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Plasma-sprayed coating of hydroxylapatite on super austenitic stainless steels
Surface and Coatings Technology 110 (1998) 4–12
Plasma-sprayed coating of hydroxylapatite on super austenitic stainless steels K.T. Oh *, Y.S. Park Department of Metallurgical Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul, South Korea Received 2 July 1997; accepted 8 June 1998
Abstract Ti, Ti alloy, Co–Cr alloy and 316L SS are used for metallic prostheses, and other various types have been developed. However, the use of 316L SS is decreasing, owing to its low pitting resistance. Ti and Ti alloy are not used in the head part of hip joints because of their low corrosion-wear resistance. However, despite this problem, Ti and Ti6Al4V are reported to be used most widely today. In this study, super austenitic stainless steel (SASS ) was used as a substrate with a very high corrosion resistance and good mechanical properties. Also, the manufacturing process of SASS is easier than that of Ti and Co–Cr alloy. HA coatings on these substrates are produced using the plasma-spraying technique. The interacting parameters have been shown to contribute complicatedly to coating properties. As the current was increased, the coating thickness gradually increased, and with an increasing number of splats. The increased spray distance augmented the melting degree of the powder that, in turn, decreased the surface roughness of its coating layer. The Ca/P weight ratio of the coating layers increased from 2.162 in the powder state to about 2.8–3.2 in the coating. Although unobserved in the powder, CaO and Ca P O were seen in the coating layer. Also a decrease in 4 2 9 HA phase fraction along with crystallinity took place. In the cases of CaO and Ca P O phases, the increase in current and spray 4 2 9 distance brought about an increase in melting amount, producing an increase in high-temperature phases other than the HA phase. From the immersion test, it was found that the longer the immersion time, the more gradual is the active reaction from the interface of the coating layer and the substrate. It was also noted that pitting was generated from an exposed part of the substrate through the connected pores. Therefore, it can be deduced that for bone ingrowth and high corrosion resistance, a graded coating that maintains a high level of surface roughness on the coating and dense around the interface is required. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Hydroxlapatite; Plasma-sprayed coating; Super austenitic stainless steels
1. Introduction Progress in material engineering has brought about changes in biological reactions and implant designs of biomaterials. Initially, various possible types of biomaterials were developed empirically. Since then, bioinert materials have been used; these do not induce any allergic reactions (positive or negative effects) in the body [1]. When using this material for implantation in the body, acrylic polymer is used for bonding to the bone. This method is called cement fixation, but it is known to be accompanied by stem release, resulting from problems such as monomer toxicity [2], polymer degradation, and mechanical failure. In order to improve these deficiencies, a bioactive ceramic coating is used that leads the bone ingrowth to the surface of the stem * Corresponding author.
[3]. This is called the cementless fixation. Outstanding examples of bioactive ceramics are tricalcium phosphate ( TCP) and hydroxylapatite ( HA). A TCP coating provides short-term benefits in the beginning, but it is difficult to maintain a long-term stability due to degradation and rapid absorption in the body. This is the reason why we use metal as substrate and coat HA on the top. They are popularly used for implants, especially in dentistry and surgery. Their benefits include fast bony adaptation, absence of fibrous tissue seams, increased tolerance of surgical inaccuracies, firm implant–bone attachment, reduced healing time, inhibition of ion release, etc. These can be seen as short-term effects. In the long term, these benefits increase bone ingrowth around the implant and become an important factor that increases stability, which establishes a bone maintenance system surrounding the coating. Ti, Ti alloy, Co–Cr alloy, and 316L SS are used as
0257-8972/98/$ – see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 53 7 - 4
K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
metal prostheses, and many other prostheses are in the process of development [4,5]. However, the use of 316L SS is decreasing due to its low pitting resistance. Ti and Ti alloy are not used in the head part of hip joints because Ti has low corrosion-wear resistance, and the hip joint’s cyclic motions can easily destroy the passive film, generating ion release until the film is repassivated. However, despite their problems, Ti and Ti–6Al–4V are reported to be used most widely today. In this study, super austenitic stainless steel (SASS), which exhibits excellent corrosion resistance, high fatigue strength and elongation, was used as the metal substrate. It is more readily manufactured than Ti alloy and Co–Cr alloy. SASS has a high corrosion-wear resistance compared to Ti and Ti6Al4V, and a high pitting resistance and elongation compared to Co–Cr alloy. This SASS is thought to be a reliable material for the prosthesis. The plasma-spray-coating method, one of the most frequently used methods because of its outstanding performance in in-vivo and in-vitro tests, is used to coat HA powder. Spray distance and plasma current were selected among the plasma spraying parameters. To understand the coating properties, we examined the surface morphology, cross-sectional microstructure, surface roughness, etc. We also attempted to ascertain the change in chemical composition of the coating according to its plasma state and phase transformation. After coating the bioactive ceramic on the metal substrate, we performed the anodic polarization test and impedance measurement to confirm the effect of restraining metal ion release.
