Interfacial interaction and its influence on the mechanical performances of hydroxyapatite through a polycrystalline model

XT Ma and L Zhang and ML Yang, PHYSICA B-CONDENSED MATTER, 594, 412338 (2020).

DOI: 10.1016/j.physb.2020.412338

A polycrystalline model is proposed in this work for hydroxyapatite (HAp) to study its microstructures and mechanical properties, as well as their variations with grain size. The polycrystalline HAp (pHAp) consists of grains with a controllable size distribution. In the equilibrium structures that are relaxed with a simulated annealing method, the boundary atoms mismatch in a disordered pattern in the interfacial region. In contrast to its crystal structure, pHAp is characterized by its isotropy in mechanical properties. The elastic constants and moduli of pHAp are in general smaller than those of crystal HAp and increase with grain size, following the reverse Hall- Petch relationship. Moreover, the uniaxial tension simulations reveal that pHAp has similar stress strain relationship in the x, y and z directions. An elastic deformation is noted at 0-2% strain, followed by a plastic deformation at 2-20% strain, which is distinct from the brittle fracture in crystal HAp. The maximum stress of pHAp decreases with decreasing grain size. A good linear dependence is noted by fitting the maximum stress and inverse square root of average grain size. Our simulations demonstrated that compared to crystal HAp, pHAp with ultra- fined grain sizes exhibits good ductility but moderate mechanical strength. Our findings would be helpful for the design of HAp-based biomaterials with controllable microstructures and mechanical performances.

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