Thermal decomposition of hydroxyapatite structure induced by titanium and its dioxide

Titanium-hydroxyapatite composites sintered at low temperature for tissue engineering: in vitro cell support and biocompatibility

Background

In clinical orthopedics, a critical problem is the bone tissue loss produced by a disease or injury. The use of composites from titanium and hydroxyapatite for biomedical applications has increased due to the resulting advantageous combination of hydroxyapatite bioactivity and favorable mechanical properties of titanium. Powder metallurgy is a simple and lower-cost method that uses powder from titanium and hydroxyapatite to obtain composites having hydroxyapatite phases in a metallic matrix. However, this method has certain limitations arising from thermal decomposition of hydroxyapatite in the titanium-hydroxyapatite system above 800°C. We obtained a composite from titanium and bovine hydroxyapatite powders sintered at 800°C and evaluated its bioactivity and cytocompatibility according to the ISO 10993 standard.

Methods

Surface analysis and bioactivity of the composite was evaluated by X-ray diffraction and SEM. MTT assay was carried out to assess cytotoxicity on Vero and NIH3T3 cells. Cell morphology and cell adhesion on the composite surface were analyzed using fluorescence and SEM.

Results

We obtained a porous composite with hydroxyapatite particles well integrated in titanium matrix which presented excellent bioactivity. Our data did not reveal any toxicity of titanium-hydroxyapatite composite on Vero or NIH3T3 cells. Moreover, extracts from composite did not affect cell morphology or density. Finally, NIH3T3 cells were capable of adhering to and proliferating on the composite surface.

Conclusions

The composite obtained displayed promising biomedical applications through the simple method of powder metallurgy. Additionally, these findings provide an in vitro proof for adequate biocompatibility of titanium-hydroxyapatite composite sintered at 800°C.

Effects of Laser Power Level on Microstructural Properties and Phase Composition of Laser-Clad Fluorapatite/Zirconia Composite Coatings on Ti6Al4V Substrates

Hydroxyapatite (HA) is one of the most commonly used materials for the coating of bioceramic titanium (Ti) alloys. However, HA has poor mechanical properties and a low bonding strength. Accordingly, the present study replaces HA with a composite coating material consisting of fluorapatite (FA) and 20 wt % yttria (3 mol %) stabilized zirconia (ZrO₂, 3Y-TZP). The FA/ZrO₂ coatings are deposited on Ti6Al4V substrates using a Nd:YAG laser cladding system with laser powers and travel speeds of 400 W/200 mm/min, 800 W/400 mm/min, and 1200 W/600 mm/min, respectively. The experimental results show that a significant inter-diffusion of the alloying elements occurs between the coating layer (CL) and the transition layer (TL). Consequently, a strong metallurgical bond is formed between them. During the cladding process, the ZrO₂ is completely decomposed, while the FA is partially decomposed. As a result, the CLs of all the specimens consist mainly of FA, Ca₄(PO₄)₂O (TTCP), CaF₂, CaZrO₃, CaTiO₃ and monoclinic phase ZrO₂ (m-ZrO₂), together with a small amount of θ-Al₂O₃. As the laser power is increased, CaO, CaCO₃ and trace amounts of tetragonal phase ZrO₂ (t-ZrO₂) also appear. As the laser power increases from 400 to 800 W, the CL hardness also increases as a result of microstructural refinement and densification. However, at the highest laser power of 1200 W, the CL hardness reduces significantly due to the formation of large amounts of relatively soft CaO and CaCO₃ phase.

