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Hydroxyapatite coating on a cobalt base alloy by investment casting
Scripta Materialia 54 (2006) 1611–1615 www.actamat-journals.com
Hydroxyapatite coating on a cobalt base alloy by investment casting J.C. Escobedo *, J.C. Ortiz, J.M. Almanza, D.A. Corte´s CINVESTAV Unidad Saltillo, Carr. Saltillo-Monterrey Km 13.5 A.P. 663, C.P. 25000, Saltillo, Coah, Mexico Received 7 September 2005; received in revised form 5 December 2005; accepted 14 December 2005 Available online 3 February 2006
Abstract A cobalt alloy was cast into preheated molds previously coated with hydroxyapatite powder. Two molds were used, one made of investment material and the other of pure graphite. Selected samples were heat treated. Both heat and non heat treated samples were immersed in simulated body fluid for 21 days at 37 °C. A ceramic layer, identified as hydroxyapatite, was formed on all the samples. A thicker layer was formed on the sample cast into the investment mold without heat treatment. A chemical interaction between the investment mold and hydroxyapatite takes place leading to a higher in vitro bioactivity. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Coating; Casting; Biomaterials; Hydroxyapatite; Cobalt alloys
1. Introduction Titanium and cobalt alloys have been widely used as hip implants under load-bearing conditions, due to their appropriate mechanical properties. However, they do not bond spontaneously to living bone [1,2]. Metallic implants are coated with a layer of a bioactive material to improve bone bonding ability [3]. Alloys coated with hydroxyapatite by plasma spraying method are already in clinical use. However, the hydroxyapatite of the coating layer differs from the bone apatite in crystallinity, structure and composition [4]. Biomimetic methods promote a bioactive surface on different materials [5]. By using biomimetic methods the feasibility of forming a bonelike apatite layer on the surface of titanium, titanium alloys, tantalum and cobalt alloys has recently been demonstrated [2,6,7]. In the case of metallic substrates, biomimetic methods lead to better results on titanium alloys than on cobalt alloys due to the different surface characteristics of these materials. *
Corresponding author. Tel.: +52 844 4389600x9623; fax: +52 844 4389610. E-mail address: [email protected] (J.C. Escobedo).
Thus, the bonelike apatite layer formed on the surface of cobalt alloys is thinner and less adherent than that observed on the surface of titanium alloys. Recently, an original surface treatment to improve the bioactivity of titanium was reported. In the study, pure titanium was cast into a graphite mold in which the cavities were coated with hydroxyapatite powder. The obtained specimens, with ceramic particles identified as hydroxyapatite and CaO embedded on the surface, was more bioactive than pure titanium [8]. The possibility of applying a similar treatment to cobalt alloys is interesting for several reasons: (i) almost all cobalt alloy prostheses are manufactured by investment casting; (ii) in spite of the fact that titanium alloys are widely used in the manufacture of hip prostheses due to its high biocompatibility, cobalt hip implants are preferred because of their wear behavior, which reduces the contact stresses with the ultra high molecular weight polyethylene acetabular cup [9]; (iii) this method promises to improve the adherence of the hydroxyapatite coating formed after the application of a biomimetic method, leading to the possibility of implanting this prosthesis without the use of any cement. However, some aspects have to be considered: the interaction between hydroxyapatite and investment
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.12.059
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mold materials during the casting and the effect of the heat treatment to which the cobalt alloy prostheses are subjected after the casting. The main aim of this work was the production, by using the investment casting technique, of the cobalt base alloy samples (alloy specified as F75 according to the American Society for Testing and Materials, ASTM F-75) with hydroxyapatite embedded in its surface and the evaluation of the in vitro bioactivity of the cast samples using simulated body fluid (SBF). 