OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 1 2022 The surface roughness was studied by 3D profi lometry with a WYKO NT-1100 device. 3D profi lograms were obtained and the average values of the parameter Ra (the arithmetic mean deviation of the profi le) were determined according to the analysis of three surface areas with dimensions of 0.9×1.2 mm and 42.5×55.8 μm. Based on the 3D profi lometry results, the surface microprofi le smoothing coeffi cient was calculated based on the approach proposed in [26]: 100 % t b Ra t Ra Ra Ra (1) where Rat – is the surface roughness after previous (turning) processing; Rab – is the surface roughness after diamond burnishing. Themeasurement of the surfacemicrohardness was performedwith the AHOTECHEcoHARDXM1270C microhardness tester at loads of 0.49 N (50 gf) and 1.96 N (200 gf) on the Vickers indenter. Using the results of surface micro-durometry, the hardening coeffi cient was calculated based on the dependence: HV HV HV 100 %, HV b t t (2) where HVb is the microhardness after surface diamond burnishing; HVt is the initial microhardness of the turned surface. The change in microhardness in the surface layer depth was determined with a cross-section using a SHIMADZU HMV-G21DT microhardness tester under load on a Vickers indenter of 0.245 N (25 gf). Using the Tescan VEGA II XMU scanning electron microscope, the samples surface and the structure of the near-surface layers in cross-sections were studied. Results and discussion In Fig. 2 and Fig. 3a, the results of optical 3D profi lometry of the sample surface after turning and dry diamond burnishing in the zones of 0.9×1.2 mm in size are presented. It can be seen that diamond burnishing led to a signifi cant smoothing of the initial surface roughness and a corresponding decrease in the value of the arithmetic mean deviation of the Ra profi le. As the burnishing force increased from 100 to 150 N, the average value of the roughness parameter Ra decreased from 0.21 to 0.10 μm. A further increase in the burnishing force to 175 and 200 N, on the contrary, caused an increase in the average value of Ra to 0.11 and 0.17 μm, respectively (see Fig. 3a). Calculation by formula (1) showed (Fig. 3b) that in the process of diamond burnishing of an austenitic steel disc in the range of the forces under study, the smoothing coeffi cient δRa ranges from 79 to 90 % with a maximum in the case of using a load Fb = 150 N. Thus, according to the criterion of the profi le arithmetic mean deviation, the specifi ed most favorable normal load mode provides smoothing of the microprofi le by 90 % formed by fi nishing turning (Ra = 1.0 μm), and results in nanoroughness (Ra = 100 nm) even on relatively extended surface areas with dimensions of 0.9×1.2 mm. The surfacemicroprofi le analysis taken in the process of 3Dprofi lometry at microsections with dimension 42.5×55.8 μm showed that, in contrast to the surface after turning with characteristic unidirectional large protrusions and depressions (Fig. 4a), isolated depressions are observed on the entire area of the burnished surface (Fig. 4b-d). With burnishing force of 100 N, these depressions take a shape elongated in the direction of tool movement (Fig. 4b). As burnishing force increases, the size of depressions decreases signifi cantly, it acquires a rounded or oval shape; its distribution becomes more uniform, and the number of depressions increases (Fig. 4c, d).At the same time, the depression depth seems to decrease with an increase in burnishing force, as evidenced by a continuous decrease in the values of the roughness parameter Ra (see Fig. 4 b-d). The presence of the revealed depressions on the burnished surfaces may be due to the insuffi cient amount of the burnishing force and the surface profi le depressions left from the previous turning (Fig. 5a). In particular, this reason can justify the presence of elongated extended depressions on the surface burnished with a minimum investigated force of 100 N (see Fig. 4b). On the other hand, an increase in the number of
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