Hot Isostatic Pressing (HIPing) of FeCralloy®-Reinforced Hydroxyapatite

The goal of this study was to produce hydroxyapatite (HAp), a bioactive biomaterial, in a decomposition-free form with fracture toughness comparable to bone by metal fibre-reinforcement. This goal was ultimately achieved. Glass encapsulation of FeCralloy®-reinforced HAp was an unsuccessful technique due to the excessive low-temperature volatilisation, which aerated the glass. Therefore a graphite/stainless steel encapsulation system was used in the present study. Hot isostatic pressing enabled the production of fully dense decomposition-free HAp with toughness improvements of 14 times (FeCralloy® fibres, optimally 15 vol%), comparable to cortical bone. Further, it was found that the HAp decomposition temperature was higher at 100 MPa (the HIPing pressure) than for pressureless sintering. Addition of the FeCralloy® fibre additive induced significant plastic deformation and ductile fracture of the hydroxyapatite.

Comparative evaluation of biocompatibility of dense nanostructured and microstructured Hydroxyapatite/Titania composites

This work deals with the biocompatibility of dense nano- and micro-structured Hydroxyapatite/Titania composites prepared by two step and conventional sintering, respectively. By application of two step sintering, it was shown that the final grain size of HA-15 wt.% TiO2 is maintained lower than 100 nm while by the application of conventional sintering it reaches higher than 100 nm. Biocompatibility of the dense bulks was evaluated by cell attachment and proliferation experiments. Cell morphology, and viability on each nano- and micro-structured Hydroxyapatite/Titania composites were examined at different time points. The nanostructured HA/Titania dense bulk exhibited higher cell viability than a microstructured one. In addition, the effects of ionic products from nano- and micro-structured bulk dissolution on osteoblasts were studied. The MTT test confirmed that the products from nanostructured HA/Titania dense bulk significantly promoted osteoblast proliferation within a certain concentration range.

Preparation and Characterization of (HAp/SiO2)/Ti Biocomposites

(HAp/SiO2)/Ti biocomposites were prepared by the powder metallurgy method. The phase compositions and the in vitro bioactivity of such biocomposites were systematically characterized. The XRD result shows that the phase compositions of (HAp/SiO2)/Ti composites are mainly composed of Ca4O(PO4)2 (TTCP), Ti, TiO2 and CaO. The synthesized (HAp/SiO2)/Ti biocomposites exhibit a good bioactivity, for example, after the samples are immersed in SBF solution only for 24 hours, the bone-like layer consisting of spherical apatite crystal clusters has deposited on the surface of the samples. The density and thickness of the apatite layer increases with increasing immersion time. The formation process and mechanisms of bone-like apatite layer are also discussed.

Tetracalcium phosphate: Synthesis, properties and biomedical applications

Monoclinic tetracalcium phosphate (TTCP, Ca(4)(PO(4))(2)O), also known by the mineral name hilgenstockite, is formed in the (CaO-P(2)O(5)) system at temperatures>1300 degrees C. TTCP is the only calcium phosphate with a Ca/P ratio greater than hydroxyapatite (HA). It appears as a by-product in plasma-sprayed HA coatings and shows moderate reactivity and concurrent solubility when combined with acidic calcium phosphates such as dicalcium phosphate anhydrous (DCPA, monetite) or dicalcium phosphate dihydrate (DCPD, brushite). Therefore it is widely used in self-setting calcium phosphate bone cements, which form HA under physiological conditions. This paper aims to review the synthesis and properties of TTCP in biomaterials applications such as cements, sintered ceramics and coatings on implant metals.

Characterization of sintered titanium/hydroxyapatite biocomposite using FTIR spectroscopy

Fourier transform infrared (FTIR) spectroscopy was employed to characterize the phase changes of hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2), HA) in a titanium/HA biocomposite during sintering. The effects of sintering temperature and the presence of Ti on the decomposition of HA were examined. It was observed that pure HA was stable in argon atmosphere at temperatures up to 1,200 degrees C, although the dehydroxylation of pure HA was promoted by the increase in sintering temperature. In the Ti/HA system, on the other hand, the presence of Ti accelerated dehydroxylation and the decomposition of HA was detected at a temperature as low as 800 degrees C. Tetracalcium phosphate (Ca(4)P(2)O(9), TTCP) and calcium oxide (CaO) were the dominant products of the decomposition, but no tricalcium phosphate (Ca(3)(PO(4))(2), TCP) was detected due to phosphorus diffusion and possible reactions during the thermal process. The main decomposed constituents of HA in Ti/HA system at high temperatures (> or =1,200 degrees C) would be CaO and amorphous phases.

