JEOS RP ISSN03 | Page 208

J. Eur. Opt. Society-Rapid Publ. 22, 21( 2026) 201

Figure 2. Digital microscope image of Vickers indentation( A) R2-C4( HV0.5) and( B) R19-C7( HV0.6) in the initial state.

Table 1. Measured diagonal length and calculated penetration depth of Vickers indentations.

cracks as two typical damages [ 76, 77 ] in the area of Vickers indentations are observed on the surface as shown in Figure 2 on indentation R2-C4( A) and R19-C7( B).

The length of the diagonals of the Vickers indentations d Vickers was measured directly on the Vickers tester. The penetration depth h of the Vickers indenter in the material was calculated using the known geometry of the Vickers indenter according to equation( 2). Both values are shown in Table 1. h ¼

d Vickers p 2 ffiffi

2 tan ð 136 = 2Þ: ð2Þ

The evolution of an exemplary indentation morphology at position R2-C4 is shown in Figure 3 for selected etching depths. In addition to the grayscale images obtained from optical dark field microscopy, the corresponding WLI measurements are shown in false-color for comparison. It can be stated that the first etching attack takes place in the center of the indentation. The central summit, which is apparent in the initial view in Figure 3A, can no longer be observed after etching to a depth of 1.5 lm; instead, a hole-shaped indentation geometry is present in Figure 3B. This shape is retained for the next etching steps. However, in the dark field image, the cone crack that was formed below the surface during the creation of the indentation( see Fig. 3B) becomes more visible. After reaching an etching depth of 10.6 lm the radial expansion of the indentation becomes noticeable, as shown in Figure 3C. It is reasonable to assume that the indentation has opened along the cone crack. The mechanism has not yet been fully clarified, but a kind of“ chipping” of the surface due to the etching attack is suspected. Probably, the radial cracks do not influence the evolution of the indentation’ s topography in a driving manner, since no example of extension of those cracks after PJE was found.

The fracture behavior of fused silica under Vickers indentation has been extensively investigated to understand SSD mechanisms. Among the observed crack types – radial cracks and cone cracks – cone-shaped cracks typically exhibit the greatest subsurface depth. Hagan et al. [ 76 ] experimentally demonstrated that cone cracks initiate near, but outside the contact region of the Vickers indenter and propagate into the bulk material at a characteristic angle, resulting in a subsurface depth significantly exceeding that of other crack types. Lawn and Wilshaw [ 77 ] provided a theoretical framework explaining that, due to the brittle nature and minimal plasticity of fused silica, cone cracks form primarily under tensile stresses beyond the contact zone and dominate the subsurface fracture morphology. Li et al. [ 78 ] experimentally studied the effects of densification on fracture behavior in fused silica under Vickers indentation. Their results confirmed that cone cracks are the primary crack type, exhibiting greater penetration depths than other cracks, thus reinforcing the critical role of cone cracks in SSD. Tomić et al. [ 79 ] further supported these findings trough numerical phase-field modeling, simulating the initiation and propagation of cone cracks from the contact periphery into the bulk. Their results emphasized the dominant role of cone cracks in subsurface damage generation by indentation. Michel et al. [ 80 ] investigated the effect of temperature on hardness and indentation cracking in fused silica, finding that cone cracks consistently form as the earliest and deepest cracks across a broad temperature range. Collectively, these experimental, theoretical, and numerical studies converge on the conclusion that