OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 a b Fig. 5. The microstructure of the inner layer of the coatings (1 in Fig. 2): a – composition 1; b – composition 2 a b Fig. 6. Fragments of diffraction patterns of the inner layer of the coatings under study: а – composition 1; b – composition 2 second layer and even the third one. Under severe operating conditions, which are typical for piercing mandrels, the outer oxide layer will gradually fail. At high temperatures, the recovery of the oxide layer due to iron oxidation in the third (metal-oxide) layer of the coatings is to be expected. The oxide layer can be restored by additional heat treatment, e.g., by surface heating with a gas or plasma torch. The porosity of the third layer will intensify oxygen penetration into the coating and increase the surface participating in the oxidation reaction. As the outer layers wear, nickel and chromium contained in the second and first inner layers become involved in the oxidation process. A decrease in the coating wear rate with transition to the inner layers is to be expected. This will be owing to the strengthening phases (carbides, silicides, and borides) having high hardness and increasing the hardness of the first inner layer (Fig. 7). The dendritic structure of the molten initial powder particles that have solidified before colliding with the substrate surface (Fig. 8) should help to slow down the wear of piercing mandrels during operation. In the coating of composiFig. 7. Microhardness distribution through the coating thickness
RkJQdWJsaXNoZXIy MTk0ODM1