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J Dent Res 82(10): 833-837, 2003
© 2003 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Influence of Post-deposition Heating Time and the Presence of Water Vapor on Sputter-coated Calcium Phosphate Crystallinity

Y. Yang1, K.-H. Kim1,2, C.M. Agrawal3,4, and J.L. Ong1,4,*

1 The University of Texas Health Science Center at San Antonio, Department of Restorative Dentistry, Division of Biomaterials, MSC 7890, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900;
2 Kyungpook National University, College of Dentistry and Institute of Biomaterials Research and Development, Department of Dental Biomaterials, 2-101 Dongin Dong, Jung-Gu, Daegu 700-422, Korea;
3 The University of Texas at San Antonio, College of Engineering, 6900 N. Loop 1604, San Antonio, TX 78249-0619; and
4 The University of Texas Health Science Center at San Antonio, Center for Clinical Bioengineering, Department of Restorative Dentistry, Division of Biomaterials, MSC 7890, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900;

*corresponding author, ong{at}uthscsa.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Extensive research suggested that calcium phosphate (CaP) coatings on titanium implants are essential for early bone response. However, the characterization of CaP crystallinity and the means to control coating crystallinity are not well-established. In this study, the effect of a 400°C heat treatment for 1, 2, or 4 hours, and in the presence or absence of water vapor, on CaP crystallinity was investigated. Scanning electron microscopy indicated dense as-sputtered coatings. Increase in coating crystallinity was observed to be consistent with the increasing number of PO4 peaks observed as a result of different heat treatments. In addition, x-ray diffraction analyses indicated amorphous as-sputtered coatings, whereas crystalline CaP coatings in the range of 0-85% were observed after different post-deposition heat treatments. It was concluded that the presence of water vapor and post-deposition heat treatment time significantly affect the crystallinity of CaP coatings, which may ultimately affect bone healing.

KEY WORDS: calcium phosphate coating • heat treatment • x-ray diffraction • crystallinity • Fourier transform infrared spectroscopy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Plasma-sprayed hydroxyapatite (HA) and calcium phosphate (CaP) coatings are successfully used in dental and orthopedic implant therapy. Despite their rapid deposition rate and sufficiently low cost, problems associated with plasma-sprayed coatings include poor adhesion between the coatings and substrates and alterations in HA or CaP structure (Yang et al., 2003a,b,c; Yang and Ong, 2003). Several other deposition processes, such as sputtering and high-velocity oxy-fuel combustion spraying, have been tested (Yang et al., 2003a). It was concluded that sputtering may be the method of choice for controlling the physical and chemical properties of HA or CaP coatings on implant surfaces.

In addition to the coating process, extensive in vitro and in vivo research also suggested that CaP coatings are essential for early bone performance when compared with Ti implants (Overgaard et al., 1997, 1999; Ong et al., 1998, 2002; Ferraz et al., 1999; Blokhuis et al., 2000; Moroni et al., 2002; ter Brugge and Jansen, 2002). The presence of water vapor during heat treatment has been reported to affect the crystallinity of plasma-sprayed HA coatings (Cao et al., 1996; Chen et al., 1997; Tong et al., 1997). In our attempt to control the crystallinity of sputtered CaP coatings in this study, we evaluated the effects of heat treatment time and the presence of water vapor during heat treatment on the crystallinity of CaP coatings.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Materials Preparation
Titanium (Ti) grade 2 (Metal Samples, Munford, AL, USA) disks, 15.30 mm in diameter and 2 mm thick, were used in this study. We prepared surfaces by wet-grinding with 240-, 400-, and 600-grit silicon carbide paper to a surface roughness (Ra) of 0.37 ± 0.01 µm. These surfaces were ultrasonically de-greased in acetone and ethanol for 10 min each, with de-ionized water rinses between applications of each solvent. Passivation was accomplished by exposure of the surfaces to a 40% nitric acid solution at room temperature for 30 min (ASTM F86-76), rinses with de-ionized water, followed by air-drying. The cleaned surfaces were then sputter-coated with a thin CaP layer.

Sputter Coating
Sputtering was performed with the use of a CMS-18 radiofrequency magnetron sputtering system (Kurt J. Lesker Company, Clairton, PA, USA) and a 4-inch-diameter sintered HA target (0.25 inches thick) bonded to a copper backing (Target Materials, Inc., Dayton, OH, USA). The base pressure in the sputtering chamber was <= 5 x 10-6 torr. Sputtering was accomplished with the use of a process pressure of 1.0 ~ 1.5 mbar and a power of 200 W for 7 hrs at a coating rate of 60 nm per hour. Fig. 1Go displays a representative fracture surface morphology of as-sputtered CaP coatings on a slide glass substrate. The dense and continuous as-sputtered coating was observed to consist of nano-particles of approximately 5 nm in diameter and in the range of 50 to 100 nm in diameter in the subsurface and surface, respectively.



