PSI - Issue 81

Valeriy Lazaryuk et al. / Procedia Structural Integrity 81 (2026) 529–535

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A total of twenty grooves (Fig. 3) were produced under constant laser power settings of 24, 32, 40, 48, 56, 64, and 72 W. The scanning speed was varied from 15 to 300 mm/s, which allowed us to compare how different levels of heat input and exposure time affected groove geometry, thermal damage, and the formation of micro-cracks.

Fig. 2. MIM-10 metallographic microscope used for optical inspection of laser-scribed grooves.

Fig. 3. Glass specimen with laser-scribed grooves produced at constant power and different scanning speeds.

Fig. 1. MTech L640 Optima - CO 2 laser processing system equipped with a RECI W2 sealed-tube CO 2 laser source (100 W).

3. Results and discussion The damage observed along the scribing line is dominated by a molten groove (Fig. 4), produced by localised melting of the glass surface under CO 2 -laser irradiation. The groove exhibits a characteristic profile across the entire range of scanning speeds. The groove surface consistently shows regularly spaced transverse thermal cracks extending along the scribing path (Fig. 4, a). The molten groove, the subsurface thermal crack beneath it (Fig. 4, b-c), and the surrounding HAZ (Fig. 4, d), form a coherent fracture system.

a)

b)

d)

c)

Fig. 4. General morphology of the molten groove.

Surface thermal cracks originate at the groove edges and fall into two principal categories: transverse and longitudinal. Transverse thermal cracks are oriented perpendicular to the direction of beam travel and appear in characteristic clusters, in which a primary crack, branched secondary cracks, and, at higher thermal loads, network or cellular crack patterns can be distinguished. These features are clearly visible in Fig. 5. Longitudinal thermal cracks occur along the groove axis and within the HAZ and are associated with stress generated during localised heating and subsequent cooling of the surface, as shown in Fig. 6. A subsurface thermal crack is consistently present beneath the molten groove (Fig. 4, b), forming a continuous fracture plane. After partial detachment of the molten layer, pulse-derived micro-craters (Fig. 7, b-c) become visible on the underlying surface and can be interpreted as imprints of individual laser pulses in a mixed continuous-pulsed operating regime. The HAZ surrounding the molten groove is characterised by optical contrast, local colour variations and microstructural non-uniformity (Fig. 4, d). Within this region, additional defects are observed, including internal spherical cavities, interference rings produced by residual stress fields and localised areas of network or cellular cracking. These features, grouped in Fig. 7, complement the fracture morphology of the molten groove and define the full system of laser-induced defects along the scribing line.