Composition–Structure–Property Relationships in PLA-nHA-Mg Hybrid Biocomposites: Effect of Ceramic–Metallic Reinforcement Ratio on Mechanical and Physicochemical Performance

Poly(lactic acid) (PLA) hybridized with nano-hydroxyapatite (nHA) and elemental magnesium (Mg) offers a route to tune load-bearing behavior while retaining a biodegradable polymer matrix. This study compares neat PLA, binary PLA-10Mg and PLA-10nHA controls, and three ternary hybrids containing a constant 10 wt.% total reinforcement at Mg:nHA ratios of 1:1, 3:1, and 1:3. Tensile, compressive, and three-point flexural responses were interpreted together with differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy. Ten specimens were tested per formulation in each mechanical assay (five initial specimens and five confirmatory specimens), and the mechanical data were consolidated across all ten specimens and are reported as an arithmetic mean ± standard deviation (SD). The recorded response was loading-mode-dependent: PLA-5Mg-5nHA combined a tensile stress of 52.0 ± 1.56 MPa with the largest tensile peak displacement ratio of 23.42 ± 0.70%; PLA-7.5Mg-2.5nHA showed the largest flexural peak load of 490.4 ± 14.71 N; and PLA-2.5Mg-7.5nHA combined 52.8 ± 1.58 MPa tensile stress, 93.7 ± 2.81 MPa compressive stress, and 431.5 ± 12.95 N flexural peak load. DSC showed a high-temperature heat-flow event at 365.6–372.6 °C under the stated mixed-purge conditions; this is treated as an operational thermal marker and not a melting or decomposition temperature. XRD remained matrix-dominated with reinforcement-sensitive features, while FTIR showed composition-dependent shifts in the C-O/C-O-C region. The results indicate that Mg:nHA partitioning alters the recorded loading-mode response within the tested 10 wt.% reinforcement window, while particle-scale mechanisms and biological performance require further validation.