2. Materials and methods 2.1. Specimen preparation The molds were designed to obtain 20 specimens of 1.5 cm in diameter and 0.5 cm in height. The shell investment mold fabrication procedure was according to the supplier’s instructions (Ransom and Randolph). In order to study the alloy–mold interaction, one mold was made of investment material (50% ZrSiO4, 50% SiO2) and a second mold was made of investment material with cavity walls of pure graphite. Before assembling the molds, the cavity walls were coated, using a paintbrush, with water moisturized hydroxyapatite (Aldrich, with an average particle size of 38.41 lm). The mold was dried at 200 °C. The cobalt alloy ASTM F-75 was melted in a high frequency vacuum furnace (INDUCTOHERM, Power Track 30) under an ultra high purity argon atmosphere (84099.77 Pa) to avoid excessive oxidation and poured at 1600 °C into the preheated molds (980 °C). Selected cast specimens were subjected to a heat treatment at 1220 °C for 1 h in air followed by water quenching [10]. This heat treatment of partial dissolution of carbides is necessary to improve the ductility of the alloy. In order to avoid excessive oxidation and decarburizing of the specimens during the heat treatment, their surface was protected with a layer of hydroxyapatite and then the specimen was coated with graphite paste. After the thermal treatment, the specimens were cleaned with compressed dry air. 2.2. Immersion of the specimens in simulated body fluid A solution with an ionic concentration nearly equal to that of human blood plasma (SBF) was prepared. The solution was prepared by dissolving appropriate amounts of reagent grade chemicals of sodium chloride (NaCl), sodium hydrogen carbonate (NaHCO3), potassium chloride (KCl), dipotassium hydrogen phosphate (K2HPO4 Æ 3H2O), magnesium chloride hexahydrate (MgCl2 Æ 6H2O), calcium chloride dihydrate (CaCl2 Æ 2H2O), sodium sulfate (Na2SO4) and tris-hydroxymethyl aminomethane ((CH2OH)3CNH2) into deionized water and buffered to pH 7.25 at 36.5 °C with hydrochloric acid [11]. Each specimen was immersed in 150 ml of SBF solution at 37 °C for 21 days. The solution was renewed every 7 days.
2.3. Characterization methods The surface of the metallic samples was analyzed by using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). 3. Results and discussion Fig. 1A shows the surface of the sample cast into an investment mold in which the cavity walls were coated with hydroxyapatite (HA) powder before casting. It was possible to observe an appreciable amount of ceramic particles on the surface of the sample. A transverse view of the sample (Fig. 1B) showed that the particles were embedded in the surface. The EDX analysis of the ceramic particles (Fig. 1C) showed the presence of Ca, P, Si and Zr. The value of the Ca/P ratio was 2.2. The presence of Si and Zr indicated that investment material was present in the ceramic particles. Fig. 1D shows the surface of the sample cast into a graphite mold in which the cavity walls were coated with HA powder before casting. In this case, the observed ceramic particles are also embedded in the surface (Fig. 1E). The EDX analysis of the ceramic particles (Fig. 1F) showed the presence of Ca and P (Ca/P ratio = 1.9). Fig. 2 shows the effect of heat treatment on the ceramic particles embedded in the surface of the sample cast into an investment mold. After heat treatment the ceramic particles were partially sintered (Fig. 2B). The EDX analysis indicated, according to the value of the Ca/P ratio (2.7), that some chemical changes occurred in the ceramic particles after heat treatment. A similar behavior was observed on the ceramic particles embedded in the surface of the heat treated samples cast into the graphite mold. Fig. 3 shows the surface of the sample cast into an investment mold in which the cavity walls were coated with HA (A–C) and the surface of the sample cast into a graphite mold in which the cavity walls were coated with HA (D–F), both after 21 days of immersion in SBF. After this period of immersion a homogeneous ceramic layer was observed on the samples. The EDX analysis of the layers showed Ca/P ratios of 1.57 and 1.78, respectively, corresponding to a substituted apatite. Fig. 4 shows the surface of the sample cast into an investment mold in which the cavity walls were coated with HA and heat treated (A–C) and the surface of the sample cast into a graphite mold in which the cavity walls were coated with HA and heat treated (D–F), both after 21 days of immersion in SBF. After this period of immersion a ceramic layer was observed on the samples. The EDX analysis of the layers showed the presence of Ca and P with Ca/ P ratios of 1.51 and 1.40, respectively. It was observed that the more homogeneous and thicker layer was formed on the sample cast into the investment mold in which the cavity walls were coated with HA without heat treatment, while the more inhomogeneous and thinner layer was formed on the sample cast into the graphite mold in which the cavity walls were coated with HA and heat treated.