2. Experimental methods The metal substrate, super austenitic stainless steel, which has a chemical composition of 23Cr– 22Ni–0.289N–6.5Mo–Fe, is manufactured in the rod by the investment casting. It then undergoes a homogenization heat treatment for 2 h at 1180 °C and is made into a button with a thickness of 15 mm. The HA powder used in this study is represented by Ca (PO ) (OH ) , 10 46 2 and its Ca/P ratio is 1.67 (at.% ratio) [6 ]. The unit cell of HA is a hexagonal structure with a= ˚ and c=6.881 A ˚ , shaped like rhombic prism, 9.432 A and space group of HA crystal is P6 /m. In Fig. 1, the 3 SEM photograph of HA powder is shown. The powder bought and used is AMDRY 6021. This powder was distributed to be about 45–165 mm in size. Before placing into a powder-feeding device, the powder was treated in a dehydrating process for 2 h at 100 °C to remove any moisture and impurities in the powder. METCO Company’s device was used for the plasma spraying. The electric power supplier was of the MCN type and supplied the electric power of 40 kW. For the gun, the MBN type was used, and to prevent any possible damage
5
Fig. 1. SEM photograph of HA powder.
caused by the high temperature during the experiment, a cooling water flow system was set up around the electrode. The 4MP-dual powder feeder was used, with argon as the powder’s carrier gas. The substrate firstly went through grit blasting to activate the surface, and then it was cleaned and fixed on to the substrate holder. The substrate was preheated using the plasma-spraying gun. After that, the coating process was continued by injecting the powder through a powder injector. For each electric current, hydrogen was used as a secondary gas at a flow rate of 15 SCFH, and the primary gas used was argon. Among the plasma spray process variables, changes to the current and spray distance had the greatest effects. Table 1 shows the experimental parameters in detail. A scanning electron microscope was used to examine the surface morphology, and an optical microscope was used to examine the cross-sectional microstructure. Meanwhile, to prevent damage of the pores in the coatings, the specimen was put under epoxy immersion before the polishing. The etching time in 1% phosphoric acid was varied according to its coating thickness. To quantify the surface morphology, surface roughness was measured. Also, to measure the change in chemical compositions of the coatings, EDS ( Voyager model, Noran) was used, and to confirm the phase transformation and to measure the fraction of phases, an X-ray diffractometer was used. Finally, to evaluate corrosion resistance of the coated substrates in the corrosive medium, an anodic polarization test and impedance measurement were carried out. Hank’s solution, known as human-body-simulated solution, was applied, and the experiment was carried out at a temperature of 37 °C. In an anodic polarization test, Hank’s solution was deaerated by purging with N gas, and a saturated 2 calomel electrode (SCE ) was used as the reference electrode. The potential scan rate was 60 mV min−1.
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K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
Table 1 Experimental parameters of HA plasma spray coating Sample number
Primary gas flow rate (SCFH )
Current (A)
Powder feed rate (g min−1)
Spray distance (mm)
NA1 NA2 NA3 NB1 NB2 NB3 NC1 NC2 NC3
100 100 100 100 100 100 100 100 100
300 300 300 370 370 370 450 450 450
25 25 25 25 25 25 25 25 25
60 100 140 60 100 140 60 100 140
3. Results and discussion 3.1. Characteristics of HA coating The HA coating used for artificial hip joints makes the bone grow towards the stem of the artificial hip joint and, after implantation, is dissolved only on the surface area. Therefore, maintenance of an adequate level of surface roughness and pore on the coating to accelerate bone ingrowth is important to maintain a strong adhesion between the bone and the implant. Also, phases other than HA either lower the biocompatibility during implantation or become absorbed in the body more easily than HA. Therefore, it is necessary to control the variables to restrict the production of other phases, and it is advisable also to maintain the high crystallinity of the HA phase itself. Fig. 2 shows the surface morphology and crosssectional microstructure photographs of the coating. We can see how partially melted particles and pores change their distribution according to the experimental conditions. In the cross-sectional microstructure photographs, the molten particles collide on the surface of the substrate or just beneath the coating and create splat, which forms layers. Also, we can observe the existence of unmelted particles on the coating. Since the surface morphology depends on the plasma flame’s temperature and velocity, particle’s residual time in the flame and cooling process in the flight, we have to measure the surface roughness in order to evaluate quantitatively such phenomena. The surface roughness shows an increase in roughness (R ) of 2–3 mm in the grit-blasting a state to 6–11 mm after coating. Fig. 3 is the surface roughness of each specimen according to the current and spray distance. The spray distance was changed when the NA specimen’s current was set to 300 A. The result showed a gradual decrease in unmelted particles on the surface as the spray distance was increased. An increase in spray distance meant that the sprayed particle had a longer residual time in the plasma flame, but it was only a minor change. In the cross-sectional microstructural photograph of the NA specimen, with increas-
ing spray distance, the coating thickness appeared to decrease. As a result, if we increase the spray distance while plasma-spraying the HA powder, the 300 A current brings about a decrease in coating efficiency. For the NB specimens, the spray distance was varied at a current of 370 A. An examination of the surface morphology of NB1 (60 mm) and NB2 (100 mm) reveals the formation of pores on the surface that are connected to the substrate. However, in the case of NB3 (140 mm), the coating is comparatively smooth. Compared to NB2 and NB3, the NB1 specimen has more particles that are partially melted. These results coincide with the surface roughness results showing 11.00, 8.21 and 6.72 mm for NB1, NB2, and NB3, respectively. As for NA, the increase in spray distances in the NB condition provided a longer residual time in the flame, and, unlike NA and NC, NB with a current of 370 A showed a better melting process. This condition produces a large change in surface roughness with spray distance. In such conditions, the level of current necessary for melting is satisfied, and this is why the degree of melting reacts sensitively to the change in spray distance. From the above results, we can say that the predominant effect of spray distance was greater than that of the current. From surface morphologies in NC specimens, a large amount of splat can be observed in the coating due to complete melting. Also, despite the change in spray distance, the surface roughness and coating thickness appeared to be consistent. It seems that the current of 450 A forms a thinner and denser splat than that at 300 and 370 A, and the coating appeared to be thicker. For the evaluation of porosity of inner coatings, we have used epoxy immersion on the coating to prevent the damage to the pores caused by polishing. For each specimen, we used a point count method to evaluate porosity. The result is shown in Fig. 4. In the crosssectional microstructural photograph (Fig. 2), the pores appear as several black spots; the black stripes are oxides and the white area between the stripes is a splat. Results from porosity measurements and surface roughness show that a longer spray distance results in a lower porosity.
