Formation of microstructure of plasma-arc coatings obtained using powder wires with steel skin and B₄C + (Cr, Fe)₇C₃ + Al filler

The processes of phase interaction occurred during high-speed plasma-arc spraying (PAS) between a steel shell and a powder filler (70B₄C + 20(Cr, Fe)₇C₃ + 10Al % wt.) of a wire-anode are analysed. The direction of the ther-modynamic 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 micro-structure of the coatings is etched in the Nital reagent. Auger analysis and study of microstructures are performed using a Jeol Jamp-9500F Auger mi-croprobe. The features of structure formation of the plasma-arc coatings are studied under various spraying conditions. Better coatings with lamellar structure, low porosity (∼1%), a large number of dispersed hardening phases (Fe₃(B, C), Fe₂B; (Cr, Fe)(B, C)₂AlB₂; FeCr; B₄C, Al₂O₃) in a doped ferrite ma-trix and high microhardness (∼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 ex-pands the field of their practical application.