249
Structure, solubility and bond strength of thin calcium phosphate coatings produced by ion beam sputter deposition J.L. Ong*, L.C. Lucas*, W.R. Lacefieldtand
E.D. Rigneyt
t Dental Biomaterials and $ Materials Science & Engineering, Departments of *Biomedical Engineering, University of Alabama at Birmingham, University Station, Birmingham, Alabama 35294, USA
Ion beam sputter deposition was used to produce thin calcium phosphate coatings on titanium substrates. Structure, solubility and bond strength of the as-sputtered and heat treated coatings were evaluated. X-ray diffraction (XRD) analysis of the heat treated coatings revealed a hydroxyapatite-type structure. The heat treated coatings were found to have significantly lower solubility as compared to the amorphous as-sputtered coatings. Although the crystalline coatings exhibited the lowest solubility, in general, the bond strengths were lower for the heat treated coatings. Keywords: Received
Calcium 26 February
phosphates,
coatings,
1991;
11 June
revised
Calcium phosphate 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’-‘. Currently, plasma spraying is the primary method used commercially to produce ceramic coatings on metallic implants. However, problems such as low bond strength and non-uniformity in coating density are often observed with this coating techniqueg. A recent study on plasma-sprayed hydroxyapatite on Ti-6Al-4V reported an average bond strength of 6.7 f 1.5 MPa”. Using a post heat treatment of 96O”C, bond strengths were increased in a range of 15.0 to 26.0 MPa, but there is concern about the effects of the high temperature on the metallic substrates”. An alternative coating method, ion beam sputter deposition, is being investigated as a means for producing biocompatible ceramic coatings on metallic implants With this technique, an ionized gas is used to sputter atoms from a ceramic target. These sputtered atoms build up on the metallic substrates placed in the path of the sputtered material. Previous studies have reported that an improvement in some of the properties of calcium phosphate coatings can be achieved using the ion beam sputter deposition process”, “. In particular, sputter coatings with bond strength values as high as 47.3 MPa have been reportedI’. However, because a thickness of 2 pm or less is practical with the sputter coating process, there is concern with the solubility of the sputtered Correspondence
0142-9612/92/040249-06
properties
1991; accepted
12 August
1991
that the structure of the coatings13. It is hypothesized calcium phosphate coatings determines the dissolution rate in solution. In this study, heat treatments were used to crystallize the calcium phosphate coatings produced by single ion-beam deposition. Full characterizations of both the as-sputtered and heat treated coatings as to structure, bond strength and solubility were conducted in this study.
MATERIALS AND METHODS Substrates Wrought commercially pure titanium (ASTM F76) was mechanically ground with 240, 400 and 600 grit silicon carbide paper and polished using 0.3 pm alumina powder. These substrates were then ultrasonically degreased with benzene, acetone and ethanol for 10 min each. Following the cleaning procedure, half of the substrates were passivated with 40% nitric acid for 30 min (ASTM F86). The remaining substrates did not receive the nitric acid passivation treatment and were referred to as non-passivated specimens. The nonpassivated specimens were only used for the bond strength tests.
Target The target material used during ion-beam position was a hydroxyapatite-fluorapatite
to Dr J. Ong.
0 1992 Butterworth-Heinemann
material
Ltd
Biomaterials
sputter detarget, con-
1992, Vol.
13 No. 4
250
Calcium phosphate coatings: J. L. Ong et al.
_
sisting of two materials; fluorapatite powders (X-ray diffraction pattern matched JCPDS # 34-0011) of 325 mesh and a hydroxyapatite powder disk (chemistry was confirmed with XPS), from Lifecore Biomedical, Inc. (Minneapolis, MN, USA). The hydroxyapatite powder disk was hot-pressed and sintered by Cerac, Inc. (Milwaukee, WI, USA) at 1150°C.Fluorapatite powders were applied to the surface of the target disk prior to sputtering.
Single ion beam sputter deposition A schematic of the ion beam sputter coater used in this study is shown in Figure 3. The titanium substrates were fixed on to the rotating stage of the sputtering system (Commonwealth Scientific, Alexandria, VA, USA) with a conductive adhesive containing silver particles. The vacuum chamber was evacuated to a minimum base pressure of 6.0 X 10e6Torr. High purity argon (99.999%) was backfilled into the chamber, bringing the pressure to z 4 X 10e4 Torr. Prior to the deposition process, the titanium substrates were sputter cleaned using a second ion beam. The substrates were placed in the path of the ion beam, and sputter-cleaning was achieved for 30 min at 40.7 mA and 500 eV (Figure la). After cleaning, the titanium substrates were moved into position to be coated with the sputtered material (Figure lb). The sputter deposition process was accomplished at a chamber pressure of z 4.0 X 10m4 Torr, with an ion-
f-7
IRotating stage
I
Ion source Titanium substrate
a
f
Post deposition
Ion source
b Figure1 Schematic representation of ion beam sputter deposition: a, sputter cleaning prior to sputter deposition; b, single ion beam sputter deposition. 1992, Vol. 13 No. 4
heat treatments
Previous studies have shown that as- sputtered calcium phosphate coatings are amorphous or only diffusely crystalline 12. In an attemp t to obtain crystallinity in the sputtered coatings, the coated samples were randomly selected for post-deposition heat treatments, conducted at 600°C in air for 1 h. These samples were then again randomly selected for either quenching in deionized water or furnace cooling until room temperature was reached (z 4 h).
Scanning
electron
microscopy
@EM)
The commercially pure titanium samples of 1 cm in diameter and 2 mm thick were coated, and the coated samples placed in the SEM vacuum chamber (ISI-lOOB, International Scientific Instruments, Inc., Pleasanton, CA, USA) tilted at 30” and subjected to an electron beam at 15 KeV. To prevent charging, the samples were coated with a thin layer of carbon. SEM was used to determine if any surface defects existed for either the as-sputtered or heat-treated coatings.
X-Ray diffraction
(XRD)
X-ray diffraction was used to evaluate the structure of the as-sputtered and heat treated coatings. For these analyses, the commercially pure titanium coupons (5.1 X 2.54 cm) were coated. A Siemens diffractometer using Cu K, radiation having energies of 40 KeV and 30 mA was used. Three samples for each treatment were analysed and data were collected from 5” to 95O 28 at 0.1 degree per min scan rate, for a total of z 15 h. Crystalline coatings were identified by matching the peaks with standard synthetic hydroxyapatite (JCPDS # g-0432). As in other studies on hydroxyapatite, lattice parameters and cell volume with one standard deviation were calculated based on the 002 reflections for the a spacing and 300 reflections for the c spacing’4”5. The lattice spacings and volume were statistically analysed using the Least Squares Means.