K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
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Fig. 2. SEM photographs of HA coating surfaces and OM photographs of cross-sectional microstructure.
Comparing the cross-sectional microstructural photograph of NA1, NB1, NC1, which all have a spray distance of 60 mm, the coating thickness increases but the splat thickness decreases with current. This shows
that the current plays an important role in controlling the coating thickness and surface morphology. The Ca/P weight ratio in the coating increased from 2.162 (Ca: 40 wt%, P: 18.5 wt%, Ca/P at. ratio=1.67)
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K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
Fig. 3. Changes in roughness with distances for different currents.
Fig. 5. XRD diffraction pattern of (a) HA powder and (b) HA coating, NB1.
Fig. 4. Changes in porosity with distances for different currents.
in the powder state to about 2.8–3.2. The reason for the increase of Ca/P wt ratio is thought to be selective evaporation of a component of HA powder sprayed into the hot flame. For the evaluation of newly formed phases and for HA’s phase fraction and crystallinity [7], an XRD analysis was performed (Fig. 5 shows the results). CaO and Ca P O phases were seen in the 4 2 9 coating, but these were not seen in the used powders. Also, with decreasing HA phase fraction, a general decrease in peak intensity and increase in FWHM were clearly seen. The FWHM increase is a result from both fine grain size and increase in microstrain that exists in the grain [8]. The well-known characteristics of a plasma-sprayed coating are fine grain size and decrease in crystallinity caused by rapid solidification of its exposure in the atmosphere [9]. In this study, even in the plasma-sprayed coating, decreased HA crystallinities were observed after spraying. Decreased crystallinity causes rapid HA absorption in the body and accelerates the growth of fibrous tissue during implantation. A number of researchers [10] reported a decrease in crystallinity after plasma spraying and recommended a postheat treatment to improve the crystallinity. Integrated
intensities for the peaks of HA, CaO, Ca P O phases 4 2 9 have been measured and are shown in Fig. 6. In the CaO and Ca P O phases, the degree of melting 4 2 9 increases with increasing current and spray distance, thereby increasing the production of the high-temperature phase (other than the HA phase). As seen in the CaO–P O phase diagram, the plasma flame trans2 5 formed the HA phase into stable CaO at high temperature. Some studies [11] have reported that during plasma-spray HA coating, the HA and b-C P that 3 existed in the powder are transformed to CaO, a-C P, C P compounds. This reaction can be explained 3 4 by the following equation: 1st step: Ca (PO ) (OH ) 2b-Ca (PO ) +Ca (PO ) 10 46 2 3 42 3 42 · CaO+2H O above 1050 °C 2 2nd step: b-Ca (PO ) a-Ca (PO ) 3 42 3 42 Ca (PO ) · CaOCa (PO ) +CaO above 1350 °C. 3 42 3 42 From the above reactions, it can be seen that as soon as the particles were injected into the plasma flame, the particle reached a temperature of more than 1350 °C. 3.2. Corrosion resistance evaluation of HA-coated substrate We compared the corrosion resistance of a HA-coated Ti alloy, 316L SS, and super austenitic stainless steel using the anodic polarization test (APT ) and observed
K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
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Fig. 7. Anodic polarization curves for an HA-coated sample on super SS, 316L SS, Ti alloy in Hank’s solution.
Fig. 8. Equivalent circuit model of coated metal/solution interface.
Fig. 6. Changes in phase content for (a) CaO and (b) Ca P O with 4 2 9 distances for different currents.
their corrosion behavior according to their immersion time using the impedance technique. Fig. 7 shows the result of an anodic polarization test for the plasma-sprayed NB1 on the Ti alloy, 316L SS, super austenitic stainless steel substrates. From the experiment performed in Hank’s solution at 37 °C, the three types of substrates all showed a low current density of between 1 and 10 mA cm−2. To observe the corrosion behavior of the substrate through the connected pores, we carried out impedance measurements in the Hank’s solution and 3.5% NaCl solution to accelerate corrosion. Also, to investigate the dissolution of the passive, the impedance measurement was carried out by loading the d.c. potential. From this result, we have used each reaction element to compose the equivalent circuit [12] shown in Fig. 8. R denotes the resistance of the surface p charge transfer reaction, C is the capacitance by the dl formation of double layers on the metal/electrolyte interface, Z is the Warburg impedance of the diffusion w in the electrode, R is the electrolyte resistance passing po through the pore, and C is the capacitance of the po coating itself [13]. Fig. 9 is a linear extrapolation showing the change in
open circuit potential in relation to immersion time in Hank’s solution and 3.5% NaCl solution. If we evaluate the line slope to the change in open circuit potential in the two types of solutions, the open circuit potential in 3.5% NaCl solution decreases on a large scale compared to that in Hank’s solution. The difference between two experimental solutions is the concentration of the Cl− ion. The substrate surface at 3.5% NaCl solution therefore indicates that it is activated more by the Cl− ion and that it can be very sensitive to Cl− ion concentration. The results of impedance measurements according to different immersion times in Hank’s solution are transformed into a Nyquist plot in Fig. 10. In the early stages of the immersion, only the exposed parts of the substrate react purely through the connecting pores. This means that the reaction area is so small that the diameter of the semicircle enlarges, but gradually, the semicircle becomes smaller in diameter owing to the increased activated surface. Fig. 11 shows the results of impedance measurements in Hank’s solution with a simulation fitting program with the presented equivalent circuit. R continues to decrease until after 74 h of immersion p time and then remains consistent afterwards. This phenomenon can be due to the fact that such an electrode reaction was dominated by a corrosion reaction at the interface, due to an increase in active surface area or dissolution of the passive film in the early stages of
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K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
Fig. 11. Variation of polarization resistance with immersion time in Hank’s solution.
