OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 2 2023 bonding to the substrate. The absence of cracks, as well as carbide particles crumbled out during spraying, indicates a high cohesive bonding. All coatings have a layered structure typical for thermal spraying. It should be noted that the resulting coatings are characterized by a significant difference in the mass fraction of carbides. Fig. 2, a–c (top row) shows coatings obtained at a spraying distance of 170 mm, and Fig. 2, d–f (bottom row) – at a distance of 250 mm. The arc current was also changed during spraying: 140 A (Fig. 2, a, d), 170 A (Fig. 2, b, e) and 200 A (Fig. 2, c, f). It can be seen that the spraying distance, as well as the arc current, have a significant effect on the amount of carbides. The dependence of the mass fraction of carbides on the spraying modes is shown in Fig. 3. The amount of WC and W2C decreases with increasing arc current and spraying distance. This is because the arc current increases, therefor the plasma flow temperature also increases, which leads to heating of WC particles to higher temperatures. The maximum amount of carbides (49 %) is observed in coatings formed using the 170/140 mode, the minimum (25 %) is in the 250/200 mode. Fig. 4, a shows the SEM image of the coating obtained by the BSE mode. It can be seen that the WC particles are located inside the splats and have different size (points 4 and 5 in Fig. 4, a). There are also areas without WC particles in the coatings (points 1–3 in Fig. 4, a). Depending on the time of exposure to high temperatures on the tungsten carbide particles, the degree of its decomposition will be different. It is known that when heated in a plasma jet, WC particles begin to melt and tungsten and carbon atoms diffuse into the liquid cobalt matrix. When this molten material is cooled at rates much higher than the critical ones, an amorphous or nanocrystalline supersaturated Co(W,C) solid solution is fixed. The scheme (Fig. 4b) shows that the degree of decarburization of WC particles is not the same in different splats. Thus, the particles practically do not melt in splats with a darker matrix (point 5 in Fig. 4, a) in contrast to splats with a lighter matrix (point 4 in Fig. 4. a). Depending on the amount of tungsten and carbon dissolved in cobalt, the matrix is characterized by different shades of gray. According to the X-ray microanalysis data (Table 2), the lighter areas (point 1 in Fig. 4, a) contain more tungsten, while the darker areas (point 3 in Fig. 4, a) contain less. The obtained data are in good agreement with the data of [5]. Fig. 3. Dependence of the WC+W2C mass fraction on spraying modes a b Fig. 4. SEM image (a) and scheme (b) of plasma WC-Co coating
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