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In the past ZERODUR® was mainly used for mirror and substrate applications, where mechanical loads were given by its
own weight. Nowadays substrates become more sophisticated and subject to higher stresses as consequences of high
operational accelerations or vibrations. The integrity of structures such as reticle and wafer stages e.g. must be
guaranteed with low failure probability over their full intended life time. Their design requires statistically relevant
strength data and information.
The usual way determining the design strength employs statistical Weibull distributions obtained from a set of
experimental data extrapolating the results to low acceptable failure probability values. However, in many cases this led
to allowable stress values too low for the intended application. Moreover, the experimental basis has been found to be
too small for reliable calculations.
For these reasons measurement series on the strength of ZERODUR® have been performed with different surface
conditions employing a standardized ring-on-ring test setup. The numbers of specimens per sample have been extended
from about 20 to 100 or even much more. The results for surfaces ground with different diamond grain sizes D151, D64
and D25 as well as for etched surfaces are presented in this paper.
Glass ceramics like all glassy materials exhibit some strength reduction when being exposed to loads above a tensile
stress threshold over long time periods. The strength change of ZERODUR® with time will be discussed on the basis of
known and newly determined stress corrosion data.
The results for samples with large numbers of specimens contribute new aspects to the common practice of extrapolation
to low failure probability, since they provide evidence for the existence of minimum strength values depending on the
structures surface conditions. For ground surfaces the evidence for minimum strength values is quite obvious. For etched
surfaces minimum values are to be expected also. However, here closer observation is still needed. The systematic
deviations from Weibull distributions lie below about 5 % failure probability and thus could not be seen in small samples
as they were common in the past.
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