Fig. 9. Variation of open circuit potential with immersion time in (a) Hank’s solution and (b) 3.5-wt% NaCl solution.
Fig. 10. Nyquist plot of HA-coated sample with increasing immersion time in Hank’s solution.
immersion, and then by Warburg impedance or electrolyte resistance through the connected pores. Fig. 12 shows the surface morphology after impedance measurements at each immersion time. In the case of immersed samples in Hank’s solution, immersion for 2 h produced a clean surface but pits were created after immersion for 246 h. Also, as the length of time was increased, the
distribution of pits gradually increased. More pits were observed in the specimens immersed at 3.5% NaCl solution. Consequently, when R decreases at an early p stage of immersion, there is a corrosion reaction due to surface activation of pitting on the coating and substrate interface [14]. Also, when R is consistent, the predomip nant reaction of the electrode can be explained as a result of a reaction by diffusion through the inner connected pores. The connected pores, blocked by the corrosion product, become affected more by the diffusion in the connected pores or the solution than by corrosion on the interface. This means that the Warburg impedance increased with immersion time. Fig. 13 shows the results of impedance measurements according to the d.c. potential. It shows how the increase in potential gradually led to a decrease in diameter of the semicircle. Fig. 14 illustrates the change in R . It p can be seen that R indicates the surface reaction, which p decreased until it reached 600 mV and then stayed consistent. The result suggests a similarity to the anodic polarization test which, in fact, demonstrated an increase in current density from 800 mV to 1200 mV. From this result, we can recognize pit formation from the surface activation due to the passive film damage. Fig. 15 shows a scanning electron microscope photograph showing the above result. The pits were seen on the surface of the substrate, although the HA-coated super stainless steel had a higher corrosion resistance. These pits may be formed through the connected pores. This is why a dense coating is required on the interface between the coating and the substrate. After the bone growth, a certain amount of HA component must be maintained. Therefore, we have to maximize the control of the coating’s chemical composition and also allow it to dissolve easily, but not completely, in the body despite all other participates. We have to raise the crystallinity of HA phase as much as possible to accelerate the bone ingrowth. In addition,
K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
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Fig. 13. Nyquist plot of HA-coated sample with increased d.c. potential in Hank’s solution.
Fig. 14. Variation of polarization resistance with d.c. potential in Hank’s solution.
Fig. 12. SEM photographs of surface of immersed samples (×1000): (a) Hank’s solution for 2 h, (b) Hank’s solution for 246 h, and (c) 3.5% NaCl solution for 295 h.
to restrain any metal-ion release that occurs when the substrate is exposed to body fluids through the coating’s connected pore, a dense coating on the coating interface will be needed. Also, with the increase of surface area on the coating surface and the interlocking effect, in order to increase the adhesion between the bone tissue and the implant, graded coating must be carried out.
4. Conclusions In implanting an artificial hip joint into the body for bone ingrowth, we have used a plasma spray method to produce the HA coating. At the same time, we have changed the current and spray distance, which are the
most effective variables among the plasma-spraying process parameters, so as to help in our understanding of the coating characteristics. These variables were shown to contribute to the coating characteristics reciprocally. (1) As the current was increased, the coating thickness increased and thinner splats were formed. It was also noted that the number of layers were increased. The increase in spray distance increased the degree of particle melting and decreased the surface roughness. It also tended to decrease the porosity of the inner coating. (2) The weight ratio of Ca/P on the coating increased from 2.162 in the powder state to about 2.8–3.2. This is why we are able to predict the phase transformation on the coating. Now we can observe the CaO and Ca P O on the coating, which could not 4 2 9 be seen in the powder. With a decrease in the HA phase fraction, the crystallinity also fell. According to the reports, because of this, the HA was absorbed rapidly in the body during implantation and accelerated the growth of fibrous tissue, thereby
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K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
crystallinity has to be maximized. As for the CaO and Ca P O phases, the increases in current and 4 2 9 spray distance brought about an increase in the degree of melting. Consequently, the high-temperature phase, excluding the HA phase, increased. As seen in the CaO–P O phase diagram, the HA phase 2 5 was transformed into CaO, which is stable at high temperatures. The immersion test showed that a reaction took place at the interface between the coating and the substrate after prolonged immersion. Summing up, the graded coating must be applied to HA coatings for biomedical application.
References
Fig. 15. SEM photographs of surface of impedance-tested samples at variable potentials in Hank’s solution (×1000): (a) d.c. 600 mV and (b) d.c. 800 mV.
decreasing the adhesion with the bone. To improve adhesion strength with the bone, the HA coating’s compositional change must be minimized and the
[1] J.F. Kay, CRC Handbook of Bioactive Ceramics, Vol. 2, CRC Press, Boca Raton, Fl, USA, 1990, p. 111. [2] H.G. Willert, J. Ludwig, M. Semlitsch, J. Bone Joint Surg. 56A (1974) 1368. [3] K. Hayasch, N. Matsuguchi, K. Uenoyama, T. Kanemaru, Y. Sugioka, J. Biomed. Mater. Res. 23 (1989) 1247. [4] W. Rostoker, J.O. Galante, J. Biomech. Eng. 101 (1979) 2. [5] P. Ducheyne, G.W. Hastings, Metal and Ceramic Biomaterials, Vol. 1, 2, CRC Press, Boca Raton, FL, 1984. [6 ] M. Jarcho, C.H. Bolen, M.B. Tomas, J. Mater. Sci. 11 (1976) 2027. [7] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., AddisonWesley, MA, 1977. [8] D.S. Rickerby, A.M. Jones, B.A. Bellamy, Surf. Coat. Technol. 37 (1989) 111. [9] R. McPherson, Surf. Coat. Technol. 39–40 (1989) 173. [10] K.A. Khur, P. Cheang, Proc. NTSC, 1993, p. 347. [11] P. Ducheyne, J. Cuckler, S. Radin, E. Nazar, CRC Handbook of Bioactive Ceramics, Vol. 2, 1990, CRC Press, Boca Raton, Fl, USA, p. 123. [12] P.E. Arnvig et al., 2nd Plasma-Technik Symposium, Vol. 3, 1990, p. 331. [13] T.A. Strivens, C.C. Taylor, Mater. Chem. 7 (1982) 199. [14] A.A. Ashary, R. Tucker, Jr, Surf. Coat. Technol. 39–40 (1989) 701.
