The processes of phase interaction occurred during high-speed plasma-arc spraying (PAS) between a steel shell and a powder filler (70B$_4$C + 20(Cr, Fe)$_7$C$_3$ + 10Al % wt.) of a wire-anode are analysed. The direction of the thermodynamic reactions between the components of the wire with the formation of new phases (boron cementite, iron boride) using the method of differential thermal analysis is established. Coating is performed using the developed in the Paton IEW institute installation PLAZER 30PL-W in the inert argon gas. The phase composition of the starting materials and the resulting coatings are studied using a DRON-UM1 diffractometer. Differential thermal analysis is performed using a VDTA-8M device in helium environment. The microstructure of the coatings is etched in the Nital reagent. Auger analysis and study of microstructures are performed using a Jeol Jamp-9500F Auger microprobe. The features of structure formation of the plasma-arc coatings are studied under various spraying conditions. Better coatings with lamellar structure, low porosity ($\sim$1%), a large number of dispersed hardening phases (Fe$_3$(B, C), Fe$_2$B; (Cr, Fe)(B, C)$_2$AlB$_2$; FeCr; B$_4$C, Al$_2$O$_3$) in a doped ferrite matrix and high microhardness ($\sim$7.4 GPa) are obtained at a higher heat input (plasma torch current—240 A). During a spraying in this mode, the amount of boron cement increases sharply (up to 28.4% wt.). This phase, formed during the spraying process, becomes the main one in the coating. Aluminium, due to its fusibility, contributes to the activation of the processes of interaction of the components of the wire and reduces the porosity of the coatings. The formation of a solid solution of chromium, aluminium and boron in iron, which occurs during PAS, is a prerequisite for ensuring the heat resistance of the developed coatings (up to temperature 1300°С), which significantly expands the field of their practical application.