Silicon Carbide Mirror

Silicon carbide (SiC) is an ideal material for larger telescopes and high-speed scanning systems due to its non-toxicity, exceptional thermomechanical stability and resistance against oxidation and chemical degradation.

This paper presents a deterministically manufactured 4.03 m SiC aspheric mirror with low-temperature PVD cladding. It illustrates how quantitative test data from generation (mm precision) to polishing (nm precision) can be seamlessly combined to allow for excellent error convergence and control.

Lightweight

Space telescope mirrors must withstand extremely high gravitational loads while remaining light to remain structurally sound, which requires using materials with high specific stiffness such as silicon carbide (SiC). Unfortunately, large mirrors made from SiC can be time-consuming to produce due to needing to grind and polish their surface to optical precision – a process which may take months or even years!

Innovative fabrication techniques can significantly decrease production times for larger mirrors. Since the late 80s, US laboratories have been exploring hot pressing techniques for composite SiC mirrors as a faster and simpler method to produce them. This method used a mixture of SiC powder and phenolic resin which was applied to both sides of a mirror core. This technique enabled fabrication of a 1.3 m secondary mirror for Subaru telescope that weighed 185 kg with an areal density of 105 kg/m2. Corning’s second generation composite replication technology employs abrasive water jetting and other technologies to produce lightweight cores which are then bonded onto the front and back plates of a mirror to form a sandwich-structure ULE mirror with an areal density as low as 37 kg/m2.

Reaction sintering of SiC is another promising approach to creating high-performance SiC mirrors, which involves using degreased green bodies composed of SiC powder mixed with phenolic binder, combined with high temperature vacuum sintering furnaces to produce sintered bodies with perfect geometric forms, minimal shrinkage and outstanding mechanical and optical properties.

To increase mirror stiffness, support must be provided at both axial and radial locations. One way of doing this is through designing its back in a lotus pattern to increase area moment of inertia and increase bending resistance. A recent study evaluated this method on 2 m SiC lightweight mirrors optimized for ground-based telescopes; their reduced mass allowed them to achieve up to 40% mass savings compared with traditional passive Zerodur mirrors of similar size.

High Stiffness

One of the key characteristics of any mirror is its stiffness. This feature enables it to maintain its shape during high-speed scanning, helping laser scanning systems achieve maximum performance levels. SiC’s superior stiffness over traditional glass mirrors (which tend to have higher densities and greater mass) also represents an attractive feature.

Silicon carbide’s combination of strength and stiffness make it ideal for many applications, including laser scanning. Laser scanners used in printing, welding, cutting and drilling require mirrors capable of supporting high scan speeds needed by these applications; classic galvanometer scan heads typically feature 50 mm beam apertures that require rigid mirrors to support this rate of scanning.

Silicon carbide is an ideal material to construct this type of mirror due to its ability to withstand both high speeds and temperatures encountered when scanning systems are employed. Furthermore, silicon carbide’s excellent thermal conductivity allows it to effectively dissipate heat while helping ensure stable optical performance throughout a broad temperature range.

Silicon carbide offers many advantages; however, large aspheric mirrors constructed from this material have been challenging to fabricate due to challenges in several key areas. Primarily these include mirror blank preparation, asphere fabrication and testing issues as well as print-through effect issues related to test procedure fidelity/precision, stress/density of cladding process stress/density density/cladding process density as well as dynamic range required for deterministic testing procedures.

Due to these obstacles, large space-based telescope systems have found it challenging to achieve desired performance. But thanks to advanced manufacturing techniques, Zygo Corporation has discovered an alternative method for fabricating lightweight aspheric mirrors out of silicon carbide using nontraditional substrates provided by POCO Graphite Inc, and their advanced deterministic finishing technology from Zygo.

This approach has enabled the production of a 4 m aspheric silicon carbide mirror with excellent figure-sharpness (FSF) specifications of 10-15nm RMS and 5-6nm MSF error. Furthermore, this mirror has only 0.094 wave root-mean-square roughness.

High Thermal Conductivity

Silicon carbide (SiC) is an outstanding polycrystalline material with strong mechanical and thermal properties, making it the go-to substrate material for optical mirrors operating in harsh environments. Resistant to chemical degradation and having a low coefficient of expansion means this stable substrate won’t shift shape as environmental conditions change – something especially crucial when large telescopes must move to capture different positions for observations.

SiC is distinguished from glass or glass-ceramics by its ability to be machined into complex three-dimensional forms and assembled to form optomechanical components with ease, making lightweight telescope designs with maximum aperture possible. Furthermore, due to its superior thermal conductivity and thermal expansion capabilities it has been successfully replaced beryllium in high speed laser scanning systems without impacting dynamic performance.

Fabricating large-diameter SiC mirrors poses a considerable difficulty due to their design requirement that they be structurally split into multiple preforms before joining using brazing or diffusion welding techniques. Due to mismatches between joining surfaces and base material, additional stresses are introduced into the mirror and production methods can only be utilized for applications that don’t demand precision accuracy or structural integrity.

Mersen Boostec has developed an alternative production process capable of mass producing a 4.03 m SiC aspheric mirror blank in one piece. Their new manufacturing technique relies on gel casting technology with water-soluble room temperature vanishing mold and crack-free drying followed by reaction bonding procedure to stitch segments of lightweight green body together into one monolithic blank. This innovative approach overcomes limitations associated with existing ceramic forming and sintering technologies and opens the way to larger, more integrated optics designs.

After being formed into a blank, this product undergoes rigorous tests to ascertain its suitability for conditions in orbit. As can be seen from the graph below, 4.03m SiC aspheric ceramic mirror blank displays outstanding stability under combined loads from both gimbals and vacuum pressures.

The mirror displays excellent axial and radial stability, further attesting its suitability for high-energy laser (HEL) applications. This performance can be directly attributed to its robust silicon carbide structure that has been optimized to withstand space’s unique stresses.

Good Optical Properties

Silicon carbide’s superior strength, rigidity and thermal conductivity make it the ideal material for large telescope mirrors, yet machining their complex surfaces often takes months or even years to produce them. To speed this process up, scientists have come up with an additive manufacturing technology which uses digital models to stack materials layer by layer into physical objects layer by layer – providing personalization, rapidity and economy over traditional ceramic forming and sintering processes.

Mirrors play an instrumental role in the optical performance of telescopes. To ensure optimal results at high laser powers, mirrors must have excellent shape stability and surface quality to avoid distortion, particularly at higher laser power levels. In order to do this, silicon carbide offers lower thermal expansion coefficient than its ceramic counterparts such as fused silica or beryllium; making it the ideal material choice for space conditions where temperatures can become extreme.

SiC mirrors not only possess excellent structural properties, but they also possess exceptional optical characteristics. Their elastic properties rival glass while being more resistant to deformation allowing designers to make thinner mirrors that reduce both weight and cost of telescope systems.

To take full advantage of these qualities, it is critical that optics manufacturers recognize the specific mechanical requirements of large optical systems. For instance, Galvanometer mirror designers must pay special attention to how these parameters influence resonance of the entire system and consider both elastic properties of the mirror itself as well as geometries of all parts involved in resonance testing.

Avantier’s innovative production process can produce customized silicon carbide mirrors to meet the precise requirements of telescopes, scanning systems and space missions. Their production methods utilize precision machining and state-of-the-art metrology equipment to ensure mirrors of only the highest quality are produced. Avantier can manufacture mirrors of various sizes and shapes; additionally they offer various options for optical coatings to meet these requirements.