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glass ceramic coatings onto Ti-6A1-4V substrates
SIIilfI E
COATINGS ELSEVIER
Surface and Coatings TechnologyI02 (1998) 191-196
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Adhesion and microstructural characterization of plasma-sprayed hydroxyapatite/glass ceramic coatings onto Ti-6A1-4V substrates P . L . Silva
a,b,*
J . D . S a n t o s a,b F . J . M o n t e i r o a.o, J . C K n o w l e s d
a [NEB-Instituto de Engenharia Biomddica, Prafa Coronel Yacheco 1, 4050 Porto, Portugal b ISEP-hTstituto Superior de Engenharia, R. de S. Tom~, 4200 Porto, Portugal Departamento de Engenharia Metal~rgica, FEUP, Universidade do Porto, R.dos Bragas, 4099 Porto Codex, Portuga/ d Eastman Dental hzstitue, Biomaterials Departement, 256 Gray's Inn Road, Lotzdon, WC1XSLD, UK
Received 7 March 1997; received in revised form 6 August 1997; accepted 7 August 1997
Abstract
Commercial hydroxyapatite (HA), Calo(PO4)6(OH)2, powders were reinforced with 4% of a CaO-P205 based glass during its sintering process. DoubIe layered coatings were obtained by Plasma Spraying and composed of 50 gm commercial HA, followed by a 50 gm (HA)/P2Os-CaO based glass composite coating. Samples were characterized by X-ray diffraction (XRD) analysis and it was shown that they were constituted of an HA matrix with small traces of tricalcium phosphate (/3-TCP) phase. The XRD spectra were quite similar to those of commercial HA coatings. For coating to substrate adhesion two different tests were performed: adhesion tests according to ASTM C 633-79 and ASTM D I002-83 standards. Higher values were obtained for ASTM C 633-79 than for ASTM D I002-83, 35 and 15 MPa, respectively. This behavior is discussed in terms of the different stress effort applied to specimens. The results obtained with ASTM C 633-79 show that these double layered satisfy the stress requirements for load bearing biomedical applications. © 1998 Published by Elsevier Science S.A. Keywords: Plasma spraying; Coating; X-ray diffraction
1. Introduction
H A has been extensively studied and used for biomedical applications [1-4]. The reasons for this interest include a chemical composition very close to that of the inorganic part of bone and excellent biocompatibility in comparison with other implant materials [5]. However, it has been shown that the mechanical properties of HA, in particularly its fracture toughness and strength [6,7], should be improved if used in load bearing applications. The main purpose of applying HA coatings on Ti-6A1-4V alloys has been to keep the mechanical properties of the metallic substrate and, at the same time, to take advantage of the coating biocompatibility and chemical similarity with bone. In this work, HA, reinforced with a CaO-P205 glass was used as a surface layer, i.e. the material that primarily makes contact with tissues and organic fluids. * Corresponding author.
This was the top layer of a double coating containing as a first undercoat a usual HA layer, adherent to the Ti-alloy substrate. In previous work [8], it was shown that by applying such reinforcement the mechanical properties of HA were improved, mainly its fracture toughness and biaxial bending strength. Also, the glass chemical composition was adjusted, so that the composite chemical composition would approach, as closely as possible, the chemical composition of the mineral part of bone [8-10]. Therefore NaaO, MgO and K20 oxides were incorporated into the P2Os-CaO based glass. It is well known [11] that one of the main concerns when using plasma spraying techniques, is the determination of the coating to substrate adhesion. The way by which a coating adheres to a substrate is very complex and it is not fully understood. Several theories about the mechanisms of adhesion exist, but there is no single coherent explanation for all adhesion behaviors [ 12, 13]. Many factors seem to influence the establishment of coating-to-substrate adhesion: (1) mechanical anchorage; (2) van der Waals physical interaction forces;
0257-8972/98/$19.00© 1998 Published by ElsevierScienceB.V. All rights reserved. PII S0257-8972(97) 00576-8
I92
P.L. Sih'a e~ al, / Smface and Coatings Technology 102 (1998) I91-]96
(3) chemical interaction and (4) metallurgical processes [ 11, 12]. Due to the difficulty in quantifying this property, many methods have been studied and applied [t3]. In this case two standard methods [11-14] were chosen, allowing for the cross-evaluation of the results.