Solubility
Titanium
Biomaterials
beam energy of 1000 eV and 40.7 mA. Sputter deposition was carried out at the rate of 0.15 pm per hour, resulting in a coating thickness of ” 0.6-lprn for each sample.
evaluations
Commercially pure titanium (1 cm in diameter; 2 mm thick) were coated for this test. As-sputtered (27 samples) and 54 heat treated samples (27 quenched and 27 furnace cooled samples) were placed in a saline solution (0.9% NaCl) and buffered to a pH of 7.4 for a 9 week period at room temperature. The experiment was conducted under a biological hood, with UV light on to prevent any bacterial or fungal contaminations. Three heat treated samples from each treatment were removed weekly, whereas three as-sputtered samples were removed hourly. It was speculated that the as-sputtered coatings would have higher dissolution rates than the heat treated samples’? therefore, the as-sputtered coatings were evaluated at much shorter time periods. Calcium phosphate coated titanium specimens which were not placed in the saline solution served as controls for the solubility evaluations. All tested samples were analysed
Calcium
phosphate
coatings:
251
J.L. Ong et al.
using energy dispersive spectroscopy or EDS (0 PGT Imagist 4000, Princeton Gamma-Tech, Princeton, NJ, USA) to determine if the sputtered coatings still remained on the titanium substrates. The equipment was calibrated with a standard apatite. The mean Ca/Ti ratio with one standard deviation was determined for each sample and was plotted against time in the saline solution.
Z-Axis bond strength
tests
Commercially pure titanium (0.64 cm diameter; 2 mm thick] were coated for bond strength evaluation. A total of 30 passivated and 30 non-passivated titanium substrates were sputter coated. Ten samples from the passivated and non-passivated titanium substrates were randomly assigned to be in the as-sputtered group, whereas the remaining samples were randomly selected to receive the required post deposition heat treatment. A Sebastian Five adhesion testing apparatus (Quad Group, Spokane, WA, USA] was used. For the tests, 3.6 mm X 12 mm stainless steel pull studs [Quad Group, Spokane, WA), pre-coated with a thermal curing epoxy, were clipped to the centre of each sample and cured in the oven for 1 h at 15O’C. After curing, the samples were inserted into the machine platen of the Sebastian Five and gripped. When activated, the stud was pulled down against the platen support ridge until failure occurred either in the coating or the epoxy bond. The tensile force required to cause failure was registered by the machine. Bond strength data were statistically analysed using the Duncan’s Multiple Range Test. Reflected light photomicrographs of all failed samples were taken to determine the location of the bond failure. Analysis of the failed samples using EDS was also conducted to determine the presence of calcium and phosphorus.
Figure 2 Scanning electron micrograph of the furnace cooled coating (original magnification X1000). The arrow shows a micro-crack on the coating.
90
z 0
80
I
70
36 ;
60
30
50
24
40
18
30
12
20
6
10
n
-5
10
15
Degrees
RESULTS Scanning
20
electron
microscopy
Using SEM, the deposited coatings were found to be relatively uniform for the as-sputtered, quenched and furnace cooled coatings. No surface defects were observed in the as-sputtered and quenched coatings; however, micro-cracks [Figure 2) were observed in the furnace cooled coatings. Complete coverage of the titanium substrates was achieved.
25
30
bp
0 35
28
Figure 3 X-ray diffraction scan of quenched coating and furnace cooled coating on passivated titanium substrate. Table 1 Lattice spacings furnace cooled coating
and volume
of quenched
and
(A31
Samples
a (A)
c (A)
Volume
Quenched Furnace cooled
9.36 f 0.01 9.35 + 0.02
6.83 f 0.007 6.82 f 0.02
518.48 + 1.57 515.84 + 3.73
X-Ray diffraction The as-sputtered coatings did not produce any crystalline diffraction peaks other than for the titanium substrate. The absence of sharp peaks on the X-ray patterns indicated an amorphous structure for the as-sputtered coatings. Sharp peaks were registered for the heat treated coatings, indicating the presence of crystalline structures. The strong reflections [between 20 = 5’ and 20 = 35”) are shown in Figure 3. The coatings’ peaks >35” (213) were masked by the strong titanium peaks. Computer matching indicated that a hydroxyapatite-type structure was present in the heat treated coatings. Since reflections >35O (20) were masked by the strong titanium peaks, the a- and c- lattice parameters (Table 2) were based on the 300 and 002 reflections, respectively.
At an a level of 0.05, the Least Squares Means indicated no significant difference in the a-lattice parameters, c-lattice parameters, and lattice volume between the quenched and furnace cooled coatings.
Solubility
evaluation
The as-sputtered coatings experienced the highest dissolution rate, with complete dissolution occurring within the first 4 h. The average Ca/Ti ratio of the as-sputtered coating for the control coatings which were not exposed to saline solution was 0.274. After 1 h, a 73% reduction in the Ca/Ti ratio was observed. By the second hour, a 97% reduction in Ca/Ti ratio was determined. No calcium was Biomaterials 1992, Vol. 13 No. 4
252
Calcium phosphate coatings: J.L. Ong ef ai.
50
40 (0 % 30 “‘ 20
1
0
I
I
I
2
4
6
I
a
10
10
Weeks
Figure 4 Solubility data showing the change in Ca/Ti ratio of the heat treated Ca-P coatings over time. The error bars indicate one standard deviation: El, quenched: 0, furnace cooled.
detected after 3 h in saline, and by the fourth hour, all of the noticeable as-sputtered coating had dissolved. Referring to Figure 4, the heat treated coatings dissolved at a significantly reduced rate as compared to the as-sputtered coatings. The Ca/Ti ratio of the quenched coatings placed in the saline solution remained relatively stable throughout the test. The furnace cooled coatings exhibited a gradual reduction in the average Ca/Ti ratio. Unlike the quenched coatings, the average CaA’i ratio for the furnace cooled coatings gradually decreased to 0 after 9 weeks in the saline solution, indicating the total degradation of the coatings. Therefore, with respect to the rate of solubility, there was a significant difference between the furnace cooled and quenched coatings.
