Silicon Carbide Mirror

Silicon carbide (SiC) is an exceptionally lightweight yet strong material commonly found in space telescope mirrors. In this article we’ll uncover its fascinating science!

Manufacturing large optical class SiC aspheric mirrors presents several key challenges, including casting complexity, accuracy and efficiency of aspheric fabrication, print-through effects, precision measurements, stress and denseness issues with the cladding process, self-weight deflection decoupling issues and self-weight deflection decoupling issues. CIOMP created a processing chain to deterministically fabricate a 4.03m RB-SiC mirror.

Lightweight

Silicon Carbide (SiC) makes an excellent material for lightweight mirrors used in high-speed scanning systems, helping them reach faster scan rates while increasing productivity. Thanks to its superior strength and thermal stability, SiC mirrors are ideal for dynamic optomechanical systems due to their lightweight nature; plus their optical transparency and thermal conductivity allow light to pass easily through them – ideal features when used for laser scanners, optical fiber communications systems or any other application requiring lightweight mirrors.

Polycrystalline and amorphous SiC are two primary varieties. Polycrystalline is more frequently produced due to its lower production costs; however, amorphous silicon carbide offers greater ductility and flexibility in lightweight mirror applications due to its more ductile nature than polycrystalline. No matter which form is chosen for production, however; all techniques must ensure optimal quality such as reaction bonding and chemical vapor deposition.

Reaction bonding involves creating a mixture of alpha and beta silicon carbides with phenolic resin for infiltration by liquid silicon. Once this reaction bonding process has taken place, dense SiC structures are produced, ideal for space telescopes or any application where large and complex designs must be lightweight yet resistant to environmental conditions such as environmental degradation. Reaction bonding has proven itself particularly adept at producing lightweight designs in large volumes that resist oxidation and chemical degradation – ideal for producing lightweight mirrors used in space applications or space telescopes.

Utilising this technique, a lightweight 2 m SiC mirror with lotus back structure was produced, featuring an areal density of 38 kg/m2. This weight reduction was over 50% lighter than passive Zerodur glass-ceramics of comparable size while maintaining RMS surface figure error and mass less than 6nm. Modal tests on its assembly confirmed it met specifications.

Researchers evaluated the structural characteristics of a 2 m mirror to optimize its design using topology optimization and parameter optimization techniques. They defined its support positions (axial and radial), their effects on structural parameters, as well as sensitivity analyses of major and non-major structural parameters with regards to surface figure error RMS/mass ratio and mass, to find optimal dimensions. This approach helped determine optimal dimensions.

High Stiffness

Silicon carbide may not immediately come to mind as an optical material, but after years of work refining manufacturing techniques it has now become an optical grade mirror material that’s ideal for laser scanning systems. Thanks to this advancement it can now produce mirrors at much larger sizes than was ever previously possible – an advantage in high performance large telescopes or laser scanning systems where weight and stiffness of mirror are crucial components.

Material has excellent tensile strength and stiffness, making it a superior choice for space-based instruments’ lightweight mirror substrates. Furthermore, its thermal properties and non-toxicity make it suitable as replacements for beryllium mirrors in high-speed laser scanning systems without impacting dynamic performance.

Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) has unveiled an entire processing chain capable of manufacturing large SiC mirrors – and has already demonstrated it can produce a 4.03m aspheric mirror. Involvement includes reaction-bonding for creating mirror blanks and CNC generating of an aspherical surface and stress lap grinding/polishing to achieve high error convergence rates, MRF polishing with CCOS surface figure correction technology and low temperature PVD Si cladding on its surface. SAP testing to guide mirror figuring covers an extensive mm-to-nm range and is combined with CGH null interferometry data fusion techniques in order to disengage air turbulence from fabrication residuals and make accurate measurements.

Boostec’s expertise lies in machining SiC for structural components and fabricating entire instruments from this material, as well as developing bonding and bolting techniques to assemble SiC parts with other materials such as metals or glass-ceramics. In addition, its research and development efforts focus heavily on improving processes and quality products – ultimately their goal being high-quality optics that meet future application demands – they have already delivered prototype instruments such as solar sail and cell demonstrator which should launch into space by 2022.

High Thermal Stability

Silicon carbide boasts exceptional thermal deformation resistance, making it an excellent material for large mirrors in harsh environments and at elevated temperatures. Furthermore, its low thermal expansion reduces risk of surface damage during polishing or use – an attribute crucial for aerospace, semiconductor lithography and astronomy applications.

Hardness and strength properties help the SiC mirror’s stability; these properties make it resistant to vibrations and environmental stresses such as air turbulence. As a result, its optical surface remains stable with high tolerances, while being easy to work with means that polishing times are much quicker compared to other materials.

SiC mirrors boast high thermal conductivity and low coefficient of expansion, making them suitable replacements for traditional metals and glass-ceramics in high-performance laser scanning systems that demand dynamic performance. SiC can offer higher stiffness and lower weight than beryllium mirrors, enabling designers to increase system speed without compromising optical performance.

SiC can easily be polished to achieve optical quality that exceeds that of beryllium, making it suitable for creating ultralightweight optics with precision surfaces suitable for many different applications.

Manufacturers seeking to ensure the quality of SiC mirrors must test them under varied conditions in order to guarantee its quality, requiring an automated system which measures lateral surface deformation and spectral response from UV to IR wavelengths, compensating for mechanical vibrations and air turbulence, while simultaneously detecting any fabrication errors.

Changchun Institute of Optics, Fine Mechanics and Physics of Chinese Academy of Sciences has taken steps to overcome these difficulties by creating a complete processing chain for 4 m class SiC mirrors using reaction bonding method preparation, aspherical surface computer numerical control generation, CCOS polishing and ion source-assisted electron beam cladding.

High Durability

Silicon carbide’s durability makes it an excellent material choice for optical mirrors. Able to withstand harsh environmental conditions like temperature extremes and vibration, SiC offers superior strength and stiffness over glass-ceramic substrate materials like glass mirrors. Furthermore, its 3D formability facilitates integrated optomechanical systems as well as ultralightweight mirrors fabricated using SiC. Furthermore, its polycrystalline structure facilitates efficient polishing for precision optics resulting in higher precision optics than its beryllium-based alternatives without compromise in dynamic performance – something neither glass nor glass-ceramic could achieve.

However, traditional production options for SiC ceramics restrict large aperture mirror fabrication due to machining limitations and shrinkage issues in both green state and after sintering. To address these challenges, POCO Graphite Inc. and Zygo Corporation created a novel fabrication method that utilizes deterministic manufacturing with high precision test data such as polishing, null interferometry and phase deflectometry testing data derived through generation, polishing and phase deflectometry testing of composite mirror blanks produced from RB SiC as well as an advanced optical processing technique which enable rapid production of high performance optical systems.

Key components of this process are sintering and joining of the composite. Sintering uses a re-melting reaction of carbon binder with composite to produce SiC product with identical properties to body substrate, eliminating thermal mismatch at joining seams while producing high dimensional stability and mechanical strength composite.

At present, the sintering process is used to produce monolithic SiC mirrors up to 2m in diameter; however, its high shrinkage ratio and risk of light weighting restrict their use to far-infrared and submillimeter band telescopes only. Furthermore, brazing techniques may add unnecessary stresses onto clad surfaces which might reduce performance of these mirrors.

To overcome these limitations, the 4.03 m class mirror was structurally broken up into twelve preforms. A proper joining technique is essential to creating a reliable and robust system, as the quality of joining surfaces determines how much shrinkage occurs; unfortunately, current joining methods require the pre-clad surface be precisely machined which limits their use to low accuracy applications only.