2. Materials and methods
glue /ine
/,6ram ,, ~1, Z5,4mm_
I
] sheararea
~
area
12.7ram
in test
~ ' " ~
127mm
The P2Os-CaO based glass was prepared from reagent grade chemicals and its chemical composition is shown in Table 1. The glass was wet-mixed and milled with HA powder using methanol in a ball mill pot for 12 h. 4 wt.% glass addition was used. Mixed powders were then dried and isostatically pressed at 200 MPa, The method used for composite preparation has been fully described elsewhere [8]. Cylindrical shaped samples were sintered at 1300 °C with 1 h dwell time and naturally cooled inside the furnace. After sintering the cylinders were once again milled, for 6 h, using an agate ball mill pot. The powders were then sieved to obtain a grain size distribution suitable for plasma spraying. Two kinds of HA were used for the base coatings. One, called HA-P, was supplied by Metco-Plasma-Technik (CH) with the reference code, AMDRY AM 6021. This was a powder specifically prepared for plasma spraying and was used "as received". The other, called HA-I, was supplied by Plasma Biotal and was conditioned to be used in plasma spraying. Composite powders were also conditioned using the same procedure. Plasma spraying was performed using automated equipment from Plasma Technik, under atmospheric conditions. After substrate preparation, consisting of grit blasting with A1203 spheres, and chemical degreasing the Ti-6A1-4V alloy with trichlorethylene, a 50 btm HA coating was sprayed, followed by the deposition of 50 btm HA/composite coating. Using a scanning electron microscope (SEM) it was not possible to distinguish the two layers. To analyze the two different coatings by SEM, a sample was prepared using the previous procedure and including, between the HA and the HA/glass layer, a very thin carbon coating obtained by sputtering. Both kinds of coatings, HA-P/composite and HA-I/composite, were characterized by XRD analysis and compared with an XRD spectrum of a 100 btm commercial HA coating.
177mm Fig. 1. Test specimenshape and dimensionsaccordingto ASTM D 1002. For the adhesion experiments, 10 samples of each coating were used for each one of the two tests. ASTM D 1002 standard testing was used to determine shear strength according to Fig. 1 and Eq. (1). The glue used to join the test specimens was Plasmatex Klebbi, from Plasma Technik. In order to obtain good alignment of specimens, the jig shown in Fig. 2 was used. Specimens were placed in the testing machine so that the outer 25 mm of each end was in contact with the jaws and the long axis of the test specimen coincided with the direction of applied pull stress, through the center line of the grip assembly. The load was applied at the rate of 1 mm/min. The load at failure was recorded, and the tested specimens analyzed by SEM. ASTM C 633-79 testing was performed according to Fig. 3 and Eq. (1). The testing specimens were joined under the same conditions, using a dynamometric screw driver and applying a 80 N force. With this type of experiment it is possible to obtain coating adhesion under stresses normal to the surface. The glue used to assemble both specimens (one that had the plasma sprayed coating, and the counterpart) was the same used for the ASTM D 1002 tests. The load was applied at a rate of 1 mm/min and after failure the samples were observed by SEM. Tests results
Table 1 Glass chemicalcomposition(mol%) P2Os
CaO
Na20
K20
MgO
35
35
i0
10
10
grips
Fig. 2. Gig used for specimensglueing.
t93
P. L, Silva et al. / Smface and Coatings Tec/mology 102 (1998) 191-196
Coa~ng
a) Specin
Fig. 3. Assembly view for the ASTM C 633-79 experiments.
b)
were expressed in terms of stress adhesion according to Eq. (1): ,Ta-
Fmax
A
(1)
Where ~a=adhesion stress, Fmax=maximum load and A = specimen surface area. Fracture surfaces were observed by SEM together with image analysis technique in order to determine what type of failure mechanism took place: cohesive, adhesive or fracture through the glue. For image analysis, three samples of each kind were analyzed.