Z-Axis bond strength test Figure 5 shows the bond strength data for the assputtered and heat treated coatings. For the passivated titanium substrates, the highest values were observed for the as-sputtered coatings. Average bond strengths (It one standard deviation) of 36.0 + 8.2 MPa were measured. In contrast to the as-sputtered coatings, the quenched and the furnace cooled coatings exhibited average bond strengths (+- one standard deviation) of 17.9 f 6.5 MPa and 9.0 rtr 2.9 MPa, respectively. For the non-passivated titanium substrates, the quenched coatings exhibited the highest interfacial bond strength, with an average (rt one standard deviation) of 45.6 + 25.1 MPa. Average bond strength (+ one standard deviation) for both the assputtered and furnace cooled coatings on non-passivated was 39.5 + 17.7 MPa titanium substrates and 8.0 + 6.8 MPa respectively. Duncan’s Multiple Range Test was performed to determine if significant differences existed for the various coatings (Table 2). At an a level of 0.05, the nonpassivated quenched, non-passivated as-sputtered and passivated as-sputtered coatings were not significantly different. Also, the passivated quenched, passivated furnace cooled, and non-passivated furnace cooled coatings were not significantly different. There were no Biamateria~s
1992, Vol.
13 No. 4
0
Q
AS
FC
Treatments Figure 5 Bond strength of the as-sputtered (AS), quenched (Q) and furnace cooled (FC) coatings. The error bars indicate one standard deviation: H, passivated; U, non-passivated.
Table 2
Average bond strength values
Treatments
Bond strength (IWPa)
Non-passivated quenched Non-passivated as-sputtered Passivated as-sputtered
45.82 39.48 38.04
Passivated quenched Passivated furnace cooled Non-passivated furnace cooled
17.88 9.05 8.02 I
Vertical lines on the right of the Table divide the data according to its statistical differances.
significant differences in the means within each group, but data between groups were significantly different, Differences in failure modes were observed between the as-sputtered, quenched and furnace cooled coatings. The reflected light photomicrograph in Figure 6 shows Wallner lines” radiating out from the centre of the fracture surface. This type of failure is typical for brittle fracture and was observed for all as-sputtered coatings, regardless of whether the titanium substrate was passivated or not. Failure always occurred at the titanium/ coating interface. Quenched coatings typically exhibited failure within the coatings. EDS analysis of the tested quenched coatings indicated the presence of calcium and phosphorus on the fracture surfaces. Wallner lines were observed in the tested region of the furnace cooled coatings indicating failure primarily occurred at the coating/substrate interface. However, in selected regions of the fracture surface, failures occurred within the coatings, as indicated by the presence of calcium and phosphorus.
DISCUSSION Calcium-phosphorus coatings potential to improve prosthetic
are bioactive and have the adhesion in the biological
Calcium
phosphate
coatings:
J. L. Ong et al.
Figure6 Photomicrographs of the as-sputtered coating (original magnification X40) showing Wallner fracture lines (shown by arrows) radiating out from the centre (C) of the tested region. The dark regions are remanents of the coating after testing.
environment: therefore, it is imperative that careful characterization of the coatings be conducted. The interaction with biological tissues will be dependent on the structure and surface chemistry of the coatings, their structures, as well as other properties such as bond strength and dissolution behaviour. The use of XRD revealed no reflections on the assputtered coatings other than the reflections for titanium substrate, indicating the as-sputtered coatings to be amorphous. With post deposition heat treatments, diffraction peaks from the coatings were observed, indicating a hydroxyapatite-type structure. The difference in lattice parameter between the heat treated films and reported hydroxyapatitel’ could be due to the differences in chemistry”. These differences in structure are suggested to be due to the post deposition heat treatments and the amounts of carbon present”. As compared to the as-sputtered coatings, an increase in the carbonate or carbon concentration has been reported in the post deposition heat treated coatings due to time in the furnacel’, lg. “. Many investigators have suggested that carbonate (CO,) present in the coatings produced a contraction of the a-lattice parameter’52 21-23. The contraction of a-lattice parameters in the post deposition heat treated coatings has suggested that the carbonate was located at the oblique face of the phosphate (PO,) tetrahedron’” 23. Furthermore, being in the furnace for an hour, there could be further loss of hydroxyl (OH) ions, thereby causing further contraction of the lattice parameters. The solubility of the coatings is in part dependent on whether the coatings are crystalline or amorphousz4. The amorphous as-sputtered coatings completely dissolved rapidly within the first 4 h of the test, whereas the dissolution rate of the heat treated coatings was much lower. Neuman and Neumanz5 reported that extremely small crystals are not restrained by interionic attraction of the ions: therefore, it would not be surprising that the amorphous as-sputtered coatings dissolved rapidly. The heat treatments significantly reduced the solubility of the coatings, with the quenched coatings exhibiting the lowest dissolution rates. The big standard deviation
253
observed in the analysis was the result of the uneven thickness associated with the coatings. This uneven thickness was observed even though the ion beam sputter coater was equipped with a rotating substrate stage to average the coating thickness. With the presence of micro-cracks as seen for the furnace cooled specimens using SEM, a faster dissolution of the furnace cooled coatings as compared to the quenched coatings would exist. In general, the as-sputtered coatings exhibited higher bond strengths than the heat treated coatings. Unfortunately, the amorphous coatings resulting from the sputtering process possess such high dissolution rates that they will not be acceptable for biomedical applications. Interestingly, the high bond strengths were observed for the quenched calcium phosphate sputtered coatings on the non-passivated titanium substrate. The bond strength of the quenched coatings on non-passivated titanium substrates was significantly higher than for the quenched coatings on passivated titanium substrates. Since passivation treatment was to enhance the oxide film, the non-passivated titanium will have a thinner oxide layer. These difference in thickness of the oxide layer suggested a possible explanation for the difference in bond strength between these quenched coatings. Golightly et al. have reported the growth of a lateral oxide layer within the existing oxide during heat treatmentz6. This presence of a lateral oxide layer was reported to have caused localized weakening within the existing oxide. Since a thinner oxide layerwas present on the non-passivated titanium substrates before heat treatment and high bond strength values were observed, the growth of lateral oxide within the existing oxide film was suggested to have little effect on the bond strength of the coatings. Thus, this suggested the necessity to be within a critical oxide thickness for high bond strength values. However, further study needs to be done to substantiate the model. As in the quenched coatings, the suggestion of having lateral oxide formation could also occur in the furnace cooled coatings, thereby causing localized weakening of the bonding. Thus, low bond strength values for the furnace cooled coatings on passivated titanium substrates were observed. However, unlike the quenched coatings on non-passivated titanium, high bond strength values were not observed for the furnace cooled coatings on non-passivated titanium. This low bond strength value was attributed to the presence of micro-cracks, as observed using SEM. CONCLUSIONS Thin, amorphous calcium phosphate coatings can be deposited on metal substrates by a process of ion beam sputter deposition. The coatings can then be subjected to post deposition heat treatments to alter their structures and properties. Characterizations of the sputter deposited calcium phosphate coatings in both the as-sputtered and heat treated states by SEM, XRD, bond strength and solubility tests were conducted in this study. The following conclusions were made from this study: 1. Complete coverage of the titanium substrates using the ion beam deposition process can be achieved. Biomaterials
1992, Vol. 13 No. 4
Calcium
254
2. Energy is required for crystalline formation in the amorphous as-sputtered coatings. This energy can be provided by post deposition heat treatments. 3. Solubility of the coatings was significantly reduced by crystalline formation in the coatings. 4. The high bond strength of the as-sputtered calcium phosphate coatings may be decreased in some cases by post deposition heat treatments. In summary, the use of ion beam sputter deposition as a method for coating orthopaedic and dental metallic implants was supported by this study. Depending on the nature of the substrate surface and the types of heat treatment provided, properties of the coatings such as adhesion and solubility can be optimized.
