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Materials Letters 62 (20

A heat treatment method for obtaining a bioactive cobalt base alloy

Juan Carlos Ortiz ⁎, Dora Cortés, José Escobedo, José Almanza

Cinvestav—Unidad Saltillo, Carr. Saltillo-Monterrey Km 13.5, A. P. 663, 25000 Saltillo, Coah, México

Received 6 July 2007; accepted 13 August 2007Available online 19 August 2007

Abstract

In the aim to promote bioactivity on a cobalt base alloy (ASTM F-75), as-cast substrates were heat treated in contact with bioactive material.Polished as-cast samples were encapsulated in a powder mixture of hydroxyapatite–bioactive glass (HA–BG). The heat treatment of theencapsulated metallic substrates was performed at 1220 °C for 1 h. For the in vitro bioactivity assessment, heat treated samples were immersed insimulated body fluid (SBF) for 21 days. In order to evaluate the effect of the heat treatment on the possible transformation of the HA–BG mixturea ceramic specimen was also prepared. The metallic surfaces, before and after immersion in SBF, were analyzed by Scanning Electron Microscopy(SEM), Energy Dispersive X-Ray analysis (EDX) and X-Ray Diffraction (XRD). Small ceramic agglomerates, composed mainly of Ca, P, O, Siand Na, were observed on the metallic surface before immersion in SBF. After 21 days, a continuous bone-like apatite layer was formed. This mayindicate that the heat treated material is potentially useful for bone replacement and tissue regeneration under highly loaded conditions. The HA–BG mixture transformed partially to β-tricalcium phosphate (β-TCP) after heat treatment.© 2007 Elsevier B.V. All rights reserved.

Keywords: Cobalt alloy; Bioactive materials; Heat treatment; Bioactivity

1. Introduction

Metallic biomaterials are widely used in the fabrication ofhip replacement prostheses due to their appropriate biocompat-ibility and high mechanical properties. However, these materialsare classified as bioinert materials since they form no chemicalbond with bone in vivo [1]. On the other hand, specific glassesin the system Na2O–CaO–SiO2–P2O5 and hydroxyapatite bondto living bone, but they cannot be used for high loadedapplications due to their low mechanical properties [2, 3].

An alternative to obtain an implant showing both bioactivityand appropriate mechanical properties is the hydroxyapatitecoating on metallic materials. For this purpose, several physicaland chemical techniques have been developed, such as plasmaspraying [4], electrophoretic deposition [5], thermal spraying[1], biomimetic process [6], sol–gel coating [7], hot isostatingpressing [8] and pre-calcification of the metallic surface [9].

⁎ Corresponding author. Tel.: +52 844 3489623; fax: +52 844 4389610.E-mail addresses: carlos.ortiz@cinvestav.edu.mx (J.C. Ortiz),

dora.cortes@cinvestav.edu.mx (D. Cortés), jose.escobedo@cinvestav.edu.mx(J. Escobedo), manuel.almanza@cinvestav.edu.mx (J. Almanza).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.08.029

Most of the physical techniques lead to coatings that differ instructure, crystallinity and composition to those of bone apatite[10].

The most common fabrication process of cobalt alloyimplants (specifically the ASTM F-75 alloy) is the investmentcasting technique followed by a heat treatment at 1220 °C. Thepurpose of this heat treatment is to obtain a homogeneouscarbides distribution in the cobalt matrix [11].

In this work, the feasibility to obtain a bioactive surface onthe ASTM F-75 alloy during heat treatment has beeninvestigated. Metallic samples were heat treated at 1200 °Cfor 1 h in contact with a mixture of 70wt% of hydroxyapatite–30wt% of bioactive glass. The aim of using this mixture was topromote an incipient melting of the glass and thus, a strongbond between the bioactive materials and the metallic surface.