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Fig. 1. A–C: sample cast into an investment mold coated with HA; D–F: sample cast into a graphite mold coated with HA. A and D: surfaces; B and E: transverse views; C and F: EDX elemental spectra.
Fig. 2. Sample cast into an investment mold coated with HA; A: before and B: after the heat treatment, C: EDX elemental spectrum of the ceramic particles after heat treatment.
Fig. 5A–D shows the XRD patterns of the surface of the samples after 21 days of immersion in SBF. It is possible to observe from these patterns the effect of the mold material and heat treatment on the characteristics of the ceramic layers. Peaks corresponding to HA can be observed for all the samples analyzed. The higher intensity HA peaks were observed in the pattern corresponding to the sample cast into the investment mold in which the cavity walls were coated with HA (Fig. 5A) and the lower intensity HA peaks were observed in the pattern corresponding to the sample cast into the graphite mold in which the cavity walls were coated with HA and heat treated (Fig. 5D). Peaks corresponding to the metallic alloy were not observed in the pattern corresponding to the sample cast into the investment mold in which the cavity walls were coated with HA (Fig. 5A). The intensities of HA peaks in the XRD patterns are proportional to the thickness of the layer formed on the surface. The amount of metallic oxides increased on the heat treated samples (Fig. 5B and
C). The results obtained using XRD are in agreement with those obtained by SEM and EDX. In the temperature range 950–1500 °C, commercial HA can form the following calcium phosphates: HA, b-tricalcium phosphate (TCP), a-TCP, tetracalcium phosphate (TTCP) and oxyapatite. Additionally, carbonated HA can form CaO and HA [12]. The purity, composition and particle size of the apatite before the heat treatment, the heat treatment temperature and conditions (water pressure, time of treatment, etc.) have an effect on the type and amount of phases formed. When the molten alloy was poured into the mold, HA reached a temperature close to 1600 °C for a very short period of time, forming b-TCP, a-TCP and CaO. The presence of tricalcium phosphate was confirmed by XRD (Fig. 5A). The Ca/P measured ratio of the ceramic particles after pouring was higher than 1.67, confirming the presence of CaO. After heat treatment (1220 °C, 1 h), the percentage of transformation rises dramatically.
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Fig. 3. A–C: sample cast into an investment mold coated with HA after 21 days of immersion in SBF; D–F: sample cast into a graphite mold coated with HA after 21 days of immersion in SBF. A, B, D and E: surfaces; C and F: EDX spectra of the ceramic layers.
Fig. 4. A–C: sample cast into an investment mold coated with HA and heat treated after 21 days of immersion in SBF; D–F: sample cast into a graphite mold coated with HA and heat treated after 21 days of immersion in SBF. A, B, D and E: surfaces; C and F: EDX spectra of the ceramic layers.
In addition to the new phases formed due to the effect of the heat treatment on commercial HA, a chemical interaction of HA and its decomposition products with the investment mold and the alloy takes place. Water is formed from HA and it interacts with elements of the alloy, forming metallic oxides. The formation of Cr2O3 and Cr3O4 after heat treatment was confirmed by XRD (Fig. 5B and D). HA and its decomposition products also interact with the investment mold materials (50% ZrSiO4, 50% SiO2).
According to the thermodynamic calculations (Eqs. (1) and (2)), the formation of calcium silicate from CaO and ZrSiO4 or SiO2 is possible. CaO þ ZrSiO4 ! CaSiO3 þ ZrO2 DG1600 C ¼ 22:58 Kcal ð1Þ CaO þ SiO2 ! CaSiO3
DG1600 C ¼ 21:36 Kcal
ð2Þ
The presence of CaSiO3 was confirmed by XRD (Fig. 5A).
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Fig. 5. XRD patterns of samples before and after immersion in SBF. INV: cast in an investment mold; HT: heat treated; G: cast in a graphite mold; 21SBF: immersion for 21 days in SBF.
