Strength of Zerodur for mirror applications: follow-up

Zerodur® is a well-known glass-ceramic used for optical components because of its unequalled dimensional stability under thermal environment. In particular it has been used since decades in Thales Alenia Space's optical payloads for space telescopes, especially for mirrors. The drawback of Zerodur® is however its quite low strength, but the relatively small size of mirrors in the past had made it unnecessary to further investigate this aspect, although elementary tests have always shown higher failure strength. As performance of space telescopes is increasing, the size of mirrors increases accordingly, and an optimization of the design is necessary, mainly for mass saving. Therefore the question of the effective strength of Zerodur® has become a real issue. Thales Alenia Space has investigated the application of the Weibull law and associated size effects on Zerodur® in 2014, under CNES funding, through a thorough test campaign with a high number of samples (300) of various types. That test campaign demonstrated a strength in fast fracture higher than 40 MPa ([1], [2]) for the tested surface finish, thus allowing much more versatility in the designs than the previously accepted strength limit (10 MPa). Another concern had however been raised: glasses are known to be susceptible to sub-critical crack growth, i.e. slow propagation of cracks under loads below fracture toughness, thus reducing fast fracture strength capabilities (since failure is linked to sudden propagation of those cracks). Taking into account data from literature, no effect was expected on Zerodur® in the conditions of use for space applications, but the very high variability of data made those computations not reliable enough. A dedicated test campaign was therefore defined in order to assess this effect and its consequences in conditions as representative as possible of real conditions. In this paper we show the outcome of this test campaign: the effect of subcritical crack growth is confirmed to be negligible and the minimum strength of 40 MPa is confirmed. In time, Zerodur® strength seems to even increase but this phenomenon was not investigated in the study.