J. Eur. Opt. Society-Rapid Publ. 22, 21( 2026) 203
Figure 5. Evolution of R19-C7 indentation( HV0.6): microscopic dark field images are shown for selected PJE process steps with etching depths of d.( A) Indentation prior to PJE.( B)–( D) Indentation after PJE steps # 2, # 3, and # 6.
Figure
6. SSD depth of Vickers indentations for different process states measured by OCT considering the PJE removal depth( n indicates the number of measurements for each column).
relative to the surface for each specific measurement. The determined SSD 99. 9 values differ by only a few micrometers, which corresponds well with the material removal depths achieved by the PJE process. For column 2, the removal depths after PJE # 3 and PJE # 5 were approximately 5.0 lm and 7.7 lm, respectively. Due to the applied area threshold of 0.1 % the given maximum of the color scale is higher than the SSD 99. 9 values obtained. As shown in Figure 7A, the maximum SSD depth of the initial indentation was 53.0 lm is due an imaging artifact, which is not included in the given evaluation threshold.
As the wedge etching removal increased from row 1 to 19, as shown in Figure 6, fewer SSD was detected with OCT( given by n), as more defects were eliminated during the PJE process, particularly starting from row 10. After PJE # 3, the mean SSD depths for Vickers indentation made with HV0.5 and HV0.6 were 31.1 ± 3.7 lm and 36.4 ± 6.7 lm, respectively. This trend was confirmed after PJE # 5, where similar mean SSD depths were observed, as shown in Table 2.
As a plausibility check, the SSD measurements are compared with distinct experiments from literature. Experimental data published by Suratwala et al. [ 81 ] includes directly comparable crack depths measured in Vickers experiments with HV0.5, yielding crack depths of 35.9 lm which aligns well with our measurement results listed in Table 2. The experiments conducted by Michel et al. [ 80 ] also show excellent agreement in both size and distribution, reporting a crack depth of 33 ± 7 lm for indentation tests under a load of 5 N at 20 ° C.
To further validate the OCT measurements, the evolution of SSD depth was additionally determined depending on etching depth resulting from wedge etching. As described in Section 2.3 the progress of the void volume VV of the indentation and the roughness parameter Sv were analyzed. VV rises with increasing etching depth, which is expected from the chemical etching processes. However, the increase is non-uniform during the whole process. Figure 8 presents this progress exemplary for three indentations. The calculation of the slope using equation( 4), where VV is the void volume and d the etching depth reveals a prominent peak that can be observed for every indentation.
Slope ¼ VV PJE n
� VV PJEn�1 d PJEn � d PJEn�1
: ð4Þ
As shown in the section before, the morphology of the indentation changes significantly. Obviously, anisotropic etching behavior prevails as long as SSD is present. So far, it is postulated that the significant increase of the void volume at a specific etching depth indicates the influence of disturbance in the material, emerging from SSD. The morphological evaluation is shown exemplary in the previous section, where the volumetric extension is visualized in Figures 3B – 3C. This behavior is comparable to the findings in wet chemical etching, where anisotropic etching of the disturbed material was detected as well [ 82 – 84 ]. After the complete removal of SSD isotropic etching continues and the slope shown in Figure 8( A) approaches zero.