The detrimental effect of the heat treatment at 1220 °C (1 h) on the bioactivity of the alloys is due to the decomposition of HA into compounds such as b-TCP, a-TCP, TTCP and CaO, compounds with less bioactivity than HA. The positive effect of using investment molds instead of graphite molds can be attributed to the presence or formation of compounds containing silicon such as SiO2, Ca2SiO4 and CaSiO3. In spite of there not being evidence of the presence of silanol groups (Si–OH) at the SBF– compound interface which act as nucleating sites for HA, it has been reported that these groups are present in systems that contain SiO2 [5,13]. 4. Conclusions A cobalt base alloy was cast into a mold in which the cavities were coated with hydroxyapatite powder. Bioactive ceramic particles were embedded on the surface of the alloy. A dense and homogeneous bonelike apatite layer was formed on the as cast cobalt alloy surface after 21 days of immersion in SBF. A decrease in bioactivity was
observed in the heat treated samples due to the decomposition of HA. References [1] Noort RV. J Mater Sci 1987;22:3801. [2] Miyazaki T, Kim HM, Miyaji F, Kokubo T, Nakamura T. Bioceramics 10. Oxford: Elsevier Science; 1997. p. 11. [3] Hench LL. Mater Sci Forum 1999;293:37. [4] Sun L. J Biomed Mater Res 2001;58:570. [5] Abe Y, Kokubo T. J Mater Sci Mater Med 1990;1:233. [6] Kim HM, Miyaji F, Kokubo T, Nakamura T. J Ceram Soc Jpn 1997;105:111. [7] Cortes DA, Escobedo JC, Nogiwa A, Mun˜oz R. Mater Sci Forum 2003;442:61. [8] Sohmura T, Tamasaki H, Ohara T, Takahasi J. J Biomed Mater Res 2001;58:478. [9] Long M, Rack HJ. Biomaterials 1998;19:1621. [10] Caudillo M, Herrera-Trejo M, Castro MR, Ramı´rez E, Gonza´lez CR, Jua´rez JI. J Biomed Mater Res 2001;59:378. [11] Kokubo T, Kushitani H, Sakka S. J Biomed Mater Res 1990;24:721. [12] De Groot K, Klein CPAT, Wolke JGC, de Bliek-Hogervost JMA, editors. Handbook of bioactive bioceramics, vol. 2. Boca Rato´n, FL: CRC Press; 1990. p. 3. [13] De Aza PN, Guitian F, De Aza S. Scripta Mater 1994;31:1001.
Hydroxyapatite coating on a cobalt base alloy by investment casting J.C. Escobedo *, J.C. Ortiz, J.M. Almanza, D.A. Corte´s CINVESTAV Unidad Saltillo, Carr. Saltillo-Monterrey Km 13.5 A.P. 663, C.P. 25000, Saltillo, Coah, Mexico Received 7 September 2005; received in revised form 5 December 2005; accepted 14 December 2005 Available online 3 February 2006
Abstract A cobalt alloy was cast into preheated molds previously coated with hydroxyapatite powder. Two molds were used, one made of investment material and the other of pure graphite. Selected samples were heat treated. Both heat and non heat treated samples were immersed in simulated body fluid for 21 days at 37 °C. A ceramic layer, identified as hydroxyapatite, was formed on all the samples. A thicker layer was formed on the sample cast into the investment mold without heat treatment. A chemical interaction between the investment mold and hydroxyapatite takes place leading to a higher in vitro bioactivity. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Coating; Casting; Biomaterials; Hydroxyapatite; Cobalt alloys
1. Introduction Titanium and cobalt alloys have been widely used as hip implants under load-bearing conditions, due to their appropriate mechanical properties. However, they do not bond spontaneously to living bone [1,2]. Metallic implants are coated with a layer of a bioactive material to improve bone bonding ability [3]. Alloys coated with hydroxyapatite by plasma spraying method are already in clinical use. However, the hydroxyapatite of the coating layer differs from the bone apatite in crystallinity, structure and composition [4]. Biomimetic methods promote a bioactive surface on different materials [5]. By using biomimetic methods the feasibility of forming a bonelike apatite layer on the surface of titanium, titanium alloys, tantalum and cobalt alloys has recently been demonstrated [2,6,7]. In the case of metallic substrates, biomimetic methods lead to better results on titanium alloys than on cobalt alloys due to the different surface characteristics of these materials. *
Corresponding author. Tel.: +52 844 4389600x9623; fax: +52 844 4389610. E-mail address: [email protected] (J.C. Escobedo).
