Abstract
The present study aims at a sol-gel processing of hydroxyapatite (HA) using precursors of calcium nitrate tetrahydrate and triethyl phosphate dissolved in 2-methoxy ethanol. Firstly, for the aged and slow-dried precursor, XRD pattern showed major peaks of HA and a very weak CaO peak. This minor CaO can be easily and completely washed out just by using distilled water. Secondly, during aging and drying, 1H NMR spectra showed hydrolyzed species: PO(OEt)2(OH) and PO(OEt)(OH)2. In solid-state MAS 31P NMR spectra, dried gels calcined at 250oC, 350oC and 600oC showed a conversion from gel to glassy phosphate, then HA ceramic. Thirdly, the dried gels were calcined by a rapid-thermal-calcine (RTC) heating (100~600 oC/min) and a conventional-furnace-calcine (CFC) heating (1.67 oC/min), respectively. The onset temperature of HA crystallization is lower in RTC, 350oC, as compared to 600oC in CFC, revealed by XRD analyses. Pyrolytic-GC/MS results combined with previous ones lead to models that RTC can catastrophically remove organic portion of the gel networks, leading to a porous surface morphology and a collapse of gel networks at local areas, so that HA crystallite nucleation is facilitated due to intimate contacts among inorganic species at lower temperatures. On the other hand, slow evolution of organics during CFC leads to carbonaceous residues that isolate the inorganic species and inhibit nucleation of HA crystallites until at a higher temperature. Finally, HA coatings were deposited on Ti-6Al-4V substrates and subjected to a pre-calcined at 400oC. After five repetitions of coating procedures the coatings were then RTC at 600oC. The formation of HA was confirmed by XRD analysis. The residual organics, as revealed by XPS spectra of a coating calcined at 400oC, might retard the formation of calcium phosphate phase, thus only small amount of calcium phosphate phase is present. After calcining at 600oC, calcium phosphate phase is the only one identifiable in XPS spectra. The adhesive strength of the five-layer coating on the substrate is around 60 MPa.