Atomistic simulation and electron microscopy to characterize grain boundaries in doped beta-tricalcium phosphate ceramics

The limitations and potential complications correlated with autogenous bone grafts have raised interest in the development of synthetic bone graft substitutes. ß-tricalcium phosphate (ß-TCP) is one of the most promising materials for synthetic bone graft substitutes because it is readily resorbed and replaced by new bone. However, even though ß-TCP has been clinically used since 1920, various studies have reported inconsistent biological responses for this bioceramic. This is most likely because the mechanisms by which ß-TCP is resorbed into the human body are still far from being completely understood. In order to better understand the in vivo behavior of ß-TCP, this thesis sets out to explore aspects of grain boundaries and ion-doping in ß-TCP via atomistic simulations and electron microscopy. The knowledge gained from this aims towards achieving a better understanding of how they in turn influence the resorption and healing processes of ß-TCP bone graft substitutes.For correct atomistic simulations, the ß-TCP crystal lattice presents a particular challenge. There is an incommensurate aspect of its structure tied to a specific, partially-filled calcium site in the conventionally-defined unit cell. Before considering the simulations of doped structures and interfaces, detailed atomistic simulations are made in order to study the arrangements of calcium ion occupations of this site, going from the scale of the unit cell up to large supercell systems.A framework is detailed for simulating ß-TCP as interface structures, which allows us to study the distribution of dopants across them. The behavior of Sr ions was investigated using a recently-developed Monte Carlo method. The preliminary results of this framework give access to interfacial energies and enthalpies of segregation for interface structures. The results suggest that Sr-segregation can be expected for certain ß-TCP grain boundaries but probably not all, and that the relative level of segregation will actually decrease as total dopant concentration in the ceramic is increased. Complementary microstructural characterizations were carried out using analytical electron microscopy. Those identified did not show any apparent segregation.This thesis also involved other electron microscopy analyses of various ß-TCP samples. This was done successfully and the results are highlighted in recent joint publications. Also, high spatial resolution electron microscopy analyses were carried out on a 5.00% Cu-doped sample. We observed the formation of a secondary phase, copper oxide, the presence of which influences the ceramic's biocompatibility, since it is correlated to a strong cytotoxicity that killed osteoclasts during cell culture medium tests. Beyond these secondary phases, this sample showed a uniform distribution of elemental Cu across grains and grain boundaries, similarly to the 5.00% Sr-doped sample.This thesis has developed a methodology for investigating ß-TCP at an atomistic level, from both a structural and microstructural point of view. Further developments and simulations are needed, but the methodology has been shown to be robust and reliable, such that it creates many avenues for future studies. From the electron microscopy front, the beam sensitivity introduced challenges to our investigations, but global powder and ceramic characterization was successful. Further work on how to better sample and characterize grain boundaries in these important biomaterials is needed.