Glass Ceramic ZERODUR Research Articles

Surface integrity and material removal mechanisms of Zerodur glass-ceramics by Gallium infiltration in high-pressure polishing

The core of the production of ultra-precision optical glass-ceramics are the reduction of high-stress concentration and micro-crack growth during polishing. This work aimed to study the effect of gallium solidification in the interfacial micro-structure on the micro-crack growth and the effect of the mechanical properties of glass-ceramics with gallium under different polishing stresses. In this study, a new polishing method was introduced. This innovative technique involves the infiltration of liquid gallium as a filler into the sub-surface of the workpiece. The gallium subsequently solidifies within the micro-cracks of the workpiece, effectively offset surface defects and improving the polishing outcome. The results of the gallium infiltration polishing method show that glass-ceramics workpieces with defects are repaired by solidified gallium, allowing them to be polished at higher pressures, reducing polishing time by 21.4% compared to low-pressure polishing. In addition, the gallium penetration polishing method dispersed the stress concentration near the microcracks and inhibited the growth of micro-cracks during the polishing process, which has been verified by the surface morphology analysis of the workpieces.

Comprehensive review of the effects of ionizing radiations on the ZERODUR® glass ceramic

The low thermal expansion glass ceramic ZERODUR® has a long and successful history in spaceborne optical applications. This material is especially used where precise shape invariance is required, i.e., for the mirror substrates dimensional stability when subject to temperature gradients and transients. In space, temperature may not be the exclusive driving force impacting the form stability; influence from ionizing radiations require considerations. The real impact of ionizing radiation on ZERODUR® has become a matter of reconciling, on the one hand, in situ experience, e.g., that the secondary mirror of low Earth orbit (LEO) Hubble Space Telescope crossing the South Atlantic Anomaly or the overall optics of geosynchronous equatorial orbit (GEO) Chandra X-ray Observatory are not reporting any specific problems related to dimensional stability at the optical form level. On the other hand, finite element simulation based on early lab experiments of ZERODUR® compaction are suggesting the opposite. This debate was brought to the forefront with the SILEX mission, where radiative ageing models were significantly overestimating the deformation experimentally observed on the lab replicas and were in even stronger disagreement with the observations collected over the mission. It has been speculated that an erroneous form factor in the physical model used to derive the phenomenological compaction law was responsible for these discrepancies. Following this hypothesis, we readdressed the effect of ionizing radiation induced by γ, electron, and proton fluences on ZERODUR® compaction. For each of these, we present and discuss the irradiation source, the experimental setup, the sample design, and the measurement procedure as well as the observations. Consistent with the feedback gathered over many different space missions, we confirm that the compaction observed is significantly smaller than the estimations available in the prior literature.

Final report of SIM.L-K7.2016

Main textThis report describes the results of the bilateral comparison SIM.L-K7.2016 carried out in 2016 - 2017 between the pilot INRIM (Italy) and INTI (Argentina). The scale is made of a glass ceramic Zerodur. The total length of main graduation is 280 mm. The measurand is the deviation from the nominal distance between the center line position of the reference line [position 0] and the center line position of the measured line.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Investigation of Characterization of Zerodur Glass Ceramics Used in Aerospace to Predict the Relationship between Surface Roughness and Subsurface Damage

Due to the brittle nature of the optical parts used in the grinding process to obtain a good surface quality, surface roughness and subsurface damage occur after the machining. These subsurface damages must be detected and destroyed with the help of post-process processes such as polishing. Because detecting these damages is time-consuming and costly, many researchers have tried relationship between subsurface damage and surface roughness since it is easier to detect and measure. In this study, Zerodur glass ceramic material characterization tests were carried out to predict the surface roughness and damage under the surface, and the material properties were determined to be specific to the sample. The grinding parameters taken from the previous studies and the values given for surface roughness and subsurface damage were examined with the help of the Lambropoulos theoretical model. The results theoretically showed that with the characterization of material properties different from those given in the literature, even if the parameters used in the experimental study do not change, much more severe subsurface damage will be created on the sample, and the load value applied to the material will increase significantly. For this reason, it is crucial to determine the material's mechanical properties by performing characterization tests before all material grinding and post-processing processes.

