Cellulose nanocrystals (CNCs) derived from cellulose are being explored for practical industrial applications due to their high crystallinity and low density, making them ideal for lightweight and durable composites. In this study, researchers investigated high-hardness, self-healing materials by modifying CNCs with aminopropyltriethoxysilane (APTES) to enhance their dispersion in organic solvents. The CNCs were then combined with polyurethane to create a composite with increased hardness through condensation polymerization and interconnection of various components. The resulting self-healing composite coating exhibited a high pencil hardness of HB and demonstrated a healing rate of over 70% when monitored with a confocal microscope. Furthermore, a salt spray test confirmed that iron plates coated with this self-healing composite displayed excellent corrosion resistance, even after experiencing scratches and self-healing processes.