## 3 Quality Control of Silicon Carbide Printed Part Formation
### The Impact and Control of Key Printing Parameters on Green Part Formation Quality
#### 3.1.2 Uniform and Dense Powder Bed
Hydrophilic powders tend to adsorb more binder, forming a uniform wetting layer that promotes bonding between powder particles. However, excessive hydrophilicity may cause over-adsorption of the binder during the printing process, leading to insufficient flowability or excessive expansion, which can result in uneven powder spreading and defects in the new powder layers. In contrast, hydrophobic powders have a weaker ability to adsorb binder, and the binder's penetration capability in the powder layers is poor, resulting in weak bonding between layers. Additionally, binder droplets that remain too long in the powder layer may form \"balling defects\" [83]. Therefore, to reduce surface defects in the powder layer caused by binder-powder interaction, it is necessary to develop appropriate control strategies based on the hydrophilicity of the powder. For example, when using hydrophilic powders, water-based binders containing more hydrophilic groups can be selected [31], while for hydrophobic powders, organic binders with stronger hydrophobic properties can be chosen. Alternatively, surface pretreatment of the powder (such as acid washing or modified coatings [84]) can improve its hydrophilicity, thereby optimizing the interaction with the binder and improving print quality. Research indicates that the particle size of the powder is closely related to the resolution of the formed products [85]. Although traditional equipment can form larger-particle powders (\(>\)200 \(\upmu\)m), the application of fine and ultrafine powders (\(\leq\)120 \(\upmu\)m) is more common in practice to avoid secondary processing of high-strength SiC products and to meet the urgent demand for high-precision, high-quality complex structural SiC ceramic materials in critical fields such as national defense and industry. However, finer powders do not necessarily lead to better printing outcomes. Excessively fine powders may cause flowability issues that affect printing accuracy. Research has shown that fine powders have low flowability, making it difficult to achieve uniform spreading, which can create pores during the printing process and adversely impact structural density and dimensional accuracy [86]. [PERSON] [87] defined the concept of the number of viscous particle bonds, categorizing powders into viscous and non-viscous types. This classification helps explain the phenomenon that, as particle size decreases, the number of viscous particle bonds increases, leading to a higher tendency for powder agglomeration, which results in poor powder flowability [88]. This cohesive powder tends to form agglomerates within the powder bed, which can induce defects and heterogeneous pore structures, consequently compromising the uniformity and compactness of the powder bed. Therefore, improving the flowability of fine SiC powders to minimize particle agglomeration during spreading represents a critical measure for achieving uniform and dense powder beds. Research on non-cohesive non-spherical powders indicates that the ratio of bulk density to tapped density decreases with increasing sphericity [89]. Powders with a Hausner ratio of less than or equal to 1.25 are considered freely flowing, while the Hausner ratio of cohesive and non-flowing powders exceeds 1.40 [87], which indicates that, the closer the shape of the powder particles approaches sphericity, the better the flowability of the powder. Based on this theory, researchers including [PERSON] [45] developed SiC fiber-reinforced SiC composites using BJ AM combined with a polymer infiltration and pyrolysis process. Spherical SiC powders were prepared using ball milling, spray drying, and thermal plasma treatment. Characterization of the powders shows that, compared to irregularly shaped SiC powders, the use of spherical SiC powders results in higher fracture toughness and hardness.