Breakthrough in Photopolymerization 3D-Printed Ceramic Core Technology

As the demand for precision casting of complex internal structures in turbine blades continues to rise, 3D printing has demonstrated unique advantages in the fabrication of ceramic cores. However, challenges such as weak interlayer bonding, mechanical anisotropy, and high sintering shrinkage have long hindered the industrial application of this technology.

Recently, Professor Xu Xuqing’s research team from Chang’an University, in collaboration with the Beijing Institute of Aeronautical Materials, published a study in Ceramics International titled "In-situ grown mullite whiskers zippering the printing layers in silica-based ceramic cores through vat photopolymerization 3D printing." 

This research presents a new method. It uses in-situ grown mullite whiskers to "zipper" print layers together. This approach improves both the strength and size accuracy of ceramic cores.

In their study, the team incorporated aluminum fluoride (AlF₃) as a precursor into a photopolymerization-based 3D-printed silica ceramic slurry. During sintering at 1200°C, a gas-solid reaction facilitated the in-situ growth of mullite (3Al₂O₃·2SiO₂) whiskers.This reaction mechanism allows AlF₃ and SiO₂ to move in vapor form. This process helps form mullite nuclei. These nuclei grow mainly along the [001] direction. They turn into rod-like single crystals that are about 500 nm wide, as shown in Figure 2. At an AlF₃ content of 9 wt%, the whiskers exhibit an optimal morphology and distribution (Figure 3d), effectively bridging adjacent printing layers like a "molecular zipper" (Figure 4d) and mitigating the layered structural defects inherent to traditional 3D-printed components.

Fig. 1. Schematic diagram of the in-situ growth of mullite whiskers.

Fig. 2. (a, b) TEM image of in situ mullite fibers; (c) SAED image of in-situ mullite whisker; (d) transmission electron microscope (TEM) image showing the elements of (e) Si (f) Al (g) O.

Fig. 3. SEM images of sintered ceramic core with(d) 9 wt%

Fig. 4. SEM images along the print layer of sintered ceramic core (c, d) with 9 wt% AlF3

2. Experimental Design and Methodology

2.1 Slurry Preparation and Printing Parameters

The research team utilized photopolymerization 3D printing technology to fabricate silica-based ceramic cores, with the slurry formulation detailed in Table 1. The base materials consisted of fused silica (D50 = 6 μm, 85 wt%), mullite (20 μm, 5 wt%), and corundum (16 μm, 10 wt%), which were uniformly dispersed using a triaxial motion mixer.

The photosensitive resin system comprised polyurethane acrylate (PUA) prepolymer, reactive diluents (TMPTA/HDDA/IBOA), and the photoinitiator TPO-L (Table 2).  To help grow mullite whiskers, we added aluminum fluoride (AlF₃, 0–12 wt%) as a precursor. Vanadium pentoxide (V₂O₅, 2 wt%) acted as a fluxing agent. After ball milling for six hours, we got a uniform slurry with a solid content of 56 vol% (Figure 5).

Table 1. Detailed information of the raw materials.

Table 2. Detailed information of the photosensitive resin.

Fig. 5. XRD pattern of raw materials: (a) Fused silica; (b) Al2O3; (c) Mullite; (d) AlF3; (e) V2O5.

2.2 Vat Photopolymerization Process

The fabrication process was carried out using a digital light processing (DLP) photopolymerization 3D printer (Autocera-M), with a single-layer exposure time of 3 seconds (25 mW/cm²) and a layer thickness of 100 μm. After printing, the green bodies were ultrasonically cleaned in alcohol. 

Debinding was performed in a stepwise manner at 262°C, 365°C, and 505°C for 1 hour at each stage, followed by sintering at 1200°C for 6 hours with a heating rate of 2°C/min. To simulate casting conditions, the sintered samples were further treated at 1540°C for 0.5 hours (Figure 6).