Semi-ductile cutting regime technology for machining Zerodur glass-ceramic microstructures

Zerodur glass-ceramic has potential uses in many fields from aerospace to electro-optical industries due to its extremely low thermal expansion coefficient. However, being extremely hard and brittle makes it difficult to process using traditional machining methods. This paper deals with a novel hybrid technology for machining 3-D microstructures using Zerodur glass-ceramic. Based on a Semi-ductile cutting depth, experiments are conducted using different cutting modes: regular cutting, current-feedback cutting, and a current-feedback control strategy with high-frequency vibration-assistance. High-frequency vibration-assisted cutting creates extremely shallow microcracks on the surface of the glass-ceramic that facilitate the removal of material. It was also found that the designed diamond tool with a −70° rake angle and negative rake angle of abrasive-grains can produce an exceptional negative rake-angle press-cut effect helping to suppress brittle-fracturing from occurring. Experimental results show that the proposed ‘high-frequency vibration-assisted cutting’ mode with a ‘self-controlling cutting force’ can automatically adjust the tool feed-rate on-line, increasing the material removal rate and protecting the glass-ceramic against brittle-fracturing. A 3-D microstructure made of Zerodur glass-ceramic is successfully machined under semi-ductile regime cutting, confirming the feasibility of the proposed technology, and opening-up the potential for micromachining hard-brittle materials such as Zerodur glass-ceramics for commercial applications.

Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 1

Observational astronomy has striven for better telescopes with higher resolutions from its start. This needs ever-larger mirrors with stable high precision surfaces. The extremely low expansion glass ceramic ZERODUR® enables such mirrors with more than 50 years of significant improvements in size and quality since its development. We provide a survey of the progress achieved in the last 15 years. The narrowest coefficient of thermal expansion (CTE) tolerance is now ±7 ppb / K. It is possible to adapt the material for lowest expansion to temperature-time courses given for special environments. At cryo temperatures, expansion is low and adaptable. Improved measurement capabilities allow for absolute CTE uncertainty of 3 ppb / K and reproducibility of 1 ppb / K (2σ). The influence of ionizing radiation on the surface figure integrity is subject to new investigations. Improved measurement capabilities increase the reliability of structure designs. Some outstanding examples are given for applications of ZERODUR in astronomy and in the very important high technology industry. The progress of thermal expansion homogeneity, the mechanical strength of ZERODUR, production capabilities in melting, precision machining, light-weighting, and dimensional metrology is presented in the second part of the paper.

Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 2

Observational astronomy has sought better telescopes with higher resolution from its beginning. This needs ever-larger mirrors with stable, high-precision surfaces. The extremely low-expansion glass ceramic ZERODUR® has enabled such mirrors for more than 50 years with significant improvements in size and quality since then. We provide a survey of the progress achieved in the last 15 years. Equally important as the thermal expansion coefficient CTE is its homogeneity. The CTE variation in 4-m mirror blanks lies below 5 ppb / K in radial and axial directions on large and short scales. Improved measurement capabilities allow reduced bias, which in the past made variations look greater than they were. Isotropy and uniformity of ZERODUR are outstanding. A method for lifetime calculation increases reliability considerably with respect to mechanical loads. The production and metrology capability and capacity are greatly extended. Surface figure and texture of large blanks allow starting directly with polishing. Filigree structures with up to 90% weight reduction are well-suited for space mirrors. The progress with low thermal expansion and its measurement, the insensitivity of ZERODUR against ionizing radiation in space, and outstanding application examples are presented in the first part of our review.

Femtosecond-laser-written integrated photonics in bulk glass-ceramics Zerodur

We demonstrated fabrication of waveguide and directional coupler in bulk glass-ceramics Zerodur, through femtosecond laser direct-writing. Single mode waveguide with propagation loss of no more than 1.5 dB/cm at wavelength of 777 nm was achieved. We expect this will lead to the deployment of Zerodur integrated photonics in fields like space-borne astronomy, ultra-precision metrology, quantum technologies and fundamental physics investigation, for both cost and technical advantages.