ACKNOWLEDGEMENT This study was funded by the Whitaker the National Science Foundation.
11
12
13
14
15
16
Foundation,
and
17 18
REFERENCES 19 1
2
3
4
5
6
7 8
9
10
Geesink, R.G.T.. de Groot, K. and Klein, C.P.A.T., Chemical implant fixation using hydroxyl-apatite coatings, Clin. Orthop. 1967, 225,147-170 de Lange, G.L. and Donath, K., Interface between bone tissue and implants of solid hydroxyapatite or hydroxyapatite-coated implants, Biomaterials 1989,10,121-125 Hayashi, K., Uenoyama, K., Matsuguchi, N. and Sugioka, Y., Quantitative analysis of in vivo tissue responses to titanium-oxide- and hydroxyapatite-coated titanium alloy, 1. Biomed. Mater. Res. 1991, 25,515-523 Nery,E.B., Lynch, K.L., Hirthe, W.M. and Mueller, K.H., Bioceramic implants in surgically produced infrabony defects, 1. Periodontol. 1975, 46,328-347 de Groot, K., Use of hydroxylapatite in oral surgery, in Seventh Cimtec World Ceramic Congress Abstracts, 1990, p 121 Fisher-Brandies, E., Jaw augmentation with hydroxylapatite, in Seventh Cimtec World Ceramic Congress Abstracts, 1999, p 121 Raja Rao, W. and Boehm, R.F., A study of sintered apatites, 1. Dent. Res. 1974, 53,1351-1354 Koch, B., Wolke, J.G.C. and de Groot, K., X-ray diffraction studies on plasma-sprayed calcium phosphate-coated implants, I. Biomed. Mater, Res. 1990, 24, 655-668 coatings, in BioLacefield, W.R., Hydroxylapatite ceramics: Material Characteristics Versus In Vivo Behavior (Eds P. Ducheyne and J.E. Lemons], The New York Academy of Science, New York, 1988, 72-80 Filiaggi, M.J. and Pilliar, R.M., Interfacial characterization of a plasma-sprayed hydroxyapatite/Ti-6Al-4V implant system, in Transactions of the Tenth Annual Meeting of the Canadian Society for Biomaterials 1989, pp 23-25
Biomaterials
1992, Vol. 13 No. 4
20
21
22
23
24
25
26
phosphate
coatings:
J.L. Ong et al.
Ong, J.L., Properties of calcium-phosphate thin films produced by ion-beam deposition, MA Thesis University of Alabama at Birmingham, 1999 Rigney, E.D., Characterization of ion-beam sputter deposited Ca-P films, Ph.D. Dissertation University of Alabama at Birmingham, 1989 Lacefield, W.R., Rigney, E.D., Lucas, L.C., Ong, J. and Gantenberg, J.B., Ion sputter deposition of Ca-P coatings onto metallic implants, in Ceramics in Substitutive and Reconstructive Surgery (in press) LeGeros, R.Z. and Trautz, O.R., Apatite crystallites: effects of carbonate and morphology, Science, 1967, 1409-1411 LeGeros, L.Z., LeGeros, J.P., Trautz, O.R. and Shirra, W.P., Conversion of monetite, CaHPO,, to apatites: Effect of carbonate on the crystallinity and the morphology of the apatite crystallites, Adv. X-ray Anal. 1979, 14, 57-66 Long, L.K., Corrosion evaluations of hydroxylapatite sputter coated implant materials, Senior Design Project University of Alabama at Birmingham, 1989 Doremus, R.H., in Glass Science, John Wiley, New York, USA, 1973, p 292 Young, R.A., Biological apatite versus hydroxyapatite at the atomic level, Clin. Orthop. Rel. Res. 1975, 113, 249-262 Harris, L.A., Ong, J.L., Lucas, L.C., Lacefield, W.R. and Rigney, E.D., ESCA analyses of passivated titanium and Ca-P surfaces, in Transactions of the Sixteenth Annual Meeting of the Society for Biomaterials 1990, p 44 Ong, J.L., Harris, L.A., Lucas, L.C., Lacefield, W.R. and Rigney, E.D., XPS characterization of ion beam sputter deposited calcium phosphate coatings, I. Am. Ceramic Sot. (in press) Dana, J.D., Apatite Series, in System of Mineralogy 7th Edn, Vol. II, [Eds C. Palache, H. Becman and C. Frondel) John Wiley, Chapman and Hall, London, 1951 van Raemdonck, W., Ducheyne, P. and de Meester, P., Calcium phosphate ceramics, in Metal and Ceramic Biomaterials, Vol. II: Strength and Surface, (Eds P. Ducheyne and G.W. Hastings] CRC Press, Boca Raton, FL, USA, 1984 LeGeros, R.Z., Trautz, O.R., LeGeros, J.P. and Klein, E., Carbonate substitution in the apatite structure (11, Bull. Sot. Chim. Fr. 1968, 1712-1718 LeGeros, R.Z., Parsons, J.R., Daculsi, G., Driessens, F., Lee, D., Liu, ST., Metsger, S., Peterson, D. and Walker, M., Significance of the porosity and physical chemistry of calcium phosphate ceramics, biodegradation-bioin Bioceramics: Material Characteristics resorption, Versus In Vivo Behavior (Eds P. Ducheyne and J.E. Lemons], The New York Academy of Science, New York, USA, 1988, pp 266-271 Neuman, W.F. and Neuman, M.W., Solubilities, in The Chemical Dynamics of Bone Mineral, University of Chicago Press, Chicago, USA, 1958 Golightly, F.A., Scott, F.H. and Wood, G.C., The influence of yttrium additions on the oxide-scale adhesion to an iron chromium-aluminum alloy, Oxid. Metals 1976, 10,163-187
Structure, solubility and bond strength of thin calcium phosphate coatings produced by ion beam sputter deposition J.L. Ong*, L.C. Lucas*, W.R. Lacefieldtand
E.D. Rigneyt
t Dental Biomaterials and $ Materials Science & Engineering, Departments of *Biomedical Engineering, University of Alabama at Birmingham, University Station, Birmingham, Alabama 35294, USA
Ion beam sputter deposition was used to produce thin calcium phosphate coatings on titanium substrates. Structure, solubility and bond strength of the as-sputtered and heat treated coatings were evaluated. X-ray diffraction (XRD) analysis of the heat treated coatings revealed a hydroxyapatite-type structure. The heat treated coatings were found to have significantly lower solubility as compared to the amorphous as-sputtered coatings. Although the crystalline coatings exhibited the lowest solubility, in general, the bond strengths were lower for the heat treated coatings. Keywords: Received
Calcium 26 February
phosphates,
coatings,
1991;
11 June
revised