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  		Figure 1. Representative scanning electron micrograph of a continuous and dense as-sputtered calcium phosphate (CaP) coating on glass substrate. The as-sputtered CaP coating was observed to consist of nano-particles approximately 5 nm in diameter and in the range of 50 to 100 nm in diameter in the subsurface and surface, respectively.

 
After being sputtered, samples were either left as-sputtered or heat-treated at 400°C for either 1 hr, 2 hrs, or 4 hrs, and in the presence or absence of water vapor in a Thermolyne 48000 furnace (Barnstead International, Dubuque, IA, USA).

X-ray diffraction
A D8 Advanced x-ray diffractometer (Bruker, Madison, WI, USA), equipped with a single Gobel mirror to yield a diffracted parallel beam while removing the Kß radiation, was used to characterize the structure of coatings. Using a grazing incidence attachment, a 0.35° soller slit, and a LiF (100) flat crystal monochromator to improve resolution and peak-to-background ratios, we analyzed triplicate coatings using Cu K{alpha} radiation at 40 KV and 30 mA. Triplicate coatings per treatment group were scanned from 25° to 35° 2{Theta} at a scan rate of 0.1° per min. Crystalline peak area in the 25° to 35° range was calculated, and we quantified the percent crystallinity of the coatings by correlating the crystalline peak area to the known HA crystallinity standard curve, derived by mixing various ratios of 100% crystalline and amorphous commercial HA powder (Hitemco Medical Applications, Inc., Bethpage, NY, USA).(AQ) Percent crystallinity of the coating was analyzed by ANOVA, and differences were considered significant if P < 0.05.

Fourier Transform Infrared (FTIR) Spectroscopy
Structural and molecular composition of coatings and sputtering target were evaluated by means of a model 550 Magna-IRTM FTIR (Thermo Nicolet, Madison, WI, USA). Using a resolution of 1 cm-1 and a scan number of 32, we analyzed triplicate coatings per treatment group from 400 cm-1 to 4000 cm-1. Control Ti disks were used for background collection. For the CaP coatings, data collection was performed with the use of a 80° grazing angle accessory.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
X-ray diffraction
X-ray diffraction analyses of the HA target, as-sputtered coatings, and heat-treated CaP coatings in the presence or absence of water vapor are shown in Fig. 2Go. It was observed that the HA target consisted mainly of an apatite-like phase, with the beta-tricalcium phosphate as a minor phase. The most intense peak for the HA target was observed to be at 31.9° (2{Theta}). As-sputtered coatings were observed to be amorphous, whereas heat-treated coatings were observed to exhibit an increase in coating crystallinity. Coatings after 1 hr of post-deposition heat treatment in the presence and absence of water vapor were observed to exhibit only one small crystalline peak at 32.9° (2{Theta}). The most intense peaks for CaP coatings after 2 hrs and 4 hrs of heat treatment in the presence of water vapor were observed to be at 32.9° (2{Theta}) and 25.9° (2{Theta}), respectively.



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  		Figure 2. Representative x-ray diffraction patterns of as-sputtered calcium phosphate (CaP) coatings, hydroxyapatite (HA) target, and sputtered CaP coatings after a 400°C heat treatment at different heating times in (A) the absence of water vapor and (B) the presence of water vapor. The different surfaces on both (A) and (B) were (1) no heat treatment (as-sputtered), (2) heat treatment for 1 hr, (3) heat treatment for 2 hrs, (4) heat treatment for 4 hrs, and (5) HA target.

 
Percent crystallinity of the HA target, as-sputtered coatings, and heat-treated CaP coatings is shown in the TableGo. In the presence or absence of water vapor, the CaP crystallinity was observed to increase significantly with heat treatment time. Coatings after 1 hr of heat treatment in the presence (2.8 ± 0.6%) or absence (1.9 ± 0.4%) of water vapor were observed to exhibit poor crystallinity. In comparison with one-hour heat treatments, an increase in coating crystallinity was observed after 2 hrs’ heat treatment in the presence (63.9 ± 10.8%) and absence (32.5 ± 9.9%) of water vapor. Increasing the heat treatment time to 4 hrs resulted in the coatings exhibiting a crystallinity of 85.0 ± 5.3% in the presence and 65.8 ± 8.5% in the absence of water vapor. As compared with coatings with the same heat treatment time but in the absence of water vapor, coatings heat-treated in the presence of water vapor were observed to increase the degree of coating crystallinity significantly (P < 0.02 and 0.03 at two- and four-hour heat-treatment time, respectively).


