Round-robin test of Subsurface Damage characterization in ZERODUR® using non-destructive optical coherence tomography

The glass ceramic ZERODUR (R) with its extremely low thermal expansion finds widespread application in precision optics and astronomical instruments. It has been used for several very large telescope mirrors, as its unique properties make it indispensable in environments demanding stringent dimensional stability. As for all brittle materials, mechanical grinding processes involved in manufacturing introduce subsurface damage (SSD), which compromises the optical and mechanical performance of workpieces. Beyond its implications for quality, SSD is often a significant cost driver, necessitating expensive finishing processes. Characterizing SSD and determining its depth at various manufacturing stages from pre-grinding to final polishing are critical for ensuring the quality and reliability of optical components. Conventional measurement techniques predominantly involve destructive methods such as taper polishing or cleaving the surface, posing limitations in assessing SSD without compromising the integrity of the workpiece. Moreover, these methods are often time-consuming and labor-intensive. In a collaborative study, we conducted a round-robin test to establish and validate the efficacy of non-destructive optical coherence tomography (OCT) as a new technique for characterizing SSD depths in ground ZERODUR (R). The aim is to provide a first comprehensive assessment of the capability of OCT in non-destructively quantifying SSD in glass ceramics. The methodology involves comparing measurements obtained from two distinct high-resolution OCT systems applied to identical ZERODUR (R) samples machined with selected process parameters. Furthermore, the study investigates the influence of different grain sizes on the SSD characteristics by comparing measurements across samples processed with varying grain sizes. By conducting the round-robin test, we seek to demonstrate the applicability of OCT as a non-destructive alternative for SSD characterization in ground ZERODUR (R). Successful validation of OCT extended to a variety of relevant optic materials such as ZERODUR (R) would enhance overall quality control processes, offering a rapid and non-invasive means of assessing SSD depths and ensuring the integrity of critical optical components.