Calcium phosphate 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’-‘. Currently, plasma spraying is the primary method used commercially to produce ceramic coatings on metallic implants. However, problems such as low bond strength and non-uniformity in coating density are often observed with this coating techniqueg. A recent study on plasma-sprayed hydroxyapatite on Ti-6Al-4V reported an average bond strength of 6.7 f 1.5 MPa”. Using a post heat treatment of 96O”C, bond strengths were increased in a range of 15.0 to 26.0 MPa, but there is concern about the effects of the high temperature on the metallic substrates”. An alternative coating method, ion beam sputter deposition, is being investigated as a means for producing biocompatible ceramic coatings on metallic implants With this technique, an ionized gas is used to sputter atoms from a ceramic target. These sputtered atoms build up on the metallic substrates placed in the path of the sputtered material. Previous studies have reported that an improvement in some of the properties of calcium phosphate coatings can be achieved using the ion beam sputter deposition process”, “. In particular, sputter coatings with bond strength values as high as 47.3 MPa have been reportedI’. However, because a thickness of 2 pm or less is practical with the sputter coating process, there is concern with the solubility of the sputtered Correspondence
0142-9612/92/040249-06
properties
1991; accepted
12 August
1991
that the structure of the coatings13. It is hypothesized calcium phosphate coatings determines the dissolution rate in solution. In this study, heat treatments were used to crystallize the calcium phosphate coatings produced by single ion-beam deposition. Full characterizations of both the as-sputtered and heat treated coatings as to structure, bond strength and solubility were conducted in this study.
MATERIALS AND METHODS Substrates Wrought commercially pure titanium (ASTM F76) was mechanically ground with 240, 400 and 600 grit silicon carbide paper and polished using 0.3 pm alumina powder. These substrates were then ultrasonically degreased with benzene, acetone and ethanol for 10 min each. Following the cleaning procedure, half of the substrates were passivated with 40% nitric acid for 30 min (ASTM F86). The remaining substrates did not receive the nitric acid passivation treatment and were referred to as non-passivated specimens. The nonpassivated specimens were only used for the bond strength tests.
Target The target material used during ion-beam position was a hydroxyapatite-fluorapatite
to Dr J. Ong.
0 1992 Butterworth-Heinemann
material
Ltd
Biomaterials
sputter detarget, con-
1992, Vol.
13 No. 4
250
Calcium phosphate coatings: J. L. Ong et al.
_
sisting of two materials; fluorapatite powders (X-ray diffraction pattern matched JCPDS # 34-0011) of 325 mesh and a hydroxyapatite powder disk (chemistry was confirmed with XPS), from Lifecore Biomedical, Inc. (Minneapolis, MN, USA). The hydroxyapatite powder disk was hot-pressed and sintered by Cerac, Inc. (Milwaukee, WI, USA) at 1150°C.Fluorapatite powders were applied to the surface of the target disk prior to sputtering.
Single ion beam sputter deposition A schematic of the ion beam sputter coater used in this study is shown in Figure 3. The titanium substrates were fixed on to the rotating stage of the sputtering system (Commonwealth Scientific, Alexandria, VA, USA) with a conductive adhesive containing silver particles. The vacuum chamber was evacuated to a minimum base pressure of 6.0 X 10e6Torr. High purity argon (99.999%) was backfilled into the chamber, bringing the pressure to z 4 X 10e4 Torr. Prior to the deposition process, the titanium substrates were sputter cleaned using a second ion beam. The substrates were placed in the path of the ion beam, and sputter-cleaning was achieved for 30 min at 40.7 mA and 500 eV (Figure la). After cleaning, the titanium substrates were moved into position to be coated with the sputtered material (Figure lb). The sputter deposition process was accomplished at a chamber pressure of z 4.0 X 10m4 Torr, with an ion-
f-7
IRotating stage
I
Ion source Titanium substrate
a
f
Post deposition
Ion source
b Figure1 Schematic representation of ion beam sputter deposition: a, sputter cleaning prior to sputter deposition; b, single ion beam sputter deposition. 1992, Vol. 13 No. 4
heat treatments
Previous studies have shown that as- sputtered calcium phosphate coatings are amorphous or only diffusely crystalline 12. In an attemp t to obtain crystallinity in the sputtered coatings, the coated samples were randomly selected for post-deposition heat treatments, conducted at 600°C in air for 1 h. These samples were then again randomly selected for either quenching in deionized water or furnace cooling until room temperature was reached (z 4 h).
Scanning
electron
microscopy
@EM)
The commercially pure titanium samples of 1 cm in diameter and 2 mm thick were coated, and the coated samples placed in the SEM vacuum chamber (ISI-lOOB, International Scientific Instruments, Inc., Pleasanton, CA, USA) tilted at 30” and subjected to an electron beam at 15 KeV. To prevent charging, the samples were coated with a thin layer of carbon. SEM was used to determine if any surface defects existed for either the as-sputtered or heat-treated coatings.
X-Ray diffraction
(XRD)
X-ray diffraction was used to evaluate the structure of the as-sputtered and heat treated coatings. For these analyses, the commercially pure titanium coupons (5.1 X 2.54 cm) were coated. A Siemens diffractometer using Cu K, radiation having energies of 40 KeV and 30 mA was used. Three samples for each treatment were analysed and data were collected from 5” to 95O 28 at 0.1 degree per min scan rate, for a total of z 15 h. Crystalline coatings were identified by matching the peaks with standard synthetic hydroxyapatite (JCPDS # g-0432). As in other studies on hydroxyapatite, lattice parameters and cell volume with one standard deviation were calculated based on the 002 reflections for the a spacing and 300 reflections for the c spacing’4”5. The lattice spacings and volume were statistically analysed using the Least Squares Means.
