Interaction of hydroxyapatite-titanium at elevated temperature in vacuum environment

Yunzhi Yang^a, Kyo-Han Kim^{a, b}, C. Mauli Agrawal^{c, d} and Joo L. Ong<sup>, , a, d

a</sup> Department of Restorative Dentistry, Division of Biomaterials, The University of Texas Health Science Center at San Antonio, MSC 7890, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
^b Department of Dental Biomaterials, College of Dentistry and Institute of Biomaterials Research and Development, Kyungpook National University, 2-101 Dongin Dong, Jung-Gu, Daegu 700-422, South Korea
^c College of Engineering, The University of Texas at San Antonio, 6900 N. Loop 1604, San Antonio, TX 78249-0619, USA
^d Center for Clinical Bioengineering, The University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA

Received 20 February 2003;  accepted 4 September 2003. ; Available online 3 December 2003.
Biomaterials
Volume 25, Issue 15 , July 2004, Pages 2927-2932

1. Abstract

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°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
-tricalcium phosphate phase. Metallic Ti was not observed in the in-vacuum heat-treated HA-Ti specimens. However,
-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.

Author Keywords: Hydroxyapatite; Titanium; Heat treatment; X-ray diffraction; Scanning electron microscopy

2. Article Outline

1. Introduction

2. Materials and methods

2.1. X-ray diffraction

2.2. FTIR spectroscopy

2.3. Scanning electron microscopy/energy dispersive spectroscopy

3. Results

4. Discussion

5. Conclusion

Acknowledgements

References

3. 1. Introduction

Calcium phosphate (CaP) ceramic coatings and synthetic hydroxyapatite (HA) are being used in an attempt to obtain better stabilization of metallic implants in the bone and surrounding tissue [1, 2, 3, 4, 5, 6, 7 and 8]. At present, most commercial HA- or CaP-coated implants are produced using plasma spray technology [1]. However, due to the very high temperature produced by the plasma flux, the starting HA powder undergoes full or partial melting during plasma spraying. This semi or fully molten HA are carried by the plasma to the implant substrate at very high speed. After deposition on the implant surface, temperature of the HA fall rapidly to the temperature of the substrates. During this process, majority of the HA was reported to loose its hydroxyl (OH) group and was transformed into oxyhydroxyapatite (OHA), while some decomposed into tetracalcium phosphate (TCPM), calcium phosphate and calcium oxide (CaO) [9, 10 and 11]. A partially amorphous phase was produced by rapid solidification of the melted outer layer of HA particles striking a cool substrate. The amorphous phase in as-received coatings was reported to compose of oxyapatite [12]. As a result, the final plasma-sprayed HA coatings on implant surfaces consisted of HA,
-tricalcium phosphate (
-TCP),
-tricalcium phosphate (
-TCP), TTCP and some amorphous phases. To eliminate residual stresses and obtain high crystallinity, a post spraying heat treatment in a controlled atmosphere was performed. This treatment transformed amorphous HA to crystalline HA. Generally, the post spraying heat treatments were done at temperatures ranging between 600°C and 1000°C under vacuum conditions at less than 10⁻³ Pa (10⁻⁵ torr) for 30 min to 4 h [13, 14 and 15]. Our previous study indicated that the substrates had an important role on the HA and CaP coatings, and the adhesive, compositional and structural properties of HA or CaP coatings were suggested to be critically governed by the interactions between HA particles and Ti implant substrates [16]. In an attempt to understand the interactions between HA coating and Ti substrate at the coating-metal interface, the interactions between Ti and HA mixture heated at 1100°C in a vacuum environment were investigated. X-ray diffraction (XRD), scanning electron microscope (SEM), and energy disperse spectra (EDS) were used in this study to characterize the heat-treated HA-Ti composite materials.

4. 2. Materials and methods

The starting HA powder was synthesized by chemical precipitation at 95°C at a pH 9.0-11.5 and was characterized by chemical analysis, XRD, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM). The specific surface area and the particle size of the starting powder were examined by a rapid specific surface analyzer (2200A, Micromeritics Company, USA) and a particle size analyzer (SA-CP3, Shimadzu), respectively [17]. It was observed from the chemical analysis, XRD, and FTIR that the synthesized powder was in the phase structure form of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and the Ca/P ratio was 1.65-1.67. The average particle size of HA powder was 1.44 
m. Commercially pure Ti powders (cp Ti, 99.3%, Chinese Nonferrous Metal Institute) used in this study were obtained commercially from the Chinese Nonferrous Metal Institute, and the size of Ti powder was in the range of 10-80 
m.