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  		Table. Crystallinity (mean ± SD; n = 3) of Calcium Phosphate (CaP) Coatings after a 400°C Heat Treatment at Different Heating Times in the Presence and Absence of Water Vapor
 
Fourier Transform Infrared (FTIR) Spectroscopy
The FTIR analysis of the HA target, as-sputtered coatings, and heat-treated CaP coatings is shown in Fig. 3Go. Analysis of the HA target indicated the presence of hydroxyl (OH) peaks at 3572 cm-1 and phosphate (PO4) peaks at 1084 cm-1, 964 cm-1, 617 cm-1, 585 cm-1, and 458 cm-1. Additional peaks observed on the HA target were 656 cm-1, 1176 cm-1, and 3647 cm-1.



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  		Figure 3. Representative Fourier transform infrared spectrum of as-sputtered calcium phosphate (CaP) coatings, hydroxyapatite (HA) target, and sputtered CaP coatings after a 400°C heat treatment at different heating times. The different surfaces were (1) no heat treatment (as-sputtered), (2) heat treatment for 1 hr in the absence of water vapor, (3) heat treatment for 2 hrs in the absence of water vapor, (4) heat treatment for 4 hrs in the absence of water vapor, (5) heat treatment for 1 hr in the presence of water vapor, (6) heat treatment for 2 hrs in the presence of water vapor, (7) heat treatment for 4 hrs in the presence of water vapor, and (8) HA target. The arrow indicates the OH peak at 3572 cm-1, and the asterisk indicates the PO4 peaks at 1084 cm-1, 964 cm-1, 617 cm-1, 585 cm-1, and 458 cm-1.

 
No OH peak was observed in as-sputtered coatings and CaP coatings heat-treated for 1 hr in the absence and presence of water vapor. For CaP coatings heat-treated for 2 and 4 hrs in the presence and absence of water vapor, an OH peak at 3572 cm-1 was observed. The number of PO4 peaks was observed to increase with increasing heat treatment time. Two PO4 peaks, at 547 cm-1 and 458 cm-1, were observed in as-sputtered coatings and CaP coatings heat-treated for 1 hr in the absence and presence of water vapor. In addition to these peaks, an additional PO4 peak at 573 cm-1 was also observed for coatings heat-treated for 2 hrs in the absence of water vapor, whereas additional PO4 peaks at 1084 cm-1, 964 cm-1, 617 cm-1, and 585 cm-1 were observed for coatings heat-treated for 2 hrs in the presence of water vapor and for coatings heat-treated for 4 hrs in the presence or absence of water vapor. The C-H peak at 2849 cm-1 and the adsorbed water molecule peaks at 3365 cm-1 and 3735 cm-1 were also observed for CaP coatings heat-treated for 2 hrs in the presence of water vapor and for coatings heat-treated for 4 hrs in the presence or absence of water vapor. In addition, the presence of water molecules (3365 cm-1, 3735 cm-1) and other CaP compounds such as oxyhydroxyapatite and amorphous CaP (1253 cm-1, 1158 cm-1, 915 cm-1, 812 cm-1) was also observed for all coatings.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Extensive in vitro and in vivo research suggested that CaP coatings are essential for early bone performance when compared with Ti implants (Overgaard et al., 1997, 1999; Ong et al., 1998, 2002; Ferraz et al., 1999; Blokhuis et al., 2000; Moroni et al., 2002; ter Brugge and Jansen, 2002). Previous characterizations indicated that as-sputtered coatings were amorphous (Ong et al., 1992) but displayed an HA-type crystalline structure after heat treatments (Ong and Lucas, 1994). In this study, the effect of heating treatment time and the presence of water vapor on the degree of crystallinity of sputtered CaP coatings were evaluated.

