Mechanical enhancement of hydroxyapatite via carbon and boron nitride nanotubes: a molecular dynamics study

Context: This study explores the mechanical limitations of hydroxyapatite (HAP), a critical bioceramic in bone tissue engineering and orthopedic implants, which is limited by its brittleness and low mechanical strength. By reinforcing HAP with carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs), the mechanical performance of HAP was significantly enhanced. The inclusion of CNTs led to a 25% increase in ultimate tensile strength (UTS), peaking at 9.18 GPa, while BNNTs improved ductility with a maximum UTS of 8.75 GPa. Toughness, representing the material’s energy absorption capacity, reached 15.8 kJ/m² in CNT-reinforced composites and 9.3 kJ/m² in BNNT-reinforced composites, emphasizing their distinct reinforcement contributions. The study highlights the potential of CNT-BNNT combinations, achieving a synergistic balance of strength, ductility, and toughness. Methods: The study employed molecular dynamics simulations to model and analyze the mechanical behavior of nano-reinforced HAP. Simulations were performed using the LAMMPS software, with the CVFF-Interface Force Field for HAP and the AIREBO potential used to model carbon interactions in CNTs. BNNTs were simulated using the Tersoff potential to account for interactions between boron and nitrogen atoms. The effects of nano-reinforcements on the mechanical properties of HAP were evaluated through tensile stress–strain curves, which quantified improvements in Young's modulus, ultimate tensile strength (UTS), and strain at UTS. Additionally, combinations of CNTs and BNNTs in varying ratios were simulated to assess synergistic interactions, while different inclusion levels were investigated to understand their impact on the composite’s mechanical performance. Toughness values, representing the material's energy absorption capacity, were calculated by integrating the area under the stress–strain curves up to failure, providing deeper insights into the ductility and energy dissipation characteristics of the reinforced HAP composites.