Solubility
Titanium
Biomaterials
beam energy of 1000 eV and 40.7 mA. Sputter deposition was carried out at the rate of 0.15 pm per hour, resulting in a coating thickness of ” 0.6-lprn for each sample.
evaluations
Commercially pure titanium (1 cm in diameter; 2 mm thick) were coated for this test. As-sputtered (27 samples) and 54 heat treated samples (27 quenched and 27 furnace cooled samples) were placed in a saline solution (0.9% NaCl) and buffered to a pH of 7.4 for a 9 week period at room temperature. The experiment was conducted under a biological hood, with UV light on to prevent any bacterial or fungal contaminations. Three heat treated samples from each treatment were removed weekly, whereas three as-sputtered samples were removed hourly. It was speculated that the as-sputtered coatings would have higher dissolution rates than the heat treated samples’? therefore, the as-sputtered coatings were evaluated at much shorter time periods. Calcium phosphate coated titanium specimens which were not placed in the saline solution served as controls for the solubility evaluations. All tested samples were analysed
Calcium
phosphate
coatings:
251
J.L. Ong et al.
using energy dispersive spectroscopy or EDS (0 PGT Imagist 4000, Princeton Gamma-Tech, Princeton, NJ, USA) to determine if the sputtered coatings still remained on the titanium substrates. The equipment was calibrated with a standard apatite. The mean Ca/Ti ratio with one standard deviation was determined for each sample and was plotted against time in the saline solution.
Z-Axis bond strength
tests
Commercially pure titanium (0.64 cm diameter; 2 mm thick] were coated for bond strength evaluation. A total of 30 passivated and 30 non-passivated titanium substrates were sputter coated. Ten samples from the passivated and non-passivated titanium substrates were randomly assigned to be in the as-sputtered group, whereas the remaining samples were randomly selected to receive the required post deposition heat treatment. A Sebastian Five adhesion testing apparatus (Quad Group, Spokane, WA, USA] was used. For the tests, 3.6 mm X 12 mm stainless steel pull studs [Quad Group, Spokane, WA), pre-coated with a thermal curing epoxy, were clipped to the centre of each sample and cured in the oven for 1 h at 15O’C. After curing, the samples were inserted into the machine platen of the Sebastian Five and gripped. When activated, the stud was pulled down against the platen support ridge until failure occurred either in the coating or the epoxy bond. The tensile force required to cause failure was registered by the machine. Bond strength data were statistically analysed using the Duncan’s Multiple Range Test. Reflected light photomicrographs of all failed samples were taken to determine the location of the bond failure. Analysis of the failed samples using EDS was also conducted to determine the presence of calcium and phosphorus.
Figure 2 Scanning electron micrograph of the furnace cooled coating (original magnification X1000). The arrow shows a micro-crack on the coating.
90
z 0
80
I
70
36 ;
60
30
50
24
40
18
30
12
20
6
10
n
-5
10
15
Degrees
RESULTS Scanning
20
electron
microscopy
Using SEM, the deposited coatings were found to be relatively uniform for the as-sputtered, quenched and furnace cooled coatings. No surface defects were observed in the as-sputtered and quenched coatings; however, micro-cracks [Figure 2) were observed in the furnace cooled coatings. Complete coverage of the titanium substrates was achieved.
25
30
bp
0 35
28
Figure 3 X-ray diffraction scan of quenched coating and furnace cooled coating on passivated titanium substrate. Table 1 Lattice spacings furnace cooled coating
and volume
of quenched
and
(A31
Samples
a (A)
c (A)
Volume
Quenched Furnace cooled
9.36 f 0.01 9.35 + 0.02
6.83 f 0.007 6.82 f 0.02
518.48 + 1.57 515.84 + 3.73
X-Ray diffraction The as-sputtered coatings did not produce any crystalline diffraction peaks other than for the titanium substrate. The absence of sharp peaks on the X-ray patterns indicated an amorphous structure for the as-sputtered coatings. Sharp peaks were registered for the heat treated coatings, indicating the presence of crystalline structures. The strong reflections [between 20 = 5’ and 20 = 35”) are shown in Figure 3. The coatings’ peaks >35” (213) were masked by the strong titanium peaks. Computer matching indicated that a hydroxyapatite-type structure was present in the heat treated coatings. Since reflections >35O (20) were masked by the strong titanium peaks, the a- and c- lattice parameters (Table 2) were based on the 300 and 002 reflections, respectively.
At an a level of 0.05, the Least Squares Means indicated no significant difference in the a-lattice parameters, c-lattice parameters, and lattice volume between the quenched and furnace cooled coatings.
Solubility
evaluation
The as-sputtered coatings experienced the highest dissolution rate, with complete dissolution occurring within the first 4 h. The average Ca/Ti ratio of the as-sputtered coating for the control coatings which were not exposed to saline solution was 0.274. After 1 h, a 73% reduction in the Ca/Ti ratio was observed. By the second hour, a 97% reduction in Ca/Ti ratio was determined. No calcium was Biomaterials 1992, Vol. 13 No. 4
252
Calcium phosphate coatings: J.L. Ong ef ai.
50
40 (0 % 30 “‘ 20
1
0
I
I
I
2
4
6
I
a
10
10
Weeks
Figure 4 Solubility data showing the change in Ca/Ti ratio of the heat treated Ca-P coatings over time. The error bars indicate one standard deviation: El, quenched: 0, furnace cooled.
detected after 3 h in saline, and by the fourth hour, all of the noticeable as-sputtered coating had dissolved. Referring to Figure 4, the heat treated coatings dissolved at a significantly reduced rate as compared to the as-sputtered coatings. The Ca/Ti ratio of the quenched coatings placed in the saline solution remained relatively stable throughout the test. The furnace cooled coatings exhibited a gradual reduction in the average Ca/Ti ratio. Unlike the quenched coatings, the average CaA’i ratio for the furnace cooled coatings gradually decreased to 0 after 9 weeks in the saline solution, indicating the total degradation of the coatings. Therefore, with respect to the rate of solubility, there was a significant difference between the furnace cooled and quenched coatings.
