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Figure 10. Determination of the SSD depth:( A) Dependency of the IF to the calculated SSD depth and visualization of the resulting anisotropic removal topography given for( B) Sample # 1 and( C) Sample # 2 applying IF > 1.5.
obtained after PJE using equation( 5). A volumetric view of topographies after anisotropic removal is shown in Figures 10B and 10C.
4.2 Scratch test sample
The evaluation method described above can not only be applied for defined static indentations shown in the previous results, but also with common scratches. In order to simulate realistic defect structures, they were generated using a scratch tester. The dynamic impact on the surface caused by a Rockwell tip can be used to model the formation of scratches caused by abrasive grains during the grinding of optical glass. The indicated mean and standard deviation of IF are determined for a smaller measurement matrix than provided by the original measurement. Here, two aspects must be considered. First, automatic alignment, as described in detail in Section 3.2, results in columns and rows filled with zeros. Since the scratches in the measurement field are oriented along y = 0, zero columns in the measurement image matrix influence the determination of the IF. Therefore, a reduction of the measurement matrix in x-direction is necessary. Furthermore, the extent of the scratch is low compared to the y-dimension of the measurement and would therefore bias the IF mean due to an enlarged area of isotropic removal( IF 1). Hence, the measurement matrix has to be cropped in the y-direction. In order to obtain statistically reliable IF values, all z exp and z calc matrices( see Eq.( 2)) were reduced from 512 512pxto476 160 px.
Again, the results of the IF calculation are compared to the slope of the VV shown in Figure 11A. Even for SSD caused by defined scratches, the highest impact of anisotropic removal( maximum of IF value) is present for the highest slope value( at etching removal of 12 lm). According to equation( 5), a value SSD = 16.12 lm is determined. Selected surface topography measurements are shown in Figures 11B – 11D. These demonstrate the significant expansion of the SSD for a surface that was previously only minorly damaged( Fig. 11B) and correlate with the plots given in Figure 11A. Figure 11B depicts scratch damage with circular structures visible at x > 100 lm. This plastic deformation is accompanied by crack formation and thus can also cause SSD [ 23 ] as determined with higher etching removal and verified by the corresponding IF values.
4.3 Conventionally ground sample
Finally, the described approach can also be applied to determine the SSD depth of mechanically ground and polished