2. Experimental

2.1. Specimen preparation

The Co base alloy samples were obtained by investmentcasting using a high induction furnace. The alloy was cast at

Fig. 1. Ceramic sample (70wt%HA–30wt%BG) after heat treatment: (A) SEM analysis (1000×), (B) EDX spectrum and (C) XRD patterns before and after heat treatment.

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1600 °C into investment preheated molds (980 °C). The as-castalloy was machined into disks with dimensions of 1.5 cm indiameter and 0.5 cm in thickness. The samples were groundwith SiC papers ranging from 80 to 2400 grit and polished using30 μm and 10 μm diamond pastes. After polishing, the sampleswere gently washed for 5 min in ethanol using an ultrasonicbath.

2.2. Bioactive mixtures

The bioactive glass (BG) used in this study was preparedfrom a mixture of reagent grade chemicals of SiO2, Na2O, CaOand P2O5 (Sigma-Aldrich). The aim was to obtain a glass with aclose composition to that of Bioglass® 45S5 [2]. The mixturewas heated at 1360 °C and poured in an ice-cooled stainlesssteel plate. The glass was crushed in a laboratory planetary-typeagate ball mill obtaining an average particle size of 36.75 μm.The hydroxyapatite (HA) powder (ALDRICH®, averageparticle size of 38.41 μm) was used as provided.

The HA–BG mixture was ball-milled in a polyethylene jarwith alumina balls for 5 h in acetone. The encapsulating process

was performed by placing the HA–BG mixture between thepolished areas of two metallic specimens. A uniaxial pressure of65 MPa was applied to the non-polished faces of the substrate.In the aim to avoid decarburation of the alloy, the specimen wasgenerously painted with graphite paste. Ceramic specimenswere also prepared from the HA–BG mixture and subjected tothe same uniaxial pressure (65 MPa).

2.3. Heat treatment

The encapsulated metallic specimens and the ceramicsamples were heat treated at 1220 °C for 1 h followed byquenching in water [11]. Finally, the specimens were washedwith deionized water, dried in air at 130 °C for 2 h and stored ina desiccator before testing.

2.4. In vitro bioactivity assessment

To assess bioactivity a simulated body fluid with an ionicconcentration nearly equal to that of human blood plasma (SBF)was prepared according to the procedure described by Kokubo

Fig. 2. Metallic sample encapsulated in 70wt%HA–30wt%BG: (A, B) SEM analysis after heat treatment; (C) EDX spectrum of the area shown inFig. 2B; (D, E) SEManalysis after 21 days of immersion in SBF; (F) EDX spectrum of the area shown inFig. 3E; (G) XRD pattern after immersion.

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[12]. Appropriate amounts of reagent grade chemicals of NaCl,NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2·2H2O,Na2SO4 and tris-hydroxymethyl aminomethane (CH2OH3-

CNH2) were dissolved into deionized water and buffered topH 7.25 at 36.5 °C with hydrochloric acid.

Each of the metallic substrates was immersed in 150 ml ofSBF for 21 days at 37 °C. After the immersion period, thesamples were rinsed with deionized water, dried and stored.

2.5. Characterization methods

The heat treated metallic samples, before and afterimmersion in SBF, as well as the ceramic specimens afterheat treatment, were analyzed by scanning electron microscopy,SEM (Philips, model XL 30 ESEM), energy dispersive X-ray

analysis, EDX (software Genesis de EDAX) and X-raydiffraction, XRD (Xpert Philips, model PW3040).

3. Results and discussion

3.1. Ceramic samples

Fig. 1 shows the SEM, EDX and XRD analysis of the ceramicsample (HA–BG) after heat treatment. A coalescence of the ceramicparticles is observed (Fig. 1A). The bioactive glass starts to melt closeto the heat treatment temperature (1270 °C) [13], therefore this glassmelts partially during heat treatment leading to a link among theceramic particles. According to the EDX analysis (Fig. 1B), elementssuch as Ca, P, Si, Na, O and Al were detected on the ceramic surface.The pick corresponding to Al is due to the alumina crucible used duringheat treatment.

