OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. 7 No. 2 2025 Fig. 6. Typical micrographs of wear and failure areas of ceramic cutting inserts without cutting edge preparation: а – general view; b, c – crater on the rake face; d – major cracks а b c d beyond the line of the chip contact length with the cutting insert, therefore, no traces of chip contact with the rake surface are observed. As a result of tool material fatigue due to periodic twitching and tearing of the contact surface of the chip near the cutting edge, defects accumulate, which develop first into a micro- and then into a macrocrack running parallel to the rake surface. This macro-crack even extends to the back surface, although the relief of the resulting micro-crater indicates a predominant crack development towards the rake surface. There is no adhesion of the machined material on the rake surface, indicating a low coefficient of friction in this area (which is typical for ceramic cutting materials) and no adhesion beyond the brittle fracture region. No traces of abrasive wear are also observed, which is due to the high hardness of the tool material. The microcrater surface is also free of particles of the workpiece material, although its relief contributes to their retention (Fig. 6, a, zone 1). This indicates high shear strength of the near-cutting part of the chip, or at least that the shear strength there is greater than the contact tangential stresses on the chip-tool surface. When the micro-crater increases to 1.2 mm (Fig. 6, a, zone 2), additional delamination of the tool material occurs, i.e., several radial micro-craters are formed. Here, particles of the workpiece material are observed in stagnant zones, although in very small volumes. In our opinion, this is due to the increase in the depth of these zones, which contributes to its retention in the form of micro-particles. At high magnification, small particles (a black spot of about 0.15 mm) of the workpiece material can be seen on the micro-crater surface (Fig. 6, b). Although the micro-crater surface has a basic radial shape, it also contains additional lines of brittle delamination of a complex shape. At even greater magnification (Fig. 6, c), deep main cracks
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