Thus, the bonelike apatite layer formed on the surface of cobalt alloys is thinner and less adherent than that observed on the surface of titanium alloys. Recently, an original surface treatment to improve the bioactivity of titanium was reported. In the study, pure titanium was cast into a graphite mold in which the cavities were coated with hydroxyapatite powder. The obtained specimens, with ceramic particles identified as hydroxyapatite and CaO embedded on the surface, was more bioactive than pure titanium [8]. The possibility of applying a similar treatment to cobalt alloys is interesting for several reasons: (i) almost all cobalt alloy prostheses are manufactured by investment casting; (ii) in spite of the fact that titanium alloys are widely used in the manufacture of hip prostheses due to its high biocompatibility, cobalt hip implants are preferred because of their wear behavior, which reduces the contact stresses with the ultra high molecular weight polyethylene acetabular cup [9]; (iii) this method promises to improve the adherence of the hydroxyapatite coating formed after the application of a biomimetic method, leading to the possibility of implanting this prosthesis without the use of any cement. However, some aspects have to be considered: the interaction between hydroxyapatite and investment
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.12.059
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mold materials during the casting and the effect of the heat treatment to which the cobalt alloy prostheses are subjected after the casting. The main aim of this work was the production, by using the investment casting technique, of the cobalt base alloy samples (alloy specified as F75 according to the American Society for Testing and Materials, ASTM F-75) with hydroxyapatite embedded in its surface and the evaluation of the in vitro bioactivity of the cast samples using simulated body fluid (SBF). 2. Materials and methods 2.1. Specimen preparation The molds were designed to obtain 20 specimens of 1.5 cm in diameter and 0.5 cm in height. The shell investment mold fabrication procedure was according to the supplier’s instructions (Ransom and Randolph). In order to study the alloy–mold interaction, one mold was made of investment material (50% ZrSiO4, 50% SiO2) and a second mold was made of investment material with cavity walls of pure graphite. Before assembling the molds, the cavity walls were coated, using a paintbrush, with water moisturized hydroxyapatite (Aldrich, with an average particle size of 38.41 lm). The mold was dried at 200 °C. The cobalt alloy ASTM F-75 was melted in a high frequency vacuum furnace (INDUCTOHERM, Power Track 30) under an ultra high purity argon atmosphere (84099.77 Pa) to avoid excessive oxidation and poured at 1600 °C into the preheated molds (980 °C). Selected cast specimens were subjected to a heat treatment at 1220 °C for 1 h in air followed by water quenching [10]. This heat treatment of partial dissolution of carbides is necessary to improve the ductility of the alloy. In order to avoid excessive oxidation and decarburizing of the specimens during the heat treatment, their surface was protected with a layer of hydroxyapatite and then the specimen was coated with graphite paste. After the thermal treatment, the specimens were cleaned with compressed dry air. 2.2. Immersion of the specimens in simulated body fluid A solution with an ionic concentration nearly equal to that of human blood plasma (SBF) was prepared. The solution was prepared by dissolving appropriate amounts of reagent grade chemicals of sodium chloride (NaCl), sodium hydrogen carbonate (NaHCO3), potassium chloride (KCl), dipotassium hydrogen phosphate (K2HPO4 Æ 3H2O), magnesium chloride hexahydrate (MgCl2 Æ 6H2O), calcium chloride dihydrate (CaCl2 Æ 2H2O), sodium sulfate (Na2SO4) and tris-hydroxymethyl aminomethane ((CH2OH)3CNH2) into deionized water and buffered to pH 7.25 at 36.5 °C with hydrochloric acid [11]. Each specimen was immersed in 150 ml of SBF solution at 37 °C for 21 days. The solution was renewed every 7 days.