Z-Axis bond strength test Figure 5 shows the bond strength data for the assputtered and heat treated coatings. For the passivated titanium substrates, the highest values were observed for the as-sputtered coatings. Average bond strengths (It one standard deviation) of 36.0 + 8.2 MPa were measured. In contrast to the as-sputtered coatings, the quenched and the furnace cooled coatings exhibited average bond strengths (+- one standard deviation) of 17.9 f 6.5 MPa and 9.0 rtr 2.9 MPa, respectively. For the non-passivated titanium substrates, the quenched coatings exhibited the highest interfacial bond strength, with an average (rt one standard deviation) of 45.6 + 25.1 MPa. Average bond strength (+ one standard deviation) for both the assputtered and furnace cooled coatings on non-passivated was 39.5 + 17.7 MPa titanium substrates and 8.0 + 6.8 MPa respectively. Duncan’s Multiple Range Test was performed to determine if significant differences existed for the various coatings (Table 2). At an a level of 0.05, the nonpassivated quenched, non-passivated as-sputtered and passivated as-sputtered coatings were not significantly different. Also, the passivated quenched, passivated furnace cooled, and non-passivated furnace cooled coatings were not significantly different. There were no Biamateria~s
1992, Vol.
13 No. 4
0
Q
AS
FC
Treatments Figure 5 Bond strength of the as-sputtered (AS), quenched (Q) and furnace cooled (FC) coatings. The error bars indicate one standard deviation: H, passivated; U, non-passivated.
Table 2
Average bond strength values
Treatments
Bond strength (IWPa)
Non-passivated quenched Non-passivated as-sputtered Passivated as-sputtered
45.82 39.48 38.04
Passivated quenched Passivated furnace cooled Non-passivated furnace cooled
17.88 9.05 8.02 I
Vertical lines on the right of the Table divide the data according to its statistical differances.
significant differences in the means within each group, but data between groups were significantly different, Differences in failure modes were observed between the as-sputtered, quenched and furnace cooled coatings. The reflected light photomicrograph in Figure 6 shows Wallner lines” radiating out from the centre of the fracture surface. This type of failure is typical for brittle fracture and was observed for all as-sputtered coatings, regardless of whether the titanium substrate was passivated or not. Failure always occurred at the titanium/ coating interface. Quenched coatings typically exhibited failure within the coatings. EDS analysis of the tested quenched coatings indicated the presence of calcium and phosphorus on the fracture surfaces. Wallner lines were observed in the tested region of the furnace cooled coatings indicating failure primarily occurred at the coating/substrate interface. However, in selected regions of the fracture surface, failures occurred within the coatings, as indicated by the presence of calcium and phosphorus.
DISCUSSION Calcium-phosphorus coatings potential to improve prosthetic
are bioactive and have the adhesion in the biological
Calcium
phosphate
coatings:
J. L. Ong et al.
Figure6 Photomicrographs of the as-sputtered coating (original magnification X40) showing Wallner fracture lines (shown by arrows) radiating out from the centre (C) of the tested region. The dark regions are remanents of the coating after testing.
environment: therefore, it is imperative that careful characterization of the coatings be conducted. The interaction with biological tissues will be dependent on the structure and surface chemistry of the coatings, their structures, as well as other properties such as bond strength and dissolution behaviour. The use of XRD revealed no reflections on the assputtered coatings other than the reflections for titanium substrate, indicating the as-sputtered coatings to be amorphous. With post deposition heat treatments, diffraction peaks from the coatings were observed, indicating a hydroxyapatite-type structure. The difference in lattice parameter between the heat treated films and reported hydroxyapatitel’ could be due to the differences in chemistry”. These differences in structure are suggested to be due to the post deposition heat treatments and the amounts of carbon present”. As compared to the as-sputtered coatings, an increase in the carbonate or carbon concentration has been reported in the post deposition heat treated coatings due to time in the furnacel’, lg. “. Many investigators have suggested that carbonate (CO,) present in the coatings produced a contraction of the a-lattice parameter’52 21-23. The contraction of a-lattice parameters in the post deposition heat treated coatings has suggested that the carbonate was located at the oblique face of the phosphate (PO,) tetrahedron’” 23. Furthermore, being in the furnace for an hour, there could be further loss of hydroxyl (OH) ions, thereby causing further contraction of the lattice parameters. The solubility of the coatings is in part dependent on whether the coatings are crystalline or amorphousz4. The amorphous as-sputtered coatings completely dissolved rapidly within the first 4 h of the test, whereas the dissolution rate of the heat treated coatings was much lower. Neuman and Neumanz5 reported that extremely small crystals are not restrained by interionic attraction of the ions: therefore, it would not be surprising that the amorphous as-sputtered coatings dissolved rapidly. The heat treatments significantly reduced the solubility of the coatings, with the quenched coatings exhibiting the lowest dissolution rates. The big standard deviation
253
observed in the analysis was the result of the uneven thickness associated with the coatings. This uneven thickness was observed even though the ion beam sputter coater was equipped with a rotating substrate stage to average the coating thickness. With the presence of micro-cracks as seen for the furnace cooled specimens using SEM, a faster dissolution of the furnace cooled coatings as compared to the quenched coatings would exist. In general, the as-sputtered coatings exhibited higher bond strengths than the heat treated coatings. Unfortunately, the amorphous coatings resulting from the sputtering process possess such high dissolution rates that they will not be acceptable for biomedical applications. Interestingly, the high bond strengths were observed for the quenched calcium phosphate sputtered coatings on the non-passivated titanium substrate. The bond strength of the quenched coatings on non-passivated titanium substrates was significantly higher than for the quenched coatings on passivated titanium substrates. Since passivation treatment was to enhance the oxide film, the non-passivated titanium will have a thinner oxide layer. These difference in thickness of the oxide layer suggested a possible explanation for the difference in bond strength between these quenched coatings. Golightly et al. have reported the growth of a lateral oxide layer within the existing oxide during heat treatmentz6. This presence of a lateral oxide layer was reported to have caused localized weakening within the existing oxide. Since a thinner oxide layerwas present on the non-passivated titanium substrates before heat treatment and high bond strength values were observed, the growth of lateral oxide within the existing oxide film was suggested to have little effect on the bond strength of the coatings. Thus, this suggested the necessity to be within a critical oxide thickness for high bond strength values. However, further study needs to be done to substantiate the model. As in the quenched coatings, the suggestion of having lateral oxide formation could also occur in the furnace cooled coatings, thereby causing localized weakening of the bonding. Thus, low bond strength values for the furnace cooled coatings on passivated titanium substrates were observed. However, unlike the quenched coatings on non-passivated titanium, high bond strength values were not observed for the furnace cooled coatings on non-passivated titanium. This low bond strength value was attributed to the presence of micro-cracks, as observed using SEM. CONCLUSIONS Thin, amorphous calcium phosphate coatings can be deposited on metal substrates by a process of ion beam sputter deposition. The coatings can then be subjected to post deposition heat treatments to alter their structures and properties. Characterizations of the sputter deposited calcium phosphate coatings in both the as-sputtered and heat treated states by SEM, XRD, bond strength and solubility tests were conducted in this study. The following conclusions were made from this study: 1. Complete coverage of the titanium substrates using the ion beam deposition process can be achieved. Biomaterials
1992, Vol. 13 No. 4
Calcium
254
2. Energy is required for crystalline formation in the amorphous as-sputtered coatings. This energy can be provided by post deposition heat treatments. 3. Solubility of the coatings was significantly reduced by crystalline formation in the coatings. 4. The high bond strength of the as-sputtered calcium phosphate coatings may be decreased in some cases by post deposition heat treatments. In summary, the use of ion beam sputter deposition as a method for coating orthopaedic and dental metallic implants was supported by this study. Depending on the nature of the substrate surface and the types of heat treatment provided, properties of the coatings such as adhesion and solubility can be optimized.
