OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 Ta b l e 1 Composition of powders being sprayed Powder composition 1, wt. % C Cr Si B Mn V Fe 1.2 3.8 2.3 3.6 1.0 1.0 Bal Powder composition 2, wt. % C Ni Si B Mn Fe 0.5 9.0 1.2 2.7 4.0 Bal Fig. 1. Morphology of powder particles for multilayer coating spraying: a – powder of composition 1; b – powder of composition 2; c – Fe powder а b c The structural features and thickness of the obtained coatings were studied on the cross cuts by means of a Tescan Vega II XMU scanning electron microscope, equipped with an Oxford HKL Nordlys F+ energy dispersive attachment, and a Neophot-21 microscope at 100- to 1,000-fold magnification. By means of a Shimadzu X-ray diffractometer using Kα Cr radiation, phase X-ray diffraction analysis of each layer was performed on the specimen surface immediately after layer application. Microhardness was measured in a Leica hardness tester with a load of 50 g. After spraying of all the three coating layers, oxidizing annealing was performed at 900 °С in order to create a dense layer of FeO+Fe2O3+Fe3O4 scale on the surface. It is this oxide layer that will act as a lubricant in the operation of piercing mandrels and as a thermal barrier decreasing the surface temperature. The average size of the powder particles ranged between 60 and 100 μm for the coating of composition 1 and between 70 and 90 μm for the coating of composition 2; the size of the Fe powder particles was 40 to 60 μm. Results and discussion As was supposed, the sprayed coatings consist of four layers (Fig. 2). The thicknesses of each layer are presented in Table 2. According to the results of the phase X-ray diffraction analysis, the surface oxide layers on both studied coatings are identical and consist of a mixture of the oxides FeO, Fe2O3, and Fe3O4 (Fig. 3). The surface layer is denoted by 4 in Fig. 2; it should play the role of a lubricant, which decreases the friction coefficient and prevents the base metal from overheating. Obviously, the presence of diffraction peaks from the Fe-based α-solid solution is due to the fact that the metal oxide layer (3 in Fig. 2) got into the analysis of the specimen surface. The structure and element distribution of this layer are shown in Fig. 4.
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