Fig6.Schematic diagram of mechanical properties test direction, the loading direction is parallel to the printed layer to evaluate the interlayer bonding strength

3. Results and Discussion

3.1 Phase Evolution and Whisker Growth Mechanism

XRD analysis (Figure 7) shows that with increasing AlF₃ content, the intensity of the characteristic peaks of mullite (PDF#15-0776) significantly increases, indicating that AlF₃ promotes whisker formation through a gas-solid reaction. In the presence of the catalyst V₂O₅, AlF₃ reacts with SiO₂ to generate gaseous AlOF and SiF₄, which subsequently form mullite nuclei in an oxidizing atmosphere and grow preferentially along the [001] direction.

TEM analysis confirms that the whiskers exhibit a single-crystal structure, with an interplanar spacing of 0.2802 nm, matching the (001) crystal plane of mullite.

Fig7.The effect of AlF₃ addition on the phase composition. The peak strength of mullite increases with the increase of AlF₃.

3.2 Interlayer Zippering Effect of Mullite Whiskers

SEM analysis (Figures 3 and 4) reveals the mechanism by which AlF₃ content influences the interlayer structure. Without AlF₃ addition, noticeable cracks appear between printed layers (Figures 4a–b). However, with 9 wt% AlF₃, the in-situ grown mullite whiskers act as a "molecular zipper," bridging adjacent layers (Figure 4d) and effectively mitigating the inherent anisotropy of traditional 3D-printed structures (Figure 8). The whiskers, approximately 500 nm in diameter, exhibit a uniform radial distribution, forming a three-dimensional reinforcement network.

Fig.4. (a-b) Interlaminar cracks are obvious when AlF₃ is not added; (c-d) Whisker suture interlaminar defects in 9wt% AlF₃ samples

Fig. 8. Diagrammatic sketch of in situ grown whisker effecting the interlayer structure with different AlF3 contents: (a) 0 wt%, (b) 3 wt%, (c) 6 wt%, (d) 9 wt%, (e) 12 wt%.

3.3 Mechanical and Dimensional Performance Optimization

Three-point bending tests (Figure 9) demonstrate that the sample with 9 wt% AlF₃ achieves a room-temperature strength of 14.4 MPa, marking a 69% improvement over the baseline group.

Fig. 9.The influence of AlF₃ content on flexural strength reaches a peak value of 14.4MPa at 9wt%.

After high-temperature casting, the strength further increases to 22.3 MPa (Figure 10b), attributed to the mullite whisker framework, which effectively suppresses the viscous flow of the silica matrix.

Fig. 10. Influence of aluminum fluoride on the (a) Shrinkage of ceramic cores in three directions after casting at 1540 °C; (b) Bending strength of ceramic cores after casting at 1540 °C

Simultaneously, the mullite whisker network impedes the densification process, reducing the sintering linear shrinkage by 73% (Figure 11a) and increasing the apparent porosity to 39.5% (Figure 11c).

Fig. 11. (a) Shrinkage of ceramic cores in three directions after sintering at 1200 °C; (b) Viscosity of the ceramic slurry and roughness; (c) Apparent porosity, water absorption, and bulk density.

Mercury porosimetry analysis (Figure 12) reveals that the whiskers modify the pore size distribution from the original 1–10 μm range to a bimodal structure (2.8/7.9 μm), optimizing permeability and leaching performance (Figure 10d).

Fig.12. Influence of aluminum fluoride content on the pore distribution of the sintered ceramic core.

4. Conclusion and Perspectives

As the demand for precision casting of complex internal structures in turbine blades continues to rise, 3D printing has demonstrated unique advantages in the fabrication of ceramic cores. However, challenges such as weak interlayer bonding, mechanical anisotropy, and high sintering shrinkage have long hindered the industrial application of this technology.

Recently, Professor Xu Xuqing’s research team from Chang’an University, in collaboration with the Beijing Institute of Aeronautical Materials, published a study in Ceramics International titled "In-situ grown mullite whiskers zippering the printing layers in silica-based ceramic cores through vat photopolymerization 3D printing." 

This research presents a new method. It uses in-situ grown mullite whiskers to "zipper" print layers together. This approach improves both the strength and size accuracy of ceramic cores.