Effects of TiO2, ZrO2 and Al2O3 dopants on the compressive strength of tricalcium phosphate

Tricalcium phosphate (TCP) powders synthesised using the Ca(NO3)2 and Ca(OH)2 routes were doped with TiO2, ZrO2 and Al2O3 in order to increase their compressive strength. An ultimate compressive strength (UCS) of 255 +/- 6 MPa was achieved for approximately 10 vol% TiO2 doping compared to 30 +/- 3 MPa for an un-doped control processed and tested in the same manner. Higher levels of TiO2 doping resulted in smaller increases in UCS with 30 and 50 vol% achieving 213 +/- 9 and 178 +/- 15 MPa, respectively. Very small amounts of Al2O3 doping (< 0.5 vol%) also resulted in a stronger materials. However, under the processing conditions employed, higher levels of Al2O3 and ZrO2 doping resulted in no beneficial effect on the UCS. Polyvinyl alcohol (PVA) was used as binding agent to facilitate processing. As expected, higher levels of PVA were associated with smaller increases in UCS. Powders synthesised using the Ca(OH)2 route had smaller particle size and resulted in larger increases in UCS compared to the Ca(NO3)2-synthesised powders. Although some powders contained alpha and beta-TCP phases, no other calcium phosphate, CaO, CaTiO3 or CaZrO3 phases were detected. In conclusion, a significant increase in the UCS of TCP was achieved by doping with approximately 10 vol% TiO2 which is expected to have little or no effect on the bioactivity or bioresorbability of the material.

Plasma-sprayed hydroxyapatite+titania composite bond coat for hydroxyapatite coating on titanium substrate

A two-layer hydroxyapatite (HA)/HA+TiO(2) bond coat composite coating (HTH coating) on titanium was fabricated by plasma spraying. The HA+TiO(2) bond coat (HTBC) consists of 50 vol% HA and 50 vol% TiO(2) (HT). The microstructural characterization of the HTH coatings before and after heat treatment was conducted by using scanning electron microscopy (SEM), electron probe microanalyser (EPMA), X-ray diffractometer (XRD) and transmission electron microscopy (TEM), in comparison with that of HT coating and pure HA coating. The results revealed that HA and TiO(2) phases layered in an alternating pattern within the HTBC, and the HTBC bonded well to HA top coating (HAT coating) and Ti substrate. The as-sprayed HT coating consists mainly of crystalline HA, rutile TiO(2) and amorphous Ca-P phase. The post-spray heat treatment at 650 degrees C for 120 min effectively restores the structural integrity of HA by transforming non-HA phases into HA. It was found that there exists interdiffusion of the elements within the HTBC, but no chemical product between HA and TiO(2), such as CaTiO(3) was formed. The cross-sectional morphologies confirmed that there is a shift towards a relatively tighter bonding from the HAT coating/HTBC interface in the as-sprayed HTH coating to the HTBC/Ti substrate interface in the heat-treated HTH coating. On quenching the coatings into water, the surface cracking indicates more apparently the positive effect of the HTBC on the decrease of residual stress in HAT coating. The in situ surface cracking also suggests that the stress on the surface of the HTH coating is stable under subjection to a repetitious heat treatment. The toughening and strengthening of HTBC is thought to be mainly due to TiO(2) as obstacles embarrassing cracking, the reduction of the near-tip stresses resulting from stress-induced microcracking and the decrease of CTE mismatch. In the HTH composite coating, the HAT coating is toughened by the decreased CTE mismatch with Ti through the addition of HTBC, which bonds well to the Ti substrate via its TiO(2) hobnobbing with the Ti oxides formed on Ti substrate.

