The paper presents the results of metallographic studies of electroerosive coatings on special steels and alloys (R6M5, 12Kh18N10T and 07Kh16N6, alloy KhN58MBYuD and bronze BrB2) obtained with use of hard wear-resistant (alloys VK8, T15K6, STIM-3V, and W, Mo, Cr), soft antifriction materials (Cu, Ni, bronze BrB2, Sn), and electroerosive alloying (EEA) with graphite. As shown, for all the materials of studied substrates, it is possible to harden their surface layers, and there are the most expedient materials of the alloying electrodes and fabrication regimes. For hardening the surface layer of the nickel alloy KhN58MBYuD, it is recommended to use an electrode from hard alloy VK8, and for beryllium bronze BrB2—from chromium. The most rational is the regime corresponding to a discharge power of 144.3 W. For reducing the hardness of the surface layer of the alloy KhN58MBYuD, it is proposed to use electrodes from copper and nickel. The microhardness in this case is of 3500 and 3710 MPa, respectively, with microhardness of the base metal of 5300 MPa. As shown, it is possible to provide low surface hardness and high coating quality in the case of electroerosive alloying at low discharge powers of the orders of 15–50 W. Based on experimental studies, the mathematical model is proposed (an equation for predicting of the surface-coating microhardness) and allows to determine the microhardness of formed coating from the energy parameters of the EEA equipment. The constants of the proposed equation are determined. The developed algorithm allows predicting the energy parameters of the EEA to fabricate alloying layer with the desired microhardness of the surface coating.