Plasma-sprayed coating of hydroxylapatite on super austenitic stainless steels K.T. Oh *, Y.S. Park Department of Metallurgical Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul, South Korea Received 2 July 1997; accepted 8 June 1998
Abstract Ti, Ti alloy, Co–Cr alloy and 316L SS are used for metallic prostheses, and other various types have been developed. However, the use of 316L SS is decreasing, owing to its low pitting resistance. Ti and Ti alloy are not used in the head part of hip joints because of their low corrosion-wear resistance. However, despite this problem, Ti and Ti6Al4V are reported to be used most widely today. In this study, super austenitic stainless steel (SASS ) was used as a substrate with a very high corrosion resistance and good mechanical properties. Also, the manufacturing process of SASS is easier than that of Ti and Co–Cr alloy. HA coatings on these substrates are produced using the plasma-spraying technique. The interacting parameters have been shown to contribute complicatedly to coating properties. As the current was increased, the coating thickness gradually increased, and with an increasing number of splats. The increased spray distance augmented the melting degree of the powder that, in turn, decreased the surface roughness of its coating layer. The Ca/P weight ratio of the coating layers increased from 2.162 in the powder state to about 2.8–3.2 in the coating. Although unobserved in the powder, CaO and Ca P O were seen in the coating layer. Also a decrease in 4 2 9 HA phase fraction along with crystallinity took place. In the cases of CaO and Ca P O phases, the increase in current and spray 4 2 9 distance brought about an increase in melting amount, producing an increase in high-temperature phases other than the HA phase. From the immersion test, it was found that the longer the immersion time, the more gradual is the active reaction from the interface of the coating layer and the substrate. It was also noted that pitting was generated from an exposed part of the substrate through the connected pores. Therefore, it can be deduced that for bone ingrowth and high corrosion resistance, a graded coating that maintains a high level of surface roughness on the coating and dense around the interface is required. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Hydroxlapatite; Plasma-sprayed coating; Super austenitic stainless steels
1. Introduction Progress in material engineering has brought about changes in biological reactions and implant designs of biomaterials. Initially, various possible types of biomaterials were developed empirically. Since then, bioinert materials have been used; these do not induce any allergic reactions (positive or negative effects) in the body [1]. When using this material for implantation in the body, acrylic polymer is used for bonding to the bone. This method is called cement fixation, but it is known to be accompanied by stem release, resulting from problems such as monomer toxicity [2], polymer degradation, and mechanical failure. In order to improve these deficiencies, a bioactive ceramic coating is used that leads the bone ingrowth to the surface of the stem * Corresponding author.
[3]. This is called the cementless fixation. Outstanding examples of bioactive ceramics are tricalcium phosphate ( TCP) and hydroxylapatite ( HA). A TCP coating provides short-term benefits in the beginning, but it is difficult to maintain a long-term stability due to degradation and rapid absorption in the body. This is the reason why we use metal as substrate and coat HA on the top. They are popularly used for implants, especially in dentistry and surgery. Their benefits include fast bony adaptation, absence of fibrous tissue seams, increased tolerance of surgical inaccuracies, firm implant–bone attachment, reduced healing time, inhibition of ion release, etc. These can be seen as short-term effects. In the long term, these benefits increase bone ingrowth around the implant and become an important factor that increases stability, which establishes a bone maintenance system surrounding the coating. Ti, Ti alloy, Co–Cr alloy, and 316L SS are used as
0257-8972/98/$ – see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 53 7 - 4
K.T. Oh, Y.S. Park / Surface and Coatings Technology 110 (1998) 4–12
metal prostheses, and many other prostheses are in the process of development [4,5]. However, the use of 316L SS is decreasing due to its low pitting resistance. Ti and Ti alloy are not used in the head part of hip joints because Ti has low corrosion-wear resistance, and the hip joint’s cyclic motions can easily destroy the passive film, generating ion release until the film is repassivated. However, despite their problems, Ti and Ti–6Al–4V are reported to be used most widely today. In this study, super austenitic stainless steel (SASS), which exhibits excellent corrosion resistance, high fatigue strength and elongation, was used as the metal substrate. It is more readily manufactured than Ti alloy and Co–Cr alloy. SASS has a high corrosion-wear resistance compared to Ti and Ti6Al4V, and a high pitting resistance and elongation compared to Co–Cr alloy. This SASS is thought to be a reliable material for the prosthesis. The plasma-spray-coating method, one of the most frequently used methods because of its outstanding performance in in-vivo and in-vitro tests, is used to coat HA powder. Spray distance and plasma current were selected among the plasma spraying parameters. To understand the coating properties, we examined the surface morphology, cross-sectional microstructure, surface roughness, etc. We also attempted to ascertain the change in chemical composition of the coating according to its plasma state and phase transformation. After coating the bioactive ceramic on the metal substrate, we performed the anodic polarization test and impedance measurement to confirm the effect of restraining metal ion release.