3. Results
SEM morphological characterization of the multilayered coating may be seen in the photomicrograph presented in Fig. 4. The line that separates the layers is the thin carbon coating obtained by sputtering. As it may be observed, both HA and composite show very similar morphologies. The two layers seems to be well bonded although presenting porosity which is characteristic of this type of coatings. Coatings chemical compositions seem to be quite similar, showing mainly calcium and phosphorus in their constitution. The top layer (HA-I/composite) also presents 1.21% magnesium and 0.35% sodimn (at.%). This results may be seen in the EDS spectra shown in Fig. 5. XRD spectra of HA and HA-I/composite coatings may be seen in Fig. 6. Both coatings present HA matrix
Fig. 4. SEM photomicrograph of the double Plasma Sprayed layer. A carbon layer (a) may be observed between the HA and HA-composite layers.
with small amounts of tricalcium phosphate, but the commercial HA coating had higher crystallinity than the HA-I + composite coating. Adhesion tests results are shown in Tables 2 and 3. Slightly higher mean values were obtained for HA-I/composite, i.e. 35 MPa for the ASTM C 633-79 tests and 15 MPa for ASTM D 1002 standard, respectively. Both coatings have shown higher resistance to unixial tensile stresses, applied in the ASTM C 633-79
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Ca
Fig. 5. EDS spectra of HA-I +composite sample.
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ib) Fig. 6. X-ray diffraction spectra for HA Plasma Sprayed coatings: (a) HA-I + composite plasma sprayed coatings; (b) HA-I plasma sprayed coatings.
test, than to shear stresses used in the ASTM D 1002 standard. Figs. 7 and 8 present examples of image analysis results applied to the samples tested according to ASTM C 633-79. The areas where some coating remained are yellow colored and the gray areas represent Ti-6AI-4V alloy substrate. Using this technique it was possible to determine that HA-I/composite failure was 86% adhesive and 14% cohesive. HA-P/composite samples have
Table 2 Stress adhesion results according to ASTM D 1002 standard
Coating
Stress adhesion (MPa)
Standard deviation
HA-I + composite HA-P + composite Glue
14.85 13.15 22.65
4.245 3.795 0.076
P, L. Silva et al. / Sm~f~tce and Coatings Technology 102 ( ! 9 9 8 ) 191-196
Table 3 Stress adhesion results according to ASTM C 633-79 standard Coating
Stress adhesion (MPa)
Standarddeviation
HA-I +composite HAP+composite Glue
35.37 33.91 70
6.478 5.556
shown 57% adhesive and 43% cohesive failure. These results were obtained from three samples of each kind and are average values. For samples subjected to ASTM D 1002 procedures, as it may be seen in Fig. 9, the coating was completely transferred to the counterpart, so that image analysis determinations were not performed.
4. Discussion For some years now, there has been on the market a substantial number of prosthesis and implants which
Fig. 7. Failure surface image analysis photomicrograph of an HA-I + composite sample.