The 80 wt% HA-20 wt% Ti mixture powder (80% HA) and 90 wt% HA-10 wt% Ti mixture powder (90% HA) were mechanically mixed. Pure HA powders and the cp Ti powders were used as controls in this study. All powders were cold dry pressed into disks with diameter 20 mm and thickness 8 mm and heat treated at 1100°C in vacuum (<6×10⁻³ Pa) for 30 min. The densities of the specimens before and after heat treatment were measured using Archimedes method.

2.1. X-ray diffraction

Using a Rigaku D/max-IIIA X-ray diffractometer (Rigaku, TX, USA), XRD analysis was used to characterize the structure of the in-vacuum heat treated HA, Ti and HA-Ti composites. Triplicate specimens were analyzed with CuK
radiation at 40 kV and 180 mA. The samples were scanned from 10° (2
) to 70° (2
) at a scan rate of 4° per minute.

2.2. FTIR spectroscopy

Structural and molecular composition of in-vacuum heat treated HA specimens and starting powders were evaluated using a Perkin-Elmer Spectrum 2000 Optica FTIR (Perkin-Elmer, Shelton, CT, USA). Using a resolution of 1 cm⁻¹ and a scan number of 32, the samples were analyzed from 400 to 4000 cm⁻¹ using a diffuse transmittance accessory. Prior to FTIR analysis, the in-vacuum heat treated HA specimens were smashed into powders and 1% of the smashed powders were mixed with FTIR grade potassium bromide (KBr) powders. To ensure uniformity in the dilution, the powder mixture was ground for 2 min. The KBr powders were also used for background collection.

2.3. Scanning electron microscopy/energy dispersive spectroscopy

Fracture surface morphology and composition of in-vacuum heat treated HA-Ti composites were evaluated. Prior to evaluation, the cross-section surface of all specimens were polished. The surfaces were then sputter-coated with a thin layer of carbon and were observed using a HITACHI X-650 SEM (Hitachi, Tokyo, Japan) at 20 kV. Elemental concentration of calcium and phosphorus were measured on different regions of the samples using a Hitachi PV9100 energy dispersive spectroscopy (Hitachi, Tokyo, Japan) attached to the SEM.

5. 3. Results

Representative XRD patterns of control HA powders and in-vacuum heat-treated HA specimens are shown in Fig. 1. It was observed that the control HA powders exhibited apatite-type structure, and the in-vacuum heat-treated HA specimens consisted mainly of an apatite-like phase, with
-TCP as a minor phase. Representative FTIR patterns for control HA powders and in-vacuum heat treated HA specimens are shown in Fig. 2. Analysis of control HA powders indicated the presence of hydroxyl (OH) peaks at 3572 and 631 cm⁻¹ and phosphate (PO₄) peaks at 1084, 962, 574, 550, 535 and 470 cm⁻¹. An additional peak at 1470 cm⁻¹ was observed for the control HA powders. In comparison to the control HA powders, the OH peak at 3572 cm⁻¹ and PO₄ peaks at 962, 631 and 574 cm⁻¹ were not observed for the in-vacuum heat-treated HA specimens. As shown in Fig. 3, SEM analysis of the fracture morphology of the in-vacuum heat-treated HA specimens showed that the specimens consisted of the 2 
m particles and were porous.

(5K)

Fig. 1. Representative XRD patterns of (a) control HA powders, and (b) in-vacuum heat-treated HA specimens. The arrow indicates the
-tricalcium phosphate (
-TCP) peak.

(6K)

Fig. 2. Representative FTIR spectrum of (a) control HA powders, and (b) in-vacuum heat-treated HA specimens.

(36K)

Fig. 3. Representative scanning electron micrograph (×2000 original magnification) of fracture morphology of in-vacuum heat treated HA specimens.