On the microstructure of biocomposites sintered from Ti, HA and bioactive glass

Sintering reactions and fine structures of the biocomposites prepared from powder mixtures of titanium ( alpha -Ti), hydroxyapatite (HA) and bioactive glass (BG) (SiO2-CaO-P2O5-B2O3-MgO-TiO2-CaF2) were investigated by X-ray diffraction and transmission electron microscopy. The results showed that complex reactions among the starting materials mainly depended on the initial Ti/HA ratios as well as the sintering temperatures. And the reaction could be expressed by the following illustrative equation: Ti+Ca10(PO4)6(OH)2-->CaTiO3+CaO+TixPy+(Ti2O)+(Ca4P2O9)+H2O.

Interaction of hydroxyapatite–titanium at elevated temperature in vacuum environment

In this study, the interaction between hydroxyapatite (HA) and titanium (Ti) at elevated temperature in vacuum environment was investigated. The 80 wt% HA-20 wt% Ti powder mixtures and 90 wt% HA-10 wt% Ti powder mixtures were dry pressed and heat-treated at 1100 degrees C in vacuum environment. HA powders and the commercially pure Ti powders were used as controls. The heat-treated samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron microscope (SEM) and energy disperse spectra. XRD and SEM indicated densification of metallic Ti specimens during the in-vacuum heat treatment. Heat treatment of HA specimens in vacuum resulted in the loss of hydroxyl groups as well the formation of a secondary beta-tricalcium phosphate phase. Metallic Ti was not observed in the in-vacuum heat-treated HA-Ti specimens. However, alpha-tricalcium phosphate, tetracalcium phosphate and calcium titanium oxide were observed for the in-vacuum heat-treated HA-Ti specimens. It was concluded that the in-vacuum heat-treatment process completely converted the metal-ceramics composites to ceramic composites.

Processing-microstructure-property relations in HVOF sprayed calcium phosphate based bioceramic coatings

Hydroxyapatite (HA) based bioceramic coatings were deposited onto titanium alloy substrates using the high velocity oxy-fuel (HVOF) spray technique. This study aimed to reveal the relations among processing parameters, microstructure, and properties of the bioceramic coatings. The processing conditions were altered through changing the starting HA powder size, content of bioinert ceramic additives or composite powder preparation techniques. Coating structure was characterized through scanning electron microscopy (SEM) and transmission electron microscopy (TEM); and the mechanical properties, Young's modulus and fracture toughness, of the coatings were evaluated through indentation techniques. Results demonstrated dominant influence of the melt state of HA powders on the phase composition of resultant coatings, and it was found that the HVOF HA coatings possess competitive mechanical properties. Furthermore, addition of titania or zirconia, as secondary phase in HA, showed promising effect on improving the mechanical properties of the HVOF HA-based coatings. Chemical reactions between HA and titania; and, HA and zirconia during coating deposition were revealed and characterized. Incorporation modes of the additives into HA and their reinforcing mechanisms were elucidated. The relationship among the processing, microstructure, and mechanical properties of the HVOF sprayed bioceramic coatings was summarily examined.

Impact formation and microstructure characterization of thermal sprayed hydroxyapatite/titania composite coatings