Fig. 8. Failure surface image analysis photomicrograph of an HA-P+composite sample,
i95
consist of calcium phosphate based coatings, mainly HA. Furthermore, more recently, some producers have developed glass coated implants. In either case, the reason behind such choice was due to the need to overcome the problems arising from the use of cemented prosthesis, as it has been thoroughly discussed in the literature. In this work, however, a new attempt was made to go a step further, with new coatings designed to tailor the needs of bonding to neighboring tissues and withstanding the contact with the physiological environment. A surface coating capable of inducing a fast response from the host in the early stages of implantation was created, and a more stable internal region showing reduced dissolution kinetics in the long term, thus allowing for the newly formed bone tissue to be fully established and fixed adequately to implants or prosthesis. To achieve this objective it was decided to go from the usual 100 gm thick (or more) HA layer, to a double layer system consisting of pure HA as a first layer, 50 gm thick, in contact with the metallic substrate and, on the top of that, another 50 Jam thick layer of HA based composite, whose bioactivity is increased with respect to pure HA, due to the introduction of a CaO-P2Os based glass. However, adequate adherence to substrate had to be a requirement, along with appropriate coating layers cohesion, so that the implants handling and performance in service would not be degraded with respect to the usual HA coated implants. Therefore, mechanical testing have been carried out to compare both types of plasma sprayed coatings. Also characterization of the coated layer was performed to allow for relationships between structure and mechanical properties. As it was clearly detected comparing Fig. 6 X R D spectra, the upper layer coating was less crystalline (more amorphous) than HA, probably due to the glass incorporation since the coating parameters were kept
Fig. 9. SEM photomicrograph of an HA-I+composite sample subjected to ASTM D i002 test.
t96
P.L. Silva et at, / N#~we and Coatings Technology ]02 (1998) 191-196
constant and the decrease in crystallinity, c o m p a r e d to that o f the powders prior to coating was minimal. Results presented in Tables 2 and 3 show that coating to substrate adhesion was quite similar for both, H A - I / C o m p o s i t e and H A - P / C o m p o s i t e . For both kinds of double layered coatings, higher adhesion values (almost twice) were determined for tensile stresses ( A S T M C 633-79) than for shear stresses ( A S T M D 1002), as the later testing induces easier crack propagation resulting from the applied shear force. Typical values o f pull-out tests applied to coatings used for load bearing applications are within 30 M P a [ 15, 16 ]. The values in this case, as they are not particularly designed for such mechanically demanding situation, m a y be considered as most satisfactory. Image analysis presented in Figs. 7 and 8 and performed on samples subjected to A S T M C 633-79 has shown that adhesion failure was 85% adhesive for H A - I / c o m p o s i t e samples, but for H A - P / c o m p o s i t e coatings this value falls to 57%. One possible explanation for this behavior may be atributted to the preparation procedure used for H A - I / c o m p o s i t e powders as referred above. The weaker particle-particle bonding o f H A - P powder may have led to a higher cohesive failure percentage than for H A - I samples. Finally, A S T M D 1002 tests showed a complete transference o f the double layer to the counterpart, as it m a y be observed in Fig. 9, indicating almost complete adhesive failure.
Acknowledgement The authors thank I N E B and I R C in Biomedical Materials for the provision of laboratory facilities,
financial support of J N I C T through Ref. P B I C / CTM/1890/95 and Marta S'~ and Barbara Silva for their immense collaboration in performing o f the adhesion tests.
References [t] H. Aoki, Science and medical applications of hydroxyapatite, Takayama Press System Center, JAAS, Tokyo, 1991, p. 165, [2] V.C. Barney, M.P. Levin, D.F. Adams, J. Periodont. 57 (1986) 764. [3] P. Ducheyne, J. Biomed. Mater. Res. Appl. Biomater. 21 (i987) 219. [4] R.Z. I_egeros, Adv. Dent. Res. 2 (1988) 164. [5] D. Williams~ Medical and Dental Materials, Pergamon Press, 1986. [6] M. Akao, K. Haoki, Kato, J. Mater. Sci. i6 ( 1981 ) 809. [7] G. With, H. Van Dijk, N. Hattu, K. Prijs, J. Mater. Sci. i6 (1981) 1592. [8] J.D. Santos, P.L. Silva. J.C. Knowles, F.J. Monteiro, J. Mater. Sci. Mater. Med. 7 ( 1996i 187. [9] C. Rey, Biomaterials 11 (1990) 14. [10] G. Evans, J. Behiri, J. Currey, B. Bonfield, J. Mater. Sci. Mater. Med. 1 (t990) 38. [11] D. Matejka, B. Benko, Plasma Spraying of Metallic Materials, Wiley, New York, 1989. [I2] S.D. Browm Thin Sol. Films 119 (1984) 127. [13] K.L. Mittak Adhesion Measurement of Thin Films, Thick Films and Bulk Coatings, ASTM STP 840. [14] ASTM D 1002, Standard Test Method for Strength Properties of Adhesive in Shear by Tension Loading, Annual Book of ASTM Standards, 1983. [t 5] ASTM C 633-79, Standard Test Method for Adhesion or Cohesive Strength of Flame Sprayed Coatings, Annual Book of ASTM Standards Section 2, 1979. [t6] J. Ruiz, J.A. Gonzklez, Rev. Metal., Madrid, 20 (1) (i984).
