## 3 Quality Control of Silicon Carbide Printed Part Formation
### Densification Post-Processing Techniques
The phenolic binder employed in BJ 3D printing can provide a certain amount of carbon (approximately 2-3% [147]) to the printed product. However, this carbon content is insufficient to react with approximately 50% of infiltrated silicon. Therefore, carbonization of the porous SiC green body prior to LSI is essential. Common carbonization methods include using carbon powder instead of SiC powder for printing [148; 149], incorporating carbon-containing mixtures into SiC printing powder [150; 151], and applying phenolic polymer impregnation and pyrolysis [152; 153; 154]. However, the aforementioned carbonization methods lack precise carbon content regulation, where excessive carbon may clog pores and impede the smooth infiltration of molten silicon during LSI. As demonstrated in the report by [PERSON] et al. [12] on composites fabricated through multiple phenolic resin impregnation-pyrolysis cycles followed by final silicon reactive melt infiltration, optimal values of density, Young's modulus, flexural strength, and thermal conductivity were achieved after two impregnation-pyrolysis cycles. Beyond two cycles, diminishing returns were observed due to pore blockage by excess carbon, resulting in reaction obstruction that generated residual porosity along with unreacted carbon and residual silicon. Therefore, ensuring equilibrium between carbon content and the reaction with liquid silicon in the carburization process while minimizing the residual silicon content as much as possible is a critical challenge that needs to be addressed in the LSI process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
[PERSON] et al. [155] utilized high-specific-surface-area porous carbon to construct an optimized silicon infiltration channel, significantly enhancing the C-Si reaction and reducing the residual silicon content to 5.2 vol%. However, the study did not report the residual carbon content. [PERSON] et al. [147] designed a phenolic resin binder impregnation (PRBI) process, where each PRBI and pyrolysis cycle contributed approximately 3% carbon, as shown in Figure 17a. With increasing numbers of PRBI cycles, the deposited carbon progressively fills the pores, resulting in significant density enhancement of the printed \"green\" samples (as evidenced in Figure 17b). This clearly demonstrates the remarkable carbon-supplying capability of repeated PRBI treatments for SiC green bodies. The residual liquid silicon content in SiC green bodies exhibited a significant decrease with an increasing number of PRBI carbonization treatments prior to chemical liquid silicon infiltration (CLSI), as clearly demonstrated in Figure 17c. This phenomenon can be mechanistically explained by the significantly increased carbon content within the pores of SiC green bodies following multiple PRBI treatments. During the subsequent CLISI process, this excess carbon chemically reacts with molten silicon to form additional SiC, which consequently reduces the residual silicon content in the resulting SiC samples. Their experimental results indicated that additional PRBI cycles (beyond three) could lead to pore blockage, thereby hindering CLISI and increasing the residual silicon content. To prevent excessive PRBI cycles from resulting in residual liquid silicon, they limited the PRBI cycles to three, obtaining half of the required carbon content. The remaining carbon  
Figure 17: Work of [PERSON]. [PERSON] et al. [147]. **(a)**. Relationship between the total weight fraction of deposited carbon and the number of impregnation cycles. **(b)**. Dependence of “green” sample density on the number of PRBI (polymer reactive binder impregnation) cycles. **(c)**. Microstructural images of SiC samples after varying cycles of PRBI treatment followed by chemical liquid silicon infiltration (CLSI), where dark regions correspond to SiC and bright regions represent residual silicon.