Formation mechanism of hydroxyapatite (HA)/titania (TiO(2)) composite coating deposited by high velocity oxy-fuel (HVOF) thermal spray process was studied, and its structural characterization was conducted and elaborated in this paper. The impact theory was employed to analyze the formation procedure of the HA/titania composite coatings. Results revealed that the crater caused by the impact of entirely unmelted TiO(2) particles on the HA matrix during coating formation was of smaller dimensions than the original size of the reinforcements. It was found that chemical reaction between the mechanically blended HA and TiO(2) powder took place exclusively during the impingement stage, and calcium titanate, CaTiO(3), was one notable by-product. The bonding between the HA matrix and TiO(2) reinforcement might have been achieved predominantly through a chemical bond that resulted from the mutual chemical reactions among the components. Differential scanning calorimetry analyses showed that the chemical reaction between HA and TiO(2) was at approximately 1410 degrees C. The TiO(2) addition was found to exert particular effects on the thermal behavior of HA at elevated temperatures, during both heating and cooling cycles. Transmission electron microscopy observation identified the chemical reaction zone between HA and TiO(2), which revealed an improved splats' interface. The reaction zone demonstrated some influence on the grain size of HA nearby during resolidification of the melted portion. A structural model was proposed to illustrate the location of the different phases in the HA/titania composite coating.

Microstructures and mechanical properties of powder injection molded Ti-6Al-4V/HA powder

Taguchi method with an L9 orthogonal array was employed to investigate the sintered properties of Ti-6Al-4V/HA tensile bars produced by powder injection molding. The effects of sintering factors at the 90% significance level: sintering temperature (1050 degrees C, 1100 degrees C and 1150 degrees C), heating rate (5 degrees C/min, 7.5 degrees C/min and 10 degrees C/min), holding time (30, 45 and 60 min) and cooling rate (5 degrees C/min, 20 degrees C/min and 40 degrees C/min) were investigated. Results showed that sintering temperature, heating rate and cooling rate have significant effects on sintered properties, whereas the influence of holding time was insignificant. It was found that a sintering temperature of 1100 degrees C, a heating rate of 7.5 degrees C/min and a cooling rate of 5 degrees C/min increased the relative density, Vicker's microhardness, flexural strength and flexural modulus. However, a further increment of sintering temperature to 1150 degrees C did not show any discernable improvement in the relative density and Vicker's microhardness, but there was a slight increase of 0.6% and 0.9% in the flexural strength and flexural modulus, respectively. Mechanically strong Ti-6Al-4V/HA parts with an open porosity of around 50% were developed.

Titanium dioxide reinforced hydroxyapatite coatings deposited by high velocity oxy-fuel (HVOF) spray

Hydroxyapatite (HA) coatings with titania addition were produced by the high velocity oxy-fuel (HVOF) spray process. Mechanical properties of the as-sprayed coatings in terms of adhesive strength, shear strength and fracture toughness were investigated to reveal the effect of the titania reinforcement on HA. Qualitative phase analysis with X-ray diffraction (XRD) showed that mutual chemical reaction between TiO2 and HA, that formed CaTiO3 occurred during coating formation. Differential scanning calorimetry (DSC) analysis of the starting powders showed that the mutual chemical reaction temperature was approximately 1410 degrees C and the existence of TiO2 can effectively inhibit the decomposition of HA at elevated temperatures. The positive influence of TiO2 addition on the shear strength was revealed. The incorporation of 10 vol% TiO2 significantly improved the Young's modulus of HA coatings from 24.82 (+/- 2.44) GPa to 43.23 (+/- 3.20) GPa. It decreased to 38.51 (+/- 3.65) GPa as the amount of TiO2 increased to 20 vol%. However, the addition of TiO2 has a negative bias on the adhesive strength of HA coatings especially when the content of TiO2 reached 20 vol%. This is attributed to the weak chemical bonding and brittle phases existing at the splats' interface that resulted from mutual chemical reactions. The fracture toughness exhibited values of 0.48 (+/- 0.08) MPa m0.5, 0.60 (+/- 0.07) MPa m0.5 and 0.67 (+/- 0.06) MPa m0.5 for the HA coating, 10 vol% TiO2 blended HA coating and 20 vol% TiO2 blended HA coating respectively. The addition of TiO2 in HA coating with the amount of less than 20 vol% is suggested for satisfactory toughening effect in HVOF HA coating.

