Silicon nitride is a ceramic material that has very interesting properties for space applications, especially for optical
instruments and telescopes. Indeed, the objective for such structures is chiefly stability to ensure performance, associated
with low mass and high stiffness to sustain launch loads. With its high strength and stiffness and low CTE, Silicon
Nitride is therefore extremely well suited for stable Space structures.
Thales Alenia Space has been using Silicon Nitride for more than a decade, and in that framework has evaluated the
ability of the material to address those needs and requirements for complex designs such as tubes, brackets, brazed
supports, beams and light-weighted plates. All these structures have been tested, qualified and are now flight proven.
In order to improve knowledge and mastering of strength properties, a characterization campaign of the material is under
way in a GSTP funded by CNES and driven by ESA. This paper describes the design of this test campaign, the choices
for the sample types and dimensions, and prediction of the expected results. In addition to the mechanical strength
testing, X-ray tomography has been implemented in order to detect flaws beforehand and to investigate the ability to
predict failure from the extracted information. This will be especially useful since verification (in particular proof tests
that are commonly used on ceramics for Space applications) is based on the relationship between strength and flaws. It is
expected that by improving this knowledge a more straightforward verification process can be derived.
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 (, ) 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.