Strength behaviour of etched Zerodur®

Abstract

Zerodur® is a well-known glass-ceramic used for optical components because of its unequalled dimensional stability under changing temperature. For example, it is used for mirrors of space telescopes. Due to the large amount of material that has to be removed during fabrication of the mirrors a rather coarse grit is used for grinding. Strength limiting defects introduced by this procedure are eliminated by removing the damaged surface layer by etching to achieve a high strength.Through a large test campaign comprising a high number of strength specimens (>160) of various types and sizes, the validity of the Weibull strength statistics was investigated. The purpose was to accurately determine the parameters of the Weibull law for Zerodur® when treated in the same conditions as mirrors.As a rule of thumb the average strength of Zerodur® is assumed to be around 10MPa. The measured strength is much higher. The strength distributions showed unexpected results, especially the absence of the expected size effect. Through a thorough fractographic analysis it was shown what kind of defects were responsible for failure of etched Zerodur® and why the prerequisites for the validity of the Weibull theory were not fulfilled in the investigated specimens.