Effect of TiO2-Ag2O additives on the formation of calcium phosphate based functionally graded bioceramics

The combined effect of titanium dioxide and silver oxide on the in situ formation of biphasic calcium phosphate ceramics was investigated. Titania (5-20 mol%) was mixed with pure hydroxyapatite (HA) or HA containing Ag2O (10-20 mol%) and was heated to 900 degrees C for 12 h. The sintered samples were found to contain tricalcium phosphate (beta-TCP) and other phases along with HA depending upon the amount as well as the type of the additives used as evidenced by X-ray powder diffraction (XRD) and fourier transform infrared (FT-IR) spectroscopic studies. Enhanced TCP formation with reduced impurity phases was observed with TiO2-Ag2O addition. In vitro solubility study in phosphate buffer at physiological conditions shows the resorbable nature of these materials. A functionally graded material (FGM) structure was formed by spreading TiO2-Ag2O mixture on the surface of the HA green compact and heating at 900 degrees C. The FGM shows gradient structure of TCP and HA from the surface to the interior of the pellet in addition to titania and silver phases.

Microstructural and in vitro chemical investigations into plasma-sprayed bioceramic coatings

Hydroxyapatite (HA) coatings plasma sprayed without and with bond coats (titania, zirconia) onto titanium alloy (Ti6A14V) substrates under both atmospheric and low pressure plasma spray conditions were investigated in terms of their microstructure and their resorption resistance during immersion in simulated body fluid (Hank's balanced salt solution). The microstructures of test samples were characterized using SEM on as-sprayed and leached surfaces and on the corresponding cross sections. Selected coating systems were studied by 2-dimensional secondary ion mass spectroscopy imaging to obtain information on plasma spray induced diffusional processes at the coating interfaces, as well as the spatial distribution of minor and trace elements. Coatings consisting of thin (10-15 microm) titania/zirconia (eutectic ratio) and titania bond coats, combined with a 150- to 180-microm thick HA top coat, yielded peel strengths in excess of 32 N/m, as well as sufficient resorption resistance.

The effect of heat treatment on the morphology of D-Gun sprayed hydroxyapatite coatings

In this study, the morphology of the Hydroxyapatite (HA) coatings sprayed on Ti alloy samples by Detonation Gun Spray (D-Gun) and the effect of aging before and after heat treatment in physiological solution were observed. Cross-sectional porosity and percentages of amorphous and crystal phase were measured using optical, electron microscopy, and X-ray diffraction analysis. Differential Thermogravimetric Analysis (DTA) was performed to estimate the glass-crystalline phase transformation temperatures. Heat-treatment at 300, 500, 700, 800 and 1200 degrees C were carried out to confirm DTA results. As a final analysis, the aging effect using Ringer's solution for 1 week on heat-treated and non-heat-treated samples was measured. It was observed that, in D-Gun sprayed samples, the cross-sectional porosity stayed in the accepted 5% range as reported for other spraying techniques.(1-5) On the other hand, surface porosity measured using the water immersion method remained in the conventional porosity limit of 15% for non-heat-treated samples. Heat-treatment had a small influence on the porosity while the crystallinity increased considerably; in addition, aging had little effect on HA crystallinity for heat treated samples. This work showed that D-gun sprayed HA coatings had lower porosity and better integrity than other coatings, due to which we can expect better performance during in vivo applications.

Characterization of hydroxyapatite film with mixed interface by Ar+ ion beam enhanced deposition

Ar+ ion beam enhanced deposition (IBED) was used to produce a hydroxyapatite (HA) film on polished titanium substrates. In this study, the HA ceramic target was sputtered by an argon-ion beam with an energy of 1.5 KeV, and the sputtered film was intermittently bombarded by energetic argon-ions at 60 KeV. An effective Ca-Ti mixed layer produced by the energetic argon-ion bombardment was confirmed by using Auger electron spectroscopy. The characteristics of the deposited films were evaluated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. XRD analysis revealed that the as-deposited film was amorphous, and a hydroxyapatite-type structure was obtained from the post-heat treatment of the deposited films. SEM observations showed that no distinct difference in surface morphology was found between the as-deposited and heat-treated samples for Ar+ IBED films, suggesting a strongly bonded HA film on the titanium substrate. In comparison with the HA target, some chemistry alterations were brought about in the deposited films, such as the incorporation of CO3, the loss of the OH groups and some distortion of the phosphate lattice.

