Effect of Pore Defects on Uniaxial Mechanical Properties of Bulk Hexagonal Hydroxyapatite: A Molecular Dynamics Study.

Hydroxyapatite (HAP) is a calcium apatite bioceramic used in various naturally-derived and synthetic forms for bone repair and regeneration. While useful for the regrowth of osseus tissue, the poor load-bearing capacity of this material relative to other biomaterials is worsened by the propensity for pore formation during the synthetic processing of scaffolds, blocks, and granules. Here we use molecular dynamics (MD) simulations to improve the current understanding of the defect-altered uniaxial mechanical response in hexagonal HAP single crystals relative to defect-free structures. The inclusion of a central spherical pore within a repeated lattice was found to reduce both the failure stress and failure strain in uniaxial tension and compression, with up to a 30% reduction in maximum stress at the point of failure compared to a perfect crystalline structure observed when a 30 Å diameter pore was included. The Z axis ([0 0 0 1] crystalline direction) was found to be the least susceptible to pore defects in tension but the most sensitive to pore inclusion in compression. The deformation mechanisms are discussed to explain the observed mechanical responses, for which charge imbalances and geometric stress concentration factor effects caused by pore inclusion play a significant role.

Study of nanoscale deformation mechanisms in bulk hexagonal hydroxyapatite under uniaxial loading using molecular dynamics.

Hydroxyapatite (HAP) is a natural bioceramic which is currently used in scaffolds and coatings for the regrowth of osseous tissue but offers poor load-bearing capacity compared to other biomaterials. The deformation mechanisms responsible for the mechanical behavior of HAP are not well understood, although the advent of multiscale modeling offers the promise of improvements in many materials through computational materials science. This work utilizes molecular dynamics to study the nanoscale deformation mechanisms of HAP in uniaxial tension and compression. It was found that deformation mechanisms vary with loading direction in tension and compression leading to significant compression/tension asymmetry and crystal anisotropy. Bond orientation and geometry relative to the loading direction was found to be an indicator of whether a specific bond was involved in the deformation of HAP in each loading case. Tensile failure mechanisms were attributed to stretching and failure in loading case-specific ionic bond groups. The compressive failure mechanisms were attributed to coulombic repulsion in each case, although loading case-specific bond group rotation and displacement were found to affect specific failure modes. The elastic modulus was the highest for both tension and compression along the Z direction (i.e. normal to the basal plane), followed by Y and X.

Thermal Conductivity of Metal-Coated Tri-Walled Carbon Nanotubes in the Presence of Vacancies-Molecular Dynamics Simulations.

Variation in the thermal conductivity of a metal-coated tri-walled carbon nanotube (3WCNT), in thepresence of vacancies, was studied using non-equilibrium molecular dynamics simulations. A Two-Temperature model was used to account for electronic contribution to heat transfer. For 3WCNT with 0.5%and 1% random vacancies, there was 76%, and 86% decrease in the thermal conductivity, respectively. In thatorder, an overall ~66% and ~140% increase in the thermal conductivity was recorded when 3 nm thick coatingof metal (nickel) was deposited around the defective models. We have also explored the effects of tubespecific and random vacancies on thermal conductivity of the 3WCNT. The changes in thermal conductivityhave also been justified by the changes in vibrational density of states of the 3WCNT and the individualtubes. The results obtained can prove to be useful for countering the detrimental effects of vacancies incarbon nanotubes.