HUI Pengxian ^{1, 2}, JI Haohao ¹, LI Xinyu ^{1, 2}, GE Ye ¹, ZHANG Jian ^{1, 2}, WANG Shiwei ^{1 ,2}

(1. Shanghai Institute of Ceramics, Chinese Academy of Sciences, State Key Laboratory of High Performance Ceramics, Shanghai 200050, China; 2. Center of Materials Science and Optoelectronics Engineering, University of

Chinese Academy of Sciences, Beijing 100049, China)

Extended abstract:

[Background and purposes] As advanced optical systems evolve toward lightweight, high resolution and high stability, cordierite ceramics have become ideal candidates for high-dimensional-stability precision optical components, due to their extremely low coefficient of thermal expansion (CTE) and excellent specific stiffness. However, traditional forming processes (such as dry pressing and isostatic pressing) struggle to achieve integrated forming of complex structures and face issues like strong mold dependency and large machining allowances. Additive manufacturing offers a new path for the near-net shaping of complex components, while screw-extrusion-based Fused Deposition Modeling (FDM) shows great potential in fabricating high-density complex-structured ceramics by directly processing feedstocks. The key to screw-extrusion FDM lies in obtaining feedstocks with excellent extrusion stability, where the backbone binder is the critical component governing the melt rheological behavior and shape-retention capability. This study was aimed to focuse on the screw-extrusion FDM process for cordierite ceramics. Based on examining the defects of single-component backbone binders, a multi-component composite backbone system (HDPE, LDPE-g-MAH, and EVA) was constructed. The regulatory effects of these components on processing adaptability and forming quality were systematically studied to facilitate the fabrication of honeycomb-structured lightweight mirrors.

[Methods] Cordierite powder was used with a solid loading of 75 wt.% (D₅₀=2.2 μm) and an organic binder content of 25 wt.%, within which the backbone binder accounted for 9.75 wt.%. The experiments were conducted for single-component backbone systems (HDPE, LDPE-g-MAH, and EVA) with four groups of composite backbone systems featuring different ratios (Groups 1–4). Feedstocks were mixed at 170 ℃ and 40 r·min⁻¹ for 60 min, then crushed into 4–8 mesh granules for printing. Printing parameters were set as follows: extrusion temperature 155 ℃, nozzle diameter 1.0 mm, layer thickness 0.22 mm and printing speed 25 mm·s⁻¹. The resulting green bodies underwent solvent debinding in n-heptane at 50 ℃ for 12 h, thermal debinding up to 600 ℃ and sintering at 1420 ℃ for 2 h. The sintered samples were further treated with Hot Isostatic Pressing (HIP) at 1380 ℃ and 180 MPa. Rheological behavior was tested using a high-pressure capillary rheometer, density was measured via the Archimedes drainage method and the surface figure accuracy and roughness of the mirrors were characterized by using a laser interferometer and a white-light interferometric surface profiler.

[Results] The HDPE system had poor interfacial compatibility with powdered feedstock, the LDPE-g-MAH system led to green bodies with obvious surface cracks and the EVA system could not be extruded under the same conditions. The composite backbone systems exhibited clear pseudoplastic rheological characteristics, with flow indices (n) ranging from 0.43 to 0.46. Group 3 (HDPE/LDPE-g-MAH/EVA mass ratio of 50:33:17) had a consistency coefficient (K) of 7244 Pa·sⁿ, while the introduction of EVA effectively reduced the flow resistance of the system. t Group 3 possessed the highest forming stability, with clear, uniform and undeformed deposited lines. The printing density of Group 3 green bodies reached 99.1%, with a flexural strength of 9.2 MPa. After sintering at 1420 ℃, the sample density was 2.44 g·cm⁻³, which was increased to 2.50 g·cm⁻³ after HIP treatment; the Young's modulus was about 125 GPa and the CTE at 22 ℃ was −0.2×10⁻⁶/K. Using the formulation of Group 3, a honeycomb-structured mirror with a diameter of about 96 mm and a weight of 82 g was successfully fabricated, achieving a lightweight ratio of 55.1%. After polishing, the surface figure accuracy (RMS) reached 0.023 λ and the surface roughness (Sa) was 0.66 nm, meeting the requirements for precision optical applications.

[Conclusions] It is confirmed that the rational design of backbone binder components can effectively improve the forming quality of highly loaded cordierite feedstocks. The composite system consisting of HDPE, LDPE-g-MAH and EVA leverages the advantages of each component. LDPE-g-MAH improves interfacial compatibility, EVA regulates flow behavior and HDPE enhances shape retention. The optimized formulation of Group 3 achieved stable printing of green bodies with high density (99.1%) and high strength (9.2 MPa). The final honeycomb lightweight mirror, after HIP treatment and precision polishing, exhibited excellent densification, surface figure accuracy (0.023 λ) and nanometer-scale roughness (0.66 nm). This technological route not only realizes the near-net shaping of lightweight cordierite ceramics but also meets the requirements for ultra-precision optical processing, verifying its application potential in the field of space optics.

Key words: cordierite ceramics; fused deposition modeling; backbone binder; lightweight mirror