Machining behaviors of glass-ceramics in multi-step high-speed grinding: Grinding parameter effects and optimization

Currently, the lack of proper manufacturing technology that takes productivity and quality into account is still considered as the bottleneck for large-scale applications of glass-ceramic. This work proposed a multi-step high-speed grinding technology for Zerodur glass-ceramics machining. Not only the experimental investigation of the effects of feed speed and grinding depth on the machining behaviors of glass-ceramics were performed, but also the theoretical analysis of machining behaviors was conducted. The effects of grinding parameter on machining behaviors were compared as well and the results showed that the effects of feed speed were greater. Based on these experimental results and the theory of brittle-to-ductile transition, a strategy for optimizing suitable grinding parameters was proposed and the optimal process combination parameters were obtained. A comparison of various existing grinding processes and the multi-step high-speed grinding technology was given in the end and verified the unique advantages and application prospects of the technology.

Effects of wheel speed on surface/subsurface damage characteristics in grinding of glass-ceramics

Glass-ceramic is widely used in optics and other areas because of its good physical and mechanical properties. However, the accompanying hard and difficult to machine characteristic brings great challenges to the commonly used grinding in industry. Previous researches on the removal mechanism of glass-ceramic are basically traditional low-speed scratch and low-speed grinding, ignoring the effect of the wheel speed on surface/subsurface damage. In this study, the effects of the wheel speed on the surface morphology, surface roughness, and subsurface damage of Zerodur glass-ceramics were studied at different wheel speeds based on a multi-step grinding process. Experimental and analytical results showed that increasing wheel speed can restrain the occurrence and development of brittle fractures due to the effects of strain rate and grinding temperature, resulting in fewer and smaller micro-pits and micro-cracks, and thus more signs of plastic flow are observed. In addition, the effects of wheel speed on surface roughness under the semi-finishing grinding stage are not as clearly defined as on that under the rough and finishing grinding stages. The smeared layers on the ground surface, which are related to the compound effects of the grinding temperature and semi-finishing grinding conditions, are recognized as the major contributor to the contradictory surface roughness results. Furthermore, the effect of wheel speed can prevent crack from nucleating and propagating, and then leads to a smaller depth of subsurface damage. Accordingly, there is a general trend toward subsurface damage depth with the increase of undeformed chip thickness, regardless of the grinding conditions. In a word, higher wheel speeds are easier to achieve ductile removal mode and conversely, the glass-ceramic is more prone to a brittle removal mode. The research is expected to provide guidance for grinding-induced damage control and the engineering application in high precision optical parts of glass-ceramic.

Minimum lifetime of ZERODUR® structures based on the breakage stress threshold model: a review

Increasing demand for using the glass ceramic ZERODUR® for optical elements with high mechanical loads called for strength data based on statistical samples larger than 20 specimens. The data now available for a variety of practical surface conditions (ground, lapped, and etched) allow stresses by factors 4 to 10 times higher than before. The larger samples revealed that breakage stresses of ground surfaces follow the three-parameter Weibull distribution. The threshold parameter of this distribution reflects the existence of an upper limit for the microcracks depth within such surfaces. It is equivalent to a minimum strength below which breakage probability is zero. Its use in the well-established crack growth theory allows calculating minimum lifetimes including fatigue using the stress corrosion constant for the prevailing environmental humidity. Long-term loading tests have confirmed the validity of the model. For fully etched surfaces, the Weibull statistics fails because in such cases failure mechanism is not unique anymore. Nevertheless, ZERODUR® with fully etched surfaces that are free from other damages still exhibit minimum breakage stress above 100-MPa tensile stress. The successful satellite mission LISA Pathfinder has confirmed the possibility to apply ZERODUR® for utmost precision experiments together with high mechanical loads.