2.3. Characterization methods The surface of the metallic samples was analyzed by using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). 3. Results and discussion Fig. 1A shows the surface of the sample cast into an investment mold in which the cavity walls were coated with hydroxyapatite (HA) powder before casting. It was possible to observe an appreciable amount of ceramic particles on the surface of the sample. A transverse view of the sample (Fig. 1B) showed that the particles were embedded in the surface. The EDX analysis of the ceramic particles (Fig. 1C) showed the presence of Ca, P, Si and Zr. The value of the Ca/P ratio was 2.2. The presence of Si and Zr indicated that investment material was present in the ceramic particles. Fig. 1D shows the surface of the sample cast into a graphite mold in which the cavity walls were coated with HA powder before casting. In this case, the observed ceramic particles are also embedded in the surface (Fig. 1E). The EDX analysis of the ceramic particles (Fig. 1F) showed the presence of Ca and P (Ca/P ratio = 1.9). Fig. 2 shows the effect of heat treatment on the ceramic particles embedded in the surface of the sample cast into an investment mold. After heat treatment the ceramic particles were partially sintered (Fig. 2B). The EDX analysis indicated, according to the value of the Ca/P ratio (2.7), that some chemical changes occurred in the ceramic particles after heat treatment. A similar behavior was observed on the ceramic particles embedded in the surface of the heat treated samples cast into the graphite mold. Fig. 3 shows the surface of the sample cast into an investment mold in which the cavity walls were coated with HA (A–C) and the surface of the sample cast into a graphite mold in which the cavity walls were coated with HA (D–F), both after 21 days of immersion in SBF. After this period of immersion a homogeneous ceramic layer was observed on the samples. The EDX analysis of the layers showed Ca/P ratios of 1.57 and 1.78, respectively, corresponding to a substituted apatite. Fig. 4 shows the surface of the sample cast into an investment mold in which the cavity walls were coated with HA and heat treated (A–C) and the surface of the sample cast into a graphite mold in which the cavity walls were coated with HA and heat treated (D–F), both after 21 days of immersion in SBF. After this period of immersion a ceramic layer was observed on the samples. The EDX analysis of the layers showed the presence of Ca and P with Ca/ P ratios of 1.51 and 1.40, respectively. It was observed that the more homogeneous and thicker layer was formed on the sample cast into the investment mold in which the cavity walls were coated with HA without heat treatment, while the more inhomogeneous and thinner layer was formed on the sample cast into the graphite mold in which the cavity walls were coated with HA and heat treated.
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Fig. 1. A–C: sample cast into an investment mold coated with HA; D–F: sample cast into a graphite mold coated with HA. A and D: surfaces; B and E: transverse views; C and F: EDX elemental spectra.
Fig. 2. Sample cast into an investment mold coated with HA; A: before and B: after the heat treatment, C: EDX elemental spectrum of the ceramic particles after heat treatment.
Fig. 5A–D shows the XRD patterns of the surface of the samples after 21 days of immersion in SBF. It is possible to observe from these patterns the effect of the mold material and heat treatment on the characteristics of the ceramic layers. Peaks corresponding to HA can be observed for all the samples analyzed. The higher intensity HA peaks were observed in the pattern corresponding to the sample cast into the investment mold in which the cavity walls were coated with HA (Fig. 5A) and the lower intensity HA peaks were observed in the pattern corresponding to the sample cast into the graphite mold in which the cavity walls were coated with HA and heat treated (Fig. 5D). Peaks corresponding to the metallic alloy were not observed in the pattern corresponding to the sample cast into the investment mold in which the cavity walls were coated with HA (Fig. 5A). The intensities of HA peaks in the XRD patterns are proportional to the thickness of the layer formed on the surface. The amount of metallic oxides increased on the heat treated samples (Fig. 5B and
C). The results obtained using XRD are in agreement with those obtained by SEM and EDX. In the temperature range 950–1500 °C, commercial HA can form the following calcium phosphates: HA, b-tricalcium phosphate (TCP), a-TCP, tetracalcium phosphate (TTCP) and oxyapatite. Additionally, carbonated HA can form CaO and HA [12]. The purity, composition and particle size of the apatite before the heat treatment, the heat treatment temperature and conditions (water pressure, time of treatment, etc.) have an effect on the type and amount of phases formed. When the molten alloy was poured into the mold, HA reached a temperature close to 1600 °C for a very short period of time, forming b-TCP, a-TCP and CaO. The presence of tricalcium phosphate was confirmed by XRD (Fig. 5A). The Ca/P measured ratio of the ceramic particles after pouring was higher than 1.67, confirming the presence of CaO. After heat treatment (1220 °C, 1 h), the percentage of transformation rises dramatically.