COATINGS ELSEVIER
Surface and Coatings TechnologyI02 (1998) 191-196
ff£##OLO /
Adhesion and microstructural characterization of plasma-sprayed hydroxyapatite/glass ceramic coatings onto Ti-6A1-4V substrates P . L . Silva
a,b,*
J . D . S a n t o s a,b F . J . M o n t e i r o a.o, J . C K n o w l e s d
a [NEB-Instituto de Engenharia Biomddica, Prafa Coronel Yacheco 1, 4050 Porto, Portugal b ISEP-hTstituto Superior de Engenharia, R. de S. Tom~, 4200 Porto, Portugal Departamento de Engenharia Metal~rgica, FEUP, Universidade do Porto, R.dos Bragas, 4099 Porto Codex, Portuga/ d Eastman Dental hzstitue, Biomaterials Departement, 256 Gray's Inn Road, Lotzdon, WC1XSLD, UK
Received 7 March 1997; received in revised form 6 August 1997; accepted 7 August 1997
Abstract
Commercial hydroxyapatite (HA), Calo(PO4)6(OH)2, powders were reinforced with 4% of a CaO-P205 based glass during its sintering process. DoubIe layered coatings were obtained by Plasma Spraying and composed of 50 gm commercial HA, followed by a 50 gm (HA)/P2Os-CaO based glass composite coating. Samples were characterized by X-ray diffraction (XRD) analysis and it was shown that they were constituted of an HA matrix with small traces of tricalcium phosphate (/3-TCP) phase. The XRD spectra were quite similar to those of commercial HA coatings. For coating to substrate adhesion two different tests were performed: adhesion tests according to ASTM C 633-79 and ASTM D I002-83 standards. Higher values were obtained for ASTM C 633-79 than for ASTM D I002-83, 35 and 15 MPa, respectively. This behavior is discussed in terms of the different stress effort applied to specimens. The results obtained with ASTM C 633-79 show that these double layered satisfy the stress requirements for load bearing biomedical applications. © 1998 Published by Elsevier Science S.A. Keywords: Plasma spraying; Coating; X-ray diffraction
1. Introduction
H A has been extensively studied and used for biomedical applications [1-4]. The reasons for this interest include a chemical composition very close to that of the inorganic part of bone and excellent biocompatibility in comparison with other implant materials [5]. However, it has been shown that the mechanical properties of HA, in particularly its fracture toughness and strength [6,7], should be improved if used in load bearing applications. The main purpose of applying HA coatings on Ti-6A1-4V alloys has been to keep the mechanical properties of the metallic substrate and, at the same time, to take advantage of the coating biocompatibility and chemical similarity with bone. In this work, HA, reinforced with a CaO-P205 glass was used as a surface layer, i.e. the material that primarily makes contact with tissues and organic fluids. * Corresponding author.