Active Optics in Astronomy: Freeform Mirror for the MESSIER Telescope Proposal

Active optics techniques in astronomy provide high imaging quality. This paper is dedicated to highly deformable active optics that can generate non-axisymmetric aspheric surfaces—or freeform surfaces—by use of a minimum number of actuators. The aspheric mirror is obtained from a single uniform load that acts over the surface of a closed-form substrate whilst under axial reaction to its elliptical perimeter ring during spherical polishing. MESSIER space proposal is a wide-field low-central-obstruction folded-two-mirror-anastigmat or here called briefly three-mirror-anastigmat (TMA) telescope. The optical design is a folded reflective Schmidt. Basic telescope features are 36 cm aperture, f/2.5, with 1.6° × 2.6° field of view and a curved field detector allowing null distortion aberration for drift-scan observations. The freeform mirror is generated by spherical stress polishing that provides super-polished freeform surfaces after elastic relaxation. Preliminary analysis required use of the optics theory of 3rd-order aberrations and elasticity theory of thin elliptical plates. Final cross-optimizations were carried out with Zemax raytracing code and Nastran FEA elasticity code in order to determine the complete geometry of a glass ceramic Zerodur deformable substrate.

Deep etching of Zerodur glass ceramics in a fluorine-based plasma

The etching of glass is still much more challenging than the deep etching of silicon. But in contrast to pure silica, most glasses are complex alloys of serval oxides including aluminium oxide. For this reason, it is quite difficult to find suitable high-rate deep dry etching processes and related masking materials. For extremely temperature-insensitive micromechanical systems it is of interest to use zero-expansion glass ceramics such as Zerodur. But the microstructure of Zerodur consists of crystalline and amorphous phases and shows a high percentage of Al2O3-bonds. This makes plasma etching challenging. Here, deep etching of Zerodur only in a fluorine-based plasma for micro-technical applications is investigated. Different process parameters such as the physical power and gas mixtures of the ICP-RIE-process have been varied. Etch rates of about 250nm/min and sidewall angles of approximately 71° were reached with a nickel mask and the etch gas SF6. The achieved total etching depth is as large as 150μm resulting in a release of microelements such as springs and gears from a Zerodur wafer.

Manufacture Technology for Large Lightweight Optical Materials

In this paper, the performance comparison between glass ceramics (Zerodur) and silicon carbide (SiC) which are two kinds of common materials used for space optical reflector is carried out, and several lightweight structure forms are analyzed. The oval plane reflector is applied in ultra-low temperature environment of space, take this kind of reflector as an example, its lightweight structure is optimized by CAD, then through finite element analysis, deformation of the planar lightweight mirror is 0.014λ (rms) in gravity condition, and deformation is 0.002λ (rms) in ultra-low temperature of-150 degrees environment. The actual lightweight processing is controled by the CNC system in a graphical way, and using chemical method to eliminate the stress and micro crack generated during processing, its final surface shape precision reaches 0.022λ (rms). Finally, this paper introduces the manufacture method of novel silicon carbide (SiC), and analyzes the current situation and development trends of the spatial lightweight reflector manufacturing technology.

Manufacturing Technology of Space Optical Materials

In this paper, the performance comparison between glass ceramics (Zerodur) and silicon carbide (SiC) which are two kinds of common materials used for space optical reflector is carried out, and several lightweight structure forms are analyzed. The oval plane reflector is applied in ultra-low temperature environment of space, take this kind of reflector as an example, its lightweight structure is optimized by CAD, then through finite element analysis, deformation of the planar lightweight mirror is 0.014λ (rms) in gravity condition, and deformation is 0.002λ (rms) in ultra-low temperature of-150 degrees environment. The actual lightweight processing is controled by the CNC system in a graphical way, and using chemical method to eliminate the stress and micro crack generated during processing, its final surface shape precision reaches 0.022λ (rms). Finally, this paper introduces the manufacture method of novel silicon carbide (SiC), and analyzes the current situation and development trends of the spatial lightweight reflector manufacturing technology.

