OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 1 2023 Tribological tests The change in the structural and phase state, as well as in the mechanical properties, also affected the results of testing the specimens for friction and wear. From the beginning of testing, the coefficient of friction coefficient (CoF) values are high for bronze C65500 specimens printed with different heat inputs (Fig. 11). Then it decreases by about 20%, after which it begins to increase monotonically until reaching the previous high values. During sliding of a specimen with a structure formed as a result of high-temperature annealing, significant fluctuations in the value of CoF are observed (reach an amplitude of ~0.2) and occupy most of the test time. Sliding of the predeformed and annealed specimen shows the CoF oscillations at the final stage of testing as high as ~0.25. The average CoF values of specimens printed with low (mode 1), medium (mode 2) and high (mode 3) heat input are 0.52, 0.39 and 0.29, respectively. For the specimen after high-temperature annealing, the CoF is 0.3, and for the sequentially deformed and annealed it is 0.34. The high amplitude of the CoF fluctuations is partly due to the test scheme. With reciprocating sliding, the sliding speed is not a constant value in all sections of the friction track. Upon reaching the final section of the friction track, it tends to zero, and then quickly recovers at the start of motion in each new sliding cycle. As a result, a slight change in friction conditions occurs at the extreme sections of the friction track, which affects the magnitude of the friction force. During sliding of specimens printed with the addition of aluminum filament, a different character of the change in the coefficient of friction is observed (Fig. 12). In the beginning, the CoF increases for ~150 seconds, which may correspond to a running-in period, and then stabilizes at a certain level. At the same time, the CoF value significantly decreased in comparison with that of bronzes printed without aluminum additives. The average CoF value is 0.184, 0.28 and 0.191 in the friction of bronze specimens printed with the addition of 10 wt.% Al, Al-5Si and Al-12Si, respectively. To explain the reasons for the CoF fluctuations, the surfaces of wear tracks on bronze specimens (Figs. 13, 15) and the surfaces of steel balls (Figs. 14, 16) were examined. On the surface of the asprinted bronze C65500 specimens, the pronounced wear tracks were formed and the surfaces of these tracks were covered with dark oxides and traces of mechanical damage. On the periphery, traces of deformation of individual sections of the material are also visible; this indicates plastic deformation of the specimens under the action of the counterbody during sliding friction. No signs of wear were found on the surfaces of the steel balls (Fig. 13), which is natural due to its significantly higher hardness compared to those of specimens printed from C65500 bronze. At the same time, Fig. 11. Change in the value of the coefficient of friction during tribological tests of C65500 specimens Fig. 12. Change in the value of the coefficient of friction during tribological tests of C65500 specimens, printed with the addition of aluminum filament
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