2. Experimental methods The metal substrate, super austenitic stainless steel, which has a chemical composition of 23Cr– 22Ni–0.289N–6.5Mo–Fe, is manufactured in the rod by the investment casting. It then undergoes a homogenization heat treatment for 2 h at 1180 °C and is made into a button with a thickness of 15 mm. The HA powder used in this study is represented by Ca (PO ) (OH ) , 10 46 2 and its Ca/P ratio is 1.67 (at.% ratio) [6 ]. The unit cell of HA is a hexagonal structure with a= ˚ and c=6.881 A ˚ , shaped like rhombic prism, 9.432 A and space group of HA crystal is P6 /m. In Fig. 1, the 3 SEM photograph of HA powder is shown. The powder bought and used is AMDRY 6021. This powder was distributed to be about 45–165 mm in size. Before placing into a powder-feeding device, the powder was treated in a dehydrating process for 2 h at 100 °C to remove any moisture and impurities in the powder. METCO Company’s device was used for the plasma spraying. The electric power supplier was of the MCN type and supplied the electric power of 40 kW. For the gun, the MBN type was used, and to prevent any possible damage
5
Fig. 1. SEM photograph of HA powder.
caused by the high temperature during the experiment, a cooling water flow system was set up around the electrode. The 4MP-dual powder feeder was used, with argon as the powder’s carrier gas. The substrate firstly went through grit blasting to activate the surface, and then it was cleaned and fixed on to the substrate holder. The substrate was preheated using the plasma-spraying gun. After that, the coating process was continued by injecting the powder through a powder injector. For each electric current, hydrogen was used as a secondary gas at a flow rate of 15 SCFH, and the primary gas used was argon. Among the plasma spray process variables, changes to the current and spray distance had the greatest effects. Table 1 shows the experimental parameters in detail. A scanning electron microscope was used to examine the surface morphology, and an optical microscope was used to examine the cross-sectional microstructure. Meanwhile, to prevent damage of the pores in the coatings, the specimen was put under epoxy immersion before the polishing. The etching time in 1% phosphoric acid was varied according to its coating thickness. To quantify the surface morphology, surface roughness was measured. Also, to measure the change in chemical compositions of the coatings, EDS ( Voyager model, Noran) was used, and to confirm the phase transformation and to measure the fraction of phases, an X-ray diffractometer was used. Finally, to evaluate corrosion resistance of the coated substrates in the corrosive medium, an anodic polarization test and impedance measurement were carried out. Hank’s solution, known as human-body-simulated solution, was applied, and the experiment was carried out at a temperature of 37 °C. In an anodic polarization test, Hank’s solution was deaerated by purging with N gas, and a saturated 2 calomel electrode (SCE ) was used as the reference electrode. The potential scan rate was 60 mV min−1.
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Table 1 Experimental parameters of HA plasma spray coating Sample number
Primary gas flow rate (SCFH )
Current (A)
Powder feed rate (g min−1)
Spray distance (mm)
NA1 NA2 NA3 NB1 NB2 NB3 NC1 NC2 NC3
100 100 100 100 100 100 100 100 100
300 300 300 370 370 370 450 450 450
25 25 25 25 25 25 25 25 25
60 100 140 60 100 140 60 100 140
3. Results and discussion 3.1. Characteristics of HA coating The HA coating used for artificial hip joints makes the bone grow towards the stem of the artificial hip joint and, after implantation, is dissolved only on the surface area. Therefore, maintenance of an adequate level of surface roughness and pore on the coating to accelerate bone ingrowth is important to maintain a strong adhesion between the bone and the implant. Also, phases other than HA either lower the biocompatibility during implantation or become absorbed in the body more easily than HA. Therefore, it is necessary to control the variables to restrict the production of other phases, and it is advisable also to maintain the high crystallinity of the HA phase itself. Fig. 2 shows the surface morphology and crosssectional microstructure photographs of the coating. We can see how partially melted particles and pores change their distribution according to the experimental conditions. In the cross-sectional microstructure photographs, the molten particles collide on the surface of the substrate or just beneath the coating and create splat, which forms layers. Also, we can observe the existence of unmelted particles on the coating. Since the surface morphology depends on the plasma flame’s temperature and velocity, particle’s residual time in the flame and cooling process in the flight, we have to measure the surface roughness in order to evaluate quantitatively such phenomena. The surface roughness shows an increase in roughness (R ) of 2–3 mm in the grit-blasting a state to 6–11 mm after coating. Fig. 3 is the surface roughness of each specimen according to the current and spray distance. The spray distance was changed when the NA specimen’s current was set to 300 A. The result showed a gradual decrease in unmelted particles on the surface as the spray distance was increased. An increase in spray distance meant that the sprayed particle had a longer residual time in the plasma flame, but it was only a minor change. In the cross-sectional microstructural photograph of the NA specimen, with increas-
ing spray distance, the coating thickness appeared to decrease. As a result, if we increase the spray distance while plasma-spraying the HA powder, the 300 A current brings about a decrease in coating efficiency. For the NB specimens, the spray distance was varied at a current of 370 A. An examination of the surface morphology of NB1 (60 mm) and NB2 (100 mm) reveals the formation of pores on the surface that are connected to the substrate. However, in the case of NB3 (140 mm), the coating is comparatively smooth. Compared to NB2 and NB3, the NB1 specimen has more particles that are partially melted. These results coincide with the surface roughness results showing 11.00, 8.21 and 6.72 mm for NB1, NB2, and NB3, respectively. As for NA, the increase in spray distances in the NB condition provided a longer residual time in the flame, and, unlike NA and NC, NB with a current of 370 A showed a better melting process. This condition produces a large change in surface roughness with spray distance. In such conditions, the level of current necessary for melting is satisfied, and this is why the degree of melting reacts sensitively to the change in spray distance. From the above results, we can say that the predominant effect of spray distance was greater than that of the current. From surface morphologies in NC specimens, a large amount of splat can be observed in the coating due to complete melting. Also, despite the change in spray distance, the surface roughness and coating thickness appeared to be consistent. It seems that the current of 450 A forms a thinner and denser splat than that at 300 and 370 A, and the coating appeared to be thicker. For the evaluation of porosity of inner coatings, we have used epoxy immersion on the coating to prevent the damage to the pores caused by polishing. For each specimen, we used a point count method to evaluate porosity. The result is shown in Fig. 4. In the crosssectional microstructural photograph (Fig. 2), the pores appear as several black spots; the black stripes are oxides and the white area between the stripes is a splat. Results from porosity measurements and surface roughness show that a longer spray distance results in a lower porosity.
