Long-Term Mechanical Stability Enhancement of Injection-Molded Biodegradable PLA/Phosphate-coated Mg Composites through Interface Control and Crystalline Structure Evolution

Polylactic acid (PLA)-based degradable implants hold excellent biocompatibility and processing advantages; however, their applications in small-sized components remain limited by insufficient crystallization, interface instability, and inadequate long-term mechanical reliability. In this study, magnesium (Mg) powder was used as a functional filler, with phosphate surface modification, and a self-designed powder-mixing injection molding system was employed to fabricate fine, rod-shaped PLA/Mg composites. The microstructure, tensile properties, creep behavior, and fatigue response were systematically evaluated. The phosphate-coated Mg powder significantly enhanced heterogeneous nucleation, while subsequent annealing promoted higher crystallinity and changes in crystalline morphology. Among all conditions, A100 exhibited the modified crystalline characteristics and improved tensile strength of the PLA/Mg composites. Compared with pure PLA, A100 demonstrated more stable time-dependent deformation and the longest fatigue life among the investigated conditions at 50 N. These enhancements are likely associated with the synergistic load-bearing contribution of the crystalline regions, constrained amorphous regions, and improved interfacial compatibility, which reduce local stress concentration and retard damage accumulation. The results demonstrate that interfacial modulation coupled with crystalline structure engineering can effectively improve the long-term mechanical stability of PLA/Mg composites under repeated loading. This study provides important mechanical insights into the future design of small-sized biodegradable implant materials.