This was the top layer of a double coating containing as a first undercoat a usual HA layer, adherent to the Ti-alloy substrate. In previous work [8], it was shown that by applying such reinforcement the mechanical properties of HA were improved, mainly its fracture toughness and biaxial bending strength. Also, the glass chemical composition was adjusted, so that the composite chemical composition would approach, as closely as possible, the chemical composition of the mineral part of bone [8-10]. Therefore NaaO, MgO and K20 oxides were incorporated into the P2Os-CaO based glass. It is well known [11] that one of the main concerns when using plasma spraying techniques, is the determination of the coating to substrate adhesion. The way by which a coating adheres to a substrate is very complex and it is not fully understood. Several theories about the mechanisms of adhesion exist, but there is no single coherent explanation for all adhesion behaviors [ 12, 13]. Many factors seem to influence the establishment of coating-to-substrate adhesion: (1) mechanical anchorage; (2) van der Waals physical interaction forces;
0257-8972/98/$19.00© 1998 Published by ElsevierScienceB.V. All rights reserved. PII S0257-8972(97) 00576-8
I92
P.L. Sih'a e~ al, / Smface and Coatings Technology 102 (1998) I91-]96
(3) chemical interaction and (4) metallurgical processes [ 11, 12]. Due to the difficulty in quantifying this property, many methods have been studied and applied [t3]. In this case two standard methods [11-14] were chosen, allowing for the cross-evaluation of the results.
2. Materials and methods
glue /ine
/,6ram ,, ~1, Z5,4mm_
I
] sheararea
~
area
12.7ram
in test
~ ' " ~
127mm
The P2Os-CaO based glass was prepared from reagent grade chemicals and its chemical composition is shown in Table 1. The glass was wet-mixed and milled with HA powder using methanol in a ball mill pot for 12 h. 4 wt.% glass addition was used. Mixed powders were then dried and isostatically pressed at 200 MPa, The method used for composite preparation has been fully described elsewhere [8]. Cylindrical shaped samples were sintered at 1300 °C with 1 h dwell time and naturally cooled inside the furnace. After sintering the cylinders were once again milled, for 6 h, using an agate ball mill pot. The powders were then sieved to obtain a grain size distribution suitable for plasma spraying. Two kinds of HA were used for the base coatings. One, called HA-P, was supplied by Metco-Plasma-Technik (CH) with the reference code, AMDRY AM 6021. This was a powder specifically prepared for plasma spraying and was used "as received". The other, called HA-I, was supplied by Plasma Biotal and was conditioned to be used in plasma spraying. Composite powders were also conditioned using the same procedure. Plasma spraying was performed using automated equipment from Plasma Technik, under atmospheric conditions. After substrate preparation, consisting of grit blasting with A1203 spheres, and chemical degreasing the Ti-6A1-4V alloy with trichlorethylene, a 50 btm HA coating was sprayed, followed by the deposition of 50 btm HA/composite coating. Using a scanning electron microscope (SEM) it was not possible to distinguish the two layers. To analyze the two different coatings by SEM, a sample was prepared using the previous procedure and including, between the HA and the HA/glass layer, a very thin carbon coating obtained by sputtering. Both kinds of coatings, HA-P/composite and HA-I/composite, were characterized by XRD analysis and compared with an XRD spectrum of a 100 btm commercial HA coating.
177mm Fig. 1. Test specimenshape and dimensionsaccordingto ASTM D 1002. For the adhesion experiments, 10 samples of each coating were used for each one of the two tests. ASTM D 1002 standard testing was used to determine shear strength according to Fig. 1 and Eq. (1). The glue used to join the test specimens was Plasmatex Klebbi, from Plasma Technik. In order to obtain good alignment of specimens, the jig shown in Fig. 2 was used. Specimens were placed in the testing machine so that the outer 25 mm of each end was in contact with the jaws and the long axis of the test specimen coincided with the direction of applied pull stress, through the center line of the grip assembly. The load was applied at the rate of 1 mm/min. The load at failure was recorded, and the tested specimens analyzed by SEM. ASTM C 633-79 testing was performed according to Fig. 3 and Eq. (1). The testing specimens were joined under the same conditions, using a dynamometric screw driver and applying a 80 N force. With this type of experiment it is possible to obtain coating adhesion under stresses normal to the surface. The glue used to assemble both specimens (one that had the plasma sprayed coating, and the counterpart) was the same used for the ASTM D 1002 tests. The load was applied at a rate of 1 mm/min and after failure the samples were observed by SEM. Tests results
Table 1 Glass chemicalcomposition(mol%) P2Os
CaO
Na20
K20
MgO
35
35
i0
10
10
grips
Fig. 2. Gig used for specimensglueing.