In the process of sputtering CaP coatings, plasma argon ions were produced with the use of working gas argon and were accelerated by electromagnetic field to bombard the HA target surface. It was reported that surface calcium and phosphorus atoms were bombarded off the target and initially deposited on substrates as CaP islands (Ong et al., 1991). The formation of these initial islands during sputtering had been attributed to the use of hollow cathode sources and the use of substrate rotation (Thornton and Hedgcoth, 1975). Over continuous sputtering time, CaP islands combined and formed the integrated CaP coating. The as-sputtered coatings were observed to be amorphous, indicating that the sputtering process resulted in a loss of the apatite-type structure found on the HA target. As observed in this study, heat treatments allowed CaP coatings to crystallize, forming an apatite-type structure. However, changes in the most intense peak between heat-treated CaP coatings and HA target suggested differences in structural orientation of the crystals. Poorly crystalline CaP coatings were observed after a 400°C heat treatment for 1 hr, as indicated by a peak at 32.9° (2{Theta}). This initial peak formation for poorly crystalline CaP coatings suggested that crystal growth initially began at the [300] crystal plane. The most intense peak remained at 32.9° (2{Theta}) after a two-hour heat treatment in the presence of water vapor. It was suggested from these observations that the [300] crystal plane was the preferred growth crystal plane for the initial CaP coatings heat-treated at 400°C.

In the absence of water vapor, crystallinity of the coatings increased with increasing heating time. The amorphous-to-crystalline transformation of CaP coatings at such a low temperature was suggested to be attributed to the nanostructure of as-sputtered coatings (Stupp and Braun, 1997). The presence of nanostructure in as-sputtered coatings was confirmed by SEM, showing a very dense coating consisting of nano-particles within the coatings. Similar to the parameters used for producing amorphous as-sputtered coatings, other reports have also indicated that nano-structural HA could be synthesized in ambient temperature and pressure (Stupp and Braun, 1997).

Using identical heating time, we observed coatings heated in the presence of water vapor to have a significantly higher degree of crystallinity compared with coatings heated without water vapor. This suggested that the presence of water vapor during heat treatments plays an important role in increasing CaP crystallinity. In addition, this observation was in concurrence with other reports indicating a significant increase in the crystallinity of plasma-sprayed HA coatings after heating in the presence of water vapor, as compared with HA coatings heated in air (Cao et al., 1996; Chen et al., 1997; Tong et al., 1997).

FTIR analysis of the HA target indicated the presence of OH peaks at 3572 cm-1 and PO4 peaks at 1084 cm-1, 964 cm-1, 617 cm-1, 585 cm-1, and 458 cm-1. Additional peaks observed on the HA target were 656 cm-1, 1176 cm-1, and 3647 cm-1 (Arends et al., 1987; Koutsopoulos 2002). As suggested by other investigators, the 1176 cm-1 peak was assigned to the presence of other CaP compounds and amorphous CaP, and the 3647 cm-1 peak was assigned to water molecules which replaced the OH groups bonded to Ca ions (Joris and Amberg, 1971).

An OH peak at 3572 cm-1 was observed for CaP coatings heat-treated for 2 and 4 hrs in the presence and absence of water vapor. It was reported that the OH group variation indicated different degrees of dehydroxylation as a result of HA decomposition during the coating or treatment process (Berry and Baddiel, 1976). As a result of decomposition, it was reported that HA converted to oxyhydroxyapatite, with the formula Ca10(PO4)6(OH)2-2xOx{delta}x ({delta} = vacancy, x < 1) (Berry and Baddiel, 1976; Tsui et al., 1998).

The PO4 peaks at 547 cm-1 and 458 cm-1 were observed in as-sputtered coatings and CaP coatings heat-treated for 1 hr in the presence and absence of water vapor. Increasing the heating time to 2 hrs in the absence of water vapor resulted in an additional PO4 peak at 573 cm-1. For coatings heat-treated for 2 hrs in the presence of water vapor and 4 hrs in the presence and absence of water vapor, additional PO4 peaks at 1084 cm-1, 964 cm-1, 617 cm-1, and 585 cm-1 were observed. The increasing number of PO4 peaks observed as a result of increasing heat treatment time or the presence and absence of water vapor was consistent with the increasing degree of coating crystallinity.

In addition to OH and PO4 peaks, other peaks at 3365 cm-1, 3735 cm-1, 1253 cm-1, 1158 cm-1, 915 cm-1, and 812 cm-1 were also observed for all CaP coatings. The peaks at 3365 cm-1 and 3735 cm-1 were suggested to be attributed to adsorbed water molecules (Joris and Amberg, 1971; Koutsopoulos, 2002), whereas the peaks at 1253 cm-1, 1158 cm-1, 915 cm-1, and 812 cm-1 were assigned to the presence of other CaP compounds, such as oxyhydroxyapatite and amorphous CaP (Tsui et al., 1998; Koutsopoulos, 2002).


   ACKNOWLEDGMENTS
 
This study was funded by grants from the NIH (Grant Nos. 1RO1AR46581 and 1S10RR016879).

Received February 24, 2003; Last revision July 22, 2003; Accepted July 25, 2003


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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