SCHOTT solutions no. 2/2013 > Glass-Ceramic

Photo: SCHOTT/C. Costard

Accurate Predictions

A new mathematical model for calculating the strength of ZERODUR® glass-ceramic components developed by SCHOTT optics expert, Dr. Peter Hartmann, provides even better performance data. Customers in such fields as aeronautics, astronomy and lithography stand to benefit thanks to the higher operational reliability, dependability and efficiency of these components.

Christine Fuhr

A rocket must travel at a minimum speed of eight kilometers per second in order to resist the earth’s gravitational pull. Lift off takes place based on the so-called recoil principle: gases are produced by burning fuel, which then propel the rocket into the sky at incredible speeds under high pressure. Thrust, acceleration, pressure, and vibration exert enormous physical forces on the rockets and instrument payloads, such as satellites or space telescopes, for example. An exact understanding of the physical properties of all the materials employed is of extreme importance for those who develop the sophisticated devices that orbit the earth, take high resolution photographs, and perform precise measurements in outer space. Ideally, they require extremely low failure probability of the respective components under maximum stress. This reduces the frustration of the failure of costly high-tech instruments to withstand extreme environmental stresses.
Measurement of a lightweight ZERODUR® mirror 1.2 meters in diameter that weighs only 45 kilograms (left). The triangular-shaped pockets on the backside allow for up to 90 percent weight reduction. High demands are placed on the strength of these filigree mirror substrates. After all, they must be able to withstand the incredibly high physical forces that result when satellites or space telescopes are launched (above). Photo: Thinkstock

Glass and glass-ceramic: brittle, yet stable and hard

Strength is a decisive factor when it comes to using brittle glass and glass-ceramic materials in more sophisticated technical applications. The glass expert and technology company SCHOTT has been working intensely for many years in the area of research and development on such topics as strength and bending stress. The goal is to make glass much stronger (see also solutions 1/2013).

Due to their strong atomic bonds, glass and glass-ceramic materials theoretically resist high bending stresses. In reality, however, these capabilities deteriorate quite considerably after shaping with diamond grinding tools introduces microcracks in the surface. If they are deep too, they can lead to breakage and thus cause entire components to fail. For this reason, it is obviously very important for developers to understand accurately when this will happen. Now, the material experts at SCHOTT are in a position to offer better data on strength for the extremely low thermal expansion material ZERODUR® glass-ceramic. In the past, the ”two-parameter Weibull distribution” method was normally used to predict the strength of a glass-ceramic. Here, the breaking stress and probability of sample failure generated in a double ring test are displayed in megapascal (MPa) and extrapolated. As SCHOTT optics expert Dr. Peter Hartmann was able to prove in his work ”ZERODUR®: Deterministic Approach for Strength Design” that included a considerable amount of sample data, this conventional approach no longer suffices, but rather must be enhanced by using a modified (Weibull) approach that takes three parameters into account.
This new approach provides a minimum strength value for defined surface conditions and allows the service life to be calculated under specified stress while taking into consideration material fatigue under stress. There is another advantage to the approach over the previous one. This model reduces statistical uncertainties and overly conservative safety facts, and thus allows substantially higher mechanical stresses to be applied to ZERODUR® components.

Customers in the fields of astronomy, aeronautics and lithography who are interested in using ZERODUR® glass-ceramic with significantly higher static and dynamic stress profiles will now be able to do so thanks to more accurate predictions of its strength.


Breakage probability for different surface conditions
The three parameter Weibull curves all drop to zero breakage probability depending on the surface conditions. The minimum values are now much higher than the previously used 10 MPa, which means that ZERODUR® can be used with higher mechanical loads in the future. Source: SCHOTT/P. Hartmann
The optics expert Dr. Peter Hartmann was awarded the 2013 SCHOTT Prize for special achievements in the area of research and development for his work on the strength of temperature resistant ZERODUR® glass-ceramic. Photo: SCHOTT/A. Sell
ZERODUR® glass-ceramic is used to manufacture filigree lightweight mirror substrates for satellites and space telescopes. Their weight can even be reduced by up to 90 percent by carving out triangular-shaped structures from the backside. ZERODUR® components also perform high-precision tasks in electronics and the semiconductor industry. They are used as mirror substrates in lithography devices for manufacturing thousands or even millions of circuits for controlling the colors that LCD monitors display, as well as in chip manufacturing, where the structural sizes will continue to become smaller and shrink to just a few nanometers in the years to come. Essentially, ZERODUR® glass-ceramic components are found in applications that require the highest precision, reliability and operational security of the components used in order to ensure that these complex processes run smoothly. ”Thanks to this optimized calculation model and reliable data on strength, we have been able to prove that ZERODUR® glass-ceramic is even stronger and more stable under severe conditions than we had first assumed. This opens up new prospects for designers in building their instruments and devices,” explains Dr. Peter Hartmann, who was awarded the SCHOTT R&D Prize for his model.

”This work documents a high degree of scientific and technical professionalism and demonstrates the outstanding material and technological expertise SCHOTT has,” explained SCHOTT Board Member Dr. Hans-Joachim Konz at the award ceremony. ”Furthermore, this work underscores our goal of offering benefits to our customers through our profound understanding of materials. This is thus an example of the successful link between science on the one hand and technical marketing on the other,” he added.

Exrtremely low thermal expansion

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