t93
P. L, Silva et al. / Smface and Coatings Tec/mology 102 (1998) 191-196
Coa~ng
a) Specin
Fig. 3. Assembly view for the ASTM C 633-79 experiments.
b)
were expressed in terms of stress adhesion according to Eq. (1): ,Ta-
Fmax
A
(1)
Where ~a=adhesion stress, Fmax=maximum load and A = specimen surface area. Fracture surfaces were observed by SEM together with image analysis technique in order to determine what type of failure mechanism took place: cohesive, adhesive or fracture through the glue. For image analysis, three samples of each kind were analyzed.
3. Results
SEM morphological characterization of the multilayered coating may be seen in the photomicrograph presented in Fig. 4. The line that separates the layers is the thin carbon coating obtained by sputtering. As it may be observed, both HA and composite show very similar morphologies. The two layers seems to be well bonded although presenting porosity which is characteristic of this type of coatings. Coatings chemical compositions seem to be quite similar, showing mainly calcium and phosphorus in their constitution. The top layer (HA-I/composite) also presents 1.21% magnesium and 0.35% sodimn (at.%). This results may be seen in the EDS spectra shown in Fig. 5. XRD spectra of HA and HA-I/composite coatings may be seen in Fig. 6. Both coatings present HA matrix
Fig. 4. SEM photomicrograph of the double Plasma Sprayed layer. A carbon layer (a) may be observed between the HA and HA-composite layers.
with small amounts of tricalcium phosphate, but the commercial HA coating had higher crystallinity than the HA-I + composite coating. Adhesion tests results are shown in Tables 2 and 3. Slightly higher mean values were obtained for HA-I/composite, i.e. 35 MPa for the ASTM C 633-79 tests and 15 MPa for ASTM D 1002 standard, respectively. Both coatings have shown higher resistance to unixial tensile stresses, applied in the ASTM C 633-79
aneer_62|Oll
IJisplay
VFS: 4000
- SpecLru,="
Livetlme: 90 Ca
Ca
Fig. 5. EDS spectra of HA-I +composite sample.
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ib) Fig. 6. X-ray diffraction spectra for HA Plasma Sprayed coatings: (a) HA-I + composite plasma sprayed coatings; (b) HA-I plasma sprayed coatings.
test, than to shear stresses used in the ASTM D 1002 standard. Figs. 7 and 8 present examples of image analysis results applied to the samples tested according to ASTM C 633-79. The areas where some coating remained are yellow colored and the gray areas represent Ti-6AI-4V alloy substrate. Using this technique it was possible to determine that HA-I/composite failure was 86% adhesive and 14% cohesive. HA-P/composite samples have
Table 2 Stress adhesion results according to ASTM D 1002 standard
Coating
Stress adhesion (MPa)
Standard deviation
HA-I + composite HA-P + composite Glue
14.85 13.15 22.65
4.245 3.795 0.076
P, L. Silva et al. / Sm~f~tce and Coatings Technology 102 ( ! 9 9 8 ) 191-196
Table 3 Stress adhesion results according to ASTM C 633-79 standard Coating
Stress adhesion (MPa)
Standarddeviation
HA-I +composite HAP+composite Glue
35.37 33.91 70
6.478 5.556
shown 57% adhesive and 43% cohesive failure. These results were obtained from three samples of each kind and are average values. For samples subjected to ASTM D 1002 procedures, as it may be seen in Fig. 9, the coating was completely transferred to the counterpart, so that image analysis determinations were not performed.
