to increase the surface hardness of AISI 304 steel by 2.2–2.7 times. With an increase in the concentration of CrB2 powder in the anode mixture, the average microhardness of the coating surface increased from 6.25 to 7.6 GPa. This can be explained by an increase in the content of chromium and boride phases in the coating. Nevertheless, mild hardness values, compared with high hardness of borides, indicate a low volume fraction of ceramic phases in the coatings. That is consistent with the phase analysis data. In general, these results are consistent with the data obtained in [9], where the microhardness of wire-arc spraying Fe87-xCr13Bx coatings increased from 7.9 to 9 GPa with increase in the boron content from 1 to 4 wt.%. The kinetics of the change in the coeffi cient of friction of the samples during the wear test under dry friction condition is shown in Figure 7, a. The average values of the coatings coeffi cient of friction were lower than those of stainless steel and were in a narrow range from 0.69 to 0.71. However, for the Cr10 and Cr15 samples deposited with a high powder content in the anode mixture, narrow ravines were observed in the friction coeffi cient curves, while the curve was smooth for the Cr5 coating. In the case of steel, a high noise level was observed on the graph of the coeffi cient of friction that is usually associated with its high plasticity and with periodic deposition and delamination of the material transferred between the rubbing surfaces [28]. Thus, in particular, in samples Cr10 and Cr15, noise can be caused by delamination of microsections of the coating due to defi ciency of plastic metal binder. The results of coating wear tests showed that the wear rate was in the range of 0.76–1.7 × 10–5 mm3/ Nm (Fig. 7, b). It was lower than that of AISI 304 steel, from 1.6 to 3.7 times. The lowest wear values were demonstrated by the Cr5 coating that is consistent with the data on the coeffi cient of friction. At a higher concentration of CrB2 in the mixture of granules, the wear of the samples increased, that is caused by a de- a b Fig. 7. Dynamics of the coeffi cient of friction from the sliding path (a) and the wear rate (б) of coatings compared to AISI304 stainless steel Fig. 6. Microhardness of coatings compared to AISI 304 steel
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