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parameter n). Thus, SSD depth values are statistically less reliable.
4 Conclusion
In this work, Vickers indentations were applied to fused silica samples, enabling the controlled and reproducible generation of localized SSD regions with well-defined geometry and thus providing a standardized model for systematic investigations of SSD depth. Non-destructive optical coherence tomography( OCT) was applied to characterize SSD depths and the morphology of surface defects induced by indentation.
In order to validate the OCT results, a destructive reference method based on iterative plasma jet etching( PJE) and topographic measurements to determine the evolution of the surface roughness parameter Sv and deriving SSD depths has been established. Although the SSD depths determined using both methods show sufficient correlation, the SSD depths measured by OCT exhibit a greater data dispersion.
Therefore, a novel hybrid approach combining PJE and OCT was employed to characterize SSD induced by Vickers indentation in fused silica glass. The experimental workflow involves selectively removing surface layers using PJE to expose the damage morphology, and performing high-resolution, 3D imaging and depth analysis via OCT. The results demonstrate that this combined approach enables reliable and even more accurate quantification of SSD depth and morphology.
The proposed hybrid methodology has important applications in both academic research and industrial practice, particularly in precision optical manufacturing. Accurate detection and quantification of SSD in fused silica and other brittle optical materials is essential for ensuring the performance, durability, and reliability of high-quality optical components, such as lenses, optical fibers, and glass-based devices. Subsurface defects can significantly affect optical performance and mechanical stability, making reliable SSD evaluation a critical requirement. From an academic perspective, this study contributes to a deeper understanding of SSD formation in hard and brittle materials. The combined use of Vickers indentation, OCT, and PJE provides a robust framework for investigating SSD with improved accuracy and can be used as a calibration tool for SSD analysis. From an industrial standpoint, the proposed method shows strong potential for quality control and process optimization in optical component manufacturing. PJE offers additional measurement benefits for OCT and an additional evaluation method based on the proposed roughness analysis. These findings are directly applicable to specific process chains in precision optics manufacturing, particularly where plasma and ion-beam techniques are already employed.
A direct comparison of SSD depths determined in this study with those measured in Vickers experiments by Suratwala et al. [ 81 ] and Michel et al. [ 80 ] shows an excellent agreement with previously recorded crack depths under similar experimental conditions.
Furthermore, this study experimentally confirms the dominant role of cone cracks in the formation of deep SSD in fused silica. Previous theoretical, experimental, and numerical studies have consistently shown that cone cracks initiate near the periphery of the contact zone and propagate into the bulk material at a characteristic angle, exhibiting significantly greater penetration depths than radial or lateral cracks. OCT-based analysis in this work reveals that the regions of maximum SSD depth correspond to the typical propagation paths of cone cracks, reaffirming their critical role in subsurface damage generation.
In summary, the developed hybrid method provides an effective and precise tool for the identification and quantification of SSD caused by Vickers indentation, particularly those governed by cone crack propagation. This approach holds substantial promise for applications in precision optical manufacturing, where the ability to detect and control subsurface defects is essential for improving the quality and reliability of optical components.
Funding
This research was funded by the German Federal Ministry for Economic Affairs and Climate Action( BMWK) within the Promotion of Joint Industrial Research Program( IGF) due to a decision of the German Bundestag. It was part of the research project 01IF22724N by the Association for Research in Precision Mechanics, Optics and Medical Technology( F. O. M.) under the auspices of the DLR Projektträger( DLR-PT). The authors gratefully acknowledge financial support by the Free State of Thuringia and the European Union( EFRE – European Regional Development Fund and REACT-EU) – grant no. 2021 FGI 0021.
Conflicts of interest The authors declare that they have no competing interests.
Data availability statement
All data generated or analyzed during this study are included in this published article.
Author contribution statement
TA and JB acquired the funding and conceptualized as well as managed the research. SF, HM and WZ wrote the original draft. WZ prepared the initial samples using Vickers indentation. SF and WZ performed OCT and DM measurements. DT and SF carried out OCT evaluations. HM performed the plasma jet etching, WLI and DM measurements. All authors were involved in methodology design and data evaluation as well as editing the manuscript. The authors have read and approved the final manuscript.
References
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