I. Synthesis of monolithic hydroxyapatite (HA) from solid metal-bearing precursors (SMP)

A novel powder-metallurgical route has been used to fabricate near net-shaped calcium hydroxyapatite. Ca₁₀(P0₄)₆(OH)₂ (HA), bodies. An intimate mixture of metallic calcium and calcium pyrophosphate. β-Ca₂P₂O₇, possessing an overall stoichiometry consistent with HA, was prepared by high-energy mechanical alloying. The malleable Ca-Ca₂P₂O₇ powder mixture was compacted, formed into desired shapes (13 mm diameter x 1-2 mm thick disks and 30 mm x 7 mm X 3-4 mm bars) by uniaxial pressing and machining, and then converted into HA through a series of isothermal heat treatments. Exposure to flowing oxygen at <600°C resulted in complete oxidation of the calcium within 3 h (at 600°C). Post-oxidation annealing at ≤1150°C in a water-vapor bearing atmosphere yielded (XRD) phase-pure HA. The reduction in solid volume associated with the oxidation of calcium (Vm[CaO] < Vm[Ca]) was offset by the increase in solid volume associated with the conversion of CaO and Ca₂P₂O₇ into HA. As a result, the overall dimensional changes upon transformation of Ca+β-Ca₂P₂O₇ precursors into HA can be relatively small. In this thesis, the fabrication of near net-shaped monolithic HA-bearing bodies from Ca+β-Ca₂P₂O₇ precursors is demonstrated. The phase and microstructural evolution at various stages of transformation are also discussed.

Superheating (600°C) within a disk-shaped specimen (14 mm diameter x 4 mm thick, 82% dense) was detected via DT analyses with thermocouple embedded within the precursor disks. The superheating (also termed thermal runaway) was caused by exothermic nature of calcium oxidation. This event might have assisted in HA transformation within the disk-shaped specimen;​ while, on the other hand, have caused catastrophic cracking of bar-shaped specimens (30 mm X 7 mm x 3 mm. 74% dense). The difference in the results is attributed to the amount of heat generated, the rate of heat dissipation, as well as the densities of the samples.

II. Synthesis of HA/Co-Cr-Mo alloy composites from solid metal-bearing precursors

The SMP approach mentioned in the previous section is applied to the fabrication of HA+ Co-Cr-Mo alloy composites. A mixture of Co-Cr-Mo powder with the precursor prepared from metallic calcium and β-Ca₂P₂O₇, targeted to yield a 75 to 25 volume ratio of Co-Cr-Mo to stoichiometric HA upon conversion, was prepared by high-energy mechanical alloying. The malleable Co-Cr-Mo/Ca-Ca₂P₂O₇ powder mixture was compacted, formed into desired shapes by uniaxial pressing and machining, and then converted into a Co-Cr-Mo/HA composite through a series of isothermal heat treatments. After sintering at 1150°C in a de-oxygenized, flowing argon atmosphere, a continuous network of the Co-Cr-Mo alloy, with HA/TCP present in the interstices between Co-Cr-Mo grains, was produced. Subsequent annealing at 850°C in a water-vapor-bearing atmosphere yielded a composite of Co-Cr-Mo alloy with phase-pure (by XRD) HA. The overall dimensional changes upon transformation of Ca+β-Ca₂P₂O₇+Co-Cr-Mo precursors into HA/Co-Cr-Mo composite were relatively small. In this thesis, the phase and microstructural evolution at various stages of transformation to HA/Co-Cr-Mo alloy composites are discussed, the mechanical strength (microhardness and flexural strength) of the annealed specimens were evaluated, the potential reaction between HA and CrO] were also investigated.

III. Kinetic studies for conversion of hydroxyapatite from calcium oxide and TCP

In order to understand the kinetics for hydroxyapatite formation from the reaction of calcium oxide and tricalcium phosphate, both planar reaction couples and powder compacts of CaO-TCP were prepared. Two Pt strips were placed between CaO and TCP in the planar reaction couples to serve as inert markers. These reaction couples were heated at 1150°C for various times in moist O₂. The results of powder compact analyses fits Carter's model very well, which indicated that the rate of HA conversion from CaO and TCP is limited by solid state diffusion. The results of the planar reaction couples indicated that the formation of new HA layer was primarily between the TCP/HA interface, which suggests that the diffusion rates of Ca²⁺ and/or OH⁻ through the HA product layer were faster than the diffusion rate of P⁺⁵ and O⁻² (or PO₄³⁻). Thus, the formation of HA from TCP and CaO were limited by the diffusion rate of Ca²⁺ and/or OH⁻ through the HA layer. Pole figure analyses of the HA layer formed between the CaO-TCP planar reaction couple indicated that the new HA layer was textured. The grains with (211) or (222) plane parallel to the HA/TCP interface were 5 to 9 times more prevalent than those with these planes oriented otherwise. In addition, there were almost no grains with (300) and (112) planes parallel to the HA/TCP interface.