A new additive manufacturing route creates carbon fiber silicon carbide mirrors that unite low weight, strength, and sub-nanometer smoothness, reaching 97 percent visible reflectance for space optics.
(Nanowerk Spotlight) Building a telescope is an act of precision balanced against weight. Every kilogram that leaves Earth must justify its place on a rocket, and every surface that gathers light must hold its exact shape through violent launch, temperature swings, and years of vacuum exposure. The mirrors that do this work are often extraordinary accomplishments of materials science. They must be stiff but light, smooth enough to reflect light without scattering, and strong enough to survive the journey into orbit.
Among the materials considered for this purpose, silicon carbide stands out. It is a ceramic that barely expands with heat, stays rigid at high temperatures, and resists wear. Yet its very strengths make it hard to shape. It is brittle, difficult to machine, and unforgiving when polished.
Traditional manufacturing can produce silicon carbide optics of high quality, but at a cost of long processing times and strict limits on shape and size. Additive manufacturing, or 3D printing, offers freedom to design lighter, more complex structures, such as thin shells and lattice cores that reduce mass while preserving stiffness.
The problem is that ceramics do not behave like plastics or metals when printed. They require high sintering temperatures and shrink unevenly, often cracking before a solid object forms. Even when a shape can be printed, polishing it to optical quality is another barrier. Uneven surfaces, residual silicon, and microstructural mismatches scatter light and limit performance. The challenge has been to link the geometric flexibility of printing with the optical perfection needed for precision mirrors.
A study in Advanced Science (“From Complex Shaping to Mirror Finish: Additive Manufacturing of Aerospace‐grade Cf/SiC Space Optics”) presents a method that closes this gap. The research describes a way to manufacture carbon fiber reinforced silicon carbide (Cf/SiC) into mirror-grade components through a single, connected process. By combining 3D printing, chemical infiltration, and thin-film coating, the team demonstrates complex lightweight structures that can be polished to sub-nanometer smoothness while maintaining strength and thermal stability.
The work matters because it brings together mechanical, chemical, and optical engineering in one coherent chain, turning what were once separate steps into a unified pathway from digital model to reflective surface.
Schematic illustration of the integrated fabrication process for optical-grade Cf/SiC composites. The process includes selective laser sintering (SLS) of composites, phenolic resin (PR) infiltration and curing, debonding, reactivemelt infiltration (RMI) with silicon particles, and surface modification via physical vapor deposition (PVD) to obtain the final optical component. (Image: Reproduced from DOI:10.1002/advs.202517980, CC BY) (click on image to enlarge)
The sequence begins with selective laser sintering, a 3D printing technique that fuses powder layer by layer. Because pure silicon carbide powder does not melt easily, the researchers mix SiC particles with phenolic resin and short carbon fibers. The resin binds the material during printing and burns away later, leaving a porous preform. This preform then undergoes polymer infiltration and pyrolysis (PIP). In this step, a liquid phenolic resin fills the pores and coats the surfaces. When heated, it decomposes into a thin layer of carbon that forms a controlled interface between the fibers and the ceramic matrix. This carbon layer improves toughness and provides a precise source of carbon for later reactions.
Next comes liquid silicon infiltration (LSI). Molten silicon is drawn into the porous body and reacts with the carbon to form new silicon carbide, sealing the structure and making it dense. The composite then receives a thin silicon film and a reflective silver layer through physical vapor deposition (PVD). These coatings create a uniform, polishable surface. The outcome is a material that combines the freedom of 3D printing with the surface quality required for precision optics.
A key parameter in this chain is carbon density, meaning the amount of carbon per unit volume before silicon infiltration. This value controls how fully the molten silicon can penetrate and react. Too little carbon leaves behind soft pockets of unreacted silicon that polish unevenly. Too much carbon blocks the flow of silicon and traps unreacted regions.
The researchers fine-tune carbon density by adjusting the phenolic resin concentration during PIP, keeping it slightly below a critical value. Microscopic analysis shows that higher phenolic content reduces residual silicon, increases newly formed silicon carbide, and lowers porosity. The resulting matrix is stronger and more uniform.
Mechanical testing confirms these changes. Flexural strength reaches 311 megapascals at 40 wt.% phenolic resin, about 24 percent higher than untreated samples. Fracture toughness rises to 4.54 MPa·m¹ᐟ² at 50 wt.%. These improvements come from stronger bonding between fibers and matrix and from a fine layer of beta silicon carbide at the interface, which helps transfer stress without brittle failure. At the highest resin levels, closed pores slightly reduce strength, matching the predicted trade-off between densification and residual voids.
Thermal measurements show that the material remains stable under large temperature swings. The coefficient of thermal expansion ranges from 2.2 to 4.6 × 10⁻⁶ per kelvin between 100 and 900 °C, low enough to prevent optical distortion. Thermal conductivity peaks at about 106 W m⁻¹ K⁻¹ at 30 wt.% phenolic, then decreases at higher concentrations as pores interrupt heat flow. The best combination of strength, toughness, and thermal behavior appears between 30 and 40 wt.% phenolic, where the structure is most continuous and dense.
Optical finishing is often the most demanding step in mirror fabrication. Polishing a surface made of both hard and soft materials can create microscopic unevenness that scatters light. The researchers address this by depositing a dense silicon layer before applying silver.
The silicon smooths local hardness differences and forms a compatible base for the reflective film. Atomic force microscopy shows that surface roughness drops to 0.44 and 0.32 nanometers for untreated and treated samples after silicon deposition. Final polishing reduces it further to 0.031 nanometers, which is essentially atomically smooth.
Reflectance averages 97 percent across the visible spectrum, matching the performance of high-end optical mirrors. Silver is chosen for its low absorption in visible light, and the silicon underlayer helps balance thermal expansion and prevents diffusion between the metal and ceramic.
Complex geometries such as hollow cubes and lattice panels retain their shape through every stage of processing. Open porosity remains below one percent, and the composite density stabilizes at about 2.88 grams per cubic centimeter. These numbers indicate a material that is light but mechanically stable enough for large optical assemblies. Interferometric tests on finished surfaces show uniform figure quality, confirming that structural stiffness and surface finish can coexist in one component.
The study provides a framework for producing lightweight, high-performance ceramic optics. Each stage addresses a distinct bottleneck: printing defines shape, carbon control guides reaction, silicon infiltration provides strength, and thin-film coating achieves reflectivity. Process parameters can be adjusted to emphasize stiffness, conductivity, or polishability. The result is a material that meets structural, thermal, and optical demands in a single integrated sequence.
Beyond mirrors, this approach can extend to other ceramic systems that require both mechanical resilience and precise surfaces. Examples include heat exchangers, high-power laser components, and protective windows for aerospace sensors. By managing interfaces and reactions from the microscopic level upward, the method demonstrates how additive manufacturing can evolve from shaping forms to engineering surfaces with near-atomic precision. This work points toward a future where printing, chemistry, and optics operate as one system rather than separate disciplines.
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Jie Yin (Shanghai Institute of Ceramics, Chinese Academy of Sciences)
, 0000-0003-3472-7083 corresponding author
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