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Fig. 2. SEM photographs of HA coating surfaces and OM photographs of cross-sectional microstructure.
Comparing the cross-sectional microstructural photograph of NA1, NB1, NC1, which all have a spray distance of 60 mm, the coating thickness increases but the splat thickness decreases with current. This shows
that the current plays an important role in controlling the coating thickness and surface morphology. The Ca/P weight ratio in the coating increased from 2.162 (Ca: 40 wt%, P: 18.5 wt%, Ca/P at. ratio=1.67)
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Fig. 3. Changes in roughness with distances for different currents.
Fig. 5. XRD diffraction pattern of (a) HA powder and (b) HA coating, NB1.
Fig. 4. Changes in porosity with distances for different currents.
in the powder state to about 2.8–3.2. The reason for the increase of Ca/P wt ratio is thought to be selective evaporation of a component of HA powder sprayed into the hot flame. For the evaluation of newly formed phases and for HA’s phase fraction and crystallinity [7], an XRD analysis was performed (Fig. 5 shows the results). CaO and Ca P O phases were seen in the 4 2 9 coating, but these were not seen in the used powders. Also, with decreasing HA phase fraction, a general decrease in peak intensity and increase in FWHM were clearly seen. The FWHM increase is a result from both fine grain size and increase in microstrain that exists in the grain [8]. The well-known characteristics of a plasma-sprayed coating are fine grain size and decrease in crystallinity caused by rapid solidification of its exposure in the atmosphere [9]. In this study, even in the plasma-sprayed coating, decreased HA crystallinities were observed after spraying. Decreased crystallinity causes rapid HA absorption in the body and accelerates the growth of fibrous tissue during implantation. A number of researchers [10] reported a decrease in crystallinity after plasma spraying and recommended a postheat treatment to improve the crystallinity. Integrated
intensities for the peaks of HA, CaO, Ca P O phases 4 2 9 have been measured and are shown in Fig. 6. In the CaO and Ca P O phases, the degree of melting 4 2 9 increases with increasing current and spray distance, thereby increasing the production of the high-temperature phase (other than the HA phase). As seen in the CaO–P O phase diagram, the plasma flame trans2 5 formed the HA phase into stable CaO at high temperature. Some studies [11] have reported that during plasma-spray HA coating, the HA and b-C P that 3 existed in the powder are transformed to CaO, a-C P, C P compounds. This reaction can be explained 3 4 by the following equation: 1st step: Ca (PO ) (OH ) 2b-Ca (PO ) +Ca (PO ) 10 46 2 3 42 3 42 · CaO+2H O above 1050 °C 2 2nd step: b-Ca (PO ) a-Ca (PO ) 3 42 3 42 Ca (PO ) · CaOCa (PO ) +CaO above 1350 °C. 3 42 3 42 From the above reactions, it can be seen that as soon as the particles were injected into the plasma flame, the particle reached a temperature of more than 1350 °C. 3.2. Corrosion resistance evaluation of HA-coated substrate We compared the corrosion resistance of a HA-coated Ti alloy, 316L SS, and super austenitic stainless steel using the anodic polarization test (APT ) and observed
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Fig. 7. Anodic polarization curves for an HA-coated sample on super SS, 316L SS, Ti alloy in Hank’s solution.
Fig. 8. Equivalent circuit model of coated metal/solution interface.
Fig. 6. Changes in phase content for (a) CaO and (b) Ca P O with 4 2 9 distances for different currents.
their corrosion behavior according to their immersion time using the impedance technique. Fig. 7 shows the result of an anodic polarization test for the plasma-sprayed NB1 on the Ti alloy, 316L SS, super austenitic stainless steel substrates. From the experiment performed in Hank’s solution at 37 °C, the three types of substrates all showed a low current density of between 1 and 10 mA cm−2. To observe the corrosion behavior of the substrate through the connected pores, we carried out impedance measurements in the Hank’s solution and 3.5% NaCl solution to accelerate corrosion. Also, to investigate the dissolution of the passive, the impedance measurement was carried out by loading the d.c. potential. From this result, we have used each reaction element to compose the equivalent circuit [12] shown in Fig. 8. R denotes the resistance of the surface p charge transfer reaction, C is the capacitance by the dl formation of double layers on the metal/electrolyte interface, Z is the Warburg impedance of the diffusion w in the electrode, R is the electrolyte resistance passing po through the pore, and C is the capacitance of the po coating itself [13]. Fig. 9 is a linear extrapolation showing the change in
open circuit potential in relation to immersion time in Hank’s solution and 3.5% NaCl solution. If we evaluate the line slope to the change in open circuit potential in the two types of solutions, the open circuit potential in 3.5% NaCl solution decreases on a large scale compared to that in Hank’s solution. The difference between two experimental solutions is the concentration of the Cl− ion. The substrate surface at 3.5% NaCl solution therefore indicates that it is activated more by the Cl− ion and that it can be very sensitive to Cl− ion concentration. The results of impedance measurements according to different immersion times in Hank’s solution are transformed into a Nyquist plot in Fig. 10. In the early stages of the immersion, only the exposed parts of the substrate react purely through the connecting pores. This means that the reaction area is so small that the diameter of the semicircle enlarges, but gradually, the semicircle becomes smaller in diameter owing to the increased activated surface. Fig. 11 shows the results of impedance measurements in Hank’s solution with a simulation fitting program with the presented equivalent circuit. R continues to decrease until after 74 h of immersion p time and then remains consistent afterwards. This phenomenon can be due to the fact that such an electrode reaction was dominated by a corrosion reaction at the interface, due to an increase in active surface area or dissolution of the passive film in the early stages of
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Fig. 11. Variation of polarization resistance with immersion time in Hank’s solution.