4. Discussion For some years now, there has been on the market a substantial number of prosthesis and implants which
Fig. 7. Failure surface image analysis photomicrograph of an HA-I + composite sample.
Fig. 8. Failure surface image analysis photomicrograph of an HA-P+composite sample,
i95
consist of calcium phosphate based coatings, mainly HA. Furthermore, more recently, some producers have developed glass coated implants. In either case, the reason behind such choice was due to the need to overcome the problems arising from the use of cemented prosthesis, as it has been thoroughly discussed in the literature. In this work, however, a new attempt was made to go a step further, with new coatings designed to tailor the needs of bonding to neighboring tissues and withstanding the contact with the physiological environment. A surface coating capable of inducing a fast response from the host in the early stages of implantation was created, and a more stable internal region showing reduced dissolution kinetics in the long term, thus allowing for the newly formed bone tissue to be fully established and fixed adequately to implants or prosthesis. To achieve this objective it was decided to go from the usual 100 gm thick (or more) HA layer, to a double layer system consisting of pure HA as a first layer, 50 gm thick, in contact with the metallic substrate and, on the top of that, another 50 Jam thick layer of HA based composite, whose bioactivity is increased with respect to pure HA, due to the introduction of a CaO-P2Os based glass. However, adequate adherence to substrate had to be a requirement, along with appropriate coating layers cohesion, so that the implants handling and performance in service would not be degraded with respect to the usual HA coated implants. Therefore, mechanical testing have been carried out to compare both types of plasma sprayed coatings. Also characterization of the coated layer was performed to allow for relationships between structure and mechanical properties. As it was clearly detected comparing Fig. 6 X R D spectra, the upper layer coating was less crystalline (more amorphous) than HA, probably due to the glass incorporation since the coating parameters were kept
Fig. 9. SEM photomicrograph of an HA-I+composite sample subjected to ASTM D i002 test.
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P.L. Silva et at, / N#~we and Coatings Technology ]02 (1998) 191-196
constant and the decrease in crystallinity, c o m p a r e d to that o f the powders prior to coating was minimal. Results presented in Tables 2 and 3 show that coating to substrate adhesion was quite similar for both, H A - I / C o m p o s i t e and H A - P / C o m p o s i t e . For both kinds of double layered coatings, higher adhesion values (almost twice) were determined for tensile stresses ( A S T M C 633-79) than for shear stresses ( A S T M D 1002), as the later testing induces easier crack propagation resulting from the applied shear force. Typical values o f pull-out tests applied to coatings used for load bearing applications are within 30 M P a [ 15, 16 ]. The values in this case, as they are not particularly designed for such mechanically demanding situation, m a y be considered as most satisfactory. Image analysis presented in Figs. 7 and 8 and performed on samples subjected to A S T M C 633-79 has shown that adhesion failure was 85% adhesive for H A - I / c o m p o s i t e samples, but for H A - P / c o m p o s i t e coatings this value falls to 57%. One possible explanation for this behavior may be atributted to the preparation procedure used for H A - I / c o m p o s i t e powders as referred above. The weaker particle-particle bonding o f H A - P powder may have led to a higher cohesive failure percentage than for H A - I samples. Finally, A S T M D 1002 tests showed a complete transference o f the double layer to the counterpart, as it m a y be observed in Fig. 9, indicating almost complete adhesive failure.
Acknowledgement The authors thank I N E B and I R C in Biomedical Materials for the provision of laboratory facilities,
financial support of J N I C T through Ref. P B I C / CTM/1890/95 and Marta S'~ and Barbara Silva for their immense collaboration in performing o f the adhesion tests.
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