Water vapour-treated hydroxyapatite coatings after plasma spraying and their characteristics

A novel way to enhance the ability of hydroxyapatite (HA) coatings in resisting degradation was revealed. The as-received plasma sprayed HA coatings were kept in water vapour at 125 degrees C, with a pressure of 0.15 MPa for 6 h; most of the amorphous phase in the coating was converted into crystalline HA and enhanced the crystallinity significantly. Meanwhile, the alpha-tricalcium phosphate, tetracalcium phosphate and CaO which decomposed from HA during plasma spraying were also transformed into crystalline HA. The dissolution experiment in distilled water at room temperature showed that the post-water vapour-treated coatings were more stable than post-heat-treated ones. The average interfacial tensile bond strength between HA and substrate before and after water vapour treatment was 45.0 and 39.1 MPa, respectively.

Influence of annealing temperature on RF magnetron sputtered calcium phosphate coatings

The effect of different annealing temperatures on the characteristics of thin calcium phosphate coatings fabricated by radiofrequency magnetron sputtering was studied. Annealing of the as-sputtered films was necessary to change the amorphous coating to a crystalline coating. The films were annealed for 2 and 4 h at 400, 600, 800, 1000 and 1200 degrees C under dry argon or argon and water vapour flow. After annealing, the structure and the chemical composition of these films were characterized with incident light microscopy, Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), and Fourier transform infrared absorption spectrometry (FTIR). Incident light microscopy showed cracks in the coatings annealed at a higher temperature than 400 degrees C. RBS revealed that the as-sputtered coatings had a high Ca/P ratio which decreased with increasing annealing temperature. After annealing at a temperature of 600 degrees C or more the XRD showed crystalline hydroxyapatite (HA) coatings. However, the second phase, present in the coatings, changed from tetra-calcium phosphate to calcium oxide to beta-tri-calcium phosphate with increasing annealing temperature. FTIR measurements showed the existence of OH- and PO- bonds in all coatings, although the PO- bonds varied for different annealed coatings, from the PO- bonds due to HA to PO- bonds due to other calcium phosphates. From the results of this study we suggest that 600 degrees C is probably the best annealing temperature to obtain a better characterization and understanding of the coating.

Effect of hydroxyapatite thickness on metal ion release from Ti6Al4V substrates

The electrochemical dissolution behaviour of Ti6Al4V alloy coated with hydroxyapatite (HA) by plasma spraying was studied in Hank's balanced salt solution (HBSS) and compared with that of polished and grit-blasted passivated surfaces. Two different nominal thicknesses of HA (50 and 200 micro m) were used. Taking a polished passivated surface as reference, grit blasting of the substrate increased the electrical charge used in the oxidation of Ti6Al4V alloy at constant potential, as a result of increased surface area. However, only HA coatings with a thickness of 200 micro m were capable of reducing the charge to values lower than those measured for polished surfaces. Electrochemical impedance spectroscopy has also shown that only 200 micro m thick coatings are effective in reducing the oxidation rate of the substrate. Furthermore, in potentiostatic experiments the 50 micro m thick coating detached from the substrate, which did not occur with the 200 micro m thick coating. However, after 6 months immersion in HBSS, detachment occurred in some regions of both coatings. No titanium, aluminium or vanadium were detected in solution by electrothermal atomic absorption spectroscopy. These data indicate that HA is an effective barrier to metal ion release, even for the thinner coatings, due to formation of metal phosphates or to incorporation of metal ions in the HA structure.