Representative XRD pattern and cross-section morphology of in-vacuum heat-treated Ti specimens are shown in Fig. 4 and Fig. 5. No difference in XRD peak position was observed for the control cp Ti powders and the in-vacuum heat treated Ti specimens. However, the full-width at half-maximum (FWHM) for heat-treated Ti was observed to be wider as compared to the control Ti powders. The in-vacuum heat-treated Ti specimens were shining, with silver white in color. The crystal grain and its boundary of in-vacuum heat-treated Ti specimens were clearly observed, with the presence of some pores.

(6K)

Fig. 4. Representative XRD patterns of (a) control titanium (Ti) powders, and (b) in-vacuum heat treated Ti specimens.

(20K)

Fig. 5. Representative scanning electron micrograph (×400 original magnification) of cross-section morphology of in-vacuum heat treated Ti specimens.

Representative XRD patterns of in-vacuum heat-treated HA-Ti specimens are shown in Fig. 6. It was observed that the in-vacuum heat-treated HA-Ti composite specimens consisted of HA,
-TCP, TTCP and calcium titanium oxide (Ca₂Ti₂O₅).¹ No Ti peak was observed in the XRD patterns of HA-Ti mixtures. In addition, the most intense XRD peak for the 80% HA composite specimen was observed to be HA, and the most intense XRD peak for the 90% HA composite specimen was observed to be CaTi₂O₅. In comparison to the in-vacuum heat-treated 80% HA specimens, the intensity of HA peaks in the in-vacuum heat-treated 90% HA specimens decreased while the intensity of TCP and Ca₂Ti₂O₅ peaks increased.

(7K)

Fig. 6. Representative XRD patterns of in-vacuum heat-treated HA-Ti specimens (a) 80% HA; (b) 90% HA. (#) HA peaks; ($) TTCP peaks; (*)
-TCP; and (+) Ca₂Ti₂O₅.

Representative fracture surface morphology of in-vacuum heat treated HA-Ti specimens are shown in Fig. 7. It was observed that the in-vacuum heat-treated HA-Ti specimens composed of three different morphologies. The first morphology observed on the specimens was granular aggregation while the second morphology was porous. The third morphology on the specimens exhibited a melted, glassy phase. The granular aggregation and porous morphologies were observed in 80%HA specimens, whereas all three different morphologies were observed in 90% HA specimens. Elemental concentrations observed in the different morphologies are listed in Table 1. In combining the XRD patterns in Fig. 6 and the SEM observations in Fig. 7, it was suggested that the areas exhibiting granular aggregation consisted of a mixture of calcium phosphate and calcium titanium oxide (Ca₂Ti₂O₅). The porous area and the area exhibiting melted, glassy phase consisted of apatite (HA),
-TCP, and TTCP.

(29K)

Fig. 7. Representative scanning electron micrograph of fracture morphology of in-vacuum heat-treated HA-Ti specimens (a) 80% HA (×1500 original magnification); (b) 90% HA (×1200 original magnification) showing different morphologies. The first morphology was the areas showing granular aggregation (A). The second morphology was the porous area (B), and the third morphology was the areas exhibiting a melted, glassy phase (C).

Table 1. Elemental composition of three different morphologies observed on HA-Ti composite specimens

The first morphology was the areas showing granular aggregation (A). The second morphology was the porous area (B), and the third morphology was the areas exhibiting a melted, glassy phase (C). The granular aggregation and porous morphologies were observed in 80% HA specimens, whereas all three different morphologies were observed in 90% HA specimens.

6. 4. Discussion

Using the HA-Ti mixture as a model, this study attempts to understand the interactions between HA and Ti at elevated temperature. In this study, four different cold-pressed green disks (HA, cp Ti, 80% HA, and 90% HA) were heat treated at 1100°C in vacuum environment for 30 min. Using HA and Ti powders as controls, all specimens were characterized using XRD, SEM and EDS.