Fig. 3. Cross-section analysis of the metallic sample after 21 days of immersion in SBF and corresponding EDX spectra.

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Fig. 1C shows the XRD patterns of the ceramic samples before andafter heat treatment. These patterns indicate the partial transformationof hydroxyapatite to β-tricalcium phosphate (β-TCP) after heattreatment. Theoretically, the hydroxyapatite transforms into β-TCP at900 °C and it is possible to find β-TCP and α-TCP above 1100 °C [3].

3.2. Metallic samples after heat treatment and after bioactivitycharacterization

Fig. 2 shows the SEM, EDX and XRD analysis of the metallicsurfaces after heat treatment and after immersion in SBF. As observed,ceramic agglomerates were attached to the oxide layer during heattreatment (Fig. 2A). Fig. 2B shows that the agglomerates are formed byfine particles with spherical morphology. The corresponding EDXspectrum (Fig. 2C) indicates that the ceramic phase was mainlycomposed of Ca, P, Si and Na. According to the analysis of the ceramicspecimen, HA and β-TCP can be found after heat treatment (Fig. 1C).Thus, during heat treatment agglomerates of HA and β-TCP can betrapped. Alloying elements (Cr and Co) were also detected, as well asAl from the alumina crucible.

Figs. 2D and E show the alloy surface after 21 days of immersion inSBF. A homogeneous ceramic layer was formed on the metallicsurface. Particles with similar morphology to those formed on theexisting bioactive systems were observed at higher magnifications(Fig. 2E). The corresponding EDX spectrum (Fig. 2F) indicates thatCa, P and O were the main elements of the ceramic layer. The Ca/Pratio was 1.38 which is in the range to the biological apatites (1.2–1.6)[14]. The pick corresponding to Mg in the EDX spectrum indicates thepartial substitution of Ca by Mg in the crystalline structure due to thepresence of Mg in the SBF [14]. This phenomenon is common whenapatite is synthesized in vitro using simulated body fluids. Sodium wasalso detected, due to its presence in the bioactive mixture. After21 days, the alloying elements were not longer detected by EDX.

Fig. 2G shows the XRD analysis of the metallic specimen after21 days of immersion in SBF. The ceramic layer was identified as apatiteby XRD, however the alloying elements were also detected.

Fig. 3 shows a cross-section analysis of a heat treated metallicsample after 21 days of immersion in SBF. The micrograph indicatesthat the thickness of the apatite layer was about 15 μm. The different

regions implicated on the system were analyzed by EDX (punctualanalysis on the center of each region). As observed, the region identifiedas (1) corresponds to the alloy; region (2) corresponds to a Cr2O3 andCoCr2O4 layer, which was detected by XRD (Fig. 2G); region (3) isformed by a mixture of oxides and bioactive material and finally, region(4) corresponds to the apatite layer.

According to these results, during heat treating of the HA–BG-encapsulated metallic samples, oxidation of the metallic surface takesplace, which leads to the trapping of ceramic agglomerates from thepartially melted ceramic mixture. As HA transforms to β-TCP, a severeoxidation occurs due to the water formed as a by-product of thedecomposition reaction [3]. This thick oxide layer (≈8 μm) withembedded ceramic particles may be detached during quenching due tothe expansion coefficients of the different materials involved. Eventhough, this layer is bioactive, it promotes the formation of a dense andcontinuous apatite layer with a variable thickness. According to theEDX spectrum of region (3), Ca is being released into the SBF, leadingto a Si-rich surface on the ceramic material and a Ca-rich SBF solution.Under these conditions the nucleation and growth of apatite occurs.

4. Conclusions

The method presented in this research seems to be promissoryto achieve a bioactive cobalt alloy. Ceramic agglomerates can beattached in the oxide layer during the conventional heat treatmentof this particular alloy. After immersion in SBF for 21 days, anapatite layer (≈15 μm) was formed on the oxide layer.

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