ZERODUR®: deterministic approach for strength design

There is an increasing request for zero expansion glass ceramic ZERODUR® substrates being capable of enduring higher operational static loads or accelerations. The integrity of structures such as optical or mechanical elements for satellites surviving rocket launches, filigree lightweight mirrors, wobbling mirrors, and reticle and wafer stages in microlithography must be guaranteed with low failure probability. Their design requires statistically relevant strength data. The traditional approach using the statistical two-parameter Weibull distribution suffered from two problems. The data sets were too small to obtain distribution parameters with sufficient accuracy and also too small to decide on the validity of the model. This holds especially for the low failure probability levels that are required for reliable applications. Extrapolation to 0.1% failure probability and below led to design strengths so low that higher load applications seemed to be not feasible. New data have been collected with numbers per set large enough to enable tests on the applicability of the three-parameter Weibull distribution. This distribution revealed to provide much better fitting of the data. Moreover it delivers a lower threshold value, which means a minimum value for breakage stress, allowing of removing statistical uncertainty by introducing a deterministic method to calculate design strength. Considerations taken from the theory of fracture mechanics as have been proven to be reliable with proof test qualifications of delicate structures made from brittle materials enable including fatigue due to stress corrosion in a straight forward way. With the formulae derived, either lifetime can be calculated from given stress or allowable stress from minimum required lifetime. The data, distributions, and design strength calculations for several practically relevant surface conditions of ZERODUR® are given.

Study of the thermal stability of Zerodur glass ceramics suitable for a scanning probe microscope frame

Abstract The work presents measurements of the length stability of Zerodur glass ceramic with temperature change. Measurement of this thermal characteristic is necessary for determination of the optimal temperature at which the Zerodur glass ceramic has a coefficient of thermal expansion close to zero. The principle of the measurement is to monitor the length changes using an optical resonator with a cavity mirror spacer made from the Zerodur material to be studied. The resonator is placed inside a vacuum chamber with a temperature control. A tunable laser diode is locked to a certain optical mode of the resonator to monitor the optical frequency of this mode. A beat-note signal from optical mixing between the laser and a stabilized femtosecond frequency comb is detected and processed. The temperature dependence of the glass ceramics was determined and analyzed. The resolution of the length measurement of the experimental set-up is on the order of 0.1 nm.

Development of a technology for joining glass-ceramics parts with zero thermal expansion

We report the results of testing samples made of glass ceramics with a zero thermal expansion coefficient. The samples consist of two parts, joined by a new technology of solid-phase sintering polished parts with vacuum-deposited aluminum coating of nanosized thickness. Also reported are data on the stability of these samples after exposure to extreme factors such as cryogenic temperature (temperature of liquid nitrogen), high temperatures of 400 and 1000°C, laser radiation, etc. The test results show that the suggested technology can be used for producing light-weight astronomical telescope mirrors of Zerodur glass ceramics. In our opinion, the suggested design of a light-weight astronomical telescope mirror with two parts joined by the new technology [the bottom part is silicon carbide (SiC) and the upper is an optical layer of glass ceramics with a thermal expansion close to that of SiC in value] is promising.

Study of chemical interaction at Al–ZERODUR interface

Effect of thermal annealing on bulk and interface properties of ZERODUR glass ceramics was investigated by Raman and X-ray spectroscopy as well as secondary ion mass spectrometry. For this purpose thin aluminium films were deposited onto the surface of ZERODUR samples prepared by mechanical polishing. Internal stresses in the bulk of investigated samples were revealed using modulation–polarization technique. The ability of chemical reactions in Al–ZERODUR system is briefly analyzed to explain experimental data. Contribution of amorphous and crystalline phases of ZERODUR to observed phenomena is discussed.

Anodic bonding using the low expansion glass ceramic Zerodur®

Using anodic bonding, a strong bond has been created between the ultralow expansion glass ceramic Zerodur® from Schott and an Al coated piece of ultralow expansion glass (ULE® from Corning). The bond strength has been tested using a tensile testing machine. The bond strength is comparable to the fracture strength of the ULE® material. Using anodic bonding it will be possible to make complicated electrostatic clamps for the next generation lithography machines based on an all ultralow expansion design.