It was observed in this study that the control HA powders exhibited apatite-type structure, whereas the in-vacuum heat-treated HA specimens consisted mainly of an apatite-like phase, with
-TCP as a minor phase. The presence of
-TCP in the in-vacuum heat-treated HA specimens indicated decomposition of the HA during sintering. Analysis of the control HA powders indicated the presence of hydroxyl (OH) peaks at 3572 and 631 cm⁻¹ and phosphate (PO₄) peaks at 1084, 962, 574, 550, 535 and 470 cm⁻¹ [17 and 18]. Additional peak at 1470 cm⁻¹ observed on the HA powder was assigned to the carbonate [17 and 18]. In comparison to control HA powder, the OH peak at 3572 cm⁻¹ and PO₄ peaks at 962, 631 and 574 cm⁻¹ were not observed for the in-vacuum heat-treated HA specimens. Such a change in composition during sintering were consistent with the structure change observed in the in-vacuum heat-treated HA specimen. In agreement with other studies, the loss of OH and the change in some phosphate structures were observed during the in-vacuum heat treatment process [11, 12, 13 and 14].

From this study, no difference in XRD peak position was observed for the control cp Ti powders and the in-vacuum heat treated Ti specimens. In our previous study using X-ray photoelectron spectroscopy, the widening in the FWHM value of the Ti 2p_3/2 peak for the Ti sample resulted from an amorphous oxide layer on Ti surfaces [19, 20 and 21]. It was also observed in Fig. 4 that the FWHM of heat-treated Ti slightly widened as compared to the Ti powders, and therefore suggesting an amorphous structure after heat-treated Ti surfaces. As suggested by other investigators, this phenomenon of having an amorphous structure after heat-treatment was observed to be attributed to the melting of Ti at high temperatures [22].

The in-vacuum heat-treated HA-Ti composite specimens were observed to consist of HA,
-TCP, TTCP and calcium titanium oxide (Ca₂Ti₂O₅) (see footnote 1). No Ti peaks were observed in the XRD patterns of HA-Ti mixtures. SEM indicated three different morphologies on the HA-Ti specimens. In combining the SEM and the elemental composition distribution observed in this study, areas exhibiting granular aggregation suggested the presence of a mixture of calcium phosphate and Ca₂Ti₂O₅. The porous area and the area exhibiting melted, glassy phase consisted of HA,
-TCP, and TTCP. These different morphologies was suggested to be due to differences in the melting point of titanium and HA and the probable fusion of HA with molten titanium at 1100°C [22]. Observations from this study suggested some possible reactions during HA and Ti interactions. With an increase in temperature in a vacuum environment, it is feasible that HA first lost part of hydroxyl groups in the following reaction [23 and 24]:

(1)

Subsequently, as shown in Eq. (2), water vapor oxidized the Ti metal at high temperature, thereby forming titanium dioxide (TiO₂):

Ti+2H2O (gas)→TiO2+2H2.

(2)

As shown in Eq. (3), the TiO₂ finally reacted with HA and thereby producing CaTi₂O₅:

Ca10(PO4)6(OH)2+2TiO2→3Ca3(PO4)2+CaTi2O5+H2O.

(3)

In addition, according to Eq. (4), it was reported that under high temperature and in vacuum, HA was induced by Ti to decompose into
-TCP and TTCP [23]:

(4)

On the other hand, at temperature higher than 1200°C in atmosphere, HA was reported to decompose into
-TCP and TTCP, with the
-TCP transforming into
-TCP [23 and 24]. In this study, the metal-ceramics composites completely changed into ceramic composites during the in-vacuum heat treating process.

7. 5. Conclusion

Commercially pure Ti specimens were observed to be densified using the in-vacuum heat treatment. On the other hand, loss of hydroxyl groups and the presence of
-TCP phase were observed after subjecting the HA specimens to a heat treatment in vacuum environment. No metallic Ti was observed in the in-vacuum heat-treated HA-Ti specimens. On the contrary,
-TCP, TTCP, and calcium titanium oxide (Ca₂Ti₂O₅) 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.

8. Acknowledgements

This study was funded by a grant from the National Institute of Health (Grant No. 1RO1AR46581).

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Corresponding author. Department of Restorative Dentistry, Division of Biomaterials, The University of Texas Health Science Center at San Antonio, MSC 7890, 7703 Floyd Curl Drive, , San Antonio, TX 78229-3900, , USA. Tel.:[phone] ; fax: [phone]

¹ PDF 25-1450.

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