ACKNOWLEDGEMENT This study was funded by the Whitaker the National Science Foundation.
11
12
13
14
15
16
Foundation,
and
17 18
REFERENCES 19 1
2
3
4
5
6
7 8
9
10
Geesink, R.G.T.. de Groot, K. and Klein, C.P.A.T., Chemical implant fixation using hydroxyl-apatite coatings, Clin. Orthop. 1967, 225,147-170 de Lange, G.L. and Donath, K., Interface between bone tissue and implants of solid hydroxyapatite or hydroxyapatite-coated implants, Biomaterials 1989,10,121-125 Hayashi, K., Uenoyama, K., Matsuguchi, N. and Sugioka, Y., Quantitative analysis of in vivo tissue responses to titanium-oxide- and hydroxyapatite-coated titanium alloy, 1. Biomed. Mater. Res. 1991, 25,515-523 Nery,E.B., Lynch, K.L., Hirthe, W.M. and Mueller, K.H., Bioceramic implants in surgically produced infrabony defects, 1. Periodontol. 1975, 46,328-347 de Groot, K., Use of hydroxylapatite in oral surgery, in Seventh Cimtec World Ceramic Congress Abstracts, 1990, p 121 Fisher-Brandies, E., Jaw augmentation with hydroxylapatite, in Seventh Cimtec World Ceramic Congress Abstracts, 1999, p 121 Raja Rao, W. and Boehm, R.F., A study of sintered apatites, 1. Dent. Res. 1974, 53,1351-1354 Koch, B., Wolke, J.G.C. and de Groot, K., X-ray diffraction studies on plasma-sprayed calcium phosphate-coated implants, I. Biomed. Mater, Res. 1990, 24, 655-668 coatings, in BioLacefield, W.R., Hydroxylapatite ceramics: Material Characteristics Versus In Vivo Behavior (Eds P. Ducheyne and J.E. Lemons], The New York Academy of Science, New York, 1988, 72-80 Filiaggi, M.J. and Pilliar, R.M., Interfacial characterization of a plasma-sprayed hydroxyapatite/Ti-6Al-4V implant system, in Transactions of the Tenth Annual Meeting of the Canadian Society for Biomaterials 1989, pp 23-25
Biomaterials
1992, Vol. 13 No. 4
20
21
22
23
24
25
26
phosphate
coatings:
J.L. Ong et al.
Ong, J.L., Properties of calcium-phosphate thin films produced by ion-beam deposition, MA Thesis University of Alabama at Birmingham, 1999 Rigney, E.D., Characterization of ion-beam sputter deposited Ca-P films, Ph.D. Dissertation University of Alabama at Birmingham, 1989 Lacefield, W.R., Rigney, E.D., Lucas, L.C., Ong, J. and Gantenberg, J.B., Ion sputter deposition of Ca-P coatings onto metallic implants, in Ceramics in Substitutive and Reconstructive Surgery (in press) LeGeros, R.Z. and Trautz, O.R., Apatite crystallites: effects of carbonate and morphology, Science, 1967, 1409-1411 LeGeros, L.Z., LeGeros, J.P., Trautz, O.R. and Shirra, W.P., Conversion of monetite, CaHPO,, to apatites: Effect of carbonate on the crystallinity and the morphology of the apatite crystallites, Adv. X-ray Anal. 1979, 14, 57-66 Long, L.K., Corrosion evaluations of hydroxylapatite sputter coated implant materials, Senior Design Project University of Alabama at Birmingham, 1989 Doremus, R.H., in Glass Science, John Wiley, New York, USA, 1973, p 292 Young, R.A., Biological apatite versus hydroxyapatite at the atomic level, Clin. Orthop. Rel. Res. 1975, 113, 249-262 Harris, L.A., Ong, J.L., Lucas, L.C., Lacefield, W.R. and Rigney, E.D., ESCA analyses of passivated titanium and Ca-P surfaces, in Transactions of the Sixteenth Annual Meeting of the Society for Biomaterials 1990, p 44 Ong, J.L., Harris, L.A., Lucas, L.C., Lacefield, W.R. and Rigney, E.D., XPS characterization of ion beam sputter deposited calcium phosphate coatings, I. Am. Ceramic Sot. (in press) Dana, J.D., Apatite Series, in System of Mineralogy 7th Edn, Vol. II, [Eds C. Palache, H. Becman and C. Frondel) John Wiley, Chapman and Hall, London, 1951 van Raemdonck, W., Ducheyne, P. and de Meester, P., Calcium phosphate ceramics, in Metal and Ceramic Biomaterials, Vol. II: Strength and Surface, (Eds P. Ducheyne and G.W. Hastings] CRC Press, Boca Raton, FL, USA, 1984 LeGeros, R.Z., Trautz, O.R., LeGeros, J.P. and Klein, E., Carbonate substitution in the apatite structure (11, Bull. Sot. Chim. Fr. 1968, 1712-1718 LeGeros, R.Z., Parsons, J.R., Daculsi, G., Driessens, F., Lee, D., Liu, ST., Metsger, S., Peterson, D. and Walker, M., Significance of the porosity and physical chemistry of calcium phosphate ceramics, biodegradation-bioin Bioceramics: Material Characteristics resorption, Versus In Vivo Behavior (Eds P. Ducheyne and J.E. Lemons], The New York Academy of Science, New York, USA, 1988, pp 266-271 Neuman, W.F. and Neuman, M.W., Solubilities, in The Chemical Dynamics of Bone Mineral, University of Chicago Press, Chicago, USA, 1958 Golightly, F.A., Scott, F.H. and Wood, G.C., The influence of yttrium additions on the oxide-scale adhesion to an iron chromium-aluminum alloy, Oxid. Metals 1976, 10,163-187