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Fig. 3. A–C: sample cast into an investment mold coated with HA after 21 days of immersion in SBF; D–F: sample cast into a graphite mold coated with HA after 21 days of immersion in SBF. A, B, D and E: surfaces; C and F: EDX spectra of the ceramic layers.
Fig. 4. A–C: sample cast into an investment mold coated with HA and heat treated after 21 days of immersion in SBF; D–F: sample cast into a graphite mold coated with HA and heat treated after 21 days of immersion in SBF. A, B, D and E: surfaces; C and F: EDX spectra of the ceramic layers.
In addition to the new phases formed due to the effect of the heat treatment on commercial HA, a chemical interaction of HA and its decomposition products with the investment mold and the alloy takes place. Water is formed from HA and it interacts with elements of the alloy, forming metallic oxides. The formation of Cr2O3 and Cr3O4 after heat treatment was confirmed by XRD (Fig. 5B and D). HA and its decomposition products also interact with the investment mold materials (50% ZrSiO4, 50% SiO2).
According to the thermodynamic calculations (Eqs. (1) and (2)), the formation of calcium silicate from CaO and ZrSiO4 or SiO2 is possible. CaO þ ZrSiO4 ! CaSiO3 þ ZrO2 DG1600 C ¼ 22:58 Kcal ð1Þ CaO þ SiO2 ! CaSiO3
DG1600 C ¼ 21:36 Kcal
ð2Þ
The presence of CaSiO3 was confirmed by XRD (Fig. 5A).
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Fig. 5. XRD patterns of samples before and after immersion in SBF. INV: cast in an investment mold; HT: heat treated; G: cast in a graphite mold; 21SBF: immersion for 21 days in SBF.
The detrimental effect of the heat treatment at 1220 °C (1 h) on the bioactivity of the alloys is due to the decomposition of HA into compounds such as b-TCP, a-TCP, TTCP and CaO, compounds with less bioactivity than HA. The positive effect of using investment molds instead of graphite molds can be attributed to the presence or formation of compounds containing silicon such as SiO2, Ca2SiO4 and CaSiO3. In spite of there not being evidence of the presence of silanol groups (Si–OH) at the SBF– compound interface which act as nucleating sites for HA, it has been reported that these groups are present in systems that contain SiO2 [5,13]. 4. Conclusions A cobalt base alloy was cast into a mold in which the cavities were coated with hydroxyapatite powder. Bioactive ceramic particles were embedded on the surface of the alloy. A dense and homogeneous bonelike apatite layer was formed on the as cast cobalt alloy surface after 21 days of immersion in SBF. A decrease in bioactivity was
observed in the heat treated samples due to the decomposition of HA. References [1] Noort RV. J Mater Sci 1987;22:3801. [2] Miyazaki T, Kim HM, Miyaji F, Kokubo T, Nakamura T. Bioceramics 10. Oxford: Elsevier Science; 1997. p. 11. [3] Hench LL. Mater Sci Forum 1999;293:37. [4] Sun L. J Biomed Mater Res 2001;58:570. [5] Abe Y, Kokubo T. J Mater Sci Mater Med 1990;1:233. [6] Kim HM, Miyaji F, Kokubo T, Nakamura T. J Ceram Soc Jpn 1997;105:111. [7] Cortes DA, Escobedo JC, Nogiwa A, Mun˜oz R. Mater Sci Forum 2003;442:61. [8] Sohmura T, Tamasaki H, Ohara T, Takahasi J. J Biomed Mater Res 2001;58:478. [9] Long M, Rack HJ. Biomaterials 1998;19:1621. [10] Caudillo M, Herrera-Trejo M, Castro MR, Ramı´rez E, Gonza´lez CR, Jua´rez JI. J Biomed Mater Res 2001;59:378. [11] Kokubo T, Kushitani H, Sakka S. J Biomed Mater Res 1990;24:721. [12] De Groot K, Klein CPAT, Wolke JGC, de Bliek-Hogervost JMA, editors. Handbook of bioactive bioceramics, vol. 2. Boca Rato´n, FL: CRC Press; 1990. p. 3. [13] De Aza PN, Guitian F, De Aza S. Scripta Mater 1994;31:1001.