Silicon carbide (SiC) is a synthetic covalent compound and a new type of engineering ceramic material. Due to its outstanding properties—including high-temperature strength, strong oxidation resistance, excellent wear resistance, thermal stability, low thermal expansion coefficient, high thermal conductivity, high hardness, thermal shock resistance, and chemical corrosion resistance—SiC ceramics are widely used in aerospace, electronics, and chemical industries. Moreover, SiC ceramics are considered highly promising for high-temperature structural components, advanced engines, heat exchangers, and high-strength wear-resistant devices, attracting significant attention from researchers worldwide.

Why Surface Modification of SiC Powders is Necessary

During the ultrafine grinding of nanoscale SiC powders, the particles experience continuous friction and impact. This process causes the accumulation of large amounts of positive and negative charges on the particle surfaces, making them highly unstable and prone to aggregation. At the same time, the powders absorb substantial mechanical and thermal energy, increasing their surface energy. To achieve a more stable state and reduce surface energy, the particles naturally tend to attract and cluster together, forming aggregates.

Surface modification is an effective way to improve the dispersibility and flowability of SiC powders, prevent aggregation, enhance the forming properties of ultrafine SiC powders, and improve the performance of the final ceramic products.

Mechanisms of Surface Modification

Surface modification of ultrafine powders involves the interaction between the powder surface and the modifying agent. This improves the wettability of the particles, enhances their compatibility with the surrounding medium, and facilitates dispersion in water or organic compounds. Modifying agents must contain functional groups that can interact effectively with the particle surface.

There are two main mechanisms:

1. Coating Modification: A layer of inorganic or organic compounds (water-soluble or oil-soluble polymers, fatty acid soaps, etc.) covers the particle surface, creating steric hindrance that prevents re-aggregation.

2. Coupling (Chemical) Modification: Chemical reactions or coupling interactions occur between the particle surface and the modifying agent. In addition to van der Waals forces, hydrogen bonding, or coordination interactions, ionic or covalent bonds may form, leading to stronger and more stable surface modification.

Common Surface Modification Methods

1. Coating Modification

Coating modification involves physically or chemically attaching a layer of modifying agent to the particle surface to change its inherent properties. Common agents include surfactants, super-dispersants, and inorganic compounds.

• Surface Adsorption Coating: Uses physical or chemical adsorption to form a continuous coating on the particle surface. This method is simple but has limited effectiveness.

• Inorganic Coating: Involves using inorganic materials that physically adhere to the particle surface, reducing surface free energy and preventing aggregation. Techniques include chemical plating, electroplating, vapor deposition, sol-gel coating, radiation, and mechanical coating.

2. Chemical Modification

Chemical modification involves a chemical reaction or adsorption between the modifying agent and the particle surface. Long-chain polymers grafted onto the powder surface can contain hydrophilic groups to improve dispersion stability in a medium. Common chemical modifiers include coupling agents, fatty acids and their salts, unsaturated organic acids, and organosilicons.

Effects of Surface Modification on Powder Properties

1. pH Influence: Surface modification can optimize dispersibility at specific pH levels, which is crucial for preparing high-solid-content ceramic slurries with uniform particle distribution.

2. Surface Properties: Powder characteristics such as surface area, surface energy, chemical composition, crystal structure, functional groups, wettability, surface charge, porosity, and lattice defects influence slurry viscosity and the maximum achievable solid content.

3. Coupling Agent Effects: Silane coupling agents, with functional groups reactive to both inorganic and organic materials, significantly enhance the dispersion and stability of SiC slurries, yielding low-viscosity, high-solid-content suspensions.

4. Molecular Structure Influence: Different modifier structures affect stability mechanisms. For example, electrostatic stabilization and steric hindrance mechanisms can optimize particle dispersion and prevent aggregation.

5. Dispersant Type and Dosage: The choice and concentration of dispersants directly impact slurry viscosity, zeta potential, and dispersion quality.

Current Challenges

While surface coating significantly improves the dispersibility, stability, and performance of ultrafine SiC powders, several challenges remain:

• Developing new, cost-effective, and easily controllable modification methods.

• Improving coating formulation, reusability, and stability for ultrafine SiC powders.

• Enhancing the wettability of SiC particles with metals in SiC-metal composites to improve corrosion resistance.

• Designing high-performance, low-cost, or multifunctional surfactants to optimize the surface treatment process.

• Establishing standardized testing and quality evaluation methods for surface-modified SiC powders.

Conclusion

Ultrafine SiC powders possess unique properties that enable broad applications in advanced materials. Surface modification changes their physical and chemical surface characteristics, improving dispersibility, stability, and performance, and enabling the development of high-performance functional ceramics. Advancements in surface modification techniques will expand the application range of nanoceramic powders and drive innovations in materials science.