Due to the outstanding thermal-mechanical properties and the high resistance to radiation, abrasion, and corrosion, SiC ceramics can be ranked as the optimal materials for the manufacture of the optics and the precision structures for space/ground-based advanced opto-mechanical systems. They fulfill the increasing demands of aperture enlargement, weight budget reduction, thermomechanical management simplification, and long-term stability. During the past three decades, ESA, NASA, JAXA, CASC, China Academy of Sciences, and so forth have been making great efforts to develop SiC components for remote sensors and telescopes for civilian and military applications at the cutting edge of the new generation optomechanical system development. The material preparation technologies and the relevant fabrication technologies, which determine the performance of the SiC components, the modules, and even the whole system, are the focus of the investigation and study.

The major concerns of the great efforts paid to the SiC preparation technologies are the accomplishment of optical surface density, the homogeneity, and the isotropy of the SiC blanks, which are essential for the optomechanical application, as well as the improvement of the thermomechanical properties such as specific modulus and thermal stability, and the manufacturability of the large-scale structural complexities.

Among various SiC preparing technologies presented in Fig. 1, the densification methods of pressureless sintering, the reaction sintering/bonding and the chemical vapor composition/converting (CVC), combining the suited forming techniques for preforms, are proven to be effective for the SiC optics and precision structures. The pressureless sintered SiC possesses relatively better mechanical performance and homogeneity. It has presented isotropy, thermostability, and machinability during the development and in-orbit services of Herschel Space Observatory's (2009) Φ3.5 m primary mirror, GAIA (2014) and Euclid's (2023) all-SiC optomechanical structures. The maximum sizes of monolithic pressureless sintered SiC (S-SiC) optics reported reach 1.7 m×1.2 m (BOOSTEC) and Φ1.5 m (Shanghai Institute of Ceramics, China Academy of Sciences). However, further enlargement encounters the difficulties of the large-size high-temperature sintering equipment construction, the high sintering shrinkage, and the resulting ununiform deformation and stress that might cause cracking. CVD or PVD cladding on the S-SiC surface is necessary for optical polishing due to the residual micropores. Typical reaction sintered/bonded SiC (RB-SiC) comprises SiC, free Si, and residual C that is detrimental to the materials. The results show that reaction sintering/bonding densification methods are suitable for various ceramic forming techniques and the shrinkages of the whole process can be kept lower than 1%. The sintering temperatures are as low as the melting point of Si and the homogeneous bonding of parts is practicable, which are the essential processes to realize the reported largest Φ4.03 m SiC mirror (Changchun Institute of Optics, Fine Mechanics and Physics, China Academy of Sciences, 2016), except for lower Young's modulus than that of single-phase SiC ceramics. The results of the previous study also demonstrated the effectiveness of the microstructure refining and the phase composition regulation on the improvement of the RB-SiC performance and optical manufacturability. SiC via chemical vapor composition/converting (CVC SiC) is a single-phase ceramic with high purity. Trex Enterprise developed the co-deposition of micro SiC powder and precursor derived SiC onto the mold along with the densification process. POCO company adapted the pure porous graphite as preforms, and the vapor SiO and Si as infiltration matters, which would react with the graphite to convert into beta SiC meanwhile promoting densification. The introduction of the heterogeneous nucleation cores of the micro SiC powders or the graphite surface increases the rate of the crystal growth via the vapor phase by 10 times more than that of the CVD process and helps to overcome the heterogeneity of the materials due to the columnar crystal growth and to reduce the stress between the interface of the sequential solidified phases, which enables the fabrication of 1.5 m class CVC SiC mirror blank. The properties of Trex's CVC SiC are as excellent as pure full-dense SiC ceramic and facilitate the direction polishing without additional surface modification for optical surface finishing. However, the deposit efficiency and the capability of the complex component fabrication are yet the bottleneck of the promotion of the CVC techniques.

As another determining factor for the performance of the SiC components, the improved structural configurations, such as topology-optimized structures (Fig. 12) and structures with the integrated cooling medium channels, exceed the capability of conventional technologies. Additive manufacturing (AM) or 3D printing techniques enable the free-form components manufacture. According to Goodman's investigation, based on the AM or 3D printing techniques, the weight reduction of the SiC optics comes up to 39% for 1-2.5 m class SiC mirrors for FIR application compared to JWST, and up to 40% of cost reduction. Investigation results show that additive manufacturing shaping combined with reaction sintering densifying is optimal for the preparation of the SiC materials for optics and precision structures. Binder jet printing, stereolithography/digital light processing, fused deposition modeling, and selective laser sintering are promising candidate methods for SiC or SiC-C preform forming. However, the problems of the lower performance compared to the materials via conventional methods, the heterogeneities of the materials, and the difficulties in non-uniform deformation control during the debonding and the reaction sintering are yet to be resolved.

The joint of SiC parts favors the large-scale optomechanical system construction less costly and risky. As a typical case of all-SiC structure, the Euclid payload demonstrated the bolt joint, epoxy bonding, ceramic bonding, and brazing of the pressureless sintered SiC parts (Figs. 17-18). The rigidity of the brazing joined components or the structural frames is more promising than that of the first two, though the bolt joint and the epoxy bonding might be realized at room temperature in a normal atmosphere and applicable for SiC and other materials. However, brazing will inevitably introduce residual stress due to the thermal mismatching of the base materials and the fillers, and due to the volume changing of the fillers during solidification. The residual stress cannot be eliminated through the post process, hence increasing the uncertainty for the dimensional stability of the precision structures. Reaction bonding techniques facilitate the homogeneous joint through Si-C reaction, which can be carried out simultaneously with the reaction sintering process and avoid the residual stress. The microstructure of the joining area can be tailored to be identical to the parent RB-SiC parts.

The advantages of the SiC materials are expected to extend to the manufacture and applications of space/ground-based large aperture photoelectric imaging systems, short wave optics for ultraviolet to soft X-ray, high power laser optics, and other precision structures such as key components in semiconductor equipment. The merits brought about include the system rigidity and the weight lessening, and the improvement of the system sensitivity and reliability, thanks to the high specific stiffness, excellent thermal stability, high resistance to abrasion, and corrosion of the SiC ceramics.

The pressureless sintered SiC, reaction sintered SiC, and CVC SiC ceramics exhibit advantages in the optomechanical system manufacture due to their thermal mechanical comprehensive properties. To further promote the application of silicon carbide in precision engineering, it is necessary to develop new fabrication methods such as additive manufacture of SiC ceramics, and advanced SiC joint technologies for the innovative structural forms within an acceptable cost space. The improvements of the material microstructures and the properties from micro to macro scale via technical breakthrough are needed in advanced material forming, densification sintering, connection technologies, and applied technologies.