Fig. 9. Variation of open circuit potential with immersion time in (a) Hank’s solution and (b) 3.5-wt% NaCl solution.
Fig. 10. Nyquist plot of HA-coated sample with increasing immersion time in Hank’s solution.
immersion, and then by Warburg impedance or electrolyte resistance through the connected pores. Fig. 12 shows the surface morphology after impedance measurements at each immersion time. In the case of immersed samples in Hank’s solution, immersion for 2 h produced a clean surface but pits were created after immersion for 246 h. Also, as the length of time was increased, the
distribution of pits gradually increased. More pits were observed in the specimens immersed at 3.5% NaCl solution. Consequently, when R decreases at an early p stage of immersion, there is a corrosion reaction due to surface activation of pitting on the coating and substrate interface [14]. Also, when R is consistent, the predomip nant reaction of the electrode can be explained as a result of a reaction by diffusion through the inner connected pores. The connected pores, blocked by the corrosion product, become affected more by the diffusion in the connected pores or the solution than by corrosion on the interface. This means that the Warburg impedance increased with immersion time. Fig. 13 shows the results of impedance measurements according to the d.c. potential. It shows how the increase in potential gradually led to a decrease in diameter of the semicircle. Fig. 14 illustrates the change in R . It p can be seen that R indicates the surface reaction, which p decreased until it reached 600 mV and then stayed consistent. The result suggests a similarity to the anodic polarization test which, in fact, demonstrated an increase in current density from 800 mV to 1200 mV. From this result, we can recognize pit formation from the surface activation due to the passive film damage. Fig. 15 shows a scanning electron microscope photograph showing the above result. The pits were seen on the surface of the substrate, although the HA-coated super stainless steel had a higher corrosion resistance. These pits may be formed through the connected pores. This is why a dense coating is required on the interface between the coating and the substrate. After the bone growth, a certain amount of HA component must be maintained. Therefore, we have to maximize the control of the coating’s chemical composition and also allow it to dissolve easily, but not completely, in the body despite all other participates. We have to raise the crystallinity of HA phase as much as possible to accelerate the bone ingrowth. In addition,
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Fig. 13. Nyquist plot of HA-coated sample with increased d.c. potential in Hank’s solution.
Fig. 14. Variation of polarization resistance with d.c. potential in Hank’s solution.
Fig. 12. SEM photographs of surface of immersed samples (×1000): (a) Hank’s solution for 2 h, (b) Hank’s solution for 246 h, and (c) 3.5% NaCl solution for 295 h.
to restrain any metal-ion release that occurs when the substrate is exposed to body fluids through the coating’s connected pore, a dense coating on the coating interface will be needed. Also, with the increase of surface area on the coating surface and the interlocking effect, in order to increase the adhesion between the bone tissue and the implant, graded coating must be carried out.
4. Conclusions In implanting an artificial hip joint into the body for bone ingrowth, we have used a plasma spray method to produce the HA coating. At the same time, we have changed the current and spray distance, which are the
most effective variables among the plasma-spraying process parameters, so as to help in our understanding of the coating characteristics. These variables were shown to contribute to the coating characteristics reciprocally. (1) As the current was increased, the coating thickness increased and thinner splats were formed. It was also noted that the number of layers were increased. The increase in spray distance increased the degree of particle melting and decreased the surface roughness. It also tended to decrease the porosity of the inner coating. (2) The weight ratio of Ca/P on the coating increased from 2.162 in the powder state to about 2.8–3.2. This is why we are able to predict the phase transformation on the coating. Now we can observe the CaO and Ca P O on the coating, which could not 4 2 9 be seen in the powder. With a decrease in the HA phase fraction, the crystallinity also fell. According to the reports, because of this, the HA was absorbed rapidly in the body during implantation and accelerated the growth of fibrous tissue, thereby
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crystallinity has to be maximized. As for the CaO and Ca P O phases, the increases in current and 4 2 9 spray distance brought about an increase in the degree of melting. Consequently, the high-temperature phase, excluding the HA phase, increased. As seen in the CaO–P O phase diagram, the HA phase 2 5 was transformed into CaO, which is stable at high temperatures. The immersion test showed that a reaction took place at the interface between the coating and the substrate after prolonged immersion. Summing up, the graded coating must be applied to HA coatings for biomedical application.
References
Fig. 15. SEM photographs of surface of impedance-tested samples at variable potentials in Hank’s solution (×1000): (a) d.c. 600 mV and (b) d.c. 800 mV.
decreasing the adhesion with the bone. To improve adhesion strength with the bone, the HA coating’s compositional change must be minimized and the
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