| Hydroxyapatite(HA) is the major inorganic components of human bone. Many attentions have been taken to study their physical and chemistry properties due to the potential orthopedic and dental applications arising from their good bioactivity and biocompatibility. The inadequate mechanical properties are the major drawbacks of HA as implant materials: the low fatigue resistance and lack of ductility, which inhibit its usage as an implant in load bearing applications. Thus, HA is generally used as coating material depositing on load bearing implants such as Ti and its alloys, to fulfill the mechanical requirements for biomedical applications by combining the mechanical strength of the implant material and biocompatibility of HA coatings. However, inherent defects of coating structure, such as great differences of physical properties and the low adhesion strength between HA coating and Ti matrix, low interface shear strength, coating stripping, cracks on the interface, and particle pieces limit their long-term performance. The critical factor to extend the lifetime of titanium alloys as implants is to strengthen adhesion between HA and Ti surfaces. Thus, it is essential to study the affinity of HA/Ti interface at the electronic level. Therefore a detailed theoretical analysis of HA/Ti interface will be of significant interest. Since adhesion is governed by chemical bonding, the problem is the electronic properties of the interface between the HA coating and the metal substrate. Therefore, a thorough understanding of the electronic properties of HA/Ti interfaces is of scientific interest. This thesis uses first-principle methods to study the adhesion strength of HA/Ti interfaces, the influence of vacancies and doping elements on the crystal stability and elastic properties of HA and the adhesion strength of the interface. Results presented here provide us a deep understanding of HA/Ti coating composites at the atom level and a guidline for to the design of new biomaterials. The main research contents are the properties of bulk HA, the surface properties of HA and Ti, and the interfaces of HA and Ti.First-principles plane-wave calculations were performed to investigate the influences of intrinsic vacancies on the stability and elastic properties of HA. Five types of vacancies, i.e. H, O, OH, Ca, and Ca2+ vacancy, were considered. Formation energies were evaluated and compared with experimental measurements. It was shown that HA with a Ca2+ vacancy is the most stable one among the considered systems. Elastic constants were estimated via curves of total energy against strain. Bulk, shear and Young’s moduli, and Poisson’s ratio are also evaluated to compare with the experimental values. The elastic properties of HA are significantly affected by the vacancy. Vacancy can soften HA via reducing its elastic moduli. The HA with Ca2+ vacancy is the softest one among the considered systems. Electronic structures of HA with vacancy are also analyzed to explain the softening mechanisms.HA minerals in human bones contain a variety of foreign ions and the dopants are often added in to enhance the bioactive properties and inhibit the inflammatory reactions of HA/Ti implants. For doped HA crystal, the influence mechanism of cations or anions on the phase stability and elastic properties were investigated. Based on first principles total energy calculations, the influence of doped ions with different concentrations, such as Zn2+, Sr2+, Mg2+, H+, Ag+, Mn2+, Y3+, Cd2+, and CO32-, on the crystal stabilities and elastic properties were investigated. The doped energies are calculated from total energies of supercells and chemical potentials for Ca2+ and foreign ions determined under chemical equilibrium between HA and its saturated solution. It is found that the substitution is significantly dependent on concentrations of doping ionic: Zn2+, Sr2+, and Mg2+ tend to exhibit more difficulty of substitution for Ca2+ at 10at% concentration while the values of doped energy are negative at 5at% concentration. The doped energies of occupying the Ca(2) sites are more negative than the Ca(1) sites for all the considered cations except for Sr2+. H+, Ag+, and Mn2+ exhibit more negative doped energies, thus, they are easier of substitution for Ca2+ in HA. Elastic constants were estimated via curves of total energy against strain for Zn2+, Sr2+, H+, Ag+, Y3+, and Cd2+ doping HA. The elastic properties of HA are significantly affected by the doped ions. Some kinds of ions can soften HA via reducing its elastic moduli. The HA with Zn at the Ca(2) site is the softest one and two H occupying Ca(1) sites is the second s oft one among the considered systems. Electronic structures of HA with doping eleents are also analyzed to explain the doping mechanisms.The surface models of the low index surfaces of HA and Ti were built based optimized crystals of HA and Ti and the surface slabs are optimized by relaxing partial atoms at the top part of the surface. The surface energies and Electronic structures of HA(0001), HA)0101(, HA)1110(, and Ti(0001) surfaces have been calculated on the optimization surface slabs. Results show that the HA(0001) surface has the lowest surface energy among the considered surfaces models and presents the most thermodynamically stable surface of HA. For lower index surfaces of HA, influence mechanism of doping ionic on the surface energy and electronic structures are studied based on doped energies of ions in HA bulk. Results illustrate that the doping ions can be divided into three categories.(Ag+ + H+),(CO32-+ H+), and two H+ substitution for Ca2+ or PO43-in HA increase the stability of HA surfaces and the doping ion at the surface zone is especially so, the surface energies of Sr2+ and Zn2+ doping HA are almost same as the undoped HA, and the energies of Mg2+,(Y3+- H+) substituted Ca2+ and(H+ + OH-) deficient HA are lower than the undoped HA, but with doping ions at the surface area decrease the stability of HA surfaces. The band gap of HA doped with(Ag+ + H+), or Zn2+ is small than stoichiometric HA. Ag-O and Zn-O are ionic bonds stronger than Ca-O bond.A basic understanding of the affinity between the HA and α-Ti surfaces is obtained through electronic structure calculations by first-principles method. The HA/Ti interfaces were constructed by two kinds of interface models, the semicycle boundary interface(denoted as SI) and the full cycle boundary interface(denoted as FI). Two methods, the full relaxation and the UBER, were applied to determine the interfacial separation and the atomic arrangement in the interfacial zone. The works of adhesion of interfaces with various stoichiometric HA surfaces were evaluated. For the HA(0001)/Ti(0001) interfaces, the work of adhesion is strongly depended on the chemical environment of the HA surface. The values are-2.33,-1.52, and-0.80 J/m2 for the none-, single-, and double-Ca terminated HA/Ti interfaces, respectively. The influence of atomic relaxation on the work of adhesion and interface separation is discussed. Full relaxation results-1.99 J/m2 work of adhesion and 0.220 nm separation between HA and Ti for the FI of 1-Ca-HA/Ti interface, while they are-1.14 J/m2 and 0.235 nm by partial relaxation. Analysis of electronic structure reveals that charge transfer between HA and Ti slabs occurs during the formation of the HA/Ti interface. The transfer generates the Ti-O or Ti-Ca bonds across the interface and drives the HA/Ti interface system to metallic characteristic. The energetically favorable interfaces are formed when the outmost layer of HA comprises more O atoms at the interface.Finally, influence mechanisms of doping elements on the affinity of HA/Ti interfaces are studied. Results show that Y, Ag and Zn doped elements increase the adhesion strengths of HA/Ti interfaces and the interface-stripping problem may be solved partially through doping this kind of elements in HA. The adhesion energy of C or Sr element doping HA/Ti interface are almost same as the undoped interfaces. However, deficient of H2 O in the c channel of HA or two H+ atoms substitution for Ca2+ and with the doping H at the interface zone will damage the affinity of HA/Ti interface. |
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